Observation Of Cloud Condensation Nuclei (CCN) In The Coastal Area Of South China And Numerical Simulation Study Of CCN Impact On Heavy Precipitation

Owing to the special geographical location and diversified surface underlying surface,the heavy precipitation in south China has always been characterized by rapid development of convection,high precipitation intensity and obvious locality.Also,the triggering and maintenance mechanism of heavy precipitation is intricate,and the in-cloud microphysical processes that play a major role in different precipitation types are different.Therefore,the numerical weather forecast of heavy precipitation is a major challenge in south China.Cloud condensation nuclei（CCN）,the basic element of cloud-/rain-forming,has a significant impact on clouds and precipitation.In numerical models,multi-source observation data is one of the important bases for parameterized physical laws.In this study,the vertical structures of CCN over southern China are investigated with two flight datasets.The results show that the CCN number concentration(N_CCN)increases first with the increasing height,peaking near 1.3 km above the ground,and then decreases sharply to a low value above 2.5km.A good agreement is obtained by employing a segmentation fitting approach to describe the vertical CCN structure,and this is different from the form of exponential decreasing or constant commonly used in numerical model.The CCN particle size distribution（CPSD）presents a unimodal mode with the peak that changes to the direction of larger diameter with growing water vapor supersaturation（SS）.As for the effects of SS,N_CCN as well as the diameter of CCN increases with the increasing SS,but the sensitivity of N_CCN to SS is still different between the two observations.The CCN spectrum is well represented by the classical Twomey power-law function of N_CCN（SS）=C·SS^k.Although the values of parameter C are 72 cm^-3 and 150 cm^-3 in the two measurements,respectively,the parameter k changes a little with its values being 1.5 or so.Compared with other regions,the higher k value indicates that there are more small hygroscopic particles in the free atmosphere.Besides,the CCN profile has been planted into the Thompson-aerosol-aware scheme for cloud droplet activation and a series of experiments have been designed for test.The simulation study of a heavy rainfall occurred in south China from May 30 to 31,2021 was carried out.The results of multi-source observation data reveal that this strong rainfall occurred in the influence of subtropical high and southwest monsoon,and the atmospheric environment with sufficient convective available potential energy and water vapor was conducive to the occurrence of heavy precipitation.Comparison of three different numerical simulation experiments,the simulation result of EXP2 which planted the observed CCN profile is closer to the data of observation.Specifically, EXP2 effectually improved the amount,location,and intensity of precipitation especially the center of heavy precipitation.As for cloud microphysical processes,the clod-cloud microphysical processes contribute a lot to this heavy rainfall,and rain cloud water and graupel are the three most important hydrometeors.The different profiles of N_CCN can alter the structures of thermodynamic,dynamic,and hydrometeor significantly by affecting the processing of vapor condensation which not only can release more microphysical latent heating but also is one of the crucial terms to form cloud water.And the cloud water involves several decisive microphysical processes for heavy rainfall,including the collision-coalescence processes between cloud water and graupel,between rain and graupel,and the melting of graupel.This study shows that the N_CCN over South China first increases and then decreases in vertical,and the optimization of cloud microphysical parameterization schemes based on the aircraft in-situ measurement is conducive to improving the numerical simulation of heavy precipitation.