不同对流参数化方案试验中凝结加热的特征及对暴雨中尺度模拟结果的影响

NUMERICAL STUDY ON CHARACTERISTICS OF CONDENSATIONAL HEATING RATES AND THEIR IMPACTS ON MESOSCALE STRUCTURES OF TORRENTIAL RAIN SIMULATIONS

使用20 km分辨率的MM5模式,分别选用KUO,GRELL,KAIN-FRITSCH和BETTS-MILLER(以下简称KU,GR,KF和BM)等4种不同对流参数化方案,对1996年8月3～4日石家庄特大暴雨过程作数值模拟试验,分析比较了4个不同试验中网格尺度(显式方案)和次网格尺度(对流参数化方案)凝结加热的水平、垂直分布和时变特征;研究探讨了凝结加热分布及差异对暴雨中尺度模拟结果的可能影响.分析显示,暴雨过程中,4个不同对流参数化方案试验所得到的次网格尺度凝结加热基本都呈单峰特征、加热峰值在对流层中层,但加热层厚度和强度在不同试验间存在差别;4个试验的网格尺度凝结加热的垂直范围表现出较好的一致性,加热重心位于对流层低层,但加热强度仍有所不同;GR和KF及BM试验的总凝结加热率的垂直分布特征主要受其网格尺度凝结加热率特征的影响、加热重心在对流层低层,而KU试验的总凝结加热率的垂直分布特征由其次网格尺度凝结加热率特征所决定、加热重心在对流层中层.研究表明,尽管4个试验在暴雨期间总凝结加热的垂直分布差异并不显著,但对暴雨中尺度模拟的影响却不能忽视.凝结加热的分布特征及演变直接影响与暴雨发生发展密切关联的物理量场的中尺度结构和演变;凝结加热对暴雨中尺度的影响具有连锁性,由加热差异波及局部环流细致结构和强度及其变化的差异,进而影响暴雨发生发展的细致特征.在20 km或更高一些分辨率的条件下,对于描述温带/中纬度暴雨的发展和结构,选用KF方案得到的模拟结果可能更具物理合理性;而KU方案模拟结果容易出现格点气柱的水汽和温度被过量调整的不合理情况.要得到一个可信的中尺度模拟结果,对降水模拟结果进行细化特征的验证、特别是随时间演变特征的验证分析是非常重要的,因为降水的细致演变特征与凝结加热及与之相联系的物理量场的中尺度演变特征密切关联.

The characteristics of condensational heating rates of both grid-scale and subgrid-scale in Shijiazhuang torrential rain event happened in Aug. 3 - 4 of 1996 are investigated using real data numerical simulation results from 4 experiments of MM5 model （△x = 20 km）, in which 4 convective parameterization schemes, KUO, GRELL, KAIN-FRITSCH and BETTS-MILLER （here after refers as to KU, GR, KF, and BM） are chosen respectively. And the impacts of condensational heating rates on mesoscale structures of the torrential rain simulations are also studied. It is found that the distributions of subgrid-scale condensational heating rates of 4 runs are similar in the vertical heating peak feature （at the middle level of the troposphere）, but are different in the heating thickness and strength; whereas those of the grid-scale from 4 experiments are quite similar in heating thickness with heating peak at the low-level of the troposphere, and there are some differences in the heating strengths too. In addition, the distributions of total condensational heating rates in GR, KF and BM runs are determined by their grid-scale heating rates, while in KU run, it is controlled by subgrid-scale heating rates. Despite the differences of the total condensational heating rates between the 4 runs in the period of the torrential rain are quite limit, their impacts on the mesoscale structures in the simulations are negligible. The differences of condensational total heating rates affect the mesoscale structures and evolutions of the torrential rain in a way of chain reaction, namely the differences of the total heating rates induce the discrepancies on mesoscale circulation over the raining region, and these local circulation discrepancies cause further the diversities of the precipitation simulations. It seems that the KF run is more reasonable in simulating the middle-latitude systems than others; whereas it is＇easy to occur in the KU run that the moisture and temperature of the grid column are adjusted excessively and unreasonably by the convective parameterization at 20 km or higher horizontal resolutions. And for a reliable mesoscale simulation, it is very important to verify the detail structures and time-series of precipitation simulations against observations because the detail structures and evolutions of the precipitation are closely related with the distributions and evolutions of the condensational heating and mesoscale circulations.