Improved Diurnal Cycle of Precipitation on Land in a Global Non-Hydrostatic Model Using a Revised NSAS Deep Convective Scheme

Abstract

In relatively coarse-resolution atmospheric models, cumulus parameterization helps account for the effect of subgrid-scale convection, which produces supplemental rainfall to the grid-scale precipitation and impacts the diurnal cycle of precipitation. In this study, the diurnal cycle of precipitation was studied using the new simplified Arakawa-Schubert scheme in a global non-hydrostatic atmospheric model, i.e., the Yin-Yang-grid Unified Model for the Atmosphere. Two new diagnostic closures and a convective trigger function were suggested to emphasize the job of the cloud work function corresponding to the free tropospheric large-scale forcing. Numerical results of the 0.25-degree model in 3-month batched real-case simulations revealed an improvement in the diurnal precipitation variation by using a revised trigger function with an enhanced dynamical constraint on the convective initiation and a suitable threshold of the trigger. By reducing the occurrence of convection during peak solar radiation hours, the revised scheme was shown to be effective in delaying the appearance of early-afternoon rainfall peaks over most land areas and accentuating the nocturnal peaks that were wrongly concealed by the more substantial afternoon peak. In addition, the revised scheme enhanced the simulation capability of the precipitation probability density function, such as increasing the extremely low- and high-intensity precipitation events and decreasing small and moderate rainfall events, which contributed to the reduction of precipitation bias over mid-latitude and tropical land areas.

摘要

在较粗分辨率的大气模式中, 积云对流参数化用于表达次网格尺度上对流的作用, 对降水日变化有显著影响. 本文基于耦合NSAS(New Simplified Arakawa-Schubert Scheme)深对流方案的全球非静力YUNMA(Yin-Yang-grid Unified Model for the Atmosphere)大气模式, 研究降水日变化模拟特征, 提出了两个新诊断闭合假设和一个对流触发函数修订, 以强调自由对流层大尺度强迫对云功能函数制造率的作用. 0.25°水平分辨率的YUNMA模式3个月批量试验结果表明, 在对流触发函数中增强大尺度平流强迫对对流触发的动力限制、 并采用合适的对流触发阈值有利于改善陆地降水日变化模拟. 改进的NSAS方案通过减少太阳辐射峰值时段对流发生频率, 能够有效推迟大部分陆地午后降水峰值, 突出易被更强的午后峰值所掩盖的夜间降水峰值. 另外, 该方案改善了降水概率密度分布, 增加极小和极高降水率事件、 减少小到中等降水事件, 减小中纬度和热带陆地降水预报偏差.