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Explicit Forecasts of Winter Precipitation Using an Improved Bulk Microphysics Scheme. Part I: Description and Sensitivity Analysis

机译:使用改进的体微物理学方案对冬季降水进行显式预报。第一部分:描述和敏感性分析

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This study evaluates the sensitivity of winter precipitation to numerous aspects of a bulk, mixed-phase mi-crophysical parameterization found in three widely used mesoscale models [the fifth-generation Pennsylvania State University―National Center for Atmospheric Research Mesoscale Model (MM5), the Rapid Update Cycle (RUC), and the Weather Research and Forecast (WRF) model]. Sensitivities of the microphysics to primary ice initiation, autoconversion, cloud condensation nuclei (CCN) spectra, treatment of graupel, and parameters controlling the snow and rain size distributions are tested. The sensitivity tests are performed by simulating various cloud depths (with different cloud-top temperatures) using flow over an idealized two-dimensional mountain. The height and width of the two-dimensional barrier are designed to reproduce an updraft pattern with extent and magnitude consistent with documented freezing-drizzle cases. By increasing the moisture profile to saturation at low temperatures, a deep, precipitating snow cloud is also simulated. Upon testing the primary sensitivities of the microphysics scheme in two dimensions as reported in the present study, the MM5 with the modified scheme will be tested in multiple case studies and the results will be compared to observations in a forthcoming companion paper, Part Ⅱ. The key results of this study are 1) the choice of ice initiation schemes is relatively unimportant for deep precipitating snow clouds but more important for shallow warm clouds having cloud-top temperature greater than ―13℃, 2) the assumed snow size distribution and associated snow diffusional growth along with the assumed graupel size distribution and method of transforming rimed snow into graupel have major impacts on the mass of cloud water and formation of freezing drizzle, and 3) a proper simulation of drizzle using a single-moment scheme and exponential size distribution requires an increase in the rain intercept parameter, thereby reducing rain terminal velocities to values more characteristic of drizzle.
机译:这项研究评估了冬季降水对在三种广泛使用的中尺度模型[第五代宾夕法尼亚州立大学-国家大气研究中尺度模型(MM5),快速更新周期(RUC)和天气研究与预报(WRF)模型]。测试了微观物理对初始冰的引发,自动转换,云凝结核(CCN)光谱,gra的处理以及控制雪和雨的大小分布的参数的敏感性。通过使用理想化的二维山脉上的水流模拟各种云层深度(具有不同的云层顶部温度)来执行敏感性测试。二维屏障的高度和宽度设计为可再现上升气流模式,其范围和大小与文献记载的冻滴情况一致。通过在低温下将水分分布增加到饱和,还可以模拟深沉的积雪。如本研究报告所述,在二维上测试了微物理学方案的主要敏感性之后,将在多个案例研究中对具有改进方案的MM5进行测试,并将结果与​​即将发表的第二篇随笔论文中的观察结果进行比较。这项研究的主要结果是:1)对于深雨雪云而言,冰启动方案的选择相对而言并不重要,但对于云顶温度高于-13°C的浅暖云而言,更重要的是2)假定的雪尺寸分布及其相关因素积雪的扩散增长,假定的gra大小分布以及将边缘积雪转化为gra的方法对云水量和冻毛细雨的形成有重大影响,并且3)使用单矩方案和指数大小对细雨进行适当的模拟分布需要增加降雨截距参数,从而将降雨终极速度降低到更多下毛毛雨的值。

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