A Study Of Cloud Microphysical Characteristics And Its Evolutionary Mechanisms In The Typhoons

With the development of numerical weather models and observation techniques,the trop-ical cyclone?TC?track and intensity forecasts have been steadily improved.However,the re-lationship between the rapid intensification?RI?in TC as well as the contribution of cloud mi-crophysical processes is still full of challenge.Besides,part due to the uncertainty of cloud microphysical parameterizations in current mesoscale numerical model,the simulated TC pre-cipitation is still barely satisfactory compared with the observations.This study focuses on the cloud microphysical processes and their internal evolutions in typhoon systems.The character-istics of hydrometeor content surrounding the rapid intensification in Super TCs that make land-fall in the mainland China during the summer and autumn seasons are compared.Moreover,the effect of vertical wind shear on the raindrop size distribution in the typhoon is discovered and used to modify the collision/breakup parameterization in microphysical scheme,which are nec-essary for understanding the evolution of cloud microphysical processes and improving the intensity and precipitation forecast in typhoon systems.Using the Joint Typhoon Warning Center?JTWC?best-track and new reanalysis data from the European Centre for Medium-Range Weather Forecasts?ECMWF?,the typhoons generated in the Northwest Pacific Ocean during 1979-2017 are divided firstly into summer and autumn typhoons.The seasonal variability of the landfalling Super typhoon's track and intensity are focused and analyzed.Although the total number of typhoons that make landfall in the mainland China in summer is two times larger than that in autumn,the latter cases are more easily to intensify to Super typhoons with shorter lifespan and stronger intensity.Up to 50%of landfall-ing Super typhoons undergo RI to reach their maximum intensity through the lifespan.During the RI period,similar evolutions of hydrometeor contents in landfalling Super typhoons are shown in both seasons.It's emphasized here that the ice-phase particles in the autumn cases exhibit notable changes during the RI period while no obvious signals exist in the summer ones.The diversities of initial intensity and intensification rate of landfalling Super typhoons shows no consistent trend,and the location where cases start intensifying as well as large-scale envi-ronmental conditions may be the factors that lead to the increase of updrafts at lower-layers in the autumn cases.The cloud droplets are transported into the mixed layer,freezing to cloud ice and releasing a large amount of latent heat that influence the middle-and upper-level instability.The correlation coefficient of hydrometeor content change before the onset of RI and the first6h intensify change of vortex indicate that the latent heat released by the phase change may only trigger the intensity change of typhoon system.In order to investigate the evolution of microphysical properties within different area and different developing stage in a super typhoon in the autumn,a simulation of Typhoon Usagi?2013?is conducted using the Weather Research and Forecasting model?WRF?with Morrison two-moment microphysical scheme.The inner-core and outer region are divided by two-fold radius of maximum wind from the storm center.It is found that solid hydrometeor content is increasingly changing during the RI period and all-size raindrops increase in the inner-core,corresponding well with the upward motion.The surface precipitation is dominantly controlled by the content and size distribution of low-level raindrops.A maximum layer of raindrop num-ber concentration is located at 1.25km which is supposed to be affect the characteristics of raindrops therein.Spatial distribution as well as transferring rate of source/sink terms of raindrops in the inner-core and outer area are different from each other.The number concentra-tion of low-level raindrops is balanced by the evaporation and self breakup.Due to the terrain,the vertical shear of horizontal wind within a landfalling typhoon has certain effects on the low-level raindrop size distribution and surface precipitation.In terms of the theoretical relationship between collisional Weber number and the diameter ratio of two raindrops when they collide,the effects of vertical wind shear on the binary raindrop collision outcomes are quantitatively investigated.Results show that strong vertical wind shear will in-crease the probability of collisional breakup when two raindrops collide with each other.Sen-sitive experiments are conducted to simulate Typhoon Nida?2016?using the WRF with Morri-son two-moment microphysics.By reducing the constant value of cloud droplet concentration from 250?88?^-3 to 30?88?^-3?NC30?and modifying the raindrop collection/breakup parame-terization as a function of vertical wind shear?Dth=max?300-4000×VWS,100?,NC30?WS?,the characteristics of raindrops as well as surface precipitation in the control and sensitive tests are compared.The vertical wind shear commonly enhances when Typhoon Nida?2016?ap-proaches the land due to the different roughness between ocean and land.An obvious decrease of raindrop size is present when the constant value of cloud droplet concentration is modified,but unrealistic large raindrops are still produced.Adding a semi-empirical factor of vertical wind shear into the microphysical scheme leads to a certain degree of improvements in simu-lating the raindrop size distribution and precipitation in Typhoon Nida.As cloud water is critical for the formation of raindrops,and aerosol has great impact on the cloud droplet number concentration and cloud water content,a simulation of Typhoon Nida using the WRF-Chem and Morrison two-moment microphysics is employed to investigate the effects of anthropogenic aerosol precursor gases on microphysics and precipitation before and after the typhoon's landfall.The sensitive experiments are conducted by reducing the emission intensity of precursor gases of sulfate?SO2??and ammonium?NH3??to one-tenth of its original level respectively,to figure out the diversity of microphysical properties,especially the cloud droplets and raindrops.Additional runs in which random perturbations on the low-level temperature in initialization field are further carried out to validate the robustness of aerosol effect on the microphysical processes and precipitation.When the typhoon locates over the ocean,larger-size cloud droplets with smaller-size raindrops and lesser precipitation are simu-lated in the SO2?and NH3?runs.The earlier enhancement of raindrops mixing ratio along with less precipitation occurs in these sensitive runs probably due to the suppression of warm-rain processes and the enhancement of convection under the effect of aerosol.As vortex makes landfalling,the opposite changes are shown as the area-averaged raindrop size and surface pre-cipitation are smaller?larger?in the SO2??NH3??run than the control run,which is likely related to the solid hydrometeor content as well as low-level humidity.Sensitive experiments with temperature perturbation indicate that the aerosol precursor gaseous has indeed effect on the cloud droplet number concentration within the vortex but large uncertainty is exist in its effect on the raindrops and precipitation.