Influence And Mechanism Analysis Of Poyang Lake On The Intensity Of Strong Convective Weather In Summer

The heterogeneity of the underlying surface of the lake and land has a huge influence on the development and evolution of severe convective weather.Poyang Lake is the largest freshwater lake in China,and the lake area has obvious monthly and seasonal variations,while the underlying surface of Weather Research and Forecast(WRF)model generally defaults to the constant lake area,which is quite different from the actual situation and leads to model forecast errors.In this paper,WRFv3.9 model was used to simulate a severe convective weather process in Poyang Lake area in the summer night of June 29,2020,which the rainfall decreased on the west bank of the lake and then increased over the lake,and through the sensitivity experiment of lake area change,we analyze the influence of Poyang Lake on the development and evolution of severe convective weather and its mechanism.The results are shown as follows:(1)This precipitation event has strong local characteristics,short duration and high precipitation intensity.It occurs under the favorable atmospheric environment of divergence in the upper troposphere,intrusion of cold air in the middle troposphere,convergence in the lower troposphere and abundant water vapor.The differences in atmospheric thermodynamic conditions before the precipitation occurred are obvious between lake and land: the 2 m temperature over the lake is significantly higher than that over land surface in summer night.The land-breeze converges and rises over the lake,while there is a sinking and divergent airflow on the shore.The lake has a higher convective available potential energy and a lower lifting condensation level compared to the shoreline.Compared with the control experiment,the precipitation in the lake-removing sensitivity experiment where the lake was replaced by the cropland increased by 20% on the west bank of the lake and decreased by 16% over the lake.To investigate the reason,it is found that Poyang Lake has a “warm lake effect” at night in summer,which causes the temperature difference between the lake and the land drives the formation of the land-breeze.On the one hand,the land-breeze motivates the secondary circulation between lake and land,which overlaps with the flow field of the convection system itself;On the other hand,the interception of the southwest warm and humid airflow brings the convergence of water vapor flux over the lake and the divergence of water vapor flux on the west bank of the lake.Combined with the heating and humidification of Poyang Lake on the lower troposphere,it changes the atmospheric instability condition and makes the atmospheric stratification over the lake tends to a more unstable state than that over the bank of the lake.The two factors together affect the thermodynamic structure of the convective system and make the it weaken over the west band of the lake and then enhanced over the lake,resulting in the precipitation intensity decreased on the west bank of the lake and then increased over the lake,which increase the difficulty of practical operational forecast.At the same time,it instructs us to focus on the possible trend of weakening before entering the lake and enhancing after entering the lake when forecasting the strong convective weather that crosses Poyang Lake at night in summer.(2)Further,four lake expansion sensitivity experiment were designed to expand the lake area to 1.5 times(EXL1.5),2.5 times(EXL2.5),3.5 times(EXL3.5)4 times(EXLS4.0),and then combined with the lake-removing sensitivity experiment and the control experiment for comparative analysis,and found that the strong convective weather is sensitive to the change of lake area.After the lake area expanded,the average precipitation increased by 7%,16%,30%,and 43% in EXL1.5,EXL2.5,EXL3.5,and EXL4.0,respectively,and the rate of precipitation increase is linearly and positively correlated with the lake area with a correlation coefficient of 0.99.The change of lake area affects the intensity of the convective weather is mainly to change the intensity of the “warm lake effect”,which affects the land-breeze and ascending motion,and then changes the physical process of convective weather development and evolution.The results reflect the importance of paying attention to the change of lake area in the operational forecast strong convective weather in the lake area,and in the numerical simulation of the strong convective weather process,if there is a large difference between the lake area and climatic lake area,the actual lake area needs to be considered to introducing into the underlying surface of the model,in order to improve the forecasting capability of the model for convective weather in the lake area.(3)The non-geostrophic wet Q vector and its divergence are used to diagnose the heavy rainfall event,and the result show that the vertical distribution of the non-geostrophic Q vector divergence can better reveal the direction,range,central location and intensity of the secondary circulation and the ascending motion of the system.In addition,it is found that the convergence area of non-geostrophic wet Q vector at 500 h Pa can better indicate the precipitation area in the next 20 minutes,and the superposition area of strong water vapor flux convergence area and non-geostrophic wet Q vector convergence area can be used to better locate the rain intensity center.The results provide a new ideas for the operational forecasting of convective weather intensity and rainfall areas in the lake area.