The Impact of Aerosols on the Radiative Forcing and Precipitation Rate in Mid-Latitude MCSs

 
Poster PDF

Authors

Stephen Saleeby — Colorado State University
Sue van den Heever — Colorado State University
Peter Marinescu — Colorado State University
Sonia Kreidenweis — Colorado State University
Paul J. DeMott — Colorado State University

Category

Convective clouds, including aerosol interactions

Description

A large portion of warm-season precipitation over the United States is generated by mid-latitude mesoscale convective systems (MCSs). These systems produce very intense rainfall events and have expansive cirrus anvil shields. Modification of microphysical processes in MCSs can impact the vertical transport of hydrometeors, which can lead to changes in convective precipitation, dynamical feedbacks, and modified radiative forcing in the cirrus anvil clouds. Anthropogenic emissions and biomass burning have the potential to introduce high concentrations of cloud droplet nucleating aerosols, which can modulate the rates of the 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 investigate the influence of aerosols on MCS microphysical and radiative characteristics. An increase in aerosol number concentration led to the production of smaller but more numerous cloud droplets. This perturbation in the cloud droplet distribution led to an increase in the rate of riming in the mixed-phase region of the clouds and an increase in the convective precipitation frequency. In addition, reduced vertical transport of cloud water to the anvil, where homogeneous freezing of cloud droplets is a major source of cloud ice crystals, led to a reduction in anvil ice mass. In spite of reduced lofting of cloud water mass in the presence of higher aerosol concentrations, many of the smaller but more numerous cloud droplets reached the anvil, led to the generation of smaller but more numerous ice crystals and greater cloud area coverage at the anvil levels. The response in the radiation budget was an increase in the cloud top albedo, a decrease in cloud top cooling, and a decrease in the net column radiative flux. This net effect led to an aerosol-induced warming, or reduced cooling, in these squall line MCSs.