Numerical Study Of A Squall Line In Eastern China

As one of the typical mesoscale convective systems,squall lines often cause huge losses of people's lives and properties due to the severe weather,such as heavy rainfall,strong winds,even hail and other disastrous weather.It is very important to study the occurrence and development mechanism of squall lines and improve the prediction ability of numerical model for the weather system.Previous studies show that the northern Jiangsu Province is the region with the highest occurrence frequency of squall lines in the eastern part of China,and the disastrous weather affects the ecomomically developed and densely populated areas,such as Jiangsu,Anhui and Shanghai.In this study,a case study of a squall line occurred here on June14,2009 is selected.Because of the great intensity and wide influence of this process,hail was observed at several stations,affecting Jaingsu,Anhui,Shandong and other places.The generation location and influence range have typical characteristics of the squall line in this area.In this study,the WRF(Weather Research and Forecasting)model combined with the observation nudging method(OBS-Nudging)was used to simulate the fine structure of midlatitude squall lines with sub-kilometer resolution.The ability and shortage of model are improved through the introduction of automatic weather stations.At the same time,the influence of terrain on the occurrence and development of squall line is studied.The main results of this study are as follows:The control experiment(CTL)simulates the evolution and basic structure of the squall line well,and the falling area of heavy rainfall is basically consistent with the observation.Based on this,the application of automatic weather stations data in the simulation is studied in this paper.It is found that the low-level relative humidity in the FNL data is lower than that observed,and that the low-level water vapor in the eastern part of China plays an important role in the formation of Convective Available Potential Energy.The sensitivity experiments of increasing low-level water vapor prove that the improvement of low-level water vapor can effectively improve the results.Using the OBS-Nudging method in the sensitivity experiment of assimilating the data from the automatic weather stations during the simulation of the model,the low-level water vapor simulation is improved,and the triggering time of convection is 1hour earlier than the control experiment,which can simulate the development process of squall line more similar to the observation.Through the simulation of the squall line by WRF model,it is found that there is a deviation in the temperature diurnal cycle simulation of WRF model.The simulated low-level daily maximum temperature lags from the actual observation for 2 to 3 hours,and the simulated temperature drop in the evening is 2-3 ? lower than the actual observation.The comparison experiment shows that the temperature exchange between the boundary layer and the free atmosphere can be weakened by changing the boundary layer parameterization scheme from YSU to MYJ,and then the simulation results can be improved to a certain extent,that is,the intensity of the simulated squall line and the low-level temperature are more similar to the actual observation.At the same time,it can also play the same role to assimilate the data of the automatic weather stations into the model without changing the boundary layer scheme.In addition to the favorable conditions such as large-scale environmental fields,the topography also plays a very important role in the frequent occurrence of squall lines in northern Jiangsu Province.In this paper,the sensitivity experiments of topography show that the mountains force the secondary circulation in the vertical direction beyond the accumulation of water vapor in the lower layer by blocking the warm and wet air flow in the lower layer,so as to enhance the vertical shear in the lower layer.The overturning and shearing of the airflow in the lower layer play an important role in the early occurrence of squall line.In the sensitivity experiment of removing the mountains,the occurrence time of convection obviously lags and the intensity weakens.The role of lakes in the development of squall line has not been well studied.Although it is generally believed that lakes can provide sufficient water vapor for convection development,this study proposed lakes through a series of sensitivity experiments on lakes in northern Jiangsu Province.This study proposes that modulation of the dynamic field affects squall line development: Hongze Lake and Gaoyou Lake in northern Jiangsu Province not only provide more favorable thermal fields such as water vapor for squall line development,but also the enhancement of low-level temperature gradients further enhances low-level vertical shear to form a power field that is more conducive to the development of squall lines.The important role of lakes in the development of squall lines is pointed out,and the reason for the frequent occurrence of squall lines in Jiangsu Province is also explained from the perspective of terrain.