How Should We Represent Downdrafts in Cloud Parameterizations?

Traditional cloud parametrizations (CPs) assume that downdrafts occur as broad compensating subsidence in the cloud free regions between updrafts. In contrast, trajectory analysis in large eddy simulations (LES) shows that > 90 % of permanent downward displacements of air parcels occur within a few kilometers of updrafts. These near-cloud downdrafts are often moister than the cloud’s far-field environment due to the evaporation of condensates, and the thermodynamic properties (and hence eddy transport) of downdrafts differ markedly from their portrayal in traditional CPs. Despite these differences, radiation of energy from near-cloud downdrafts via gravity waves results in net hydrostatic adjustments within the cloud-free region that mimics what occurs in CPs. Hence, the traditional treatment of downdraft mass flux in CPs is likely adequate.

Some CPs trigger cold pool formation by allowing free tropospheric downdrafts to penetrate the planetary boundary layer (PBL), and by using downdrafts to predict cold pool characteristics. In contrast with this assumed behavior, we find that cold pools are primarily generated via the evaporation of rain into air near the interface with the PBL and free troposphere and are largely decoupled from downdrafts in the free troposphere. Bulk updraft mass flux and rain rates are a better predictor of cold pool formation and intensity than the properties of downdrafts in the free troposphere. We conclude by suggesting ways to more realistically portray cold pools in CPs.