Lesson 9 The observed Structure of Extratropical Circulations

第九课：温带环流的观测结构

Atmospheric circulation systems depicted on a synoptic chart rarely resemble the simple circular vortices. Rather, they are generally highly asymmetric in form with the strongest winds and largest temperature gradients concentrated along narrow bands called fronts. Also, such systems generally are highly baroclinic; the amplitudes and phases of the geopotential and velocity perturbations both change substantially with height. Part of this complexity is due to the fact that these synoptic systems are not superposed on a uniform mean flow but are embedded in a slowly varying planetary scale flow that is itself highly baroclinic. Furthermore, this planetary scale flow is influenced by orography (i.e., by large-scale terrain variations) and continent-ocean heating contrasts so that it is highly longitude dependent. Therefore, it is not accurate to view synoptic system as disturbances superposed on a zonal flow that varies only with latitude and height. However, such a point of view can be useful as a first approximation in theoretical analyses of synoptic-scale wave disturbances.

在天气图上描绘的大气环流系统很少呈现简单的圆形涡旋。相反，它们通常呈现高度不对称的形态，最强的风速和最大的温度梯度集中在称为锋区的狭窄带上。此外，这些系统通常具有很强的斜压性；地势高度和速度扰动的振幅和相位随高度显著变化。部分复杂性是由于这些天气系统并非叠加在均匀的平均流场上，而是嵌入在一个缓慢变化的行星尺度流场中，而这个流场本身也具有很强的斜压性。此外，这种行星尺度的流场受到地形（即大规模地形变化）和大陆-海洋加热差异的影响，从而表现出显著的经度依赖性。因此，将天气系统视为仅随纬度和高度变化的纬向流场上的扰动是不准确的。然而，从理论分析的角度来看，将其视为对天气尺度波动扰动的一种近似方法是有用的。

Zonally averaged cross-sections do provide some useful information on the gross structure of the planetary scale circulation in which synoptic-scale eddies are embedded. Fig.1 shows meridional cross-sections of the longitudinally averaged zonal wind and temperature for the solstice seasons of (a) December, January, and February (DJF) and (b) June, July, and August (JJA). These sections extend from approximately sea level (1000 hPa) to about 16 km altitude (100 hPa). Thus, the entire troposphere is shown while the lower stratosphere is shown only for the extratropical regions. Here we are concerned with the structure of the wind and temperature fields in the troposphere.

区域平均的横截面确实提供了一些关于行星尺度环流粗略结构的有用信息，其中嵌入了天气尺度的涡旋。图 1 显示了在冬至季节（a）12 月、1 月和 2 月（DJF）以及（b）6 月、7 月和 8 月（JJA）经度平均的纬向风和温度的经向横截面。这些横截面从大约海平面（1000 hPa）延伸到约 16 公里的高度（100 hPa）。因此，整个对流层都被展示出来，而下层平流层仅在外热带地区显示。在这里，我们关注的是对流层中风和温度场的结构。

The average pole to equator temperature gradient in the Northern Hemisphere troposphere is much larger in winter than in summer. In the Southern Hemisphere the difference between summer and winter temperature distributions is smaller, owing mainly to the large thermal inertia of the oceans together with the greater fraction of the surface that is covered by oceans in the Southern Hemisphere. Since the mean zonal wind and temperature fields satisfy the thermal wind relationship to a high degree of accuracy, the maximum zonal wind speed in the Northern Hemisphere is much larger in the winter than in the summer, while there is a smaller seasonal difference in the Southern Hemisphere. Furthermore, in both seasons the core of maximum zonal wind speed (called the mean jet stream axis) is located just below the tropopause (the boundary between the troposphere and stratosphere) at the latitude where the thermal wind integrated through the troposphere is a maximum. In both hemispheres, this is about 30°—35° latitude during winter, but it moves poleward to 40°—45° during summer.

北半球对极温度梯度在冬季远大于夏季。南半球夏季和冬季温度分布之间的差异较小，主要是由于海洋的热惯性大，以及南半球表面被海洋覆盖的比例更大。由于平均纬向风和温度场在很大程度上满足热风关系，北半球的最大纬向风速在冬季远大于夏季，而南半球的季节差异较小。此外，在两个季节中，最大纬向风速的核心（称为平均急流轴）位于对流层顶下方（对流层和平流层之间的边界），在对流层中热风积分达到最大值的纬度。在两个半球中，冬季大约在 30°—35°纬度，但在夏季向极地移动到 40°—45°。

That the zonally averaged meridional cross-sections of Figure 1 are not representative of the mean wind structure at all longitudes can be seen in Fig. 2, which shows the distribution of the time-averaged zonal wind component for DJF on the 200 hPa surface. It is clear from this figure that at some longitudes there are very large deviations of the time-mean zonal flow from its longitudinally averaged distribution. In particular, there are strong zonal wind maxima (jets) near 30° N just east of the Asian and North American continents and distinct minima in the eastern Pacific and eastern Atlantic. Synoptic-scale disturbances tend to develop preferentially in the regions of maximum time-mean zonal winds associated with the western Pacific and western Atlantic jets and to propagate downstream along storm tracks that approximately follow the jet axes.

图 1 的经向平均经度剖面并不能代表所有经度的平均风结构，这在图 2 中可以看到，该图显示了 200 hPa 层上 DJF 期间时间平均的纬向风分布。从这个图中可以清楚地看出，在某些经度上，时间平均的纬向流与其经度平均分布之间存在很大的偏差。特别是在 30°N 附近，亚洲和北美大陆以东有强烈的纬向风极大值（急流），而在东太平洋和东大西洋则有明显的极小值。天气尺度的扰动往往优先在与西太平洋和西大西洋急流相关的时间平均纬向风最大值区域发展，并沿着大致遵循急流轴的风暴轨迹向下游传播。

The large departure of the northern winter climatological jet stream from zonal symmetry can also be readily inferred from examination of Fig. 3, which shows the mean 500 hPa geopotential contours for January and July in the Northern Hemisphere. Even after averaging the height field for a month, very striking departures from zonal symmetry remain. They are clearly linked to the distributions of continents and oceans. The most prominent asymmetries are the troughs to the east of the American and Asian continents. Referring back to Fig.2, we see that the intense jet at 35° N and 140° E is a result of the semipermanent trough in that region. Thus, it is apparent that the mean flow in which synoptic systems are embedded should really be regarded as a longitude-dependent time-averaged flow.

北半球冬季气候学上急流大幅偏离纬向对称性也可以从图 3 中得出，该图显示了北半球 1 月和 7 月 500 hPa 高度场的平均等高线。即使对高度场进行月平均处理后，依然存在非常显著的偏离纬向对称的现象。这些偏离显然与大陆和海洋的分布密切相关。最显著的不对称性是位于美洲和亚洲大陆东侧的槽区。回顾图 2，我们可以看到，位于北纬 35 度、东经 140 度的强急流是该地区半永久性槽区的结果。因此，很明显，天气尺度系统嵌入的平均流应当被视为与经度相关的时间平均流。

In addition to its longitudinal dependence, the planetary scale flow also varies from day to day owing to its interactions with transient synoptic-scale disturbances. In fact, observations show that the transient planetary-scale flow amplitude is comparable to that of the time-mean. As a result, monthly mean charts tend to smooth out the actual structure of the instantaneous jet stream since the position and intensity of the jet vary. Thus, at any time the planetary scale flow in the region of the jet stream has much greater baroclinicity than indicated on time-averaged charts. This point is illustrated schematically in Fig.4, which shows a vertical (latitude-height) cross section through an observed jet stream over North America. Panel (a) shows the wind and temperature cross section, while panel (b) shows the wind and potential temperature. The latter provides vivid evidence of the strong static stability in the stratosphere. It also illustrates the fact that isentropes cross the tropopause in the vicinity of the jet so that adiabatic motion can exchange tropospheric and stratospheric air in that region.

除了其经度依赖性外，行星尺度的流动还由于其与瞬时天气尺度扰动的相互作用而每天变化。实际上，观测显示瞬时行星尺度流动幅度与时间平均值相当。因此，月平均图表往往会抹平瞬时急流结构的实际情况，因为急流的位置和强度在变化。因此，在任何时候，急流区域的行星尺度流动的斜压性都比时间平均图表所显示的要大得多。如图 4 所示，这一点用北美上空观测到的急流的垂直（纬度-高度）剖面图示意。面板（a）显示了风和温度的剖面，而面板（b）显示了风和位势温度。后者生动地证明了平流层的强静力稳定性。它还说明了在急流附近等熵线穿过对流层顶的事实，从而使绝热运动可以在该区域交换对流层和平流层的空气。

At any instant the axis of the jet stream tends to be located above a narrow sloping zone of strong temperature gradient called the polar-frontal zone. This is the zone that in general separates the cold air of polar origin from warm tropical air. The occurrence of an intense jet core above this zone of large-magnitude temperature gradients is, of course, not mere coincidence but rather a consequence of the thermal wind balance.

在任何时刻，喷流的轴线往往位于一个狭窄的倾斜区域之上，该区域具有强烈的温度梯度，称为极锋区。这个区域通常将极地来源的冷空气与热带的暖空气分开。在这个大幅度温度梯度区域上方出现强烈的喷流核心，当然不是偶然的，而是热风平衡的结果。

It is a common observation in fluid dynamics that jets in which strong velocity shears occur may be unstable with respect to small perturbations. By this is meant that any small disturbance introduced into an unstable jet will tend to amplify, drawing energy from the jet as it grows. Most synoptic-scale systems in midlatitudes appear to develop as the result of instability of the jet stream flow. This instability, called baroclinic instability depends on meridional temperature gradient, particularly at the surface. Hence, through the thermal wind relationship, baroclinic instability depends on vertical shear and tends to occur in the region of the polar frontal zone.

在流体动力学中，一个常见的观察是，当喷流中出现强速度剪切时，可能会对小扰动不稳定。这意味着，任何引入不稳定喷流的小扰动都会倾向于被放大，从喷流中提取能量以便它增长。大多数中纬度的天气系统似乎是由于喷流流动的不稳定性而发展起来的。这种不稳定性称为斜压不稳定性，依赖于经度温度梯度，特别是在地表。因此，通过热风关系，斜压不稳定性依赖于垂直剪切，并倾向于发生在极地锋区。

Baroclinic instability is not, however, identical to frontal instability because most baroclinic instability models describe only geostrophically scaled motions, while disturbances in the vicinity of strong frontal zones must be highly nongeostrophic. As we shall see later, baroclinic disturbances may themselves act to intensify preexisting temperature gradients and hence generate frontal zones.

然而，斜压不稳定性并不等同于锋面不稳定性，因为大多数斜压不稳定性模型仅描述地转尺度的运动，而强锋面区域附近的扰动必须是高度非地转的。正如我们稍后将看到的，斜压扰动可能会加剧已有的温度梯度，从而产生锋面区域。

The stages in the development of a typical baroclinic cyclone that develops as a result of baroclinic instability are shown schematically in Fig. 5. In the stage of rapid development there is a cooperative interaction between the upper level and surface flows; strong cold advection is seen to occur west of the trough at the surface, with weaker warm advection to the east. This pattern of thermal advection is a direct consequence of the fact that the trough at 500 hPa lags the surface trough so that the mean geostrophic wind in the 1,000–500 hPa layer is directed across the l000–500 hPa thickness lines toward larger thickness west of the surface trough and toward smaller thickness east of the surface trough. This dependence of the phase of the disturbance on height is better illustrated by Fig. 6, which shows a schematic downstream (or west-east) cross section through an idealized developing baroclinic system. Throughout the troposphere the trough and ridge axes tilt westward (or upstream) with height, while the axes of warmest and coldest air are observed to have the opposite tilt. As we shall see later the westward tilt of the troughs and ridges is necessary in order that the mean flow give up potential energy to the developing wave. In the mature stage (not shown in Fig. 5) the troughs at 500 hPa and 1000 hPa are nearly in phase. As a consequence, the thermal advection and energy conversion are quite weak.

典型的斜压气旋发展的各个阶段如图 5 所示。快速发展的阶段，上层流动与表面流动之间存在协同作用；在表面槽的西侧可以看到强冷平流，而在东侧则有较弱的暖平流。这种热平流的模式是 500 hPa 槽滞后于表面槽的直接结果，因此在 1000-500 hPa 层的平均地转风是指向表面槽西侧较大厚度的方向，而指向表面槽东侧较小厚度的方向。扰动相位对高度的依赖性在图 6 中得到了更好的说明，图中显示了一个理想化发展中的斜压系统的下游（或东西）横截面。在对流层中，槽和脊轴随着高度向西（或上游）倾斜，而最暖和最冷空气的轴则观察到有相反的倾斜。 正如我们稍后将看到的，槽和脊的西倾是必要的，以便平均流能够将潜在能量释放给发展中的波。在成熟阶段（图 5 中未显示），500 hPa 和 1000 hPa 的槽几乎是同相的。因此，热平流和能量转换相当弱。