Convective invigoration: untangling aerosol impacts on deep convection

Wojciech W. Grabowski Senior Scientist and Section Head of Mesoscale and Microscale Meteorology (MMM) Laboratory, National Center for Atmospheric Research (NCAR), Boulder, USA

Abstract for Weekly Colloquium on Thursday, March 29, 2018 at 4 pm in Harvill 318

Formation and growth of cloud and precipitation particles (“cloud microphysics”) affect cloud dynamics and such macroscopic cloud properties as the mean surface rainfall, cloud cover, and liquid/ice water paths. However, separation the dynamical impact (i.e., convective invigoration) from purely microphysical effects (e.g., increased rainfall due to more efficient conversion of cloud condensate into precipitation) is difficult using traditional cloud-scale simulations and virtually impossible in observations. Traditional cloud simulations are not reliable because of the natural variability of a cloud or cloud field that is affected by the feedback between cloud microphysics and cloud dynamics, with simulated clouds evolving differently when a change to the cloud microphysics in introduced. A novel modeling methodology, the piggybacking, was recently developed to separate microphysical and dynamical impacts on deep convection, as well as to isolate aerosol impacts from the effects of meteorological conditions. This talk will illustrate misconceptions concerning aerosol effects on deep convection, discuss the piggybacking technique, and present model simulations that allow clear separation of dynamical and microphysical effects. The new methodology clearly shows that microphysical effects dominate aerosol impacts on deep convection with dynamical effects playing less significant role. In particular, model results question the postulated dynamical invigoration of deep convection in polluted environments. Additional simulations show that the accuracy of atmospheric observations is insufficient to separate aerosol effects from the impact of eteorological conditions.