Numerical Simulation Study Of The Heavy Rainfall Belt Of Typhoon Hato (2017) Under The Influence Of Mesoscale Topography

The research on rainfall of landfall typhoons is the focus of typhoons'research at home and abroad.When Typhoon Hato(2017)moved inland to southwest China,the rainband passed through the Dayao Mountain,producing heavy rainfall.In this paper,WRF numerical simulation and sensitivity tests were used to analyze the change characteristics of organizational structure of rainband and the physical mechanism of the influence of topography.The study showed that the terrain has an increasing effect on typhoon rainfall.There are many small and medium scale convective cells in the rainband,which spiraled into the eye wall.The convective cells were greatly developed when passing through the terrain,thus bringing local heavy precipitation distributed along the terrain.Topography plays an important role in the dynamic,thermodynamic and cloud physical processes of the development of heavy rainband.The rainband passed over the terrain with high speed,that the airflow easily passed over,producing upward movement on the windward side and downward movement on the leeward side.The warm and humid air in the lower layer was rised along the steep slope,forcing the air to move upward and transport water vapor,condensing and releasing latent heat,which further enhanced the instability and vertical movement over the windward slope.The convective unstable stratification and strong upward movement in front of the slope were conducive to the development of the convective system.The over-mountain air formed high-density flow near the summit and developed to a strong downslope flow.Due to the adiabatic warming effect of the downflow,the downstream flow is uplifted and the lee waves disturbance on the leeward side is induced.When the convective system in the rainband passed over the terrain,the typhoon precipitation cloud system and orographic precipitation cloud system synergistically promote the increase of typhoon rainfall.The vertical structure of hydrometeors in the clouds above the terrain showed that ice crystals located at the highest level,followed by snow and graupel,cloud water,and rain water.Melting ice particles and supercooled water co-exist near 0? layer.Due to the condensation process of air rising along the terrain,cloud water accumulates in front of windward slope,which is beneficial to the development of orographic precipitation cloud system.Under the influence of strong low-level wind speed,precipitation extended to the downstream,and the strong downward movement accelerated the fall of hydrometeors,so precipitation mainly concentrates on the top of the mountain.However,a large number of convective system weakened and died after passing over mountain,and fewer of them moved to the downstream of the leeward side.Only the rainfall generated by the leeward wave was limited,and the precipitation on the leeward side decreased without the influence of convection.The sensitivity tests of topographic height shows it has great influence on the intensity and distribution of orographic precipitation.The higher the topographic height is,the stronger the topographic forced uplift is as F_w is greater than 1,leading to the greater increase of typhoon precipitation.The latent heat test shows that latent heat release plays a great role in maintaining the development of upwelling motion in front of slope.When the latent heat feedback is turned off,the upwelling motion weakens and strong convection cannot continue to develop.