Aerosol Impacts on Mesoscale Convective Systems Forming Under Different Vertical Wind Shear Conditions

Following our previous study of wind shear effect on mesoscale convective system (MCS) organization under a clean atmospheric condition using the Weather Research and Forecasting model coupled with spectral-bin microphysics, we conduct sensitivity simulations by increasing cloud condensation nuclei concentration to investigate aerosol impacts on MCSs forming under different wind shear conditions. We find that increased aerosols induce stronger updrafts and downdrafts in all MCSs. The stronger updrafts and enlarged convective core area contribute to larger vertical mass fluxes and enhance precipitation. The enhanced updrafts and vertical mass fluxes indicate convective invigoration. Increased updraft speed below 8-km altitude is attributed to enhanced condensational heating, except for the weak wind shear and strong low-level shear cases in which the enhanced low-level convergence is another contributing factor. Interestingly, above 8-km altitude, we see reduced updraft speed by the increased aerosols due to reduced vertical pressure perturbation gradient force. The accumulated rainfall and mean rain rate are increased with a greater occurrence frequency of heavy rain. Larger rain rate is seen in both convective and stratiform regions. In general, we see a higher frequency of deep clouds in the polluted condition because of invigorated convection, and more stratiform/anvil clouds, but a lower frequency of shallow warm clouds, with a larger significance for more organized MCSs. The consistently invigorated MCSs by aerosols under various wind shear conditions revealed by this study have important implications to weather and climate in warm and humid regions that are influenced by pollution.