Aerosol-Cloud-Precipitation Interactions in a Closed-cell and Non-homogenous MBL Stratocumulus Cloud

A closed-cell marine stratocumulus case during the Aerosol and Cloud Experiments in the Eastern North Atlantic(ACE-ENA)aircraft field campaign is selected to examine the heterogeneities of cloud and drizzle microphysical properties and the aerosol-cloud-precipitation interactions.The spatial and vertical variabilities of cloud and drizzle microphysics are found in two different sets of flight legs:Leg-1 and Leg-2,which are parallel and perpendicular to the cloud propagation,respectively.The cloud along Leg-2 was close to adiabatic,where cloud-droplet effective radius and liquid water content linearly increase from cloud base to cloud top with less drizzle.The cloud along Leg-1 was sub-adiabatic with lower clouddroplet number concentration and larger cloud-droplet effective,but higher drizzle droplet number concentration,larger drizzle droplet median diameter and drizzle liquid water content.The heavier drizzle frequency and intensity on Leg-1 were enhanced by the collision-coalescence processes within cloud due to strong turbulence.The sub-cloud precipitation rate on Leg-1 was significantly higher than that along Leg-2.As a result,the sub-cloud accumulation mode aerosols and CCN on Leg-1 were depleted,but the coarse model aerosols increased.This further leads to a counter-intuitive phenomenon that the CCN is less than cloud-droplet number concentration for Leg-1.The average CCN loss rates are −3.89 cm^(-3)h^(-1)and −0.77 cm^(-3)h^(-1) on Leg-1 and Leg-2,respectively.The cloud and drizzle heterogeneities inside the same stratocumulus can significantly alter the sub-cloud aerosols and CCN budget.Hence it should be treated with caution in the aircraft assessment of aerosol-cloud-precipitation interactions.

Aerosol-Cloud-Precipitation Interactions in WRF Model:Sensitivity to Autoconversion Parameterization

Cloud-to-rain autoconversion process is an important player in aerosol loading, cloud morphology, and precipitation variations because it can modulate cloud microphysical characteristics depending on the participation of aerosols, and affects the spatio-temporal distribution and total amount of precipitation. By applying the Kessler, the Khairoutdinov-Kogan（KK）, and the Dispersion autoconversion parameterization schemes in a set of sensitivity experiments, the indirect effects of aerosols on clouds and precipitation are investigated for a deep convective cloud system in Beijing under various aerosol concentration backgrounds from 50 to 10000 cm^-3. Numerical experiments show that aerosol-induced precipitation change is strongly dependent on autoconversion parameterization schemes. For the Kessler scheme, the average cumulative precipitation is enhanced slightly with increasing aerosols, whereas surface precipitation is reduced significantly with increasing aerosols for the KK scheme. Moreover, precipitation varies non-monotonically for the Dispersion scheme, increasing with aerosols at lower concentrations and decreasing at higher concentrations.These different trends of aerosol-induced precipitation change are mainly ascribed to differences in rain water content under these three autoconversion parameterization schemes. Therefore, this study suggests that accurate parameterization of cloud microphysical processes, particularly the cloud-to-rain autoconversion process, is needed for improving the scientific understanding of aerosol-cloud-precipitation interactions.