Quantitative Analysis Of Physical Processes Related To Typhoon Fitow Torrential Rainfall: Based On Three-Dimensional WRF Model

By using the rainfall budget(large-scale surface rainfall budget,cloud microphysical budget,raindrop microphysical budget)and heat budget equations in the three-dimensional(3D)Weather Research and Forecasting Model(WRF)version 3.5.1,this study conducts quantitative analysis through investigating the main physical processes responsible for the torrential rainfall associated with a landfall of Typhoon Fitow(2013).According to this study,the following conclusions can be drawn.(1)This paper verifies the convective-stratiform rainfall separation method based on the large-scale surface rainfall budget.Results show mean ascending motions are prevailing in the entire troposphere in the convective region,and liquid cloud microphysics is dominant.Whereas mean descending motions occur in the mid-and lower troposphere in the stratiform region,and ice cloud microphysics is dominant in stratiform regions.The result indicates that the convective-stratiform rainfall separation method based on the large-scale surface rainfall budget is applicable to the typhoon rainfall simulated by 3D WRF model.(2)It shows that water vapor convergence leads to condensation,which produces rainfall through accretion of cloud water by rain and melting of graupel.The typhoon rainfall is further classified into convective rainfall and stratiform rainfall with convective-stratiform rainfall separation method based on the large-scale surface rainfall budget.Over convective regions,water vapor convergence generates condensation,which feeds convective rainfall and supports transport of raindrops to stratiform regions through accretion of cloud water by rain.Over stratiform regions,melting of graupel and the transport of raindrops from convective regions are major sources for stratiform rainfall.Similar conclusions can be drawn when using another two different convective-stratiform rainfall separation schemes based on rain rate and radar reflectivities.The results are further strengthened by the analysis of the data in a storm-following coordinate using the convective-stratiform rainfall separation method based on the large-scale surface rainfall budget and another two different convective-stratiform rainfall separation schemes based on rain rate and radar reflectivities.(3)The rainfall centers with high precipitation efficiency are associated with water vapor convergence,condensation,accretion of cloud water by rain,and hydrometeor convergence.Precipitation efficiency is mainly determined by hydrometeor loss/convergence.Hydrometeor loss/convergence corresponds to the hydrometeor flux convergence,which may be related to the increased vertical advection of hydrometeor in response to the upward motions and upward decrease of hydrometeor,whereas hydrometeor gain/divergence corresponds to the reduction in hydrometeor flux convergence,which may be associated with the decreased horizontal advection of hydrometeor in response to the zonal decrease in hydrometeor and easterly winds and the meridional increase in hydrometeor and southerly winds.The water vapor convergence and associated condensation do not show consistent relationships with orographic lifting all the time.(4)The inclusion of radiative effects of liquid clouds suppresses radiative cooling in liquid cloud layers via reducing outgoing radiation to ice cloud layers,whereas it enhances radiative cooling in ice cloud layers through trapping less radiation from liquid cloud layers.The enhanced radiative cooling decreases from ice cloud layers to liquid cloud layers.The suppressed stability and vertical mass convergence increase.Thus,heat divergence is weakened to warm the atmosphere,which reduces net condensation and rainfall.The inclusion of radiative effects of ice clouds suppressed radiative cooling by reducing outgoing radiation.The suppressed radiative cooling reduces from ice cloud layers to liquid cloud layers,and the suppressed instability and vertical mass convergence decrease when radiative effects of liquid clouds are present.As a result,heat divergence is strengthened to cool the atmosphere,which increases net condensation and rainfall.The suppressed radiative cooling increases temperature and reduces net condensation and rainfall when radiative effects of liquid clouds are absent.(5)Four schemes(Hsie et al.(1980),Krueger et al.(1995),Zeng et al.(2008),and Shen et al.(2014))that parameterize depositional growth of ice crystal are tested with three-dimensional Weather Research and Forecasting(WRF)model simulations of Typhoon Fitow(2013).Shen scheme produces the rainfall,minimum sea level pressure close to those by Hsie and Krueger schemes,and close to the observations.The Zeng scheme with high ice crystal concentration simulates anomalously large rain rate and weak intensities before landfall,but the Shen scheme significantly improves rain rate simulations by dramatically reducing the depositional growth of cloud ice from cloud water.The Shen scheme improves typhoon intensities through eliminating the strong cooling effects associated with the melting of graupel in the lower troposphere.