基于激光雷达遥感和参数化模式研究城市混合层的发展机制

A Study of Evolution and Dynamics of Urban Atmospheric Mixing-Layer Depth Based on Lidar Data and Numerical Simulation

利用北京大学的微脉冲激光雷达(MPL)观测的偏南气流条件下的混合层高度和夹卷层厚度探测资料,研究简单天气条件下城市混合层的发展机制并与GB94的参数化方案相互映证。通过激光雷达遥感的混合层高度和夹卷层厚度计算了混合层顶的夹卷率A,得到其平衡夹卷阶段的值为0.24。在不考虑机械混合前提下反演了地面感热通量,结果表明遥感的反演值与梯度法的计算值有系统性偏差,但总体上仍旧有较好的相关。偏差量的大小反映了影响混合层发展的机械湍流的参数B,进一步通过GB91模式的模拟确定该参数的最佳值约为3.5。在此基础上讨论了混合层顶夹卷和机械湍流对城市混合层发展的相对重要性。采用以上分析结果开展了个例模拟研究,结果表明GB91模式可以较好地模拟城市下垫面的混合层发展,但大尺度天气系统造成的垂直运动对模拟结果的影响是不容忽视的。

Mixing-layer depth and entrainment zone thickness are extracted from micro pulse lidar data sets measured at Peking University when surface wind is mainly from the south. The estimated values of the normalized entrainment zone depth are plotted versus an entrainment Richardson number, resulting in a good ordering of the data and agreement with the parameterization proposed in a study by Gryning and Batchvarova. The evolution and dynamics of mixing-layer over urban area under simply meteorological conditions are studied using a simple well-mixed boundary layer theory and the entrainment flux ratio （which is often used to simulate entrainment in atmospheric boundary-layer models） with an empirical relationship which is calculated from these two quantities. This ratio is generally believed to be 0. 24 under equilibrium conditions, but is highly variable during the morning and evening hours. Regional surface fluxes of sensible heat are retrieved based on a traditional thermodynamic model without mechanical mixture effects from the time series. The cause for differences between retrieval result and actual measurement （with a correlation coefficient of 0. 79） are discussed. It is the differences that make the quantification of the parameter B for mechanical turbulence production possible. A value of 3. 5 is confirmed from the result of GB91 numerical simulation. Preliminary analysis indicates both convective and mechanical turbulence, which can he interpreted qualitatively by parameters A and B, contribute to the growth process, in which the importance of every parameter can he estimated when the difference in retrieved surface fluxes of sensible heat is growing rapidly. In this study, five experiments are available for evaluating the simulation of the growth of the mixed layer. When the model adopts the effect of vertical motions induced by convergence or divergence in the large-scale flow field, realistic mixed layer developments with urban underground are obtained.