Transport in Stable Boundary Layers and Thin Drainage Flows

这个项目的目的是提高我们对地球表面和低层大气之间的热量、水分、动量和污染物交换的理解，当靠近地面的大气层非常稳定时。在这种情况下，风随高度和时间的变化以及大气对热量和其他物质的垂直输送不符合适用于正常白天条件的理论。白天，地面加热导致地面附近的空气变得不稳定，导致对流、发育良好的湍流、可预测的温度、风速和湿度分布，因此可预测的垂直热量和水蒸气通量。但在夜间，地表寒冷，空气稳定，湍流往往是间歇性的，由垂直风切变与地形引起的排水气流有关。最近的Case-99现场计划(堪萨斯州，1999年10月)表明，排水风通常形成在仅为平缓倾斜的地面上，并且可能仅限于大气的最低几米处。垂直热通量和其他性质是由排水水流中的剪切和湍流决定的，但这些特征并不是很清楚，因为测量结果在垂直方向上通常相差太远，无法很好地反映真实结构。在这个项目中采用了两种方法。一是分析过去实地项目的现有数据(特别是Case-99)。分析的重点是理解湍流的间歇性爆发，这些湍流是稳定条件下热量和动量涡流输送的主要原因。CASES数据使人们有可能区分由排水气流顶部的切变在当地产生的湍流和在其他地方产生并在观测地点上空平流的湍流。第二种方法是开展一个小型实地方案，重点放在细小的排水水流上，采用新的仪器，以便能够在空间和时间上进行高分辨率的测量。这些仪器包括热膜风速计、短程声波风速计和微型换能器，以及激光多普勒风速计。这项研究的结果将是改进非常稳定的边界层中的输送过程的公式，以用于天气预报和气候预报的大型数值模式。

The purpose of this project is to improve our understanding of the exchange of heat, moisture, momentum, and contaminants between the earth's surface and the lower atmosphere when the atmospheric layer near the ground is extremely stable. In these conditions, the variation of the wind with height and time and the vertical transport of heat and other material by the atmosphere do not conform to the theories appropriate for ordinary daytime conditions. During the day, surface heating causes the air next to the ground to become unstable, leading to convection, well-developed turbulence, predictable profiles of temperature, wind velocity, and humidity, and therefore predictable vertical fluxes of heat and water vapor. But at night, the surface is cold, the air stable, and the turbulence often intermittent and caused by vertical wind shear associated with terrain-induced drainage flows. The recent CASES-99 field program (Kansas, October 1999) showed that drainage winds often form over ground that is only gently sloping, and may be confined to only the lowest few meters of the atmosphere. The vertical flux of heat and other properties is determined by the shear and turbulence in the drainage flow, but these characteristics are not well known because measurements are ordinarily too widely separated in the vertical to give a good representation of the true structure. Two approaches are pursued in this project. One is to analyze existing data from past field programs (especially CASES-99). The analysis focuses on understanding the intermittent bursts of turbulence that account for much of the eddy transport of heat and momentum in stable conditions. The CASES data make it possible to distinguish between the turbulence generated locally by shear at the top of the drainage flow and that which is created elsewhere and advected over the observing site. The second approach is to conduct a small field program focussed on thin drainage flows employing new instruments to enable measurements with high resolution in space and time. These instruments include hot-film anemometers, sonic anemometers with short path lengths and miniaturized transducers, and laser-Doppler anemometry. The result of the research will be an improved formulation of transport processes in the very stable boundary layer for use in large-scale numerical models for weather forecasting and climate prediction.