Multi-scale Characteristic Analysis And Numerical Simulation Of A Heavy Rain In Northern Henan

Using the hourly merged precipitation dataset derived from the national automatic weather station and CMORPH(Climate Precipitation Center Morphing)product at a0.1° × 0.1° resolution,the National Centers for Environmental Prediction Final Analyses(NCEP-FNL)reanalysis data,the temperature of black body(TBB)data from the Himawari-8 satellite,and the mesoscale Advanced Research Weather Research and Forecasting(WRF)Model,this paper performs diagnostic analyses and numerical simulation to the heavy rainfall happening in northern Henan and south of Hebei province during 18-21 July 2016(GMT+8).The main results are as follows:(1)As for large-scale circulations,the upper trough in the western part of the heavy rainfall areas and the subtropical high in eastern part of the heavy rainfall areas were stable and inactive.Heavy rainfall occurred under the favorable weather patterns of “high pressure to the east and low pressure to the west”.A southwest vortex forming on the eastern side of the Tibetan plateau,which stagnated for a period and then moved out of the source.At the same time,there was also a vortex generated in the north of Gansu province.The two vortices were collectively wrapped in a large low pressure system.Since then,the vortex in the northern part of Gansu province moved to the east,and the centers of the two vortices merged in the northern Henan province.(2)At 200hPa,the intensity of the jet stream increased constantly,together with the enhancement of divergence in entrance region of jet stream,leading to a north-south secondary circulation hanging over Henan and Hebei province,and heavy rain occured near the upward branch.The convergence motion induced by development of the southwest vortex at 700 h Pa appeared in front of the moving direction,the intensity of which increased with the development of the low vortex.Through high-level divergence and low-level convergence motions,the high-level jets and the low-level vortex interacted with each other,which resulted in an increase of upward branch in the secondary circulation,leading to a dynamic forcing to the occurrence and development of heavy rain in northern Henan province.In addition,the rainstorm area was in a convective instability state.Water vapor from the Bay of Bengal,the South China Sea and the East China Sea converged here.All of those conditions were favorable for the generation and development of heavy rain.(3)The southwest vortex played a key role in the occurrence of heavy rainfall in northern Henan province.The vorticity budget showed that the horizontal vorticity advection mainly worked in the formative period of the southwest vortex due to the topography of the eastern plateau and the convergence of the low vortex in northern Gansu province.Subsequently,the convergence and ascending motion caused by the secondary circulation contributed most to the development of the southwest vortex through the horizontal convergence and divergence term and the vertical transportation term.The potential vorticity(PV)analysis showed that the isentropic potential vorticity can induce the gensis of southwest vortex.With the development of the PV,the southwest vortex was also developed.The downward propagation of positive PV in high level can also stimulate the generation and maintenance of the cyclonic vorticity.(4)According to the analysis of the southwest vortex track,the southwest vortex handed around for 19 hours in source region after formation and then moved northeastward.The vortex in northern Gansu province moved eastward.The two vortex centers merged at 2000 BT 19.The intensity of the southwest vortex strengthened.Since then,the southwest vortex hovered around the area of Taihang Mountain,leading precipitation to the north of Henan province.The numerical model well simulated the occurrence and development of the heavy rain in northern Henan province.Sensitivity experiments showed that a shrinking precipitation range,a reduced precipitation intensity and a deviation of precipitation center emerged with the decrease of the terrain in Taihang Mountain.Respective analysis on thermal and dynamical effects of terrain indicated that terrain had an enhanced impact on the heavy rainfall in northern Henan province.