Research On The Evolution Process And Mechanism Of Continuous Heavy Rainfall In North China On July 19

In 2016,the "7·19" local heavy rainfall in North China lasted for a long time,and the precipitation was large.The extreme characteristics of precipitation were obvious,which had a great influence on the local area,and the forecast was difficult.Based on the precipitation fusion product with a resolution of 0.1°× 0.1° per hour,ERA-interim reanalysis data with a spatial resolution of 0.25°× 0.25°,and NCEP/NCAR FNL data,the evolution process and formation mechanism of sustained heavy precipitation in North China were studied.WRFv3.8 model was used to simulate and diagnose the heavy precipitation process and low vortex development mechanism,and topography sensitivity test was conducted to further explore the role and influence of Taihang Mountain on precipitation in different stages.The conclusions are as follows:(1)The heavy precipitation occurred in the favorable circulation situation of "high in the east and low in the west",the southwest vortex moved toward the northeast,and the southwest and southeast jet at the lower level transported warm and wet air and energy.Influenced by the topography of Taihang Mountain and Yanshan Mountain,precipitation extremes in the eastern piedmont of Taihang Mountain were obvious.The continuous heavy precipitation process was mainly topographic precipitation controlled by low-level easterly risk before 12:00 on July 19,and vortex precipitation generated after the moving of vortex weather system after 12:00.Further analysis of mesoscale convective cloud cluster showed that the blackbody radiant temperature TBB(the same below)reflected the characteristics of high efficiency mixed precipitation in the middle and late period of precipitation,and there were both stratus precipitation and convective precipitation in persistent precipitation events.Atmospheric instability conditions also reflected the characteristics of precipitation in different stages.(2)The occurrence and development of precipitation was closely related to the low vortex.The terrain affected the moving path of the vortex,and precipitation in Taihang Mountain area increased with the enhancement of the vortex.Sustained heavy precipitation occurred under favorable weather background and dynamic conditions.Vorticity budget analysis showed that horizontal divergence term and torsion term made a major positive contribution to the maintenance and development of low vorticity,and with strong upward movement and vertical wind shear,the enhancement of vertical wind shear promoted the transformation of horizontal vorticity to vertical vorticity.The advection and tilting terms of the whole layer vorticity variability played a significant role,and the 850 h Pa single-layer stretch term and vertical transport term contributed positively to the vorticity growth.Precipitation occurred in the region with large apparent heat source and apparent moisture sink value at 500 h Pa.The development of low vortex showed that the latent heat released by apparent moisture sink Q2(the same below)heated the atmosphere and promoted the occurrence and development of low vortex and precipitation.The vertical transport term had the largest effect on heat source.The release of latent heat of condensation in the precipitation process,the strong convective instability in the middle and lower layers,and the convergence rise of air flow were the main factors for the maintenance of the heavy precipitation process and the occurrence and development of the vortex.(3)The analysis of the characteristics of local persistent heavy precipitation in Taihang Mountain showed that the terrain had a great effect on precipitation increase,and the height sensitivity tests of multiple terrain showed that Taihang Mountain had different effects on precipitation in two stages.The large value area of the middle troposphere of apparent heat source Q1(the same below)reflected the deep heating layer and the development of low vortex.The precipitation changed from convective precipitation with significantly different peak values of Q1 and Q2 to relatively stable non-convective precipitation.With the increase of the topographic height setting,the precipitation was confined on the east windward slope due to the blocking and forced uplift of the convective precipitation topographic in the first stage.In the second stage of vortex precipitation,the truncation effect of Taihang Mountain topography on the northeast airflow of vortex made precipitation increase on the whole,and the precipitation area moved eastward.At the same time,the path of the vortex shifted to the southeast,and the range of the vortex became more concentrated and its intensity increased significantly.The vertical transport of vorticity and the effect of torsion term were increasing,which were the important reasons for the maintenance and development of low vorticity.With the increase of terrain height,more water vapor was trapped in the east side of Taihang Mountain,which was conducive to the convergence of water vapor flux and the formation of continuous heavy precipitation process.In this paper,the process of sustained heavy rainfall in North China was simulated by numerical model,and the evolution and mechanism of precipitation were analyzed and studied.It is proved that Taihang Mountain is an important reason for the occurrence and maintenance of heavy precipitation in the same place in different stages by multiple topographic sensitivity tests,which provides theoretical basis and reference for the research and forecast of local sustained heavy precipitation.