Aerosol parameterisation in a three-moment microphysical scheme: Numerical simulation of submicron-sized aerosol scavenging

Abstract

This paper explores the scavenging of submicron aerosol particles (APs) collected by cloud water and rainwater and by cloud ice nucleation on APs. The scavenging coefficients for different collection processes were calculated and then implemented in a three-moment microphysical scheme of a three-dimensional numerical model. The concentration and mixing ratio of the APs in air and in all hydrometeors were calculated explicitly in the model. We performed several numerical experiments to test the influence of individual processes of AP collection and nucleation of cloud ice on AP scavenging, as well as the redistribution of APs among hydrometeors. We estimated AP scavenging based on the change in the total number and mass of APs in air over time or on the basis of the total number or mass of APs deposited on the ground by precipitation. We found that the initial number of APs in air is reduced mostly due to collection by cloud water. In the absence of ice nucleation on APs, their number in air during 90 min of integration is reduced by 49.73%, which is significantly larger than the reduction in their total mass in air, namely 7.74%. The decrease in the total number of APs in air is largely due to collection by Brownian diffusion, and the decrease in total mass is influenced equally by turbulent and Brownian diffusion. In addition to collection, when all ice nucleation processes are active, the total number of APs in air is reduced by 53.97%, and their total mass in air is reduced by 67.47%. Therefore, ice nucleation on APs is much more important for reducing their total mass in air than collection with liquid hydrometeors. The dominant process is depositional nucleation. The total number (total mass) of APs deposited by precipitation on the ground during 3 h divided by the initial total number (total mass) of APs in air (RAPN and RAPM, respectively) are 29.65% and 7.25%, respectively, in the case of collection without ice nucleation, and 27.63% and 14.88%, respectively, in the case of collection with ice nucleation. In the case of collection of APs by cloud water and rainwater, the RAPN decreases from 29.65% to 27.63% by including ice nucleation, and the RAPM increases from 7.25% to 14.88%.