夏季东天山中段一次强对流天气过程的数值模拟

A numerical study of the severe convective precipitation processes over the middle section of the eastern Tianshan Mountains during the summer seasons

利用常规气象观测资料、区域自动站观测资料和FY-2D卫星逐时TBB资料,采用WRF中尺度数值模式,对2011年夏季发生在东天山中段一次强对流天气过程进行数值模拟和诊断分析,研究了天山特殊地形对降水过程的动力结构、水汽输送和云降水微物理机制的影响。结果表明,西风气流东移时受东天山的阻挡,气流从东天山南北两侧绕流,北侧急流经博尔塔拉谷地越过北天山西段后,急流右侧气流反气旋转向形成北支气流;南侧急流遇吐鲁番地区反气旋系统阻挡而转向北进形成南支气流。两支气流受地形动力抬升在东天山中段北坡汇合,为此区域局地强对流降水的形成和发展提供动力条件,北支气流为主要的水汽供应源。高空西南气流引导的冰相云系与低层局地对流云在东天山中段北坡结合,分别持续提供冰晶和云水,促使云微物理过程发展旺盛,致使局地暴雨过程产生。

The Tianshan Mountains are the largest set of mountain range in middle Asia. As such,as a result of their extended zonal topography,they partially determine the distribution of the water resources in Xinjiang during the summer months,due to their capability of stimulating precipitation events. The precipitation events which are induced by the Tianshan Mountains' topography have significant benefits for the arid and sub-arid regions near Xinjiang. In this research,the aim was to explore the dynamical structures,transportation of water vapor,and microphysical mechanisms which are affected by the Tianshan Mountains' topography. A severe convective rainfall event which occurred during the summer of 2011 over the middle section of the eastern Tianshan Mountains was analyzed using the available data from ordinary observations,and automatic weather stations,along with Fengyun-2 D( FY-2 D) Temperature of Black Body( TBB) measurements. A Weather Research and Forecast( WRF)model with a nested domain setting was also employed in this study. When compared with the observational and satellite data,the WRF output was found to be nested in three sub-domains which had different spatial resolutions,and together,these data were found to reasonably recapture this particular precipitation event. Therefore,a more detailed analysis of the large-scale dynamics and microphysics was conducted in this study,in order to retrieve the features of this topographical-induced precipitation event over the middle section of the eastern Tianshan Mountains. The results showed that the eastern Tianshan Mountains had blocked one airflowmoving towards the east,and split the flowgoing around the eastern Tianshan Mountains from the northern and southern slopes. On the level of 900 to 800 h Pa,a northern jet stream converged following the Yili Valley,and then turned toward the southwest,before climbing Mount Poluokenu,which has a relatively lowaltitude in the west section of the northern Tianshan Mountains. Therefore,the northern sub-flowwas determined to have originated from a clockwise turning of the northern jet stream at the right exit of the jet flowbefore it was forced topographically by the northern slope of the middle section of the eastern Tianshan Mountains. On the level of 750 to 650 h Pa,a southwestern jet stream developed after the southern jet stream climbed the southern Tianshan Mountains to reach the Talimu Basin,and then turned to the north due to being affected by an anti-cyclonic system over the Tulufan region. It then formed a southern sub-flowwhich climbed up the southern slope of the northern Tianshan Mountains after reaching the Yanzhe Basin. These two sub-flows eventually merged at the top of the middle section of the eastern Tianshan Mountains,and together dynamically favored the development of local convections in which the northern sub-flowbecame the main source of the water vapor,which potentially caused the severe rainfall event. As can be seen from the large-scale dynamics,the high level southwestern airflowled to ice-phase cloud bodies with depths of 5 to 6 km on the northern slope of the middle section of the eastern Tianshan Mountains. These merged with the lowlevel deep convections which have already been formed there. Then,the cloud particles in the form of super cooled water were able to be uplifted to a higher level,and co-existed with the ice particles in the icephase clouds. This co-existence enabled a much stronger Bergeron process,and a riming process was determined to have occurred,which accelerated the growth of the cloud particles. This in turn caused heavy rain fall processes to occur in the area. Therefore,based on both the observations and numerical simulations,this study was able to present a better understanding of the topographical effects of the Tianshan Mountain regions on heavy rainfall events. The results of this study may also potentially act as a reference for future weather forecasts and modifications in the mountainous areas of Xinjiang.