边界层气象学课件：CH10_边界层气象学总复习.ppt

边界层气象学总复习 壹：基本概念 掌握：大气边界层、湍流闭合问题、泰勒湍流冻结假说、奥布霍夫长度、理查逊数、地表粗糙度、湍流动能、摩擦速度、湍流通量、雷诺应力、湍流强度、混合长理论、Monin-Obukhov相似理论、埃克曼螺线、埃克曼抽吸、中性边界层、对流边界层、稳定边界层、低空急流、惯性振荡理论 ABL general characteristics ABL flows are predominately turbulent. Vertical transports of momentum, energy and mass are mainly accomplished by turbulence of various scales, from millimeter up to the whole boundary-layer. Boundary-layer structure varies with time and space, especially, ABL features a distinct diurnal cycle. Diurnal evolution of ABL The formulation suggests ways in which the Reynolds stresses might be measured, but gives no indication of how to express them in terms of the mean quantities. The simplest approach is to draw an analogy with molecular viscosity and, considering a plane boundary in the xy-plane, write for the eddy stress in the x direction on a plane parallel to the boundarywhere K is the coefficient of eddy viscosity (with the same dimensions as kinematic viscosity) and is effectively defined above. Typical atmospheric values of K lie in the range 1?100 m2 s-1. These are high values when compared with the molecular viscosity of ordinary fluids (typically 10-5 m2 s-1 for gases at STP). They demonstrate the effectiveness of eddy motions compared with molecular motions in transferring momentum. Effects of Turbulence, K - theory The limitations of K theory: 1. eddy viscosities depend on the flow; 2. K not a constant in BL; 3. K theory is not accurate for large eddies. Mixing length hypothesis 1 Prt ~ 0.8 is the turbulent Prandtl number for air. For the determination of the turbulent diffusion coefficients, the mixing length parameterization is used, which is based on the work of Prandtl (1925). Mixing length hypothesis 2 Mixing length In the layer within a few tens of meters of the surface, the shearing stress is approximately constant?a layer known as the constant flux layer. A further plausible hypothesis is that the size and path of the eddies should be proportional to height above