The Atmospheric Boundary Layer
The Atmospheric Boundary Layer
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DOI：
10.1017/cbo9780511840524.010
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发表时间：
2019-01
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影响因子：
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通讯作者：
John C. Wyngaard-
中科院分区：
文献类型：
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作者：
John C. Wyngaard-
气氛。大气中受该边界影响最大的部分称为大气边界层（ABL，图 9.1），或简称为边界层。边界层的厚度在空间和时间上变化很大。通常厚度为 !1 或 2 公里（即占据对流层底部 10% 至 20%），范围可以从数十米到 4 公里或更长。湍流和静态稳定性共同在下面的边界层和上面的对流层其余部分（称为自由大气）之间夹入了一个强大的稳定层（称为顶逆温）。这个稳定的层将湍流、污染物和湿气捕获在其下方，并防止自由大气感受到大部分表面摩擦。在晴朗的天气（与高压中心相关）期间，我们习惯了由边界层物理和动力学控制的温度、湿度、花粉和风的昼夜（每日）循环。夜晚凉爽而平静；白天温暖且有风。据说，只要表面比空气温暖，例如在阳光明媚的日子里，陆地上有微风，或者当冷空气平流过较温暖的水面时，边界层就会不稳定。该边界层处于自由对流状态，具有剧烈的热上升气流和下降气流。当表面比空气冷时，例如在陆地上晴朗的夜晚，或者当暖空气在较冷的水面上平流时，边界层被认为是稳定的。中性边界层在有风和阴天的条件下形成，并处于强制对流状态。湍流在边界层内无处不在，它负责有效地分散现代生活中伴随的污染物。然而，封盖反转将这些污染物捕获在边界层内，导致我们“炖在我们自己的废物中”。地表和空气之间的湍流传播非常迅速，使空气能够快速呈现下垫面的特征。事实上，边界层的一个定义是对流层下部在大约 30 分钟或更短时间内感受到下垫面影响的部分。气团1是在不同表面上形成的边界层。邻近气团之间的温差会导致斜压，从而驱动温带气旋。边界层中的热量和湿度是对流云的重要燃料。封顶逆温抑制了雷暴的形成，从而允许自由大气中对流可用势能（CAPE）的积累。边界层中的风切变是由地面附近的阻力引起的，会产生水平涡度，对流云中的上升气流可以使水平涡度倾斜，形成龙卷风。边界层内动能的耗散可作为大型风力系统的制动器。湍流极其复杂，由称为涡流的漩涡叠加组成，这些漩涡非线性相互作用，产生准随机的混沌运动。需要无数个方程才能完全描述这些运动。因此，尚未找到完整的解决方案。但当对许多漩涡进行平均时，我们可以观察到持久的模式
atmosphere. The portion of the atmosphere most affected by that boundary is called the atmospheric boundary layer (ABL, Fig. 9.1), or boundary layer for short. The thickness of the boundary layer is quite variable in space and time. Normally !1 or 2 km thick (i.e., occupying the bottom 10 to 20% of the troposphere), it can range from tens of meters to 4 km or more. Turbulence and static stability conspire to sandwich a strong stable layer (called a capping inversion) between the boundary layer below and the rest of the troposphere above (called the free atmosphere). This stable layer traps turbulence, pollutants, and moisture below it and prevents most of the surface friction from being felt by the free atmosphere. During fair weather (associated with high-pressure centers), we are accustomed to the diurnal (daily) cycle of changes in temperature, humidity, pollen, and winds that are governed by boundary-layer physics and dynamics. It is cool and calm at night; warm and gusty during daytime. The boundary layer is said to be unstable whenever the surface is warmer than the air, such as during a sunny day with light winds over land, or when cold air is advected over a warmer water surface. This boundary layer is in a state of free convection, with vigorous thermal updrafts and downdrafts. The boundary layer is said to be stable when the surface is colder than the air, such as during a clear night over land, or when warm air is advected over colder water. Neutral boundary layers form during windy and overcast conditions, and are in a state of forced convection. Turbulence is ubiquitous within the boundary layer and is responsible for efficiently dispersing the pollutants that accompany modern life. However, the capping inversion traps these pollutants within the boundary layer, causing us to “stew in our own waste.” Turbulent communication between the surface and the air is quite rapid, allowing the air to quickly take on characteristics of the underlying surface. In fact, one definition of the boundary layer is that portion of the lower troposphere that feels the effects of the underlying surface within about 30 min or less. Air masses1 are boundary layers that form over different surfaces. Temperature differences between neighboring air masses cause baroclinicity that drives extratropical cyclones. Heat and humidity trapped in the boundary layer are important fuels for convective clouds. The capping inversion inhibits thunderstorm formation, allowing the buildup of convective available potential energy (CAPE) in the free atmosphere. Wind shear in the boundary layer, caused by drag near the ground, generates horizontal vorticity that can be tilted by the updrafts in convective clouds to form tornadoes. Dissipation of kinetic energy within the boundary layer serves as a brake on largescale wind systems. Turbulence is inspiringly complex, consisting of a superposition of swirls called eddies that interact nonlinearly to create quasi-random, chaotic motions. An infinite number of equations is required to fully describe these motions. Hence, a complete solution has not been found. But when averaged over many eddies, we can observe persistent patterns