Comparative Analysis Of The Microphysical Mechanism Of Precipitation During The 2020 Degenerate Typhoon “Mesak” And The Non-degenerate Typhoon “Bavi??

The typhoon "Maysak"(experiencing extratropical transition)and "Bavi"(a non-transitioning typhoon)in 2020 were simulated by numerical model WRF4.2.To analyze the precipitation processes,this study has divided the typhoon into two distinct regions: the central low-pressure area and the outer cloudy region.For the typhoon “Maysak”,its structural evolution and the microphysical processes of precipitation were analyzed.Thus,the impact and mechanism of the cold trough on the spatial distribution and temporal variation of precipitation intensity were obtained.Meanwhile,a comparison was made between "Bavi" and "Maysak," which have similar paths,in order to better summarize the microphysical characteristics of precipitation during the typhoon transition.The results showed that:(1)In terms of the structural evolution,the mechanism of the cold trough’s impact on Typhoon "Maysak" is as follows: In the central low-pressure area,the height of convection lowered due to the invasion of dry and cold air from the middle layer.However,the invasion also contributed to maintaining the convection intensity of the middle and lower layers.In outer cloud area of typhoon,the vertical vorticity transfer,as well as cold air invaded from lower layers to lift warm air,enhances the upward motion of the middle and upper layers.Then,the system gradually evolves into a forward-leaning convective structure.The influence of the trough on the convection of "Bavi" is relatively minor.There is no central convection height lowered process similar to "Maysak" after encountering cold air.(2)The cold trough impacts the dynamic and thermodynamic structure of the typhoon,affecting the spatial distribution of hydrometeors.The distribution characteristics of hydrometeors are as follows: in the central low-pressure area,cloud water is more concentrated towards the core,while in the outer cloud area,snow is ahead of cloud water along the outflow direction.In terms of quantitative characteristics,the intrusion of low-level dry and cold air leads to higher rates of snow desublimation and rain evaporation in the outer cloud area,while cloud water condensation efficiency is low.It results in a higher proportion of snow in the hydrometeors and a lower proportion of cloud water and rainwater.The greatest difference in structural evolution between transitioning typhoon and non-transitioning typhoon is: The process of "Maysak" transforming into an extratropical cyclone involves the development of both the central low-pressure area and the outer cloudy region.And the evolution of "Bavi" is a process where the outer cloudy area gradually dissipates,and the convective structure of the low-pressure center gradually spreads outward and tilts.The differences in the structural evolution of hydrometeors result in "Bavi" not exhibiting a distinct spatial distribution of snow ahead of cloud water.The distribution of hydrometeors is mainly concentrated around the center of minimum pressure.(3)In terms of the temporal variation of precipitation,the cold trough primarily affects the vertical velocity of “Maysak”,directly impacting the efficiency of snow desublimation and cloud water condensation.It in turn influences the efficiency of the process of snow collecting cloud water,snow collecting cloud water converted into graupel,snow melting to rainwater,rainwater collecting cloud water,graupel melting to rainwater,and ultimately lead to the increase of precipitation.The difference in microphysical process characteristics between the two typhoons can be attributed to two main factors.The first factor is the insufficient water vapor condition of "Bavi",while the second factor is the vertical velocity differences caused by structural evolution.The differences ultimately manifest in the precipitation as follows: "Bavi" did not show a significant increase in precipitation due to cold air similar to "Maysak".The precipitation trends of the central low-pressure area and outer cloudy region tended to be opposite.(4)The cloud microphysical characteristics of "Maysak" and "Bavi" exhibit certain similarities.This similarity is primarily manifested in the higher content of snow,followed by cloud water and rain,and the lower amounts of graupel and ice.Additionally,the main source of rain for both typhoons is the melting of snow and the collection of cloud water by rain.However,the specificity of the precipitation process in extratropical transitioning typhoon is evident in the following aspects: regarding spatial distribution,due to the unique structural evolution characteristics of transitioning typhoon,there are distinct spatial distributions of cloud water and snow.This ultimately leads to its distinctive precipitation pattern.In terms of temporal evolution,the influence of the cold trough causes an increase in the vertical velocity values in both the central low-pressure region and the outer cloud region of transitioning typhoon.This directly enhances the efficiency of cloud water condensation and snow desublimation,resulting in an overall increase in precipitation for transitioning typhoon.