Aerosol Impacts on Mid-Latitude MCS Precipitation Intensity and Radiative Forcing

Mid-latitude mesoscale convective systems (MCSs) generate a large portion of warm-season precipitation, produce very intense rainfall events, and have expansive cirrus anvil shields. Modification of key microphysical processes that impact the vertical transport of hydrometeors in MCSs can lead to dynamical feedbacks, changes in convective precipitation, and altered cirrus anvil radiative forcing. The introduction of high concentrations of cloud droplet nucleating aerosols, through mechanisms such as anthropogenic emissions or biomass burning, can modulate the rates of these key microphysical processes by perturbing the hydrometeor size distributions. In this study, cloud resolving model simulations of two MCSs, that occurred during the MC3E field project, were performed to examine aerosol influences on MCS microphysical and radiative properties. An increase in aerosol loading led to the production of more numerous, smaller cloud droplets. This modification of the cloud droplet distribution led to an increase in riming rate in the mixed-phase region and an increase in the frequency of convective precipitation. Further, reduced lofting of cloud water to the anvil, where homogeneous freezing of droplets is the primary source of cloud ice crystals, led to reduced anvil ice mass. In spite of reduced lofting of cloud water mass for greater aerosol concentrations, many of the more numerous, smaller droplets reached the anvil, led to the generation of more numerous, smaller ice crystals and greater areal cloud coverage in the upper anvil levels. The net radiative response was an increase in cloud top albedo, reduction in cloud top cooling, and reduced net column radiative flux, which led to an aerosol-induced warming (reduced cooling) effect in these MCSs.