A Comparison of Two Bulk Microphysics Parameterizations for the Study of Aerosol Impacts on an Idealized Supercell

Abstract

Idealized supercell storms are simulated with two aerosol-aware bulk microphysics schemes (BMSs), the Thompson and the Chen-Liu-Reisner (CLR), using the Weather Research and Forecast (WRF) model. The objective of this study is to investigate the parameterizations of aerosol effects on cloud and precipitation characteristics and assess the necessity of introducing aerosols into a weather prediction model at fine grid resolution. The results show that aerosols play a decisive role in the composition of clouds in terms of the mixing ratios and number concentrations of liquid and ice hydrometeors in an intense supercell storm. The storm consists of a large amount of cloud water and snow in the polluted environment, but a large amount of rainwater and graupel instead in the clean environment. The total precipitation and rain intensity are suppressed in the CLR scheme more than in the Thompson scheme in the first three hours of storm simulations. The critical processes explaining the differences are the auto-conversion rate in the warm-rain process at the beginning of storm intensification and the low-level cooling induced by large ice hydrometeors. The cloud condensation nuclei (CCN) activation and auto-conversion processes of the two schemes exhibit considerable differences, indicating the inherent uncertainty of the parameterized aerosol effects among different BMSs. Beyond the aerosol effects, the fall speed characteristics of graupel in the two schemes play an important role in the storm dynamics and precipitation via low-level cooling. The rapid intensification of storms simulated with the Thompson scheme is attributed to the production of hail-like graupel.

摘要

本研究利用WRF模式搭配两种云微物理方案, Thompson和CLR, 模拟理想超级单体, 了解参数化气溶胶和云的相互作用。Thompson方案预设单参气溶胶数浓度, 而CLR方案有三参的气溶胶粒径分布和完整的双参水成物, 对成云降雨的云物理过程有更详尽的描述。结果显示, 在高浓度气溶胶的环境下, 超级单体由大量的云水和雪组成, 在干净的环境下, 则有较多的雨水和软雹。两方案模拟的3 h的总降水和降水率有明显不同, CLR方案比Thompson方案少许多。在超级单体初期增**时, 云水转化为雨水的过程以及低层固态水造成的冷却是造成此差异的关键。两方案在气溶胶活化和云水转换雨水这两个过程有显著的不同, 突显了参数化方法的不确定性。除了气溶胶的作用之外, 两个方案针对软雹落速度的处理也影响低层冷却效应, 进而改变整个系统的动力和降水场。由Thompson方案模拟的超级单体因为可以产生类冰雹的固态水使单体得以快速增**。