Effects Of Cloud Condensation Nuclei Concentration On Heavy Precipitation In Convection Permitting Simulations: Sensitivity To Cloud Microphysical Schemes

Cloud condensation nuclei (CCN) play an important role of the atmospheric environment growing attention by human. Previous studies about the CCN are mostly on the effects of climate research, and now more research is concerned about the process of CCN on the impact of heavy precipitation. Effects of CCN concentration on explicit-deep convection simulations of precipitation associated with a squall line that developed in Guangdong Province, China are investigated using the Weather Research and Forecasting model. Four groups of simulations are conducted, each with a different bulk microphysical scheme (Morrison, Thompson09, Thompson07, and WDM6) and consisting of three members with low, median and high CCN concentration, respectively. Changes caused by the CCN concentration variation in the surface rainfall rates within the regions of deep convection, stratiform cloud, and shallow convection, as well as in their areas, are compared among the four groups of simulations. Other than precipitation analysis, the paper also analyzes the cloud microphysical processes and the strength of dynamic circulation results were affected by the impact of CCN concentration of four groups of simulations. The primary findings are as follows:(1) Both similarities and differences in the CCN-precipitation impacts are noted among the four groups of simulations. Different cloud microphysical processes have a direct or indirect contact with CCN concentration. The different cloud microphysical processes are associated with complex relationship. Moreover the nonlinear coupling between cloud microphysical processes and the dynamic circulation is existed. Between the different schemes, the parameters of hydrometeors spectral distribution and the expression of the details of the process have many differences. (2) The impacts are more significant when the Thompson07 and Thompson09 schemes are utilized and the least significant in the WDM6 simulations. The differences are probably related to impacts of CCN on the strength of dynamical circulation in the simulations.(3) Rain suppression during the early stages of rainfall production by the CCN concentrations enhancement is notable in all four groups of simulations. Moreover, suppression of total rainfall amount also occurs during the late stages. At the mature stage of the squall line, CCN-precipitation impacts is more complex, which is related by the strong ice phase processes. (4) All of the simulations, precipitation in the shallow convection regions is smaller than the value of the deep convections and the stratiform cloud regions three orders of magnitude. But the maximum area of the shallow convection regions is accounted for more than 40% of the total area during the entire period of the simulations. The shallow convection regions extend with the increasing CCN concentrations during the entire period of the simulations. The rainfall of the shallow convection regions are mainly control by warm rain processes in the simulations. CCN concentrations enhancement suppressed the conversion of cloud droplets to rain process. (5) In the stratiform cloud regions, rain suppression during the early stages of precipitation by the CCN concentrations enhancement. The stratiform cloud regions areas are reduced with the increasing CCN concentrations during the entire period of the simulations. The reason is also related with warm rain weaken by CCN concentrations enhancement.