Influence Of Cloud Microphysical Processes On The Triggering Of The Convective System Of Beijing's "7.20" Rainstorm

A heavy rainstorm disaster(the “7·20” rainstorm)occurred in Beijing from 18 to 19 July 2016.In the early stage,the rainstorm was sudden and local,and in the later stage,the scope of the rainstorm was expanded,resulting in large scale rainstorm.The formation stage of the rainstorm convective system was from 18 to 19 July.The Weather Research and Forecasting model,version 3.9,coupled with bulk or bin microphysics schemes,was used to simulate this process,and the rainstorm was simulated well.The interaction between dynamic convergence and the latent heat forcing associated with the microphysical processes was studied separately with bulk and bin microphysics schemes to reveal the effects of the microphysical processes upon the initiation of the mesoscale convective system.Results showed that:(1)The initiation of mesoscale convective system of rainstorm benefited from favorable environmental dynamic and thermaldynamic conditions.The convective cell was initiated and developed by the terrain convergence line and the forcing of latent heat relea se and the cold surface.The cold pool strengthened the thermodynamic compensation effect,and the thermodynamic force enhanced the dynamic effect,thus forming a good dynamic and thermaldynamic environment.Due to the different calculation methods and physical characteristics between bulk and bin scheme,the convection in bin scheme was initiated earlier in the low level,while in bulk scheme was initiated in the middle level.(2)The formation and development of convective system were closely related to the evolution characteristics of cloud water material structure.In bulk scheme,the convection developed in the environment of latent heat release in the hollow,and the content of hydrometeors increased.Rainwater evaporated and absorbed heat to form a cold pool at low level and water vapor condensed into snow,graupel and ice particles,and released heat when falling at high altitude,forming strong latent heat release which absorbed by the upper layer.Then ice phase particles turned into water particles.In bin scheme,the position of convection initiation was lower than that in bulk scheme.The rainwater mass center was mainly below the 0? layer,and the cloud water particles were more than bulk scheme.Similarly,the condensation of water vapor in the upper level caused a strong release of latent heat,and since that was basically below the 0? line,the effect of latent heat was stronger.(3)The interaction between the microphysical processes and the thermodynamic environment had an important influence on the initiation and development of the mesoscale convective system of rainstorm.According to the analysis of the mass and latent heat budget of hydrometeors,bulk and bin schemes had similar feedback processes to the environment,but the main sources a nd sinks and latent heat items were different.In bulk scheme,rainwater was mainly generated by the melting of ice phase particles,and the main heating items were deposition of graupel and collection of cloud water by graupel.The updraft increased significantly with the release of latent heat,and the hydrometeors accelerated the development and conversion.The main cooling items of latent heat were rainwater evaporation near the ground and graupel sublimation in the convection core,which strengthened the cold pool and cold surface.Rainwater particles in the bin scheme simulation mainly came from the collection of cloud water and the melting of graupel.The deposition of hydrometeors enhanced the environmental latent heat and the evaporation of cloud water and rainwater enhanced the cooling effect in the lower layer,which further strengthened the circulation combined with the latent heat in the upper layer.The microphysical processes played a positive feedback role to the circulation,and the strengthening of the circulation in turn promoted the conversion of water particles,thus increasing the rainwater.The cell initiation position of bulk scheme was higher than that of bin scheme,and the main water particle conversion and heating were completed above the 0? layer.In bin scheme,stronger latent heat and cooling effect of water particles led to stronger rainwater.