Lesson 5 Depression: Life Cycles

第五课 低压：生命循环

A depression is an atmospheric disturbance characterized by lower atmospheric pressure than in surrounding areas. In particular, the term depression here refers to such disturbances in extra-tropical latitudes. It commonly takes the form of a roughly circular area of closed concentric isobars. The flow of air around a depression is counterclockwise (clockwise) in the Northern (Southern) Hemisphere.

低气压是指大气扰动，其特征是气压低于周围地区。特别是，这里所说的低气压指的是在亚热带纬度的这种扰动。它通常呈现为一个大致圆形的闭合同心等压线区域。低气压周围的气流在北半球是逆时针方向（在南半球是顺时针方向）。

Central pressure varies typically from about 950 to 1020 hPa, but values as low as about 915 hPa have been documented. A depression can be described as “shallow” or “deep” if encircled by few or many isobars. The diameter can vary from as little as 30 km in the case of a small polar air disturbance to as much as 4000 km for a large mature depression. A typical example of a depression is shown in Fig.1. It is the size of the pressure gradient around a depression which determines the wind strength. Generally speaking, the greater the horizontal pressure difference between two places, the faster the air will move, and the stronger the wind will blow. In extreme cases, pressure differences of around 50 hPa over 500 km may result in hurricane force wind (sustained winds in excess of 32 m/s).

中心气压通常在 950 到 1020 百帕之间变化，但据记录，气压低至约 915 百帕。一种气压低压可以描述为“浅”或“深”，这取决于它被少数或多数等压线包围的情况。其直径可以从一个小型极地空气扰动的 30 公里小到一个大型成熟低压的 4000 公里大。低压的一个典型例子显示在图 1 中。决定风力强度的是低压周围的气压梯度大小。一般来说，两个地方之间的水平气压差越大，空气移动得越快，风力也越强。在极端情况下，500 公里范围内约 50 百帕的气压差可能导致飓风级别的风力（持续风速超过 32 米/秒）。

The extra-tropical depressions are associated with unsettled weather, usually accompanied by strong winds or gales, as well as much precipitation. On a global scale, depressions are responsible for much of the interchange of air and heat energy between high and low latitudes. The energy source which drives mid-latitude depressions is derived from temperature differences between these contrasting masses of air.

温带气旋通常与不稳定的天气相关，通常伴有强风或狂风以及大量降水。从全球范围来看，气旋在高纬度和低纬度之间的空气和热能交换中起着重要作用。驱动中纬度气旋的能量来源于这些对比鲜明的空气团之间的温差。

Life cycle

生命周期

The traditionally accepted life cycle model of a depression was first identified and described by Bjerknes in 1919 and has been the corner-stone of meteorological analysis ever since. This section sets out a brief outline of these well established ideas on life cycles, more detail about which can be found in many textbooks.

传统上被接受的气旋生命周期模型首次由 Bjerknes 在 1919 年确定并描述，并且自那时以来一直是气象分析的基石。本节简要概述了这些已确立的关于生命周期的理念，更多详细信息可以在许多教材中找到。

Depressions usually develop in areas of marked temperature contrast, where cold air of polar origin is juxtaposed with warm tropical air, the boundary between the two air masses is commonly known as the “polar front” (Fig .2(a)).

低压通常在明显温度对比的区域形成，在这些区域，极地来源的冷空气与温暖的热带空气相互交错，两种气团之间的边界通常被称为“极地锋”（图 2(a)）。

A typical extra-tropical depression passes through a life cycle as illustrated in Fig.2. Once cyclogenesis (the birth of a depression) has been triggered, the depression evolves through three distinct stages, the “wave depression”, “warm sector depression” and “occluding depression”. Each of these stages is discussed in turn.

一个典型的温带气旋经历如图 2 所示的生命周期。一旦气旋形成（气旋的诞生）被触发，气旋会经过三个不同的阶段，即“波浪低压”、“暖区低压”和“锢囚低压”。每个阶段依次讨论。

Cyclogenesis

气旋生成

The initial trigger for cyclogenesis is normally found in the upper troposphere, typically between 8000 to 12000 m above sea level, the trigger being divergence of air ahead of a mobile upper trough. Another common mechanism takes the form of a propagating jet stream, a particularly favorable area for cyclogenesis in the Northern Hemisphere is beneath the left-hand downstream exit of a jet stream. The start of cyclogenesis is normally observed as a localized (perhaps several hundreds of kilometers) fall of pressure along a pre-existing front.

气旋生成的初始触发通常发生在对流层上部，通常位于海拔 8000 到 12000 米之间，触发因素是移动上槽前方空气的发散。另一种常见机制是传播的喷流，北半球气旋生成的一个特别有利区域是在喷流的左侧下游出口下方。气旋生成的开始通常表现为沿着一个已有锋面局部（可能几百公里）气压下降。