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全球环流 热带气象学概论 第三章

3.0 综述

Three-cell model of the general circulation of the atmosphere

本章以回顾大气运动的一般规律开始,期间还对热带的大气运动进行了尺度分析。

本章叙述了主要的大气环流和海洋环流,同时也概述了平流层大气环流。 大气环流模型的研究是本章的一项重点部分。我们在热量响应理论的框架下对热带环流进行研究。本章对季风气候,其概念模型,季节性变化和变率进行了探讨。本章着重讨论了哈德来环流的维持、季节性迁移、北半球和南半球的差异,以及热带和中纬度地区风场的对比。

学习目的

学习完本章后,你应该理解并掌握:

  • 基本的运动方程和连续方程
  • 掌握静力方程hydrostatic equation,压高公式hypsometric equation和热力学能量方程thermodynamic energy equation
  • 在自然坐标系中,利用流线和等风速线估算散度和涡度
  • 理解基本的低纬度不同尺度系统和低纬度地区的平衡关系
  • 描述全球环流的各种模式和产生这些模式的机制
  • 描述平流层的大气环流状况
  • 描述热带对流层顶和热带深对流系统在全球规模的化学输送上的作用
  • 识别热带和亚热带半永久性的高低压系统
  • 掌握季节性迁移的热带环流系统和不同半球之间的差异
  • 理解和描述大气运动在热带和中纬度地区的异同
  • 了解维持哈德来环流的机制和它的纬度范围
  • 描述热带辐合带对大气环流的作用以及影响ITCZ位置的机制
  • 描述埃克曼输送和海洋上的上升流
  • 掌握全球海洋上层海水的流动机制和主要洋流
  • 描述当前的季风概念模型
  • 理解在热带因热力加热特征不同引起的哈德来环流和沃克环流的基础理论
  • 理解和描述亚洲季风的演变
  • 比较/对比亚洲季风与澳大利亚,非洲、美洲季风系统的异同点。
  • 描述全球季风年际变化的主要模式及其振荡的一些影响因素
  • 描述导致季节变率和全球季风间断周期的主要的影响因素
  • 描述大气环流模式的主要组成部分

 

3.1大气运动的一般规律

3.1.1大尺度运动的受力平衡方程

大气和海洋的运动规律,可以从牛顿第二定律(力=质量×加速度)出发。气象学家关注的是空气块的加速度和单位质量的力的变化率,通过对气块的受力分析,推导出了纳维-斯托克斯方程Navier-Stokes equations,这是一个跟时间的变化率有关的方程,该方程组适用于空气和水等流体。大气的受力有气压梯度力(力的指向是从高压指向低压),再加上地球的旋转效应,摩擦力和重力,旋转行星上的流体运动加速度可以改写为:

加速度=压力梯度+科里奥利力+有效重力+摩擦力

equation(1)

其中U是速度矢量(m s-1) ;p是压力(Pa) ρ是密度(kg m-3) f是科氏参数 (f = 2ΩsinΦ, 地球旋转速度的垂直分量)。Φ是地球纬度,g是有效重力(地球的重力和离心力的矢量和)。

科氏力产生的加速度与速度方向垂直(在北半球指向速度方向的右侧,在南半球指向运动方向的左侧),摩擦力Fr与运动方向相反。个别变化率D/Dt是局地变化率与平流变化率的总和equation

Schematic of horizontal forces acting on air parcels near the surface and above the friction layer.
图3.1 地面附近气块和摩擦层以上气块水平作用力示意图。

摩擦层以上(自由大气),平衡是由压力梯度力和科里奥利力维持的,这里实际风与地转风近似equation,风力的计算可由以上的平衡公式得出。 在地转平衡中,风沿着平行的等压线吹,这意味着这些风向也平行于其他场(密度、温度场)。 这个地转近似适合于赤道附近以外的大尺度运动。a而向下来到地表,由于摩擦力的增加,地表风不再是平行于压力场,而是向压力较低的地方倾斜(图 3.1,下)。

 

由气压梯度力,科氏力,向心力(负离心力)的共同作用,流体平行于高/低压中心的等压线流动。在这些流动的中心附近地区,地转风平衡近似对于实际风来说不够理想,因为这里存在弯曲的流动。由这三个力平衡得到的方程,被称为梯度风。虽然梯度风平衡只出现在流动中心附近,梯度风可被认为是可忽略摩擦力的气层上的一种近似结果,一般这一层距地面约2000英尺(600m)。

 
垂直加速度可表示为 加速度=压力梯度+重力

equation (2)

其中w是垂直速度,g是重力加速度。因为压力随高度降低,在右边的第一项是正的,-g是负的,因此产生的垂直加速度是由于他们的相对大小决定。

在气象学上的基本原则之一是守恒,在系统中除非有外部的源和汇,系统的质量是守恒的。能量守恒角动量守恒已经在第一章予以介绍,现在我们将重点关注到质量守恒,它可以表示成连续方程的形式。

Schematic representation of the continuity principle
图 3.2.连续性原理的示意图

 

在图3.2中提到了连续性原理的示意图。考虑了空气的流速、空气的流量和密度ρ。在u方向上单位面积通过的流量(质量)为ρu ,则通过A面的总流量为ρu δy δz. A面与B面之间的质量积累(相距ΔX的距离)大约是

equation (3)

其中δx δy δz 是固定体积的流体中观察到的。

因此通过体积δx δy δz的分量合成,单位体积的净流量率可表示为equation 其中U是三个方向的合速度。

由于,质量是守恒的,质量可以从系统中流出和流进,扣除这些流入流出后的就是局地时间的密度变化率equation,因此,连续方程为

equation (4a)

方程(4a)是流量形式的连续方程。展开梯度算子并除以密度ρ,(4a)改写为

equation (4b)b

这是第二形式的连续方程,被称为平流形式,

 

连续方程是一个有用的预测工具,它把密度的变化率与辐散速率联系起来。如果空气是不可压缩的,在时间变化率上它的密度是常数,单位质量的水平辐散可以通过把方程扩展成其为水平和垂直部分的连续方程计算得到。

equation (5)

因此,很明显,对于不可压缩的流体,随高度在垂直方向上的运动,其水平辐散是成比例改变的,因此,水平辐合(负辐散)必然导致流出气块的垂直运动,水平的辐散导致流入气块的垂直运动。

我们可以在考察热带大气时应用这些规律,即使它不能被认为是不可压缩的:近地面水平辐合意味着水平方向的净流入和补偿的上升气流,相应地区为低气压,容易成云降雨。相反,下沉运动导致高压,干燥和近地面的辐散。在信风带,边界层以上的下沉运动抑制强雷暴的发展,低浅的云则在潮湿的边界层顶部形成。这些说明了大气中的响应依赖于相对于边界大气的辐合辐散区的位置。

a科氏参数f在赤道上为零,因此地转风是无限大的.

b 回想一下 equation ,是欧拉时间导数,是在一个固定的位置上的变化, equation 是拉格朗日时间导数,是气块移动的测量。因此两个时间导数之间的差是密度平流equation.

3.1.2大尺度系统垂直结构的简化方程

对于天气尺度和行星尺度环流,水平尺度远远大于垂直尺度,因此可以忽略垂直加速度的变化即Dw/Dt≈0。因此,垂直压力梯度与大气重力平衡,这就是所谓的流体静力学近似或静力平衡,表示为

equation (6)

通过实验室中的实验表明,压力,密度(或特定的体积)和物质的温度可以通过状态方程相联系,其中当方程应用于气体时,被称为理想气体定律。对于干燥的空气,状态方程可以被写为:

equation (7)

其中p是压力,ρ 是密度,α 是比容度(ρ-1, 单位是 m3 kg -1),T是温度,R是气体常数,干燥气体的气体常数为(287 J kg-1 K-1).通过理想气体定律,我们可以重写流体静力学方程:

equation (8)

它可以重新整理成位势Φ关于压力和温度的函数,

equation (9)

通过对某一层气体的垂直积分,我们得到压高公式,表明两个压力面之间的厚度正比于该层的平均温度。在冷空气柱中压力的降低远比暖空气柱中压力的降低快

equation (10)

 

由于压力梯度与地转风有关,平均水平温度梯度也与该层的顶部和底部地转风有关,在两层之间的地转风差值与温度梯度成比例,可表示为热成风方程thermal wind equation,其中热成风(VT)表示地转风(Vg)的垂直切变, 因此风场的u分量,介于等压面之间的热成风可表示为

equation
equation (11)

其中T是该层的平均温度,ug是纬向地转风,ut是纬向热成风。这个简单的热成风平衡方程,最适用于解释大气环流的平均结构。贯穿整个赤道地区的平均纬向风即处于热成风平衡中。

通过能量守恒可知,大气同时也被热力学因素驱使,系统内热量的增加等于内能的变化减去所做的功(Chapter 1, Eq. (1))。对于理想气体,热力学第一定律/热力学能量方程可表示为

equation (12)

式中Q是单位质量的加热率(J kg-1 s-1),cv(717 J K-1 kg-1)是定容比热。CvT是内能,利用Cp=Cv+R (cp =1004 J K-1 kg-1),可以改写热力学能量方程

equation (13)

再利用理想气体状态方程,我们可以得到:

equation (13a)

其中Q/cp 表示非绝热加热率。

 

对于绝热过程,因为系统与外部环境无热量交换,所以Q=0。热力学能量方程13a可以整理成初始状态(p,t)、参考状态p0,和位温θ的关系,得到

equation (14)

其中温度是开尔文温度,一般p0取1000hpa,θ称为位温,表示空气块绝热膨胀或被压缩到p0所具有的温度,气块做绝热变化,位温不变。对于干空气的绝热温度递减率(温度变化随高度的变化率)可以由热力学第一定律导出,再由dQ=0静力方程得到

equation (15)

然后

equation (16)

其中Γd干绝热递减率。

3.1.3大气的大尺度结构

空气的流动遵循前面描述的基本原则,在大尺度(1000–10,000 km)运动中,空气环绕高低压区域流动 ,形成波状形式。气压场中的槽和脊类似于地形图中的山谷和山脊。 空气向低气压区辐合,在高气压区辐散,与之对应的是因质量的连续性产生的垂直运动。

下面的图3.3和动画了说明在北半球空气在低压和高压区的运动情况。在南半球,科氏力向左侧偏转,与北半球偏转方向相反。

animation showing rising air and counter-clockwise flow associated with low pressure. animation showing sinking air and clockwise flow associated with high pressure.
图 3.3 北半球低压和高压系统,流动的原理图

垂直运动导致绝热温度变化,因此高低压的位置可以给予我们云量和降水量的信息。在低压区的上升运动导致膨胀,绝热冷却和水汽凝结;而在高压区的下沉导致绝热压缩,增温和干燥天气。

Box 3-1 涡度

相对涡度

一般而言,涡度用来衡量流体局地旋转的特性。适用于任何环境下的流体流动,从气象方面看,流动指的就是风相对涡旋,equation,是流体旋转的微观度量

它的形式是一个微分、矢量形式

  • 相对涡度是由空间上的风各分量equation定义:
equation (B3-1.1)
  • 相对涡度单位从方程(B3-1.1) 的分析可知,其单位为s-1
  • 涡度定义为逆时针方向为正
  • 由于在天气尺度下,水平风通常远大于垂直风(3.1.2 节),我们近似地把其垂直分量当作相对涡度。
equation (B3-1.2)
  • 在短时雷雨大风或中尺度系统中,不能假设水平风分量远远大于垂直风。

地球涡度和科氏参数f

地球绕地轴以equation速度旋转,由于风是相对于地表面定义的,我们用地球坐标来定义:

equation (B3-1.3)

 

其中equation是指向正北的单位向量,equation是铅直向上的单位矢量(垂直于表面),Φ 是纬度

  • 由于纬度的改变会导致从地表到地球自转轴的距离的改变,所以其旋转产生的涡度也会发生变化。
  • 科里奥利参数f,是地球涡度的垂直分量(s-1),
equation (B3-1.4)
  • 在北极点,f=;在赤道f=0,在南极点f=-2Ω
  • 既然地球总是沿着相同的方向旋转,为什么越过赤道f的符号会改变。 这是因为,我们定义地球涡旋时是在地球表面定义的。现在想想如果以我们所在地点为参考系,地球的旋转是怎样的。例如,从北极点看,地球的自转是逆时针的,所以其旋转方向为正方向。从南极点,地球的自转感觉上是顺时针旋转,(用一个旋转的地球去想象),这时旋转为负方向。气旋的旋转总是和地球的自转相匹配.因此在南半球,负方向的旋转与气旋式切变相联系

绝对涡度

绝对涡度是相对涡度和地球涡度的矢量和

equation (B3-1.5)

再者,对于天气尺度的流体,我们通常关注它的垂直分量ηζa

equation (B3-1.6)

位涡

位涡Potential vorticity定义为绝对涡度除以空气柱的厚度Δz, 它的单位是m-1s-1,在绝热,无摩擦下,位涡是守恒的;如果不存在非绝热加热或湍流混合,位涡保持恒定,即是一个常数:

equation (B3-1.7)

因此,没有厚度变化的空气移向赤道,因为f是减少的,它的相对涡度必须增加,以保持位涡恒定。.如果因为流过山峰,空气的厚度变化,那么它的绝对涡度必须改变。通过诱导气旋式或反气旋式的相对涡度或改变它的纬度。(即改变f大小)

等熵位涡Isentropic potential vorticit,P,是两个等熵面(等位温面)之间的位涡,并被定义为

equation (B3-1.8)

或者写成

equaiton

其中p是压力,θ是位温,P通常被表示为单位位涡.位涡单位为10-6 K kg-1 m2 s-1

3.1.4热带系统的尺度分析

Space and time scales of dynamical processes in the atmosphere
图 3.4 大气的动力学过程中的空间和时间尺度

我们用长度,质量,时间、温度等基本量来描述不同力之间的相对重要性。图3.4显示了大气动力学过程中通过时间和空间尺度分类的例子表3.1列出了基本量的尺度

表 3.1 基本量纲
符号 变量 单位
U 水平速度 m s-1
W 垂直速度 m s-1
L 长度 m
H 高度 m
δP/ρ 水平气压变化除以密度 J kg-1 = m2 s-2,units of geopotential
T = L/U Time s

罗斯贝数

科氏力在赤道上为零,因此地转平衡在此不适用。没有地转平衡,风就不会被迫使与质量场或压力场平行.因此,在赤道或临近赤道的风通常是辐散的,风场容易向质量场适应。我们可以通过无量纲数罗斯贝数R0(惯性力与科氏力之比)评估哪里可以适用地转平衡。R0定义为:

equation (17)

当风场平行于压力场和质量场时,罗斯贝数很小,表3.2所示,当u-10m/s,l-106m时R0随纬度的变化。

表 3.2. 相同风速和空间尺度下罗斯贝数的大小
纬度, Φ 科氏力参数,f

= 2ΩsinΦ

U (m s-1) L (m) 罗斯贝数, R0 U/(f L)
45° 1.12 x 10-4 10 106 » 0.09
23.5° 6.32 x 10-5 10 106 » 0.16
3.6° 9.95 x 10-6 10 106 » 1

在中纬度地区,f随纬度的变化不大。然而在热带地区,f变得很小,其随纬度的变化较大(见表3.2)因此,我们通常假设f~Βy,其中Β=∂f/∂y.这个就是所谓的β平面近似。称赤道的流动在Β平面上,而称中纬度的流动被称为在f平面上c

需要注意的是R0除了随纬度变化,还随尺度变化。

对于一般尺度的平均风速,找出在热带地区地转平衡所需的L的范围

提示: 地转假设成立时:R0一定要小,使用R0为0.1

纬度, Φ 科氏参数,f

= 2ΩsinΦ

U (m s-1) L (m) 罗斯贝数, R0 U/(f L)
45° 1.12 x 10-4 10 0.1
23.5° 6.32 x 10-5 1 0.1
3.6° 9.95 x 10-6 1 0.1

使用同样的原则来确定长度尺度。在风速为35m/s,R0较大的情况下

纬度, Φ 科氏参数,f

= 2ΩsinΦ

U (m s-1) L (m) 罗斯贝数, R0 U/(f L)
45° 1.12 x 10-4 35 1.0
23.5° 6.32 x 10-5 35 1.0
3.6° 9.95 x 10-6 35 1.0

提示:

你应该看到,为了满足地转平衡,热带地区的大尺度流动需要有足够大的L。以使R0较小;在强风和小尺度系统中,如龙卷风,R0是很大的,且是压力梯度与向心力之间的平衡。

静力稳定度Static Stability与布伦特-维萨拉频率Brunt-VÄisalÄ frequency

布伦特-维萨拉频率N(浮力频率)是一个用来描述大气和海洋垂直运动的基本量。在静力稳定条件下,垂直移动的空气块在流体中振荡的频率。在大气中:

equation (18)

其中g是重力加速度,θ 是位温,单位k,z是高度。

对于海洋,equation,是势密度. equation由温度和盐度决定。N的变化依赖大气或海洋的稳定性。(表3.3)对于中性环境,N=0

表3.3静态稳定度和浮力频率.
静态稳定 (密度随高度减小) N2 > 0 垂直运动的气块会加速回到初始点,但如果振幅过大,则会产生振荡,频率为N.
静力不稳定 (密度随高度增加) N2 < 0 垂直运动的气块会加速远离初始点(导致对流和混合))

空气块的位移回复力跟空气块受的重力、初始层与移动到的层的密度差成比例。布伦特-维萨拉频率是两层之间重力内波的最大频率。

罗斯贝变形半径Rossby Radius of Deformation

对于受重力和旋转影响的流体,当流体的旋转效应变得跟浮力效应同等重要时,用罗斯贝变形半径λR来描述其水平长度尺度,

Scaling of the Rossby radius of deformation.
图 3.5 罗斯贝变形半径尺度

罗斯贝半径可以尺度分析为

equation (19)

其中N为布伦特-维萨拉频率,是对静力稳定度的衡量。H是大气高度,ζ是相对涡度,η是绝对涡度。通过这些典型的变量值,我们可以估算λR.

  • 在中纬度地区, λR ≈ 500 km
  • 在 10°, λR ≈ 2000 km 在纬度为10
  • 在 5°, λR ≈ 10,000 km 纬度为5
  • 当靠近赤道,它的值变得无穷大,f为0

实际应用中,λR用来评估压力或高度扰动是动力学上的是大还是小。如果它较大,他会维持一段相当长时间的扰动特性,风场会向气压场/质量场适应,如果是动力学上的小,它的作用会衰弱,高度场会向风场调整。可以在COMET模块了解更多有关罗斯贝变形半径的,地转平衡适应:http://www.meted.ucar.edu/nwp/pcu1/d_adjust/.

水平运动尺度

水平运动方程可以写成

equation (20)

展开为

equation (21)

基本量为equation
对于中纬度系统

equation

剩下的项是科氏力与气压梯度力的平衡。对于天气尺度而言,Δp/ρL ~ 10-3Δp ~ 1000 Pa即10 hPa

在5°-10°纬度

equation

剩下的是equation

为保持平衡,当尺度L ~ 10-6m,Δp/ρL ~ 10-4

在赤道地区, 5S°-5N°

equation

剩下的项是压力梯度力平流项的平衡。

 

低纬度地区的垂直结构近似。

流体静力学方程可写为

equation (22)

基本尺度: 当ρ ~ 1 kg m-3,equation

对于热带, Δp ~ 100 Pa (请看上面的尺度分析),

equation

热力学能量方程近似

热力学能量方程(13a)可以以位温θ的形式表示为:

equation (23)

展开成

equation (23a)

在没有强非绝热加热的情况下,Q/ cp ≤ 1 K/天。水平温度平流与垂直运动(w~0.3 cm s-1)平衡

温度水平扰动尺度可表示为

equation

这里,H是典型的大气高度(~ 104 m),风速U ~ 10 m s-1,重力加速度为g ~ 10 m s-2,L是典型的水平尺度

  • 对于热带地区Δθ/θ ~ 10-3
  • 对于中纬度地区f ~ 10-4 s-1, L ~ 106 m,因此 Δθ/θ ~ 10-2

热带的水平温度梯度远小于中纬度地区,因此在高湿、深对流系统中,观测到的大范围加热无法与热量水平平流相平衡.在热带天气尺度降水系统中,Q/ cp ~ 5 K /天,所以必须要有几厘米每秒的垂直运动。


c
f平面近似和β平面近似,使我们能够将我们感兴趣的地球区域作为一个切平面去考虑,:f平面将地球表面近似为一个圆柱面。β平面将地球表面近似为一个椎体。这样,β平面中地表与旋转轴的距离是不同的,因此当气块向北或向南移动时,地球自转对其的影响也不尽相同。

3.1.5自然坐标系

由于等压线分析对于分析热带地区的风不是很有效,我们使用运动学分析并依据自然坐标系定义运动。

The natural coordinate system with inset of the horizontal natural coordinate system showing the tangent, s, to a fixed curve (streamline), and the normal, n, the radius of curvature R, and angle ?, relative to a fixed direction.
图3.6自然坐标系中的水平自然坐标系:切线s(一个固定的曲线/流线),法线n,正的曲率半径R,相对于指定方向的方向角Ψ
external links 动画: 自然坐标系.

自然坐标系描述流体基于法线或切线方向及其对应曲率半径的运动,流动以流线而不是等压线的形式进行描述,表征风的瞬时方向,等风速线和瞬时风速。我们可以通过单位矢量ns构造自然坐标系,其中n是法线向量,s是切向向量。它们可以通过垂直单位向量k联系起来,n = k × s 我们可以定义一个角度Ψ, 作为曲线的切线的角度,使得流线的切线有个固定的方向。请注意Ψ 定义为逆时针为正方向,并且R > 0对于逆时针流动。自然坐标系下的连续方程为:

equation (24)

其中散度项有两部分,一个是横向辐散,代表整个流体的运动在角度上的变化。当流线向外延展呈辐散状时,也表征着气块互相远离并辐散。汇流也类似,如 3.7b所示。但需要注意的是,分流的区域并不总是辐散的区域。连续方程的第二项中,表示下游的辐散,表征速度的变化。正如(5)中,我们可以表示用垂直速度变化表示连续方程:

equation (25)
Streamline patterns showing (a) diffluence and (b) confluence. The locations of divergence and convergence relative to a wind speed maximum are shown in (c). The dot is motion out of the page, the cross is the opposite (Courtesy of Mick Pope, BOM).
图 3.7. 流线模式显示了流体的 (a) 分流和 (b)汇合。 (c)中显示了相对于最大风速区的辐合辐散位置。圆点代表流出纸面的运动,十字则表示流入纸面。(Courtesy of Mick Pope, BOM).

在自然坐标系下,由Box 3-1中的定义,涡度方程为:

equation (26)

其中R是曲率半径。U是风速。自然坐标系中的涡度是曲率涡度和切变涡度的总和(图 3.8c)。

(a,b) Streamline patterns for curvature vorticity. (c) Flow with linear shear that produces positive and negative shear vorticity. The paddle wheels and curved arrows in (c) show the relative turning produced by the linear wind shear.
图 3.8(a,b)曲率涡度的流线模式

(c)流动的切变产生正和负的切变涡度,图中的浆轮符号和弯箭头表示由风切变产生的相对转动。

3.2大气环流

大气环流是指在一个足够长的时期内全球的平均流动,这个时间长度足以消除变化多端的天气系统引起的流动异常,但是它又足够短以捕捉季节变化和月变化。大气环流的主要影响因素有:

  • 不均匀加热
  • 行星的旋转
  • 地形
  • 大气和海洋流体的动力学作用

3.2.1全球大气环流概念模型的历史演变

在1686年,埃德蒙哈德来Edmund Halley1提出了东北和东南信风理论。他推测,信风的形成是由于冷并密度大的空气流向较热地区,此后上升。为了保持平衡,上升的空气会从热的区域移出,下降,从而完成一个循环。他设想,下层东北信风与上层的西南气流平衡,下层的东南信风将由上层的西北气流平衡。乔治哈德来George Hadley(1735)2提出了一个单一的南北半球轴对称的循环,这个循环将热量从热带带到两极。开尔文Lord Kelvin(1857-1892)提出了一个更复杂的近地表环流,但保留了高空的单一的‘哈德来环流’模式。

Schematic of an aqua planet with single cell circulation.
图 3.9一个充满水的星球的单圈环流示意图。

这些早期的理论都没有考虑旋转的影响和角动量守恒(Chapter 1, Section 1.8)的作用。一个空气块跨纬度运动时需要保持它的绝对角动量不变。如果一个空气块从赤道移动到两极,它将获得一个无限大的纬向速度,以保持它的绝对角动量(Table 3.4)。

赤道上固定的空气块

MEQ = (Ωa cos(0) + u) a cos(0) (27)

= Ωa2 = 2.96 x 109 m2 s-1

Table 3.4. 不同纬度上的角动量和诱导的相对纬向风速
纬度 地球角动量 (EAM) × 109 m2 s-1 诱导的相对风速 (m s-1)
EQ (0°) EAMEQ = MEQ = 3.0 0
±30° EAM30° = 2.2 134
±45° EAM45° = 1.5 327 (~ speed of sound)
±60° EAM60° = 0.4 697
±90° EAM90° = 0 → ∞ !!

观测证实在赤道和亚热带地区约±30°之间存在哈德来环流,在1856年和1861年,威廉费雷尔提出,中纬度西风环流是由科里奥利力造成的。3在1921年,威廉发现了极地东风,并提出它们的形成来自于间接热力环流——费雷尔环流,其平衡由中纬度气旋维持。伯杰龙提出了三圈环流结构,并定义了每个环流之间的范围。因此在一个充满水的星球上出现了更复杂的大气环流,包括热带东风,中纬度西风和极地东风(图3.10).当考虑大陆的影响,发展出了更为复杂的地面模式。加入陆地后,高低压带不再连续,而是出现区域高低压中心,如中心在海洋上的副热带高压。

The three-cell circulation model for an aqua planet The three-cell circulation model for a planet with continents
图 3.10 (a)只有水面的星球三圈环流;(b)有水和大陆的行星三圈环流。
Mean vertical motion (omega, Pascal per second) for (a) Dec-Feb and (b) Jun - Aug for 1968-1995 (from NCEP Climate Atlas)
图 3.11 (a)十月到二月的平均垂直运动(ω, Pa s-1);b图是六月到八月的平均垂直运动(来自CEP Climate Atlas 1968–1995)

图3.11显示了平均经向环流。热带为上升运动,副热带为下沉运动,从而形成哈德来环流。然而这个模式不是关于赤道对称的。最强的上升运动出现在夏天的北半球大陆(图3.11b)。上升运动两侧均是下沉运动的模式只发生在南半球的夏季。受大陆的影响,北半球夏天在对流层低层产生强烈的上升运动,冬天产生强烈的下沉运动。

轴对称哈德来环流理论由Isaac Held和 Arthur Hou(1980)首次提出并发展4,是当前通行的行星大气理论。他们的基本假设是一个旋转的地球,把大气分为两层,并且通过能量守恒,角动量守恒,质量守恒来强迫出运动。参见Box 3-2了解更多理论细节。

Schematic of air moving upward and poleward in the Hadley cell to form the Subtropical Jet; air moving poleward from the equator to a smaller radius of rotation speeds up to conserve angular momentum.
图 3.12哈德来环流向上和向极地气流构成亚热带急流的原理图;

空气从赤道出发向半径较小的极地移动,为保持角动量守恒,必须增加其纬向速度。

在地表以上可不考虑摩擦力的区域,风速一般较快,对流层顶附近往往是急流的强风区域。热带气象最关心的急流是副热带急流(STJ图3.12)和热带东风急流(TEJ)

 

副热带急流大约位于北纬30°,由哈德来环流的上升气流和向极地方向的空气流动产生(图3.12)。当空气块向较小的纬圈移动时,它们的速度必然增加,以保持角动量守恒。需要注意的是,副热带急流的典型速度小于使用角动量方程(表3.4)计算的值。因为大尺度涡旋(例如气旋)会从哈德来环流中输送角动量到中纬度地区(图 1.28),并且气块会因一些小尺度扰动而速度变慢。其他机制也促使副热带急流的全球多变性,比如南北半球对流层中层的温度梯度和亚洲中部地区地形的起伏。

Equator to pole profile showing the mean positions of the subtropical and polar jet, ITCZ convection, and clouds along the polar front.Schematics of the mean meridional circulations and the jet streams
图3.13(a)赤道到北极剖面图,显示亚热带和极地急流,赤道辐合带对流,极锋云带的平均位置(b)平均经向环流和急流原理图

极锋急流形成于极锋中极寒的极地气团和较温暖的赤道一侧气团之间温度对比最强的区域。极锋急流在冬季最强,并且它有时甚至可以移到热带地区。图3.13显示了西风急流和平均经圈环流的相对位置。正如第一章中,深对流在全球的能量平衡上起到关键作用,5卡尔.古斯塔夫.罗斯贝(1941)6是第一个认识到强对流在大气环流中的明确作用(图3.13A)。有组织的强对流往往发生在高空急流的辐散区域,这个辐散区域与低空风力最大的区域即低空急流也有联系.我们将在特别关注二中学习更多有关低空急流的内容。

Box 3-2 轴对称哈德来环流:理论和假设

哈德来环流对能量和角动量向极地的输送起着非常重要的作用。哈德来环流的纬度范围跟如下因素有关:副热带地区的静力稳定度(形成了哈德来环流的外部边界),从赤道到两极的非均匀非绝热加热,地球的自转和角动量守恒。我们对哈德来环流的大多数理解来自简单的理论。根据其中一种理论4,哈德来环流中上升支的纬向风要满足角动量守恒。哈德来环流是一个能量运输的封闭系统,所以上升气流中的非绝热加热区域与下沉的非绝热冷却区域需要达到平衡。这一理论的数学表达式可以通过β-平面近似把问题变换到直角坐标系中7,此时,向极地方向的哈德来环流的表达式可以推导如下
几何假设

  • 大气垂直尺度,H,远小于地球半径,a

H << a, a = 6.37 x 106 m

  • 垂直方向上为两层
  • 地球的自转速度为常数 Ω = 7.292 x 10-5 rad s-1
  • 纬度角Φ很小,

sin(Φ) ≈ Φ, cos(Φ) ≈ 1
y = a Φ, so YC = a ΦC

利用小角度的β平面近似。在β平面作小角度近似为

f = βy β=2Ω cosΦ /a (B3-2.1)

在赤道平面上,β-平面近似可化简为

f = 2Ω y/a β= 2Ω/a (B3-2.2)

平衡假设(动力学约束)

  • 准静态equation
  • 轴对称equation
  • 静力平衡 equation(B3-2.3)
  • 热成风平衡equation(B3-2.4)
  • 绝对角动量守恒

M(Φ1) = M(Φ2), where M= (Ωa cos(Φ) + u) a cos(Φ) (B3-2.5)

热力强迫(热力学约束)

  • θE(Φ)表示等位温分布,由入射的太阳辐射(辐射平衡)的分布差异造成
  • 对热力强迫的动力响应使温度得到重新分配θ0(Φ) ≠ θE(Φ) 对热力
  • 由于太阳辐射导致的热力强迫θE(Φ)可以写为:
equation (B3-2.6)

其中,τE是弛豫时间,表示其他的强迫(运动)停止后,θE(Φ)复位到原值的时间。

其结果表示为赤道平均位温θE之差,辐射平衡和动量平衡的方程

equation (B3-2.7)

ΦH,第二个方程适用于经向变化的哈德来环流

equation (B3-2.8)

其中Hτ是热带对流层高度,θ0是全球平均温度,Δθ是赤道-极地表面位温在辐射平衡差异。是这个理论中主要的热力学因子

另一种理论把哈德来环流的纬向宽度跟角动量守恒/垂直切变导致斜压不稳定相关联。这个纬度被定义为

equation (B3-2.9)

其中ΦH是哈德来环流的纬度,He是该纬度的对流层高度,N是布伦特-维萨拉频率。在这里热力学的主要影响因子是亚热带的干静力稳定性。

为什么我们需要注意哈德来环流向极地延伸的宽度

根据卫星观测、无线电探空仪的数据和全球再分析资料表明,从1979起哈德来环流扩大了约2到4个纬度。8,9,10哈德来环流边界位置的亚热带干旱地区、沙漠和急流已经向极地方向移动。如果按照这种趋势持续下去,可能会造成重大的社会影响,如降水模式的改变使一些地方更加干旱,以及导致的植物,昆虫或动物的生态变化。

3.2.2热带和亚热带地区的路线图

World map marked with ocean currents and winds moving around semi-permanent high-pressure cells typical for the month of JanuaryWorld map marked with ocean currents and winds moving around semi-permanent high-pressure cells typical for the month of July.
图3.14(a)六月和(b)七月地球表面的平均气压系统和有代表性的风矢量场。红色虚线表示赤道槽/ITCZ(在亚洲即为季风槽)

热带大气环流由在时间上变化较小的准静态环流和发生季节性反转的季风环流主导(第3.4部分)。位于中心的是赤道槽,是由大气净加热和上升运动引发的气压低值区。

 

在北半球的夏季(七月),最突出的低压中心有印度次大陆上的热低压,非洲撒哈拉沙漠的热低压和在北美索若拉热低压。南半球的夏天(一月)最显著的低压位于南美的亚热带地区、南非和澳大利亚-印尼区域。亚热带地区占主导地位的是太平洋高压和北半球的亚速尔高压,与之对应的南半球地区上相对应的平均气压约为1020hpa,他们在各自的夏季时最强.注意到南半球有更多的纬向特征,和由于陆地较少而表现为季节性变化较弱。信风,是哈德来环流的最低空的分支,其在塑造热带地区的天气,以及与热带海洋的相互作用中起着重要的作用。它们是相当持久稳定的气流,平均风速达4-7 m s-1,在冬季有最大风速。在信风带中的云高向西,向赤道方向增加(图1.18b)(Chapter 5, Section 5.2.2.3, 图 5.16),信风受年际波动的影响,如厄尔尼诺-南方涛动(ENSO)。

经向和纬向的气压中心位置的变化受大气对非均匀表面的响应所驱使。例如,赤道槽和热带辐合带的平均位置在北半球,在北纬5°附近建立‘气象意义上的赤道’。南半球热带辐合带通常是东太平洋的春季特征,而在西大西洋只出现在夏季。然而,最新的QuikSCAT卫星的风速数据证实,存在、横跨大西洋和太平洋的南支ITCZ。我们不禁要问是什么控制ITCZ位置的经向变化?下面提供几种解释:埃克曼抽吸Ekman pumping造成的赤道太平洋冷却,有效水汽量,强大的海温梯度或海温最大值区,大陆的位置,安第斯山脉的位置。在special box讨论了一些在热带东太平洋中ITCZ位置的相关理论。在热带和中纬度地区之间是南太平洋辐合区(SPCZ)和南大西洋辐合区(SACZ),这些与年降水极大值相关(图9.13)。在东南非和印度洋上也存在间歇性的辐合区域,这些辐合区域是向极地移动的热带气旋和副热带气旋的通道,它们的位置受海温梯度和副热带高压变化的影响。

Mean wind vectors at 200 hPa in December-February (upper) and June- August (lower). Note stronger jetstreams in the winter hemisphere (magenta > 60 m s-1).
图 3.15 12月-2月(上图)和6月-8月(下图)200hpa平均风场的矢量图
注意:冬半球中较强的急流(橙色> 60 m s-1))

 

对流层高空风(图3.15)相比于地面复杂的风的模式要简单一些,呈现出更明显的纬向风。然而即使在如此高的高度上,大陆对海岸附近的高空风波动起伏和风速的变化的影响也是明显的。注意强急流来自亚洲东海岸和北美东海岸。高空风模式在南半球呈现出更多纬向风特性和更强急流,这是由于南半球的陆地面积较少。副热带急流在南半球全年出现,但是在北半球的夏季,由于南北温度梯度的降低,它将消退。在夏季,热带对流层高空出现东风急流区,不过没有西风急流那么强劲。(图3.15b)这些最大风速表示着热带东风急流(TEJ)在100-150hpa位置的建立。11在印度的中部和南部高空,出现最强风速区,可达40-50 m s-1,这个东风急流的形成是对热带陆地气团经向热量盈余的响应。这些热量有助于在上层建立高压和强劲的东风。

 Mean 200 hPa heights for August 1971-2000 (from NCEP/NCAR Reanalysis). A blue, dashed line marks the TUTT.
图3.16 1971-2000年的8月200hpa高度的平均值(来自NCEP/NCAR在分析资料)

热带对流层上部槽(洋中槽)Tropical Upper Tropospheric Troughs (TUTTs)12,(如图3.16)是在夏季热带海洋上空的半永久性特征。它们与热带的强对流天气相关,有时和热带气旋生成发生耦合。最强的热带对流层上部槽在北太平洋,北大西洋,加勒比海和墨西哥湾。东太平洋地区是南半球中最显著的TUTT区域,较弱一些TUTTs发生在澳大利亚,南美,偶尔也会在非洲出现。热带对流层上部槽将在天气尺度系统和分析中的独立章节详细介绍。

3.2.3热带与中纬度的对比

在太阳辐射年循环中,热带在全年都是接受辐射加热最多的地方,而中纬度地区在夏至点或冬至点才吸收最大的辐射。热带地区有辐射加热的盈余而中纬度以上地区是辐射赤字(图1.1)。在热带,剩余的热量通过洋流和哈德来环流向极地方向输送,最大的输送在10-15°纬度之间(图 1.12d)。而在中纬度,瞬变涡动输送大部分的热量(Section 1.4.2)。由此相比于只能从太阳辐射中获取能量而无传输的情况,实际热带地区的温度要比预期的低一些。

因为温暖的洋流和湿润的行星边界层,热带大气层有相近的热惯性。垂直风切变在热带地区较弱,大约是中纬度地区垂直风切变平均值的20%。对于相同尺度的运动和不存在强对流的情况下,赤道地区的垂直速度比中纬度地区的要小的多。

一般认为,中纬度大尺度天气是斜压baroclinic的,在热带地区,同样尺度的天气通常是满足正压barotropic条件的。斜压意味着密度是关于压力和温度的函数即ρ = ρ (p,T),比如,在一个等压面上,可以看到不同的密度和温度,等压线与等温线相交。相比之下,正压流体,密度只是压力的函数ρ = ρ (p);即,等压面与等密度面/等温度面是互相平行的。流线与等温线相互平行。中纬度涡旋运动,如中纬度气旋,是由于大气的斜压不稳定引起的。大气趋向于削减这种由于强温度梯度和密度梯度造成的不平衡状态,从而恢复到纬向环流的状态。中纬度气旋是冷中心系统,其气旋性随高度增加。相比之下,热带气旋则是暖中心系统,没有锋面,气旋式环流反而随着高度的增加而削弱。

热带环流,例如半永久性气压系统,构成大气环流的热带经向环流部分和季风系统共同解释了热带低频振荡变化13,这与中纬度大气环流的高频振荡变化的不同。

3.2.4.平流层环流Stratospheric Circulations

3.2.4.1 布鲁尔多普森环流Brewer-Dobson Circulation

Schematic of mean meridional tropospheric and stratospheric circulation at the solstice (WMO 1985 ). Much of stratospheric air enters through the tropical tropopause.
图 3.17 二至点时对流层和平流层的平均环流示意图(气象组织1985)14

有大量平流层空气进入热带对流层顶TTL。

热带对流层顶最高也最冷,在热带地区可到16-17km的高空(图 1.16).布鲁尔Brewer (1949)根据无线电探空仪中测得的湿度变化和平流层湿度低值特征阐述了气体通过热带地区的对流层顶冷点进入平流层的现象。这种在平流层的环流被称为布鲁尔-多普森环流15,16,通过空气凝结,空气变得干燥,并从热带移向极地方向,使得臭氧向冬半球极地方向输送并累积。(图3.17)布鲁尔-多布森环流受大气中的重力波buoyancy waves驱动,重力波可由空气流过高山以及强雷暴云体产生的。这种波动向上传播,逐渐加强,在平流层中部破裂,在破裂处对现有流动产生拖曳。跟对流层环流一样,平流层环流同样部分受不均匀的辐射加热驱动。在晚上平流层冷却,冬半球极地的冷却作用造成了强大的气压梯度力和流向极地的气流。

热带对流层顶层Tropical Tropopause Layer(TTL)是对流主导的对流层和平流层之间的过渡层。范围从10-12公里处气温递减率最低的层到16-17公里的冻点层(图3.18a)17
TTL扰动相对较少,这是因为对流随高度增加而减少的原因。这层受持续的化学过程作用。相比于弱对流,活跃的深对流对平流层的侵入有更大影响17。对流层顶冻点在一月到三月(北半球的冬季)最高。在六月到八月(北半球的夏季)高度最低。从1978年起,对流层顶冻点高度以每十年20米升高,并每十年下降0.5K,气压以每十年0.5hpa降低9。对于TTL的继续研究是为了了解空气进入平流层的方式,比如通过对流上升至冻点对流层顶,缓慢的辐射上升,和伴随着对流现象的增湿/干燥过程。

Schematic of the TTL, convection, and vertical transport in the tropics, the mean temperature profile where CPT is the cold point tropopause.Schematic of the TTL, convection, and vertical transport in the tropics, the mean temperature profile where CPT is the cold point tropopause.  (Lower) Important processes coupling dynamics, chemistry and cloud microphysics in the upper troposphere and lower stratosphere. The dashed green line denotes the time average tropopause. In the tropics, maximum outflow from deep convection occurs near ~12-14 km; while the cold point tropopause occurs near 17 km. Deep blue shading marks the TTL. Extratropical stratosphere-troposphere exchange occurs in tropopause folds and intrusions linked with synoptic weather systems, which transport stratospheric ozone into the troposphere. Green arrows represent transport of near-surface pollutants into the upper troposphere, strongly influencing global-scale chemistry. Adapted from Stohl et al. (2003)
图3.18(a)热带对流层顶TTL,对流和垂直输送和平均温度廓线示意图,其中CPT指冻点对流层顶的位置。 (b)对流层上层和平流层下层与动力、化学和云微物理耦合的重要过程。绿色虚线表示对流层顶在热带地区的平均位置 ,深对流最大流出的高度出现在12-14 千米,尽管对流层顶冻点出现在约17千米高度。深蓝色阴影表示着热带对流层顶层(TTL)位置,温带平流层-对流层交换发生在对流层顶褶皱和入侵区,这与天气尺度的天气系统有关,这个区域将平流层臭氧运输至对流层。绿色箭头代表近地面污染物迁移到对流层上部的过程,这个过程强烈影响着全球尺度的的大气化学变化。改自 Stohl et al. (2003)18

 

3.2.4.2准两年振荡Quasi-Biennial Oscillation (QBO)

赤道平流层有典型的纬向风准两年振荡的特征,风向从东风转向为西风。振荡是关于赤道对称的(图3.19a)并起源于约40km的高度(大概20hpa),向下传播(图3.19b)振荡周期为22-34月,其最大振幅为20-30 m s-1 (图3.19b)。准两年振荡是Reed19在1960年发现的。1961年Veryard 和Ebdon20也独立发现了这种现象。

Latitude-height distribution of the amplitude and phase of the zonal wind QBO. Amplitude (solid lines) in m s -1, phase (dashed line) at 1- month intervals with time increasing downward (Andrews et al. 1987 ).Time-height cross-section of the equatorial zonal-mean zonal winds simulated by Takahashi. Westerly (red) and easterly (blue) winds have 6 m s-1 contour interval.
图3.19(a)振幅和纬向风QBO相位的纬度-高度分布,振幅(实线)m s-1,逐月间隔的位相(虚线)随着时间推移向下传播。(安德鲁斯等人1987)21
(b)纬向风平均的时间-高度图,红色是西风,蓝色是东风(来自Baldwin等人2001)22

东风和西风的模式是不对称的。从东风转变至西风时向下传播的速度快于西风向东风的转变。西风的加速最初在赤道出现而后延伸至高纬度地区,而东风的加速则在纬度上显得较为一致。此外,西风的加速显得更强烈。更多有关准两年振荡的介绍,如它的演变,结构和有关它形成的理论,将在热带变化一章给出。

3.3 海洋环流

3.3.1 全球海洋上层和深层环流

Global upper ocean circulations and subtropical gyres; red marks the warm ocean currents and blue marks the cool ocean currents.
图 3.20 全球海洋上层洋流和副热带环流圈,红色代表暖流,蓝色代表寒流。

海洋在风应力作用下产生洋流。半永久性的地面风环流(如图3.14所示的副热带高压和信风)有助于驱动海洋环流(图3.20)。洋流可按温度可分为寒流和暖流;寒流来自极地,带来寒冷的水流;暖流从赤道流向极地,带来温暖的水流。盛行洋流控制着海表温度,使热带海洋东部及其邻近的海岸变冷,而在热带海洋西部与东海岸使海温变暖,从而形成了温度梯度。(图 1.23)

Surface and depth averaged transport currents due to Ekman spiral for deep water. This is often true for deep water but not shallow water.Conceptual image showing coastal upwelling
图 3.21(a)北半球深处水域的埃克曼螺旋Ekman spiral示意图。
(b)洋流在海面风场及其应力带动下,在南半球沿着东海岸运动示意图。

洋流的方向并不与风向完全一致。海面所受的风应力随着深度而减小,由此产生随深度速度逐渐减小并螺旋状扭转的洋流(埃克曼螺旋Ekman spiral23)(如图3.21a所示)。正如在大气中一样,水的运动也受到地转偏向力的影响,平均海面洋流方向大约与风向成45°角,北半球偏向风向的右边,南半球偏向左边。当表面的水流被移除(辐散),深层的水就会涌上来填补空缺,这个过程叫做涌升现象(图 3.21b)。

海洋环流受到温度和盐度差异的驱动(图 1.10b)。温度和盐度梯度引起密度变化和垂直环流(温盐环流)。由于较冷的海水比温暖的海水密度大,此外含盐量高的海水密度也比含盐量低的水大;所以较冷或盐度较大的海水更趋向于下沉。比如,在北大西洋,当墨西哥湾海流冷却和结冰时,引起上层海水盐度增大,海水发生下沉。图 3.22a所示的全球海洋深层大循环global ocean conveyor belt是海洋洋面与深层海水传输的合成平均状况。

小尺度的海洋波动和旋涡,统称为涡旋,这些涡旋对局地的海洋环流有重要作用。如第一章中所述,由于水的热容很大,海洋在热量的储存和释放,以及把热量从热带传到极地中扮演着至关重要的角色(1.4.2)。

The global ocean conveyor belt showing surface currents and deep currents and the subduction and upwelling zones in between. Conceptual image showing thermohaline circulation in the Atlantic Ocean
图 3.22.(a) 海洋深层大循环global ocean conveyor belt,表层洋流、深海流和之间的俯冲、上涌区域。(b)大西洋经向翻转环流Meridional overturning circulations

海洋环流也受底层地形和海陆边界的影响。图 3.23a 显示了强劲的印度西南季风和阿拉伯半岛对上层海洋的作用。海面风驱动海水向其右边移动(红色箭头)。同时离岸运动的海水导致较冷的深处海水上涌。水平方向的风切变使埃克曼流出现速度上的差异,进而导致最大风速区离岸处的海面发生辐合,引发沉降流(向下蓝色箭头);而近岸处则相反,形成上升流。

Schematic cross section of the upper ocean dynamical response to the southwesterly monsoon winds and the Arabian Peninsula (Honjo and Weller 1997). MODIS image of sea surface temperature over the Arabian Sea showing cold temperatures along the coast of Oman due to upwelling. 25 May 2003
图 3.23(a)原理图:西南季风和阿拉伯半岛地形影响下的上层海水的动力响应(Honjo and Weller 1997)24 (b)MODIS探测器图像:阿拉伯海温显示了在阿曼海岸处由于上升流导致的海温低值(25 May 2003)。

3.3.2 海洋动力学

 

除了把密度常数化的假设以外,大尺度海洋环流遵循和大气中一致的流体动力学特征(Section 3.1)。

equation (28)
equation (28)

u,v代表水平风的经向、纬向分量;p压强;f科氏力参数;F摩擦力

Schematic of general air-sea interactions involving transfer of momentum, heat, moisture, and radiation.Schematic of dynamic and thermodynamic processes in the upper ocean. Monthly mean temperature (red line) and salinity (magenta line) profile during summer and winter in the tropical south Indian Ocean.
图 3.24(a)示意图:海洋上层的动力和热力过程。(b)南印度洋夏季和冬季月平均水温(红)和盐度(蓝)廓线。注意相对于较深海水,在冬季和夏季海洋表面温度有很大的差异,这说明了表面辐射效应随着深度的快速递减。

垂直运动由温度和盐度梯度引起的密度变化所驱动。这些梯度的变化可由蒸发、降水、淡化、海冰溶解和其他一些过程引起(图 3.24a)。上层海水对辐射加热的改变比深处的海水敏感得多,正如图3.24b中看到的那样。注意相较于较深深度温度在夏天和冬天基本一致的状态,而近海面温度的差异要大得多。温跃层thermocline可以在图3.24b 识别出,它分开近表面温暖的混合层与较深较冷的海水/湖水。在海洋中,温跃层也分开近海面盐度较低和底下较高的层(注意盐度廓线在近表面的翘起)。

3.4 赤道加热的响应

热带的热量盈余驱动着大气环流。如第一章所述,太阳入射辐射是纬向对称,但净热值却不是对称的。净热值取决于许多因素,比如地表性质和云的分布。图3.25 显示了,在大陆和暖流区垂直总净热值有极大值,并且极大值的数量级要比极小值的量级要大些。

Vertically integrated net heating during (a) Dec-Feb and (b) Jun-Aug based on six years of ECMWF data. Contour interval is 50 W m-2; positive values are shaded.
图 3.25 垂直总净热值(a)12-1月 (b)6-8月,基于ECMWF六年的数据。等值线间隔50 W m-2;阴影为正值区(James 1994).7

那么对净热值的动力响应的性质有哪些呢?许多科学家都刻画了大气受赤道加热的动力响应模式,最为出名的来自Matsuno (1966)25 和 Gill (1980).26 他们发现热带环流中重要的纬向不对称性部分。利用浅水方程组,和Section 3.1的运动和连续方程,他们计算出了一种波解,这种波被之后的观测所证实。

Two-Layer Model of equatorial heating Other views of the circulations: horizontal wind vectors and vertical velocity contours; perturbation pressure contours overlaid on the same wind vectors; meridional integrated flow. The Rossby waves propagated westward and the Kelvin wave propagates eastward.
图 3.26(a)大气双层模式显示了在赤道的热源和大气的响应。
(b)环流的其他显示模式:水平风矢和垂直涡度等值线;扰动气压等值线,水平风矢;经向平均气流。罗斯贝波向西传播,开尔文波向东传播。改自 Gill (1980)26

这些方程的解是一种波动,沿着赤道传播并垂直扩展到对流层上层和平流层低层(图 3.26a)。热量的扩散在东面比西面传播得更远,但是产生的振幅西面会更大(图 3.26b)。方程的解在东边更加靠近赤道。这些沿着赤道的波动一部分向东传播,一部分向西传播。如图3.26b所示,在赤道的热源扰动产生了向东传播的开尔文波和向西传播的罗斯贝波。这些波有时与强对流相结合,特别是在印度洋和西太平洋地区,这些地方往往由于这些波动诱发热带气旋的生成。

Schematic of response to anti-symmetric heating away off the equator (a) horizontal wind vectors and vertical velocity contours; (b) perturbation pressure contours overlaid on the same wind vectors. A dominant anti-symmetric mode is a mixed Rossby-gravity wave that moves westward.
图 3.27 赤道反对称加热示意图(a)水平风矢和垂直涡度等值线;(b)扰动的气压等值线和风矢。显著的反对称模式有混合罗斯贝重力波向西移动。改自 Gill (1980)26

赤道热源的分布呈现反对称模式分布,促使赤道两侧形成相应高压和低压。在热源的西侧形成一个行星尺度的大气长波。在热量盈余区产生上升运动和气旋式环流,并且相应的在反侧形成反气旋。在东侧基本没有扰动产生的响应。显著的响应是一种反对称的赤道波,叫做混合罗斯贝重力波mixed Rossby-gravity wave(图 3.27b).我们会在热带变化一章中进一步讨论赤道的波动。

Tropical circulations that result from (a) anti-symmetric and (b) symmetric components of diabatic heating.
图 3.28 热带环流示意图:产生于赤道附近的非对称和对称非绝热加热,形成相应的哈德雷环流和沃克环流。着色背景是平均降水量,来自 the Global Precipitation Climatology Project for 1979-2008.

热带环流来自于对称和非对称加热的组合。非对称加热的一个结果是7月的哈德雷环流(图3.28蓝色箭头),这里的北半球热量盈余区有上升支,而在南半球冷却区域为下沉支。由对称加热部分形成的响应之一是沃克环流的太平洋支,这是一个东西向的环流,在温暖的西太平洋的热量盈余区有上升运动(图3.28 浅棕色箭头)。沿着赤道横跨太平洋的那个最大的环流叫做沃克环流,以纪念Sir Gilbert Walker,他通过观察澳大利亚达尔文和南太平洋的塔希提岛两地的气压振荡,确认了这个环流的存在。达尔文和塔希提岛之间的气压扰动可量化为 南方涛动指数Southern Oscillation Index (SOI)。现在广义的沃克环流指全球的平均东西向环流,由海洋大陆大面积的上升运动,南非热带,非洲热带地区之间的下沉运动等构成。

沃克环流的显著变化是厄尔尼诺南方涛动(ENSO),这是一个2-7年周期的大气海洋耦合现象。一般状况下,沃克环流在东面海洋有冷上升流,在南面有温暖的海温。在厄尔尼诺期,在东太平洋和中太平洋出现异常变暖,在西面出现异常变冷,南方涛动在这种海面增暖下发生转变。沃克环流的扰动会引起全球大气环流、降水模式、季节气候的转变。ENSO的极端冷事件称之为拉尼娜事件。


3.5 季风系统

3.5.1 季风的定义

全球大部分人口居住在季风区,所以季风的研究显得格外重要。很多季风区的社会都需要靠雨水供给的农业为他们提供食物。所以降水的量、降水时间、降水地点都非常重要。

Monsoon这个词来自于阿拉伯语’mausim’,意思为季节,这是首个用于表述在南亚和印度洋随季节转变的地面盛行风的词汇。伴随着季风的转变有不同的降水形势:在夏季伴随着登岸风和降水,在冬季伴随着干燥的离岸风。这个看似简单的图景实际上是一个复杂的系统,它的开始(爆发)、强度和撤退的时间等都是极富挑战性的预报问题。

The monsoon regions as defined by Ramage (1971).
图 3.29 Ramage所定义的季风区(1971).

Ramage经典的季风区标准(1971)27

  • 在1月和7月,盛行风转变120°+
  • 平均盛行风频率> 40%
  • 平均风速超过3 m s-1
  • 气压模式满足稳定性判据

符合上述标准定义的季风区如图 3.29所示。其中印度季风最符合这些条件。在此后数十年的时间里,季风区的范围逐渐被扩展(图 3.30)。现在全球季风系统包括在美洲夏季降水时间和风场特征与印度季风相似的区域。然而,如图3.30左边的图所示,有些区域的冬季并不满足经典的季风标准。

Monsoon Systems: OLR, 200hPa Streamlines, 850 hPa Wind Climatology (1979-1995) for the American, African, and Asian Australian Monsoons.  Top panels are peak boreal summer monsoon, Bottom panels are peak austral summer monsoon.  Small inset maps are digital elevation.
图 3.30 在各自峰值期的全球热带季风系统

3.5.2 季风演变概念模型

是什么引起了季风?额,这确实是个古老的命题。在1987年Webster28建立一个基于哈德雷1的理论,另一个哈德雷2在1735年添加了对流中的水汽反馈的理论。图3.31表述了,应用地转效应,用高湿海风原理假设的行星尺度季风环流模型。然而,关键是这个大陆热-海洋冷的模型是有明显漏洞的。因为陆地表面的温度实际上在季风期时是有所下降的(Focus Section 1.2, Box 1-2)。数十年的卫星图像观测表明,南亚季风并不是简单的局地 大陆-海洋 风。其实,季风区也属于在所有温暖热带海洋所组成的行星尺度降雨带(伴随着赤道槽)的一部分,只是在季风区,其振幅更加显著29,30。行星尺度的季风可以考虑以下基础机制:

  • 太阳辐射加热的季节振荡带来夏半球的净热值增加,从而引起赤道槽和热带辐合带的迁移
  • 在陆地和海洋的加热差异所导致的气压梯度(Halley 1686)
  • 地转导致的涡旋(Hadley 1735)
  • 对流中的水汽过程

大陆上的季风降水格外引起社会的关注,众多云系从温暖的海洋上生成并把降水带到大陆31。比如,在南亚季风建立的时期,海洋性降水带向北移动进入大陆。当全球季风系统受到行星尺度的净加热而产生响应的同时,区域的季风演变与海陆分布、海温梯度和地形因素有关。

Schematic of planetary-scale monsoon circulations driven by differential heating between warm land and cool ocean on a rotating planet. The upper panel shows a cross-section of the circulation between the ocean and the land.Schematic of planetary-scale monsoon circulations driven by differential heating between warm land and cool ocean on a rotating planet. The upper panel shows a cross-section of the circulation between the ocean and the land.
图 3.31 (上)暖大陆-冷海洋的环流剖面。(下)暖性大陆气团和冷海洋在有/无地转时的季风环流。

从概念上,由于太阳辐射加热的季节振荡对地球表面的影响,全球尺度夏季风是夏半球对正净辐射的首要响应。区域季风的演变取决于海陆分布、海温梯度和净海洋热量传输。海陆热力差异产生了海陆表面巨大的温度梯度。水平温度梯度诱发了高空的水平气压梯度和横向环流。在陆地上,传输热量的方式一般通过分子扩散完成,只有一小部分被储存起来。而混合了海洋上的水汽后,则可以更好地储存于分子中。28

环流并不是直接从海洋到达大陆的,由于有地球自转引起的科氏力,影响着风与洋流的形成和其演变的强度。最后,湿过程中的云效应、垂直涡度和云的辐射效应又反过来影响有云和无云地区的热量分布。

Schematic of the major circulations of the Indian Ocean, the planetary Hadley cell, the tropical easterly jet, and the Asian summer monsoon.
图 3.32 示意图:印度洋主要环流、行星尺度哈德雷环流、热带东风急流、亚洲夏季风。(改编自 Gerald Meehl)

南亚夏季风的主要特征有季风低压,低空登岸风和越赤道流,马斯克林高压,湿上升对流、高空高压、热带东风急流(图 3.32)32。热带东风急流是强劲的西南季风的一个高空通风系统(图 3.33)。这些区域的响应与行星尺度的大气环流相耦合(图 3.32)。

Mean wind vectors at 200 hPa (1948-2000). Color shades mark the TEJ and the schematic shows the ageostrophic motion and regions of expected upper-level divergence (rising motion) and convergence (subsidence).
图 3.33 (a)200hPa平均风矢(1948-2000)。填色区域为热带东风急流。(b)示意图:非地转运动,高空的辐合(下沉)和辐散(上升)

3.5.3 亚洲季风系统的演变

亚洲季风有着区域上的差异。最早建立的是在四月下旬孟加拉湾南部的季风,在五月初推进到东南亚、菲律宾和南印度,然后向北和西北逐渐推进扩展到大陆,在6月下旬到7月到达日本(图 3.34a)。在日本的季风到达峰值时,印度的季风已经开始南撤(图 3.34b)。按照季风演变规律的不同,亚洲季风区可分为两个差异较大但又互相联系的季风亚系统:印度夏季风/南亚季风 和 东亚季风(图 3.34)33。后者又可分为 东亚和西北太平洋季风。

(a) Mean onset date and (b) peak pentad of the Asian summer monsoon rainy season; (c) division of the Asian monsoon (Adapted from Wang and Lin 2002).
图 3.34 (a) 平均到达时间和(b)亚洲夏季风雨季峰值时间(c)亚洲季风区的划分(Adapted from Wang and Lin 2002)33
Climatological pentad (5-day) mean precipitation rate (mm/day) averaged over (a) the Indian sector (70°E-95°E) and (b) the western Pacific sector (115°E-140°E). The data used are derived from Xie and Arkin (1996)  for the period of 1979-2000. (From Wang et al. 2005)
图 3.35 (a)印度(70°E-95°E)和(b)西太平洋(115°E-140°E)5天平均降水率(mm day-1)。

数据从1979-2000,来自Xie 和Arkin(1996)34,改自Chang 等。(2005)35

南亚季风是东西向的降水带(ITCZ)季节迁移的北支(图 3.35)。降水带在北半球夏季从南半球迁移到北半球,而在北半球的冬季回迁。热带降水带在南亚季风区最北可达到20°N,而在冬季撤退到5°S36。有两个区域的降水带较为出名:加热的印度次大陆(符合早期的理论)和温暖的赤道印度洋以东洋面。这个海洋性云带即可增强,也可抑制主要季风降水。如果海洋的降水增强,则会在陆地引发下沉36。强大的温度梯度(图 3.36a)导致大尺度的气压梯度和从南印度洋到北印度洋的越赤道气流。在印度的20°N平均地面气压在冬季可达1016hPa,而在夏季风的峰值期则降到1002hPa37。强温度和气压梯度,再加上受东非山脉阻挡而转向的越赤道流形成了索马里急流。

Mean wind stress at the ocean surface showing the low-level Somali Jet which results from the strong cross equatorial pressure gradient and the high terrain of East Africa.Cross section along 5°N showing the magnitude and areal extent of the Somali jet core (data from the Japanese 25-year Reanalysis, 1979-2004).
图 3.36(a)海面平均风矢显示了索马里低空急流,这个急流由强越赤道气压梯度和东非高地的阻挡造成。(b)沿着5°N的垂直剖面,显示了急流中心的强度和范围。(数据来自Japanese 25-year Reanalysis, 1979-2004, 地形来自 NOAA NGDC)

年季风循环受跨赤道的大气和海洋热量传输支配。在夏季风期,印度洋海洋热量向南输送,冬季则向北输送(图 3.37)。向南运动的热量补偿了较冷的南印度洋,而冬季向北传输洋流使北印度洋增温。在夏季向南的暖流趋向于让北印度洋降温。海洋大气的耦合相互作用减少了海温梯度,并且增强了系统中的负反馈,由此调节了季风的季节极端性。此时南北向的梯度占主导,同时也存在着东西向的海温梯度(图 3.36a)。

Schematic of regulation of the seasonal cycle of the Indian Ocean for (a) the boreal summer (June- September) and (b) the boreal winter (December-February). Curved solid lines indicate near-surface winds forced by the large-scale pressure gradient associated with the cross-equatorial heating gradient denoted by ?warm? and ?cool?. Small blue arrows denote Ekman transport and the direction of the associated heat flux.
图 3.37 印度洋季节性循环概念图,(a)北半球夏季(6-9月)(b)北半球冬季(12-2月)。弯曲的实线表示近地面风,受大尺度气压梯度和越赤道的加热梯度所驱动。灰色箭头表示埃克曼传输和热流运动的方向。改自 Loschnigg 和 Webster (2000)38

印度和东亚季风的年周期变化是不同的,首要原因是大气对海陆差异和地形的加热响应不同。在暖大陆-冷海洋间强大的南北向梯度,在青藏高原加热下得到进一步加强,在印度产生了强大的季风。在东亚,其情况要复杂些。有来自澳大利亚和西北太平洋南北向冷暖梯度,也有受热亚洲大陆气团和较冷太平洋之间的东西向梯度。结果形成一个较弱的季风环流和伴随着热带季风环流和副热带锋区的降雨带,我们会在接下来具体介绍这一情况。

Conceptual model of the (a) mid-May and (b) mid-June monsoon onset over East Asia. Closed solid (dashed) curves mark the subtropical high before (after) onset. Light (dark) shades signify an increase (decrease) in clouds and/or evaporation. Dashed arrows show the direction of the extension of low-level westerlies (from Wu 2002).
图 3.38 概念模型 :(a)5月中旬(b)六月中旬 季风在东亚登陆。封闭实线(虚线)曲线表示副热带高压登陆之前(之后),淡(深)阴影表示云量和/或蒸发增加(减少),虚线箭头指示低空西风的方向。(Wu 2002)39

在东亚季风区,在六月中旬,相对于五月中旬的形势,云量和蒸发增加,纬向扩展的西南风进入太平洋后转变为东风(图 3.38a)。太平洋高压在五月中旬向东、向北扩展、登陆、加强(图 3.38a).一个大的温度槽在大陆取代了副热带高压脊。气流流入赤道槽,导致在ITCZ云量增加。季风槽Monsoon Trough从赤道槽向西北拓展进入大陆。东亚季风的其他特征相似于南亚季风,比如在对流层高空的热带东风急流和低空越赤道流。在高空北面是较弱的副热带高压(图 3.38)。

梅雨锋

Mean annual cycle of surface pressure over India (20N) and south Indian Ocean (20S).Conceptual model of the Meiyu-Baiu frontal cloud zone (Ninomiya, 2004 ). Note the hatched areas representing a family of cloud systems along the front, the 850 hPa low-level jet, and the mid-tropospheric maximum wind tracks (short-waves develop along this track and enhance instability and ascent).
图 3.39(a)东亚季风爆发的一般天气模式。嵌入图显示的是梅雨的相对位置。(b)梅雨锋云区模式图。(Chang et al. 2005)35注意斜线阴影区域代表了伴随着锋线的云系,850hPa低空急流,对流层中层极大风速轨迹(短波沿着这条轨迹发展并增强了不稳定性和上升运动)

对比东亚春季和初夏,一个最突出的特征就是梅雨锋,这是一个半永久准静止较弱的锋面系统,它从中国东部一直向东北延展到太平洋(图 3.39)。在北部的日本,称为Baiu(バイウ) Front。在韩国,称为Changma(장마) front(它们的相对位置如图3.39a)。梅雨锋开始于五月中旬同时向北推进,一直持续到仲夏初期。梅雨锋的关注点是它的由中尺度对流系统产生的持续强降水,此系统形成于锋线的东面(图 3.40)。不稳定层结,强上升运动和持续深对流,和低空急流共同作用,带来中国南部和孟加拉湾温暖潮湿空气,低空暖湿平流和位于副热带急流右前方强烈的高空辐散(图 3.41)。大部分的强降水在锋面的南侧和东侧,这里有较高的相对湿度(图 3.41b)。梅雨锋在中纬度的结构更加典型。间隔3天左右就有弱气旋扰动伴随着梅雨锋;它们在锋的北部边缘带来层云、雾和小雨;在锋的南面出现突发的雷暴和强降水。

Mean annual cycle of surface pressure over India (20N) and south Indian Ocean (20S).
图 3.40 10年(1998-2007)平均累积降水(a)5月11日-6月24日(b)从5月11日-6月24日梅雨期降水。注意梅雨对总降水的贡献。(改自 Xu 等 2009)40
Schematic diagram showing the flow structure of an observed Meiyu front (from Y. L. Chen et al. 1994). The thin solid line depicts the direct (D) circulation while the thin dashed line depicts the indirect (I) circulation. The heavy solid line shows the frontal position. The character J denotes the jet positions. The thick heavy line represents the tropopause boundary. Regions with relative humidity greater than 70% are shaded.
图 3.41 (a)伴随梅雨锋的MCS三维结构((来自 H. Yamada授权)41)。(b)示意图:梅雨锋结构42。细实线表示直接(D)环流,细虚线表示间接(I)环流。粗实线表示锋的位置。J表示急流位置。黑粗线表示对流层顶边界。阴影表示相对湿度大于70%的区域。

西太平洋季风槽

Schematic of the western North Pacific tropical cyclogenesis region partitioned into a monsoon trough zone and the near-equatorial ITCZ, meeting at a confluence zone (following Briegel and Frank 1997).
图 3.42 示意图:西北太平洋热带气旋生成区。分为季风槽和近赤道ITCZ,在交汇处为合流区(following Briegel and Frank 1997)43

西北太平洋季风区是地球上热带气旋发生频率最高的地方,主要是因为热带西太平洋是热带海洋最温暖的地区之一。热带气旋发生常在季风槽中(图 3.42Chapter 8, Section 8.3.2,)44 ,季风槽的位置对热带气旋活动的分布有重要影响45。在季风槽中的大尺度气旋式涡旋受低空赤道西风或西南风和副热带东风信风的驱动(图 3.42)。在季风槽东端,在向西的季风和向东的信风合流区有利于热带气旋的生成(图 3.42)。有时有反向季风槽生成45,从南海向东北扩展,这与梅雨锋方向一致(图 3.39a);在此条件下,热带气旋活动在季风期减少45。在罕见的情况中,热带气旋生成于季风槽转变为涡旋时46。虽然罕见,当变为涡旋后持续2-3周并且形成强烈的涡旋,仍然可以成为孕育热带气旋的种子46。想要了解更多关于季风槽中的热带气旋请阅读tropical cyclones章节。

东印度洋夏季风系统

东印度洋有其特征性的夏季风雷暴系统,比如苏门答腊风。苏门答腊风向东移动;在马六甲海峡时,低空的辐合与来自苏门答腊和马来西亚的陆风共同作用,形成小时间尺度的飑线47。他们在夜晚形成,在黎明时分发展到200-300km,向东移动到马来西亚和新加坡。

亚洲冬季风

因为寒冷的亚洲大陆气团和温暖的北太平洋(日本暖流/黑潮从赤道传输热量到北方)的反差,在东亚的冬季风比印度次大陆要强。另外,越赤道流被炎热的澳大利亚和相对冷的北太平洋的温度差异所加强,气流流向澳大利亚-印度尼西亚季风区。由于青藏高原阻挡了寒冷的西伯利亚气团,在印度次大陆南北向的差异有所减弱。

Differences of TRMM PR rainfall and QuikSCAT winds between boreal winter and boreal summer (DJF minus JJA). Warm colors are the boreal summer monsoon regime and cool colors are the boreal winter monsoon regime (Chang et al. 2005)
图 3.43 TRMM降水雷达数据和QuickSCAT风在北半球冬季和北半球夏季的差值(DJF减去JJA)。暖色调表示北半球夏季风模式,冷色调是北半球冬季风模式。(Chang et al. 2005)48

图3.43 显示在热带东亚,TRMM 降水雷达数据和QuikSCAT所测风场在DJF和JJA的差异。黄色-红色区域表示六到八月获得更多的雨水,而绿色-蓝色区域表示在12月到1月获得更多的雨水。在北半球冬季,降水更多的在赤道以北附近地区(比如,菲律宾和东南亚以东),但在北半球夏季,降水主要在北半球远离赤道的地方。这个不同点可以被部分地解释为,强大的北半球冬季风直接吹向海岸,因此降水可发生在靠近赤道的地方;但由于极少有海岸是面对着西南季风的风向,在北半球夏季降水较少。

海洋大陆在冬季风期,此时有最为活跃的对流,从亚洲大陆袭来冷气流成为这个地区范围广、持续长的深对流系统的不稳定机制之一。另外,天气尺度的婆罗洲漩涡(低空涡旋)49,虽然在大多数情况下为准静态,但它有时可以迁移到南海的南部。婆罗洲以西地区是中尺度对流复合体的多发地50。在离岸处东北季风和海风相互作用产生了强大的低空辐合,也有助于形成中尺度对流51

3.5.4 其他季风系统

3.5.4.1 澳大利亚-海洋大陆季风

 

澳大利亚-海洋大陆季风与亚洲季风52在季节上反相(注意观察南半球的暖季图 3.30(右侧图))。它于八月下旬的马来西亚爆发;可于一月初延伸到最南端,到达澳大利亚的北面(图 3.44)。

Monsoon onset dates defined by the threshold value of more than 30% of the mean high cloud amount for the monsoon season. Regions A and B had onset prior to 15 December and 26 December, respectively. Mean daily zonal winds and precipitation at Darwin, Australia.
图 3.44(a)季风建立的时间(由平均高云量大于30%阈值定义)。A地和B地的爆发时间分别在12月15日和12月26日之前(Adapted from Tanaka 1994)。达尔文每日平均纬向风(b)和降水(c)(来自 Drosdowsky 1996)53
.

季风环流跟随着加热最大值区的步伐,从北半球夏季的亚洲来到海洋大陆和澳大利亚北部。澳大利亚-印度尼西亚大尺度季风环流的热源太靠近赤道,受地转效应的制约,其强度并不强54。来自于亚洲大陆冷气团和暖海洋大陆之间的南北向温度梯度加强了越赤道气流。

在海洋大陆的大多数降水产生于中尺度对流系统,受海陆风或大尺度扰动的影响50。低纬度的海洋大陆靠近亚洲冷气团环流的地方会受到寒潮爆发的影响,引起大气不稳定和深对流。事实上,季风寒流与局地环流的相互作用被认为是一些不常见的近赤道热带气旋的形成因素,比如2001在新加坡1.5°N附近生成的台风画眉56。热带气旋的形成也喜欢在澳大利亚-海洋大陆季风期赤道以南的季风槽中57。在此地的热带气旋活动受年际变化的ENSO和季节内振荡的作用。厄尔尼诺让季风槽向东转向,伴随着降水和热带气旋活动。MJO活跃期产生了适宜热带气旋生成的环境58,59。了解澳大利亚-海洋大陆季风的更多内容请看由Dr. Mick Pope撰写的特别关注板块the Australian Bureau of Meteorology.

3.5.4.2 西非季风

西非季风(WAM)非常接近经典的季风标准定义,在夏季风期有潮湿,较冷的西南气流;在冬季则是干燥、暖而多尘的东北风(哈马丹风)(图 3.45)。热带锋(Intertropical Front)/热带不连续层(Intertropical discontinuity)是潮湿的西南季风和来源于撒哈拉的干热东北风的过度边界。与地面槽侧面相接的是南大西洋(圣海伦娜)和北非的副热带高压区。

OLR (shaded), 200 hPa streamlines, and 850 hPa wind vectors. Thick, black line marks the Intertropical Discontinuity (ITD) or Intertropical Front (ITF). The inset map shows the Normalized Difference Vegetation Index (NDVI) for 21-31 August 2000.
图 3.45 OLR(阴影),200hPa流线和850hPa风场。黑粗线表示热带不连续层(ITD)/热带锋(ITF)。内嵌图植被指数归一化均值差(NDVI)21-31 August 2000.

西非有显著的南北梯度,地表条件纬向均一:从几内亚湾到赤道的热带雨林,再到萨赫勒草原,最终到干燥多尘的撒哈拉地区。(内嵌图 3.45)

西非季风在五月上旬始于几内亚湾(图 3.46)。在6月下旬到7月上旬几内亚湾的雨季结束(图 3.46中的垂直黑线),降水的极大值到达萨赫勒。这个降水带的上移常称为突变,也是季风在萨赫勒爆发的信号(注意降水序列随时间的变化,其范围向高纬移动 图 3.46b)。在季风向南撤退后,八月份季风便开始了,海岸的雨季到十一月初结束。西非季风爆发日期与撒哈拉干热层的扩展有关,也许是通过北非的阿特拉斯山和阿哈加尔高原的地形60的相互作用引起。

Seasonal evolution of rainfall in 1978 based on daily rainfall observations between 10°W-10°E.
图 3.46 降水的季节演变,基于1978年日降水量观测,从10°W-10°E. (改自 Sultan 和 Janicot 2003)61

如图 3.47,在西非萨赫勒地区的月平均季风降水循环和ITF 。注意在萨赫勒地区的强降水梯度,同时萨赫勒以北大部分地区降水量极少,平均不足2mm/d,即使在季风峰值期亦如此。萨赫勒地区的特点之一就是它变化幅度极大的降水;在某一年里,降水带可位于平均位置的南边或北边。我们会在第3.5.5小节讨论西非季风的年际变化。注意降水的北界在ITF以南大约100-250km。最强的降水在ITF以南400km左右。降水和ITF纬度变化的联系,说明了ITF是季风降水推进和撤退的先导。

Long-term (1974?2003) average calendar monthly rainfall rate isohyets (mm day-1) in relation to the long-term average (1974?2003) monthly ITF position (red broken line) for April?October in the West African Sudan?Sahel zone.
图 3.47 长期(1974–2003)平均月降水率等值线,长期平均月ITF位置(红色虚线),从4月-10月,在西非苏丹-萨赫勒地区(Lele and Lamb 2010)62

西非季风的重要特征有萨赫勒热涡,冷海温和几内亚湾的强海温梯度,撒哈拉气层(SAL),热带辐合带和对流层中层的非洲东风急流(AEJ)63,64

Major large-scale features of the West African Monsoon and Tropical Atlantic.  Inset map is schematic of N-S vertical cross section along the Greenwich Meridian highlighting the moist monsoon, dry harmattan, heat low-AEJ-ITCZ system, convection, and the SAL(Major synoptic and mesoscale weather systems of the West Africa monsoon and the tropical Atlantic.
图 3.48(a)主要大尺度特征 (b)西非季风和热带大西洋主要天气尺度系统要点。
(a)的内嵌图是沿本初子午线的南北向垂直剖面图,强调了热涡—非洲东风急流-热带辐合带系统heat low-AEJ-ITCZ system,撒哈拉气层(SAL)(黄色阴影),大气边界层的经向变化。(改自Parker et al. 2005)65

非洲东风急流由热带东非经向温度梯度引发:从北边炎热干燥的撒哈拉地区到南边冷(海洋/潮湿土壤/植被)区。非洲东风急流平均最大风速在10 m s-1 约 650 hPa处。
南北向剖面(A-B)图说明在中低空温度和湿度的差异(内嵌图3.48a)。强垂直切变发生在非洲东风急流和低空西风季风间。垂直切变有利于深对流系统的组织,如图中的高大云系。不同尺度系统的相互作用是西非季风的特征之一,中尺度对流系统,如飑线,沿着/伴随着天气尺度的非洲东风波移动66,67,68(图 3.48b)。非洲东风波是非洲季风期常见的现象,每隔3-5天发生,它在非洲东风急流处有最大振幅。许多大西洋的热带气旋产生于非洲东风波中69。同时气旋发生有时是东风波与中尺度对流系统相互作用的结果(比如 图 8.24b)70,71。西非中尺度和天气尺度系统的结构、环境、和生命周期将在接下来的章节予以介绍。

3.5.4.3 美洲的季风

American Monsoon SystemsSchematic vertical section for the corresponding summer season east-west at about 30N across the NAM (left panel) and southwest-northeast across SAM region. Regions of deep convection and low-level jets are indicated (Panel for NAMS adapted from W. Higgins)
图 3.49夏季30°N北美季风区东西向剖面(左)和南美季风区西南-东北向剖面(右);(下图)季风剖面图,深对流区和低空急流(橙色圆)。(北美季风系统概念图 改自 W. Higgins, NOAA)

美洲季风在冬季没有反向的风向转变,所以它并不符合经典的季风定义。然而相对于年平均的风向,它有风距平的反转72

北美季风(NAM)的特征有:在索诺兰沙漠(Sonoran Desert)的加热;地面热涡进入潮湿的热带气团形势的建立(图 3.49 ,左)73。这个热带气团的源地有加利福尼亚湾、墨西哥湾(在850hPa上)。南美季风(SAM)接受来自热带大西洋和亚马孙盆地的水汽(图 3.49 右)74

Mean North American Monsoon onset date based on 5-day satellite estimated rainfall from Janowiak and Xie (2003).  The shading represents the standard deviation of the onset dates in days. (b) Schematic of the NAM system (Dave Gochis, NCAR).
图 3.50(a)北美季风平均爆发日期,基于5天卫星降水估计74。阴影是季风建立日期标准差。
(b)北美季风系统示意图(courtesy, Dave Gochis, NCAR).

虽然北美季风的降水大部分都在海洋中,美国西南部和北墨西哥的大部分区域每年主要降水还是来自于北美季风带来的降水(亚利桑那州大概35-45%,北墨西哥60%左右)。74季风降水大约持续100天,六月初在南墨西哥开始,在七月下旬到达它的最北端,美国中央大平原(图 3.50)。

(left) Mean South American Monsoon onset date based on occurrence of 3 mm/day of rainfall and (right) the accumulated gauge precipitation (mm) for the mature monsoon (December - February, 1979-1995)
图 3.51(a)南美季风平均爆发时间,基于3 mm day-1 降水观测。
(b)季风旺盛期累积降水(mm),12-2月,1979 to 1995.

在南美,季风开始于春季(图 3.51a)。在夏季风旺盛期,在东部和东南部达到降水累积量最大值(图 3.51)。此后,季风于秋季撤退。

3.5.5 季风的变化

变化无常的季风降水时不时导致严重的洪灾,或者干旱,这些气象灾害造成了巨大的经济损失和人员伤亡。75,76,77,78比如,在80年代,非洲萨赫勒地区的季风降水异常增多,造成了巨大的人员伤亡。季风的变化可由多种要素衡量,如降水距平;或者动力学变量如气压距平、环流级别。

图 3.52显示,主要的环流异常、南亚季风、西北太平洋夏季风。在强大的印度夏季风期,南太平洋副热带高压达到峰值,加强了越赤道气流。强西北太平洋季风中有一在热带形成的波列,穿越北太平,进入北美东部,这是热带-温带耦合和东亚季风遥相关影响的一个实例。

Schematic diagrams showing the major circulation anomalies associated with a strong Indian summer monsoon (a) and a strong western North Pacific summer monsoon (b). The lower-level and upper-level circulation anomalies are denoted by solid and dashed line, respectively. Letter ?A? and ?C? represent anticyclone and cyclone respectively.
图 3.5.2 示意图:分别与(a)强印度夏季风、(b)与强西北太平洋夏季风相关的主要环流异常。实线和虚线分别表示低空和高空环流异常。A和C代表反气旋和气旋。(改自 Wang et al. 2001)79

在印度,季风强度的变化主要用降水数据来衡量,也可用表明加热变化的垂直风切变作为指标。有许多的季风指数,但都没有受到公认。下面列举一些季风指数:

  • Webster and Yang index (WYI)-大尺度亚洲夏季风:850hPa-200hPa纬向垂直风切变距平, (40–110E, 0–20N), JJA;
  • Monsoon Hadley index (MH) – 南亚夏季风:850hPa-200hPa经向垂直风切变距平,(70–110E, 10–30N) JJA;
  • All-India monsoon rainfall (AIMR) or Indian Summer Monsoon Rain (ISMR)—印度夏季风降水:印度大陆平均降水值,JJAS;
  • Extended Indian monsoon rainfall (EIMR) – 南亚夏季风降水,包括临近海洋:(70–110E, 10–30N), JJA.

南亚夏季风降水的增加伴随着海陆温度梯度的增强80和南亚水汽通量的增加37,81(如 图 3.53)。

Schematic of the feedback between Indian and Pacific oceans and the Asian monsoon during the evolution from weak (dry) to strong (moist) monsoons
图 3.53 示意图:弱(干)和强(湿)季风年(Loschnigg et al. 2003)81

3.5.5.1 亚洲季风区的对流层准两年振荡(TBO)

对流层准两年振荡(TBO)是亚洲-澳大利亚季风年际变化的一个显著的信号。它在夏季降水和环流中有明显的表征,跟亚洲季风变化和ENSO有着一定的联系。82图 3.54描述了地面风场、对流、海温、沃克环流、温带气旋和温跃层深度的距平变化。80,83,84,85,86

Time sequence of TBO evolution showing positive minus negative TBO composites for equatorial upper-ocean temperature differences (blue-red, °C), SST (blue-red, °C), surface wind stress (scaling arrow = 0.03 N m−2), precipitation (pink-green, mm day−1), surface winds (scaling arrow = 1.0 m s−1), and schematic representation of large-scale Walker circulation anomalies for (a) DJF − 1 showing the weak Australian monsoon prior to a strong Indian monsoon, (c) strong Indian monsoon season during JJAS, (e) strong Australian monsoon following a strong Indian monsoon. Contour interval for precipitation is 0.5 mm day−1 (values between −1 and +1 mm day−1 are not plotted), for SST is 0.25°C, and for upper-ocean temperature is 0.5°C. Notice the opposite anomaly phases during the DJF-1 and DJF+1. (Meehl et al. 2003).
图 3.54 TBO演变时间序列,正值减去负值的TBO合成,包括赤道上层海温异常(蓝-红,°C)、海温(蓝-红,°C)、表面风应力((scaling arrow = 0.03 N m-2)、降水(粉-绿,mm day-1)、表面风场(scaling arrow = 1.0 m s-1)、大尺度沃克环流异常的示意图(a)DJF – 1 期在强印度季风前的弱澳大利亚季风,
(b)在JJAS时的强印度季风,(c)强澳大利亚季风,伴随着强印度季风。
降水等值线间隔为0.5 mm day-1 (-1 和 +1 mm day-1 的数值未画出),海温等值线间隔为0.25°C,上层海温为0.5°C。注意DJF-1和DJF+1的距平象限反转(改自 Meehl et al. 2003)
87

3.5.5.2 厄尔尼诺-南方涛动(ENSO)

有些全球季风的年际变化与ENSO事件有关。ENSO通过沃克环流东西向的变化和亚欧大陆的增温来影响南亚夏季风。一般来说,在厄尔尼诺事件时,雨量比平时少(负距平);在拉尼娜事件时是正距平。然而自80年代末开始,厄尔尼诺事件的影响认为正在降低88。然而事实上,在1997年那次强厄尔尼诺极端事件中,印度夏季风降水反而比平时高。89

ENSO强烈影响着东亚-西北太平洋夏季风和冬季风,虽然ENSO是在北半球的冬季达到最强。在ENSO循环中发生的热带太平洋的海温异常,诱导西北太平洋季风环流的异常。在夏季,它导致气旋-反气旋环流的东西向异常,从而引发南北向偶极子现象的发生90,在秋季异常环流的方向一般为东北-西南向。

ENSO对澳大利亚季风区有重要的影响。一般来说,厄尔尼诺事件伴随着干旱(特别是在东澳大利亚),和西太平洋和东印度洋的海温负距平。ENSO影响澳大利亚的降水和温度;伴随着ENSO循环的结束,澳大利亚北部在接下来的几个月里趋向于变暖。印度尼西亚西部对ENSO的相关较弱,但中部和东部地区在雨季时有较好的相关。91最大的相关在冬半球—在拉尼娜事件中海洋大陆的降水增加。

(left) A schematic of the SOI for a typical El Niño, together with the rainfall pattern in the Australian region.  (right) Correlation between SOI and annual rainfall for Australia. Darker colors indicate a higher correlation.
图 3.55 (左)典型厄尔尼诺年的SOI指数曲线,及相对应的澳大利亚降水模式。(右)SOI和澳大利亚年降水的相关系数。深颜色代表高相关。(由Mick Pope提供)

ENSO影响西非季风,但是相关性变化很大(从1940s以来)92。ENSO和西非季风的遥相关发生在沃克环流的调整期,这导致热带西非低空季风气流的下降/减弱,或者低空季风气流的抬升/增强。当几内亚海岸和萨赫勒的降水有一致的信号时,ENSO就有极为重要的影响。当热带大西洋比ENSO发挥更大的影响时,ENSO与降水有较弱的负相关性,此后几内亚海岸和萨赫勒有反向的降水距平信号。93

3.5.5.3印度洋偶极子现象/纬向模式

亚-澳季风变化的主要模式是印度洋偶极子/印度洋纬向模式,一个东西向的海温异常振荡,同时伴随着降水矩平的改变。94,95,96一般情况下,IOD正相态伴随着非洲的强降水,而在印尼降水减少;在IOD负相态则是反向的模式(Fig. 3.56)。一般,偶极子在初夏-仲夏有明显的开始信号。在晚秋到初冬达到其峰值,然后在北半球的冬季迅速衰退。负相位有相似的时间、量级和衰退。

Schematic of the positive and negative phases of the Indian Ocean Dipole
图 3.56 印度洋偶极子正相位和负相位。红色阴影为正海温异常,深蓝色为负海温异常。白色表示比平时更多的降水。

赤道印度洋振荡(EQUINOO)89,97是大气对印度洋偶极子的耦合效应。印度夏季风降水的极端异常现象与EQUINOO和ENSO的相互作用有关。IOD的形成机制还在讨论之中,并且它是否与ENSO完全独立,也有待研究。78,94

3.5.5.4 西非季风的年际变化

对西非季风产生关键影响的是几内亚湾海温与受热的北非大陆的热力学差异。热带大西洋对西非季风的影响比ENSO更大。海洋对西非季风的作用在地表—大气反馈中得到加强,如植被覆盖的辐射效应。98

萨赫勒降水在七十年代初出现了明显的下降(图 3.57),这种现象已经得到广泛的研究。93,98,99,100,101这种下降在八十年代初尤为明显,这与热带海洋盆地的海温异常变暖相关,特别是靠近热带印度洋和热带大西洋的区域。温暖的东大西洋会减少海陆差异和流入西非的季风。

Time series (1941?2008) of average normalized April?October rainfall departure (σ) for 20 stations in the West African Sudan?Sahelian zone (11°?18°N) west of 10°E. From
图 3.57 时间序列(1941-2008),西非10°E以西的苏丹-萨赫勒地区(11°–18°N)20个测站4-10月标准化降水距平。(Lele and Lamb (2010)62

3.5.5.5 季节内变化

季节内振荡与季风活跃期与中断期有关。在间断期,天气条件从湿润转变为相对干燥,持续1-2周甚至是30天(图 3.58)。

Precipitation and surface to 700 hPa precipitable water over the Arabian Sea and Indian west coast for 1 May to 15 Sep 1979. Notice the reduction (break) then increase (active) in rainfall amounts (Cadet 1986, Tellus).
图 3.58 1979 5月1日到9月15日阿拉伯海和印度西海岸的降水和 地面到700hPa可降水量。注意降水量的减少(间断期)和增加(活跃期)(Cadet 1986 , Tellus)102

对于南亚季风(图 3.59)

  • 活跃期 强劲西风:孟加拉湾和印度出现强降水,但在赤道相对干燥
  • 间断期 弱西风:加拉湾和印度出现降水减少,赤道潮湿
Composites of OLR anomalies (Wm?2) during (a) break and (b) active spells. Period of analysis: 1979?2007 (From Rajeevan et al. 2010)
图 3.59 在间断期和活跃期的长波出射距平(Wm-2)。1979-2007分析资料(from Rajeevan et al. 2010).103

隔2-6周,云带从赤道印度洋的北移现象也是南亚季风的季内变化的一个特征,这个特征有利于大陆上辐合形势的维持。103在南亚和东亚季风中,主要存在10-20天和30-60天的周期。104,105后者就是季内振荡的时间尺度。

季节内振荡(MJO)

MJO,热带降水和大尺度环流的30-60天振荡,在印度洋和太平洋最强,自然而然其对亚澳季风是有影响的。58,106MJO向东移动,它的循环一般起始于西印度洋的降水异常,此后在大西洋再次出现,然后又到印度洋。107,108MJO的活跃期有利于印度和澳大利亚季风的建立,而在其抑制阶段,季风建立则相当少。

Probability of weekly rainfall in highest quintile conditioned upon the phase of the MJO described by the daily values of the Real-time Multivariate MJO (RMM) series. The rainfall weeks are centered on the day upon which the RMM phase is determined. The probabilities are computed for overlapping weeks in the DJF season for 1974?99. (From Wheeler and Hendon 2004)
图 3.60 MJO各阶段每星期的可降水。实时多元MJO序列的每日资料。降水的数周都集中在RMM阶段的那些天。可降水率由1974-99年DJF交叠各星期计算得出。(来自 Wheeler and Hendon 2004)109

相较于其他地方,MJO在澳大利亚-印度尼西亚季风区起着更为重要的作用。110比如,在澳大利亚北部,MJO湿阶段的极大每周可降水率差不多是在干燥阶段的3倍(图 3.60)。在北半球冬季,111,112MJO经常在海洋大陆有极大振幅,这是驱动着全球环流的热带的一个主要热源(图 1.14a)。因此在海洋大陆受MJO加强的对流可以通过波列传播到下游区域,到达北太平洋、北美甚至到在1-2周的时间里到达欧洲。113

为什么MJO与澳大利亚季风的活跃与中断期有关呢?通过对MJO和澳大利亚季风降水的研究发现,在1987/1988有强相关性(R2 = 0.58),但在1982/83却没有(R2 = 0.09)。这个结果表明MJO在某些年份会对季风造成较大的影响,也许是跟那些年份MJO的强度有关。114

在澳大利亚-印度尼西亚季风期,MJO的一个最为重要的影响是在热带季风活动中的作用。在MJO的活跃阶段会有更多的热带气旋生成,相反在抑制阶段则生成量下降。58,59

MJO对西非季风也有影响。根据观察到的案例表明,MJO传播到西非时,有利于激发赤道波,并且增强上升气流和潮湿西风的流入。

赤道陷波

亚-澳季风和西非季风的次季节变化也与开尔文波和赤道罗斯贝波的耦合有关(图 3.26)。在西非,时间尺度为25-60天干燥/湿润期与受太平洋MJO作用的赤道开尔文(第四章 图 4.15)和罗斯贝波的耦合相关。115, 116

美洲季风的湿润和干旱期

Schematic illustration of the circulation during a dry (left) and wet (dry) surge in the Gulf of California (Wayne Higgins, NOAA)Opposite phases of the dominant mode of variability over South America during the monsoon. Thick orange arrows are the low-level jets. The red circle is where mesoscale convection will be enhanced.
图 3.61(上)加利福尼亚湾湿润和干燥期的建立(Wayne Higgins, NOAA)。(下)南美季风期变化的反相显著模式。橙色粗线表示低空急流,红圈表示中尺度对流系统加强的区域(Chang et al. 2005)35

由低空急流导致的地形抬升加强和地面加热,诱发在美洲季风系统的雷暴(图. 3.49)。像其他地方的季风,北美季风的雷暴活动也有中断期和爆发期、在爆发期,在高空西风中有一个浅槽形成,伴有强地面加热和水汽辐合,引发大气不稳定和大范围的雷暴活动。相比之下,在中断期,有来自地面太平洋高压延展到内陆所形成的高压脊,切断了南风气流并抑制对流运动的形成。

另外,关于MJO的调控,南美季风的季内震荡也受来自亚马孙水汽通量和南美辐合带强度的影响117(图 3.61, 下)。在南美辐合带SACZ降水的增强时,在副热带平原的降水也会减少。当来自亚马孙的水汽通量增加时,SACZ减弱,同时内陆的副热带平原降水增加(图 3.61,右下)。

3.6 热带环流和降水分布

在热带大尺度环流特征影响着热带大洋盆地和邻近大陆地区的降水分布。总的来说,副热带的海洋东部和邻近大陆较为干燥;向西演变,副热带高压脊和信风逆温层减弱,云层变高,降水的强度和量增加,如图 3.62所示。与这个示意图不相符的地点是印度洋,这是因为印度洋主要受西风季风的影响。

Idealized distribution of precipitation influenced by the subtropical highs, ITCZ and trade wind inversion.
图3.62受副热带高压脊、ITCZ和信风逆温层影响下理想化降水分布图

由于信风驱动造成的洋面暖池,西面的热带海洋盆地是极度潮湿的区域。热带西太平洋有着最为温暖的海水,称为热带暖池。由于温暖的海面和水汽的向下游平流输送,在附近的陆地常有降水发生。中等湿润地区在极度湿润地区的北部和南部。图3.63 说明了在7月热带大西洋与太平洋,和1月东南印度洋降水和盛行风的概念模型

Seasonal mean precipitation (shaded, 925 hPa horizontal wind vectors, and 500 hPa geopotential heights (contoured every 10 dam) for (a) Jun-Aug and (b) Dec-Feb. Precipitation data from Xie and Arkin (1997) for the period 1979-1999; wind and geopotential height data from ERA-40 for the period 1962-2001.
图3.63 季节平均降水(填色,925hPa水平风矢,500hPa位势高度(等值线/10dam))(a)6-8月(b)12—2月。降水数据来自1979-1999年,Xie 和 Arkin;风场和位势高度数据从1962-2001,来自。(来自 Chang 等人 2005)35

3.7 热带在大气环流中的角色

The role of the Tropics in the General Circulation
图 3.64 热带在大气环流的热量和动量传输中的作用示意图

热带在大气环流的作用可以由图 3.64概括。热带提供了热量盈余的热源,这驱动着全球环流。热量和动量通过哈德雷环流进行传输,这对全球热平衡和角动量平衡有至关重要的作用。最近的研究表明,哈德雷和沃克环流的强度是波动的,实际上他们在九十年代的强度有所加强118,119。科学家认为这与环流中的辐散(下沉)区和辐合区(上升)的云量、辐射、大气垂直涡度的变化有关。这种哈德雷环流和沃克环流的加强,与更多热量,长波辐射在热带大气中的逃逸和阳光反射上的减少(大概4W/m2)有关。

特别关注

关注一:全球环流的模式

3F1.1 全球模式

全球范围的地球-大气系统(包括陆地,海洋,冰层和生物圈)数值模式是以在3.1部分和3.3部分的动力学和热力学的基本原则为基础,如运动方程,连续性原则,理想气体定律,流体静力平衡和热力学定律。这些方程被转换成计算机程序并转化为全球网格上的数值点。模式的初始化是利用从地面观测,无线电探空仪,卫星,船舶,飞机的数据,然后在一个指定的时间进行数据收集和同化。图3.11是一个全球的海表温度,海冰密集度,海平面气压和低层风的模式说明。在模拟的过程中,一些流程太复杂以致不能用计算机计算,或尺度太小不能由模式的网格点表现出,或对于天气过程的知识还不足够丰富而无法精确模拟出该过程。这些过程是参数化的,这意味着,它们的数值估计来自于它们所产生的效应而不是来自模拟天气过程。例如,海洋模式就包括了一些太小而不能被网格点表示的小涡旋的参数化表示。(图3f1.2)因为我们无法观测每一个涡旋,或者在一个格点上无法表征它们。同样对于云单体也可以这么说。全球模式不能预测云单体,但尽力提供最佳接近云影响的模式。全球气候模式过程和组成的示意图在图1.15

Simulation of the global climate depicts sea surface temperatures and sea ice concentrations, sea level pressure and low-level winds, including warmer air moving north on the eastern side of low-pressure regions and colder air moving south on the western side of the lows. Such simulations, produced by the NCAR-based Community Climate System Model, can also depict additional features of the climate system, such as precipitation.
图3f1.1全球气候的模拟描绘了海洋表面温度和海冰密度,海平面气压和低层风,包括在低气压区域的东面暖空气向北移动,和在低气压的西侧较冷空气向南移动。这些模拟,由NCAR社区气候系统模式制作,也可以描述气候系统的其他特征,如降水
Conceptual cross section showing parameterized processesIllustration of all processes and physical model elements that are parameterized in numerical weather prediction models. Includes 20 different items, such as topography, deep convection, longwave radiation absorption and emission, microphysical processes, land surface processes and land use types, soil and vegetation processes, snow/water/ice at the earth surface, atmospheric radiation transfer, etc.
图3f1.2海洋和大气模型的过程参数化示意图

全球模式,也像其他数值模式一样,有很多偏差,包括:

l 初始条件:观测数据的覆盖(观测数据的多少和数据的频率)、数据的错误(仪器和代表性),质量控制,在数据同化模式和模式格点的客观性分析错误、变量缺失。

l 数值近似:方程本身的误差和它们的数值近似误差,使用的网格类型(正交的,六角形的。立方形的),空间分辨率,时间积分的步骤和程序中的错误。

l 边界条件:表面条件,垂直边界条件,地形

l 物理近似和参数化:云(积云对流),微物理过程(降水),辐射,边界层湍流

l 本身预报的局限性:流体运动是混沌的。在大气中存在各种不同尺度的运动。在模式中不能反应尺度小于观测分辨率的能量。因此,真正大气和模式大气是不同的,会随着时间推移而发散。

3F1.2气候变化和热盐环流

墨西哥湾流从热带向北大西洋运送暖流.(图3.20)这使得欧洲在它所处的纬度上仍然温暖(相较于在同一纬度的伦敦和哈德逊湾和处于同一纬度的瑞典和格陵兰岛)。墨西哥湾流受以下条件驱动:

l 来自北大西洋环流的风应力(图3.20)。

l 热盐环流:北冰洋的冷盐水的下沉(高密度海水在低密度海水中下沉),使得墨西哥湾流补充流失的海水。(图3.22b)。

 

漫长的冬夜(极夜)导致海水变冷,由于北冰洋海冰的形成造成了海水盐度的提升,从而引起水密度的增加,寒冷的水和较高的水密度驱使水俯冲。冰盖的冰冻,使得盐分从冰中析出,因而冰的形成增加了水的盐度。

预测全球变暖对热盐循环的影响有:

l 冰盖融化造成北冰洋淡水增加

l 在西伯利亚的永久性冻土和冰川融化,造成北向流动的河流,可以把大量温暖淡水带入北冰洋。低密度水抑制海水俯冲和深层寒冷水循环的供应

l 低密度流抑制海水俯冲和对海洋深层大循环的补给

l 墨西哥湾流减缓,并在低纬度就向东转向,因为它不再有被向北驱使的动力,

l 由此向北的热量输送中断,造成欧洲和北美的东北部变得更冷

由于地球大部分被海洋覆盖,海洋对气候的相互作用是也体现在改变气候上。由于风场产生洋流,气候风的多变性可以影响洋流,进而影响热量和、水汽的输送,进一步影响天气和气候,反过来影响风和洋流,等等。气候变化和洋流变化之间的反馈作用有助于锁定一定的气候模式。

3F1.3 未来亚洲夏季风变化展望

全球季风与大气-海洋-陆地系统耦合,是对热力差异的响应。因此未来任何净热量的改变,如它的空间和时间分布都将会影响到季风系统。然而,正如前面提到的,季风系统是在多个尺度中显示多变,引起任何预测方案都对其有固有的不确定性。我们对云辐射过程和大气、海洋、陆地之间的反馈的了解有限,这进一步加剧了问题的复杂化。

在全球温度增加的情况下,对未来的南亚季风的影响有

l 由于海陆温度差异的增加和潮湿空气向南亚季风区的输送增加,南亚夏季风的季节性平均降水会增加。96,120

l 由于降水量与热带太平洋海温的变化有关,南亚夏季风降水的年际变化增加.96,120

同时,最近的模拟表明,南亚夏季风环流可能会因为二氧化碳的增加而减弱121(图 3F1.4)。正如前面提到的,模式在复杂性和不确定性上各有区别,所以在不同的模式中,他们的预测也不尽相同。

Multi-model mean changes in surface air temperature (°C, left), precipitation (mm day?1, middle) and sea level pressure (hPa, right) for boreal winter (DJF, top) and summer (JJA, bottom). Changes are given for the IPCC A1B scenario, for the period 2080 to 2099 relative to 1980 to 1999. Stippling denotes areas where the magnitude of the multi-model ensemble mean exceeds the inter-model standard deviation
图3F1.3 多模式平均地表温度变化(℃,左)、降水量(mm每天,中)和海平面气压(hpa,右),北半球冬季(DJF,上图)和夏季(JJA,底部)。变化由IPCC A1B方案预测给出,2080到2090年期间相对于1980到1999年。点表示的区域是多模式全体平均超过了模式间标准差的地方
Projected future changes in South Asian summer precipitation and monsoon onset date. This study found that rising future temperatures could lead to less rain and a delay in the start of monsoon season by up to 15 days by the end of the 21st century (Ashfaq et al. 2009)
图 3F1.4 未来南亚夏季降水的变化和夏季风爆发日期的预测。这个研究发现未来气温上升可能会导致降水减少和季风爆发的延迟,在21世纪末可能延时长达15天(阿什法克,2009)121

关注二:热带低空急流

低空急流(LLJs)是对流层低层风速极大值区域,Stensrud (1996)122定义低空急流如下:

l 最大风速的狭长区低于700hPa;

l 垂直风廓线靠近急流中心时风速增加,在它上面减少;

l 急流侧边存在水平风切变,例如急流边缘风速减弱。

Regions of LLJs and frequent occurrence of mesoscale convective systems (from Stensrud 1996).
图 3F2.1. 区域(阴影部分)和中尺度对流系统(矩形)的出现频率122

低空风速极大的区域是低空水汽的通道,用以供给对流和降水(图 3F2.1)。122,42,123低空急流的变化可以对区域降水造成很大的影响。

其中最著名的热带急流是索马里急流,这是一个最大风速在850hPa附近的西南急流,它向南印度季风输送水汽(图3.22)。这个急流由越赤道流被东非高原阻挡而向西流动形成(图3.36)在东亚,持续的强对流和强降水天气出现在850hPa低空急流存在的地方出现深对流和强降水天气。并与梅雨或梅雨锋相互作用(图3.39, 3.41a)。41,42南美低空急流(图3.49)是南美降水过程的一个重要组成部分,它的位置变化和强度变化对南美暖季降水有很大的影响。123

The Caribbean LLJ in February and July and the vertical profile of the zonal winds averaged for the column 71°–76°W and 12°–16°N (Adapted from Muñoz et al. 2008)
图3 F2.2. 加勒比低空急流和71°–76°W 和 12°–16°N.平均纬向风的垂直廓线(改编自Muñoz等,2008)12

一个有关信风带中低空急流的例子――加勒比低空急流(CALLJ),它的一支最大风速在925hpa,在二月和七月盛行在加勒比海上空12-14°N和70-75°W之间(图3F2.2)124。这支急流向北美洲的中部和北部输送大量的水汽。124,125

业务重点

季节性气候学知识

l 热带和亚热带地区的路线图3.22

l 热带降水模式3.6

流线分析

l 确定辐散辐合区 3.15

l 识别曲率涡度和切变涡度 ,图3.8

区域性季风的爆发和演变

l 季风演变的概念模型,3.52

l 亚洲夏季风爆发 图3.34

l 澳大利亚-海洋大陆季风爆发 图3.44

l 北美季风爆发 图3.50A

l 南美季风爆发 图3.51

l 西非季风爆发 图3.46,3.47

 

季风系统

l 梅雨或梅雨锋和天气系统,3.53

l 东亚冬季风对流3.53

l 海陆季风对流3.541,3.53

l 主要的西非季风天气系统3.542

季风区和热带气旋起源

l 季风槽和热带气旋成因3.5.3

l 澳大利亚海洋大陆季风,热带气旋3.5.4.1

l 西非和大西洋 3.542

 

预测季风间断

l 南亚在活跃季风期与中断季风期的风的转变

l MJO对季风的影响 3.5.5.5

l 美洲季风的潮湿和干旱期3.5.5.5

 

季风的年变化

l 对流层准两年振荡(TBO)3.5.5.1

l ENSO对亚洲季风的影响3.5.5.2

l 印度洋偶极子或带状模式3.5.5.3

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108. Matthews, A. J., 2000: Propagation mechanisms for the Madden-Julian Oscillation. Quart. J. Roy. Meteor. Soc., 126, 2637-2651.

109. Wheeler, M. C., H. H. Hendon, 2004: An all-season real-time multivariate MJO index: Development of an index for monitoring and prediction. Mon. Wea. Rev., 132, 1917-1932.

110. Wheeler, M. C., K. M. Weickmann, 2001: Real-time monitoring and prediction of modes of coherent synoptic to intraseasonal tropical variability. Mon. Wea. Rev., 129, 2677-2694.

111. Chen, B., M. Yanai, 2000: Comparison of the Madden-Julian oscillation (MJO) during the TOGA COARE IOP with a 15-year climatology. J. Geophys. Res., 105, 2139-2149.

112. Hsu, H., M. Lee, 2005: Topographic effects on the eastward propagation and initiation of the Madden-Julian Oscillation. J. Climate, 18, 795-809.

113. Shapiro, M. A., A. J. Thorpe, 2004: THORPEX International Science Plan. Vol. WMO/TD-No.1246 WWRP/ THORPEX, No.2, World Meteorological Organization, 51 pp.

114. Slingo, J., D. Rowell, K. Sperber, and F. Nortley, 1999: On the predictability of the interannual behaviour of the Madden-Julian Oscillation and its relationship with El Niño. Quart. J. Roy. Meteor. Soc., 125, 583-609.

115. Lavender, S. L., A. J. Matthews, 2009: Response of the west African monsoon to the Madden-Julian Oscillation. J. Climate, 22, 4097-4116.

116. Janicot, S., B. Sultan, F. Mounier, N. M. Hall, S. Leroux, and G. N. Kiladis, 2009: Dynamics of the west African monsoon. Part IV: Analysis of 25-90-day variability of convection and the role of the Indian monsoon. J. Climate, 22, 1541-1565.

117. Paegle, J. N., L. A. Byerle, and K. C. Mo, 2000: Intraseasonal modulation of South American summer precipitation. Mon. Wea. Rev., 128, 837-850.

118. Chen, J., B. Carlson, and A. Del Genio, 2002: Evidence for strengthening of the tropical general circulation in the 1990s. Science (Wash.), 295, 838-840.

119. Wielicki, B., T. Wong, R. Allan, A. Slingo, J. Kiehl, B. Soden, C. Gordon, A. Miller, S. Yang, D. Randall, F. Robertson, J. Susskind, and H. Jacobowitz, 2002: Evidence for large decadal variability in the tropical mean radiative energy budget. Science (Wash.), 295, 841-844.

120. Meehl, G., J. Arblaster, 2003: Mechanisms for projected future changes in south Asian monsoon precipitation. Clim. Dyn., 21, 659-675.

121. Ashfaq, M., Y. Shi, W. Tung, R. J. Trapp, X. Gao, J. S. Pal, and N. S. Diffenbaugh, 2009: Suppression of south Asian summer monsoon precipitation in the 21st century. Geophys. Res. Lett., 36.

122. Stensrud, D. J., 1996: Importance of low-level jets to climate: a review. J. Climate, 9, 1698-1711.

123. Vera, C., J. Baez, M. Douglas, C. Emmanuel, J. Marengo, J. Meitin, M. Nicolini, J. Nogues-Paegle, J. Paegle, O. Penalba, P. Salio, C. Saulo, M. Silva Dias, P. Silva Dias, and E. Zipser, 2006: The South American low-level jet experiment. Bull. Amer. Meteor. Soc., 87, 63-77.

124. Munoz, E., A. Busalacchi, S. Nigam, and A. Ruiz-Barradas, 2008: Winter and summer structure of the Caribbean low-level jet. J. Climate, 21, 1260-1276

125. Cook, K. H., E. K. Vizy, 2010: Hydrodynamics of the Caribbean low-level jet and its relationship to precipitation. J. Climate, 23, 1477-1494.

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A

Absolute angular momentum
For the atmosphere, the absolute angular momentum, per unit mass of air, is the sum of the angular momentum relative to the earth and the angular momentum due to the rotation of the earth.
Absolute vorticity
See Vorticity.
Absorber
Anything that retains incident electromagnetic radiation due its physical composition.
Absorption
The process by which incident radiant energy is retained by a material due to the material’s physical composition.
Absorption band
A portion of the electromagnetic spectrum where radiation is absorbed and emitted by atmospheric gases such as water vapor, carbon dioxide, and ozone.
African easterly wave
A trough or cyclonic curvature maximum in the trade-wind easterlies. The wave may reach maximum amplitude in the lower middle troposphere.
Aggregation
The clumping together of ice crystals after they collide.
Anomaly
The deviation of a quantity over a specified period from the normal value for the same region. For example, El Niño is identified by sea surface temperature anomalies.
Atlantic Multidecadal Oscillation (AMO)
A natural oscillation of the North Atlantic SST between warm and cool phases. The SST difference between these warm and cool phases is about 0.5°C and the period of the oscillation is roughly 20-40 years (the period is variable, but is a few decades long). Evidence suggests that the AMO has been active for at least the last 1,000 years.
Attenuation
Any process in which the intensity of radiation decreases due to scattering or absorption.
Atmospheric Window
A portion of the electromagnetic spectrum where radiation passes through the atmosphere without absorption by atmospheric gases such as water vapor, carbon dioxide, and ozone.
Available potential energy (APE)
The portion of the total potential energy available for adiabatic conversion to kinetic energy. The total potential energy is a combination of the APE and the potential energy representing the mass distribution needed to balance the mean atmospheric motions.

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B

Backscatter
That portion of radiation scattered back toward the source.
Baroclinic
Dependence on the horizontal temperature contrast between warm and cold air masses., In a baroclinic atmosphere, the geostrophic wind varies with height in direction as well as speed and its shear is a function of the horizontal temperature gradient (the thermal wind equation).
Barotropic
The atmosphere has the same horizontal structure at all levels in the vertical. This is equivalent to the absence of horizontal temperature gradients.
Barotropic-Baroclinic Instability
Barotropic and baroclinic instability analyses are used to explain the growth of a small perturbation to the flow. A perturbation growing due to baroclinic instability draws its energy from the available potential energy (APE). A perturbation growing due to barotropic instability draws its energy from the kinetic energy of the background flow. A perturbation growing through both APE and mean kinetic energy conversion to kinetic energy of the growing system (intensifying the system) is developing through combined barotropic baroclinic instability.
Best track
As defined by the National Hurricane Center, it is a subjectively-smoothed representation of a tropical cyclone’s location and intensity over its lifetime. The best track contains the cyclone’s latitude, longitude, maximum sustained surface winds, and minimum sea-level pressure at 6-hourly intervals. Best track positions and intensities, which are based on a post-storm assessment of all available data, may differ from values contained in storm advisories. They also generally will not reflect the erratic motion implied by connecting individual center positions fixed during operations.
Beta (β) effect
Denotes how fluid motion is affected by spatial changes of the Coriolis parameter, for example, due to the earth’s curvature. The term takes its name from the symbol β representing the meridional gradient of the Coriolis parameter at a fixed latitude. The asymmetric flows resulting from the interaction of the vortex with the changing Coriolis parameter is known as the β-gyres.
Beta (β) plane
An approximation of the Coriolis parameter in which f = f0 + βy, where β is a constant. The Coriolis parameter is assumed to vary linearly in the north-south direction. The term takes its name from the symbol β representing the meridional gradient of the Coriolis parameter at a fixed latitude.
Blackbody
An object that absorbs all incident radiation and emits the maximum amount of energy at all wavelengths.
Blended precipitation estimate
An estimate that is derived by combining low earth-orbiting microwave measurements, which have high resolution but low frequency, with the more frequently available geostationary IR.
Bow echo
An organized mesoscale convective system, so named because of its characteristic bow shape on radar reflectivity displays. Bow echoes are typically 20–200 km long and last for 3–6 hours. They are associated with severe weather, especially high, straight-line surface winds, which are the result of a strong rear-inflow jet descending to the surface.
Brightness temperature
The Planck temperature associated with the radiance for a given wavelength.

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C

Center
Location of the vertical axis of a tropical cyclone, usually defined by the location of minimum wind or minimum pressure. The cyclone center position can vary with altitude.
Cloud track winds
Winds derived from tracking movement of cloud elements using IR and water vapor images from geostationary satellites.
Conditional Instability of the Second Kind (CISK)
A theory for tropical cyclone development that relates boundary layer moisture convergence (driven by Ekman pumping) to the potential for tropical cyclone intensification. As the storm intensifies, the moisture convergence must increase, providing a feedback to the system. As with WISHE, CISK relies on the presence of an incipient disturbance.
Coordinated Universal Time (UTC)
Same as Zulu (Z) and Greenwich Mean Time (GMT).
Coriolis parameter, f
A measure that is twice the local vertical component of the angular velocity of a spherical planet, 2Ω sinφ, where Ω is the angular speed of the planet and φ is the latitude.
Cyclogenesis
The formation of a cyclone.
Cyclone
An closed circulation of low pressure, rotating counter-clockwise in the Northern Hemisphere and clockwise in the SH.
Cyclone Phase Space (CPS)
A concise, three-parameter summary of the structure of a storm. It can be used to describe the structure of any synoptic or meso-synoptic cyclone.

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D

Deposition
The process by which molecules are changed from the vapor phase directly to the solid phase, such as from water vapor to ice.
Doppler Effect
The apparent shift in the frequency and wavelength of a wave perceived by an observer moving relative to the source of the wave.
Doppler radar
Radar that uses the Doppler effect to detect radial velocity of targets based on the phase shift between the transmitted pulse and the received backscatter.
Dvorak Technique
a classification scheme for estimating the intensity of TCs from enhanced IR and visible satellite imagery. It is the primary method of estimating intensity everywhere, except the North Atlantic and North Pacific where aircraft reconnaissance is routine.

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E

Eddy angular momentum flux (EAMF)
Flux (net transport) of angular momentum into a circle centered on the storm. If EAMF is positive, the flow inside the circle will become more cyclonic; negative EAMF render the system less cyclonic (more anticyclonic). See Box 8-6 for a definition and discussion of angular momentum in tropical cyclones.
Ekman layer
Thin horizontal layer of water at top of the ocean that is affected by wind. That layer has a force balance between pressure gradient force, Coriolis force and frictional drag.
Ekman pumping
The force balance determining the vector wind is modified by friction at the Earth’s surface. The addition of friction changes the force balance to slow the winds and change their direction: winds now flow into a low and out of a high pressure system. Winds flowing into a low because of friction are forced upwards and out of the boundary layer. This process is known as Ekman pumping.
El Niño-Southern Oscillation (ENSO)
An oscillation of the ocean-atmosphere system in the tropical Pacific which affects global weather and climate. El Niño, the warm phase of ENSO, is a quasi-periodic (2-7 years) warming of ocean surface waters in the equatorial and eastern tropical Pacific and an eastward shift in convection from the western Pacific climatological maximum. Changes occur in the tropical trade easterlies, vertical wind shear, and ocean height. Cool ocean temperature anomalies are observed in the tropical western Pacific extending eastward into the subtropics of both hemispheres. “La Niña” refers to the less intense, anomalous cool phase of ENSO. The Southern Oscillation refers to the atmospheric pressure difference between Darwin and Tahiti that is correlated with El Niño.
Electromagnetic (EM)
Energy carried by electric and magnetic waves.
Emission
The process by which a material generates electromagnetic radiation due to its temperature and composition.
Emissivity
The emitting efficiency of an object compared to an ideal emitter (or blackbody). A blackbody has an emissivity of one.
Emitter
Anything that radiates measurable electromagnetic radiation.
Empirical Orthogonal Function (EOF)
See Principal Component Analysis.
Energy
The capacity to do work or transfer heat. Measured in SI units as Joules.
Entrainment
The integration of unsaturated environmental air into the turbulent cloud-scale circulation. The antonym of entrainment is detrainment.
Explosive Deepening
A decrease in the minimum sea-level pressure of a tropical cyclone of 2.5 hPa hr-1 for at least 12 hours or 5 hPa hr-1 for at least six hours.
Extratropical
A term used to indicate that a cyclone has lost its “tropical” characteristics. The term implies both poleward displacement of the cyclone and the conversion of the cyclone’s primary energy source from the release of latent heat of condensation to baroclinic processes.It is important to note that cyclones can become extratropical and still retain winds of hurricane or tropical storm force. Given that these dangerous winds can persist after the cyclone is classified as extratropical, the Canadian Hurricane Centre (for example) follows them as “Former hurricane XXX.”

Extratropical Transition (ET)
The evolution of a poleward-moving initially tropical cyclone resulting in an extratropical cyclone. In the process of this evolution the energy source of the storm shifts from latent heat release to baroclinic development.
Eye (of tropical cyclone)
The approximately circular area of light winds at the center of a tropical cyclone. It is surrounded entirely or partially by clouds in the eyewall.
Eyewall / Wall Cloud
The full or partial ring of thunderstorms that surround the eye of a tropical cyclone. The strongest sustained winds in a tropical cyclone occur in the eyewall.

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F

Field of View (FOV)
Generally associated with the ground resolution from the detector standard viewing location, field of view is the solid angle through which a detector observes radiation.
Fraction of Photosynthetically Active Radiation (FPAR)
An index that measures how much sunlight the leaves are absorbing.
Frequency
The number of recurrences of a periodic phenomenon per unit time. The frequency, v, of electromagnetic energy is usually specified in Hertz (Hz), which represents one cycle per second.
Fujiwhara Effect
The mutual advection of two or more nearby tropical cyclones about each other. This results in cyclonic rotation of the storms about each other.

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G

Gale Force Wind
A sustained surface wind in the range 17 m s-1 (39 mph, 63 km hr‑1 or 34 knot) to 24 m s-1 (54 mph, 87 km hr‑1 or 47 knot) inclusive, and not directly associated with a tropical cyclone.
Geostationary or Geosynchronous orbit
An orbit whose rotation period equals that of the Earth. The altitude of a geostationary orbit is approximately 35,800 km. Its orbit keeps it above a single point on the equator.
GOES
Geostationary Operational Environmental Satellite (operated by NOAA).
GOES Precipitation Index
An estimate of precipitation that uses 235K as the IR temperature with the best correlation to average precipitation for areas spanning 50-250 km over 3-24 hours.
GPS
Global Positioning System, a network of defense satellites established in 1993. Each satellite broadcasts a digital radio signal that includes its own position and the time, accurate to one billionth of a second. GPS receivers use the signals to calculate their position to with a few hundred feet.
GPS radio occultation
The technique by which satellite receivers intercept signals from GPS and infer the deviations in the signal’s path caused by temperature and moisture gradients.
Gravity waves
Oscillations usually of high frequency and short horizontal scale, relative to synoptic- scale motions, which arise in a stably stratified fluid when parcels are displaced vertically. Gravity is the restoring force.
Greenwich Mean Time (GMT)
Mean solar time of the meridian at Greenwich, England, used as the basis for standard time throughout most of the world. Also referred to as Zulu (Z) and Coordinated Universal Time (UTC).

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H

Hadley Cells
Circulation cells in which air rises in the ITCZ, sinks into the subtropical highs, and returns to the equatorial low along the trade winds. George Hadley proposed a model (1735) of the global atmospheric circulation with rising motion at the equator, where there is surplus heating, and sinking motion at the poles, where there is net cooling. Hadley’s model did not account for the Coriolis effect, which leads to average westerly motion in the mid-latitudes. The Hadley model does explain the circulation within 30 degrees of the equator.
Horizontal Convective Rolls
Lines of overturning motion with axes parallel to the local surface. These rolls result from a convective instability (high density over low density – often corresponding to cool air over warm) and can mix strong winds from above down towards the surface.
Hurricane
A tropical cyclone in which the maximum sustained surface wind (using the local time averaging convention) is at least 33 m s-1 (74 mph, 119 km hr-1 or 64 knot). The term “hurricane” is used for in the Northern Atlantic and Northeast Pacific; “tropical cyclone” east of the International Dateline to the Greenwich Meridian; and “typhoon” in the Pacific north of the Equator and west of the International Dateline.

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I

Inertial period
The time taken to complete one rotation. In the tropical cyclone this is calculated by dividing the circumference at the radius of interest (commonly, the radius of maximum winds) by the wind speed at that radius.
Infrared (IR)
Electromagnetic energy within the wavelength interval generally defined from 0.7 to 100 microns.
Irradiance
The energy per unit time incident upon a unit area of a given surface, measured in SI units as Wattsm-2.
Insolation
The incoming solar radiation that reaches the earth and its atmosphere.
Intensity
The peak sustained surface wind in the region immediately surrounding the storm center, or the minimum central pressure measured in the eye.
Intertropical Convergence Zone (ITCZ)
The zone where the northeast and southeast trade winds converge. It is marked by low pressure, rising motion, and thunderstorms, which occur with strong surface heating. Its latitudinal position shifts in response to the solar maximum and heating response of the surface. It is recognized in satellite images as a band of thunderstorms across the tropics. It is often, but not always, co-located with the zone of low pressure known as the “Equatorial Trough”.
Intraseasonal
Varying on time scales shorter than one season.

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J

Joule
SI unit of energy equal to 0.2389 calories.

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K

Kelvin waves
At the equator, eastward propagating waves with negligible meridional velocity component and Gaussian latitudinal structure in zonal velocity, geopotential, and temperature, symmetric about the equator.

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L

Landfall
The intersection of the surface center of a tropical cyclone with a coastline. Because the strongest winds in a tropical cyclone are not located precisely at the center, it is possible for the strongest winds to be experienced over land even if landfall does not occur.
Leaf Area Index (LAI)
The ratio of green leaf area to the total surface area occupied by vegetation.
Longwave (LW)
Electromagnetic energy lying in the wavelength interval generally defined from 4.0 microns to an indefinite upper limit.
Low earth orbit (LEO)
An orbit that is located at an altitude generally between 200 and 1000 km.
Low earth orbit satellite
A satellite that has a low earth orbit. Most have paths crossing the poles and can provide synchronous observations (e.g., the NOAA series or Defense Meteorological Satellite Program systems). The TRMM is an LEO satellite that orbits between ±35º latitude.

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M

Madden-Julian Oscillation (MJO)
Tropical rainfall exhibits strong variability on time scales shorter than the seasonal. These fluctuations in tropical rainfall often undergo a 30-60 day cycle that is referred to as the Madden-Julian Oscillation or intraseasonal oscillation. The MJO is a naturally occurring component of the Earth’s coupled ocean-atmosphere system that significantly affects the atmospheric circulation throughout the global tropics and subtropics.
Maritime Continent
The region of Southeast Asia that comprises many islands, peninsulas, and shallow seas (including countries such as Indonesia, Malaysia, Papua New Guinea, and the Phillipines and covers approximately 12°S to 8°N, 95°E to 150°E).
Meridional
North-south, crossing latitudes; by convention the meridional wind from the south is positive.
Mesoscale
Spatial scale of 100-1000 km and temporal scale of hours to a day; between synoptic and convective scale. Tropical clouds are most often organized into mesoscale systems.
Mesoscale convective complex (MCC)
A large, quasi-circular mesoscale convective system that produces heavy rainfall and severe weather. In some MCCs, a mid-tropospheric vortex forms and remains after the deep convection has dissipated.
Mixed Rossby-Gravity (MRG) Wave
A divergent Rossby wave, resulting from conservation of potential vorticity and buoyancy forcing. These waves propagated westward along the equator. Meridional velocity is symmetric about the equator. Zonal wind, temperature, and geopotential area antisymmetric about the equator.
Monochromatic
Of or pertaining to a single wavelength, or in practice, perhaps a very narrow spectral interval.
Monsoon
A term whose roots are from the Arabic for “season”, it is a seasonal wind reversal. The monsoon has inflow to a surface heat low and an offshore flow from high pressure during the winter when the land cools relative to the ocean. The Indian monsoon is the most prominent but it has been recognized that that monsoon region extends from Southeast Asia to West Africa. The summer monsoon is a vital source of moisture; its arrival, duration, and amount of precipitation modulates the economies of these regions.
Monsoon Gyre
A closed, symmetric circulation at 850 hPa with horizontal extent of 25° latitude that persists for at least two weeks. The circulation is accompanied by abundant convective precipitation around the south-southeast rim of the gyre.
Monsoon Region
Refers to the combination of features including a monsoon trough, confluence zone, and the ITCZ.

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N

Nadir
The satellite viewing angle directly downward (viewing zenith angle = 0 degrees). Also used to refer to the sub-satellite point location.

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0

Ocean conveyor belt
The name given to summarize the pattern of global ocean currents. The surface ocean currents generally transport warm salty water polewards, out of the tropics. The water cools as it moves polewards, becoming increasingly dense (remember that salty water is more dense than fresh water). This water sinks in the North Atlantic and also in the Southern Ocean near Antarctica. The deep water currents transport the water around the globe until it rises to the surface again, once more part of the surface ocean currents.
Opaque
A physical description of a material which attenuates electromagnetic radiation.
Optical depth
A measure of the cumulative attenuation of a beam of radiation as a result of its travel through the atmosphere.

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P

Pacific Decadal Oscillation (PDO)
The PDO is a basin-scale pattern of Pacific climate variability; PDO climate anomalies are most visible in the North Pacific and North American regions, with secondary features in the tropics. The phases of the PDO persist for 20-to-30 years. Causes for the PDO have not yet been explained.
Planck’s Law
An expression for the variation of monochromatic radiance as a function of wavelength for a blackbody at a given temperature.
Planetary Boundary Layer (PBL)
The layer of the atmosphere that extends upward from the surface to heights of 100 to 3000 m. The boundary layer is directly influenced by surface forcing such as friction, heating, and evapotranspiration.
Polar orbit
An orbit whose path crosses the polar regions. This type of orbit is located at an altitude generally between 200 and 1000 km, and can provide sun-synchronous observations.
Polar Orbiting Environmental Satellite (POES)
A satellite which has a polar orbit, such as the NOAA series or Defense Meteorological Satellite Program systems.
Potential evapotranspiration
A measure of the maximum possible water loss from an area under a specified set of weather conditions.
Potential Intensity (PI)
The largest possible intensity (maximum wind, minimum pressure) expected to be possible for a particular tropical cyclone.
Potential vorticity
A scalar measure of the balance between the vorticity and the thermal structure of the atmosphere.
Principal component analysis
A mathematical technique for identifying patterns in data by reducing multidimensional data to a smaller number of dimensions. A number of variables that are (possibly) correlated are transformed into a new coordinate system. The transformation identifies the components that account for variability in the data. The first principal component often accounts for the most of variability in the data. Also known as Empirical Orthogonal Function (EOF) analysis.

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Q

Quasi-Biennial Oscillation (QBO)
An oscillation in the lower stratospheric zonal winds averaged around the equator. It is typically diagnosed from the zonal winds between 30-70 hPa (although it is evident as high as 10 hPa). The QBO has a varying from about 24 to 30 months. The zonal winds change by about 40 m s-1 between the maximum easterly and maximum westerly phase.

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R

Radar (Radio Detection And Range)
An instrument that detects objects remotely by transmitting high-frequency pulses to the atmosphere and measuring the “backscatter” or echoed pulses from that object. Weather radar transmits microwave (mm-cm) pulses; the returned signal is interpreted to determine where it is precipitating.
Radiance
A measure of radiant intensity produced by a material in a given direction and per unit wavelength interval, measured in Watts/m 2 /steradian/micron. Monochromatic radiance is the most fundamental unit measured by satellite instruments.
Radiation
Energy transferred by electromagnetic waves.
Radius of Maximum Winds
The distance from the center of a tropical cyclone to the location of the cyclone’s maximum winds. In well-developed systems, the radius of maximum winds is generally found at the inner edge of the eyewall.
Rapid Deepening
A decrease in the minimum sea-level pressure of a tropical cyclone of 1.75 hPa hr-1 or 42 hPa for 24 hours.
Recurvature
The poleward motion of a tropical cyclone taking it from the mean tropical easterlies to the midlatitudes westerlies. This change in the advection of the storm results in curvature in the storm track.
Reflection
The process by which incident radiation is scattered in the backward direction (backscattered).
Reflectivity
The fraction of incident radiation reflected by a material.
Relative vorticity
See Vorticity.
Remnant Low
Used for systems no longer having the sufficient convective organization required of a tropical cyclone (e.g., the swirls of stratocumulus in the eastern North Pacific).
Retrieval
The process or end result of a process where physical quantities such as water vapor, temperature, and/or pressure are extracted from measurements of total upwelling radiance to space; here involving the GOES sounder.
Riming
The formation of ice by the rapid freezing of supercooled water drops as they impinge upon an object such as an ice crystal or aeroplane wing.
Rossby Radius of Deformation
The Rossby radius is the critical scale at which rotation becomes as important as buoyancy, which allows an initial disturbance to be sustained. It is a function of the absolute vorticity, stability, and depth of the disturbance. When a disturbance is wider than LR, it will persist; systems that are smaller than LR will dissipate.
Rossby Wave
A planetary wave, resulting from conservation of potential vorticity. Gradients of potential vorticity provide a restoring mechanism to allow propagation of the waves. This text focuses on Rossby waves centered on the equator equatorial (n=1) Rossby waves.

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S

Saffir-Simpson scale
A scale that links the observed damage and the effects of wind, pressure and storm surge that could lead to such damage. Initial wind damage scale was defined by Herbert Saffir and later expanded by Robert Simpson to include storm surge.
Scattering
The process by which a material interacts with and redirects incident radiation (in any given direction).
Scatterometer
A radar that infers near-surface wind velocity by sending pulses of microwave energy to the ocean surface and measuring the backscatter from small-scale waves. Scatterometry wind retrievals can be ambiguous during rain, since rain creates additional backscatter and attenuates the radar beam.
Shortwave (SW)
Electromagnetic radiation generally defined as having a wavelength shorter than 4.0 microns.
Size
The mean radius of a tropical cyclone enclose by winds of at least 17 m s-1. Size may also be defined as the outer closed isobar of the surface pressure.
Solar declination angle
The angle between the rays of the Sun and the equatorial plane of the Earth. It is zero during an equinox and 23.5° during a solstice.
Southern Oscillation Index (SOI)
The normalized difference in sea level pressure between Darwin, Australia and Tahiti, French Polynesia.
Specific humidity
The mass of water vapor per unit mass of air (including water vapor), usually denoted by q and measured in units of grams per kilograms.
Spectral
A descriptor for radiometric quantities or measurements which have a limited wavelength range.
Split window
A pair of regions of the electromagnetic spectrum which are closely located in wavelength, but have slightly different attenuation characteristics. Used to denote the 11- and 12-micron regions in which greater water vapor attenuation at 12 microns causes slightly different brightness temperatures.
Stefan-Boltzmann Law
The energy emitted per unit area (from all wavelengths and represented by the area under the blackbody curve) is proportional to the 4 th power of the absolute temperature
Steradian
The unit of measure of solid angles, equal to the angle subtended at the center of a sphere.
Storm Surge
An abnormal rise in sea level accompanying a tropical cyclone or other intense storm, and whose height is the difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone. Storm surge is usually estimated by subtracting the normal or astronomic high tide from the observed storm tide.
Storm Tide
The actual level of sea water resulting from the astronomic tide combined with the storm surge.
Subtropical Cyclone
A non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones.The most common type is an upper-level cold low with circulation extending to the surface layer and maximum sustained winds generally occurring at a radius of about 100 miles or more from the center. In comparison to tropical cyclones, such systems have a relatively broad zone of maximum winds that is located farther from the center, and typically have a less symmetric wind field and distribution of convection.

A second type of subtropical cyclone is a mesoscale low originating in or near a frontolyzing (dying frontal) zone of horizontal wind shear, with radius of maximum sustained winds generally less than about 50 km (30 miles). The entire circulation may initially have a diameter less than 160 km (100 miles). These generally short-lived systems may be either cold core or warm core.

Subtropical Depression
A subtropical cyclone in which the maximum sustained surface wind speed does not exceed 17 m s-1 (39 mph, 63 km hr‑1 or 34 knot).
Subtropical Storm
A subtropical cyclone in which the maximum sustained surface wind speed is at least 17 m s-1 (39 mph, 63 km hr‑1 or 34 knot).
Synthetic Aperture Radar (SAR)
Works like other radars except that it has very fine resolution in the azimuthal direction. It synthesizes the fine resolution normally achieved with a large antenna by combining signals from an object along a radar flight track and processing the signals as if obtained simultaneously from a single large antenna. The distance over which the signals are collected is known as the synthetic aperture.

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T

Trade Winds
Prevailing easterly winds flowing from the subtropical highs that affect equatorial and subtropical regions. Trade winds are mostly east to northeasterly in the Northern Hemisphere and east to southeasterly in the Southern Hemisphere. During the monsoon, easterly trades are replaced by mostly westerly winds.
Transmission
The process by which incident radiation propagates forward through a material.
Transpiration
The process by which water vapor enters the atmosphere through the stomata in the leaves of plants.
Thermocline
The inversion layer separating the near-surface warm waters from the colder, deeper layers of oceans and lakes. It is about 1km deep and is thermally stratified. In the ocean, it also separates the fresher waters near the surface from the saltier waters below.
Tropical Cyclone
A warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center. Once formed, a tropical cyclone is maintained by the extraction of heat energy from the ocean at high temperatures and heat export at the low temperatures of the upper troposphere. In this they differ from extratropical cyclones, which derive their energy from horizontal temperature contrasts in the atmosphere (baroclinic effects). Also see Hurricane.
Tropical Cyclone Season
The portion of the year having a relatively high incidence of tropical cyclones. Also known as “Hurricane Season” or “Typhoon Season”.
Tropical Depression
A tropical cyclone in which the maximum sustained surface wind speed is not more than 17 ms-1 (39 mph, 63 km hr‑1 or 34 knot).
Tropical Disturbance
A discrete tropical weather system of apparently organized convection – generally 185 to 550 km (100-300 n mi) in diameter – originating in the tropics or subtropics, having a nonfrontal migratory character, and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field.
Tropical Storm
A tropical cyclone in which the maximum sustained surface wind speed ranges from 17 ms-1 (39 mph, 63 km hr‑1 or 34 knot) to 33 ms-1 (74 mph, 119 km hr-1, 64 knot).
Typhoon
See Tropical Cyclone and Hurricane.

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U

Ultraviolet (UV)
Electromagnetic radiation of shorter wavelength than visible radiation but longer than x-rays (approximately 0.03 to 0.4 microns)

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V

Visible
The region of the electromagnetic spectrum which is detectable to the human eye (approximately 0.4 to 0.7 microns).
Vorticity
The local rotation of the flow, calculated as the the curl (cross product) of the vector wind. Vorticity has units of inverse seconds (s-1).“Relative vorticity” is the vorticity calculated for the observed winds. It is called “relative” since the winds are the flow relative to the Earth’s rotation.

The vertical component of the vorticity vector is most often used since it is much larger than the other vorticity components. This is because the horizontal winds in tropical cyclones are much greater than the vertical wind component.

“Absolute vorticity” is the vorticity calculated for the total motion of the atmosphere the combination of the observed winds and the Earth’s rotation.

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W

Walker Circulation
The east-west circulation cells that form along the equator in response to differential surface heating.
Warning
A warning that sustained winds exceeding the threshold for either tropical storm or tropical cyclone and associated with such a storm are expected in a specified coastal area in 24 hours or less.
Watch
An announcement for specific coastal areas that either tropical storm or tropical cyclone conditions are possible within 36 hours.
Wavelength
The distance a wave will travel in the time required to generate 1 cycle, denoted by λ. A length measured from the midpoint of a crest (or trough) to the midpoint of the next crest (or trough).
Wavenumber
The reciprocal of the wavelength, denoted by κ.
Water Vapor Channel (or water vapor IR channel)
A spectral band in which the radiance is attenuated by water vapor. This usually refers to the 6.7 micron channel in this module.
Weighting function
A mathematical expression representing the relative radiance contribution provided from a given level of the atmosphere (usually a function of atmospheric pressure).
Wind-Induced Surface Heat Exchange (WISHE)
A tropical cyclone development theory based on a conceptual model of a tropical cyclone as an atmospheric Carnot engine. Consistent with its Carnot engine roots, WISHE relates (i) fluxes of heat and moisture from the ocean surface and (ii) the temperature of the tropical cyclone outflow layer to the potential for continued storm development. The fluxes increase with surface wind speed providing a feedback to the system. As with CISK, WISHE relies on the presence of an incipient disturbance.
Wind profiler
Vertically pointing radar which operates on the same principle as horizontally-scanning Doppler radar; provides best measurements of vertical air motion inside convective storms
Wien’s Displacement Law
The wavelength of maximum blackbody emission is inversely proportional to its absolute temperature.

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X

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Y

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Z

Zonal
East-west, crossing longitudes; by convention, the zonal wind from the west is positive.
Zulu (Z)
Used to represent the same clocktime at GMT and UTC. See Greenwich Mean Time (GMT), or Coordinated Universal Time (UTC)

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