PNNL

Abstract

We investigate a significant model-observation difference found between cloud-base vertical velocity for continental shallow cumulus simulated using large-eddy simulations (LES) and observed by Doppler Lidar measurements at the U.S. Southern Great Plains. The LES cloud-base vertical velocity is dominated by updrafts that are consistent with a general picture for convective clouds, but are inconsistent with Doppler lidar observations that also show the presence of considerable downdrafts. The underestimation of simulated downdrafts is found to be a remarkably robust feature, being insensitive to various numerical, physical, or dynamical choices. We find that simulations can more closely reproduce observations only after improving the model physics to use size-resolved microphysics and horizontal longwave radiation, both of which modify the cloud buoyancy and velocity structure near cloud sides. The results suggest that these physical treatments are needed for the proper simulation and subsequent parameterization development of shallow cumulus vertical transport.