Parameterizations of different hydrometeor spectral relative dispersion in the convective clouds

Spectral relative dispersion of different hydrometeors is vital to accurately describe sedimentation. Here, the Weather Research and Forecasting model with spectral bin microphysics is used to simulate convective clouds in Shouxian of Anhui province in China to study the spectral relative dispersion of different hydrometeors. Firstly, regardless of clean or polluted conditions, the relative dispersion of ice crystal spectra and its volume-mean diameter are negatively correlated, while the relative dispersion of other hydrometeor spectra is positively related to their respective volume-mean diameter. The correlations for cloud droplets and raindrops are affected by the process of collision–coalescence; the correlations for ice crystals, graupel particles, and snow particles could be affected by the deposition, riming, and aggregation processes, respectively. Secondly, relative dispersion parameterizations are developed based on a comprehensive consideration of the relationships between the relative dispersion and volume-mean diameter under both polluted and clean conditions. Finally, the relative dispersion parameterizations are applied to terminal velocity parameterizations. The results show that for cloud droplets, ice crystals, graupel particles, and snow particles, assuming the shape parameter in the Gamma distribution is equal to 0 underestimates the shape parameter and overestimates the relative dispersion; and for raindrops, assuming the shape parameter is equal to 0 is close to the relative dispersion parameterizations. The most appropriate constant shape parameters are recommended for different hydrometeors. The relative dispersion parameterizations developed here shed new light for further optimizing the terminal velocity parameterizations in models.摘要离散度的诊断对模式中沉降过程的准确描述至关重要. 本文利用WRF模式结合谱分档方案模拟安徽寿县地区的对流云, 研究不同水成物的离散度. 首先, 无论在清洁还是污染条件下, 除冰晶谱的离散度与体积平均直径间呈现负相关关系外, 云滴, 雨滴, 霰粒子与雪粒子谱离散度与体积平均直径呈现正相关关系; 云滴和雨滴受碰并过程影响, 冰晶, 霰粒子和雪粒子分别受凝华过程, 淞附过程和聚并增长影响. 其次, 综合考虑污染与清洁条件下离散度和体积平均直径之间的相关关系, 建立了离散度的参数化方案. 最后, 把该离散度方案应用到下落末速度的参数化方案中, 结果表明, 对于云滴, 冰晶, 霰粒子和雪粒子, 在Gamma分布中假设谱形参数等于0会低估谱形参数而高估离散度. 对于雨滴而言, 假设谱形参数等于0与参数化方案结果接近. 针对不同的水成物, 给出了最合适的谱形参数定值. 本文发展的离散度方案为进一步优化模式中下落末速度参数化方案提供参考.