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，当接近地面的大气层非常稳定时。在这些条件下，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普通白天条件的理论。在白天，地表加热使靠近地面的空气变得不稳定，导致对流、充分发展的湍流、可预测的温度、风速和湿度分布，因此可以预测热量和水蒸气的垂直通量。但在夜间，地面很冷，空气稳定，湍流通常是间歇性的，由垂直风切变引起，与地形引起的排水流有关。最近的CASES-99野外项目（1999年10月，堪萨斯州）表明，排水风通常只在平缓倾斜的地面上形成，并且可能仅限于大气中最低的几米。垂直的热通量和其他特性是由排水流中的剪切和湍流决定的，但这些特性并不为人所熟知，因为测量通常在垂直方向上太分散，无法很好地表示真实的结构。本项目采用了两种方法。一种是分析过去实地项目（特别是case -99）的现有数据。分析的重点是理解在稳定条件下热量和动量的涡流传输中，间歇性的湍流爆发。case数据可以区分由排水流顶部的局部剪切产生的湍流和在其他地方产生并在观测地点平流的湍流。第二种方法是进行一个小型的现场项目，重点关注稀薄的排水流，使用新的仪器来实现高空间和时间分辨率的测量。这些仪器包括热膜风速计、具有短路径长度和小型化换能器的声波风速计和激光多普勒风速计。研究结果将改进非常稳定边界层中输送过程的公式，用于天气预报和气候预测的大尺度数值模式。

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.