On simulated trade-wind cumulus convection and cold pools

Observations of organized precipitating trade-wind cumuli show convective invigoration on the downwind side of their cold pools. We study convection and cold pools using nested-WRF simulations of 19 January 2005, a day from the Rain in Cumulus over the Ocean experiment. First, the sub-cloud updrafts downwind and near the cold pool boundaries are statistically compared to updrafts not associated with cold pools. Updrafts near cold pool outflows are more moist and possess higher vertical velocities than the other updrafts, are correlated to cold pool expansion rate, and are associated with higher cloud liquid water paths. The lifted air remains downstream, and is able to access the moist air ahead of the cold pool outflow, because the circulation vorticity within cold pool boundary is typically weaker than that of the environmental wind.

We then examine how different microphysics scheme change the characteristics of simulated trade-wind cumulus convection and cold pools. The initial simulation, relying on the hybrid single-moment Thompson scheme, is compared to an additional nested-WRF simulation using the double-moment Morrison microphysics scheme. The higher autoconversion rate in the Thompson scheme produces more rain at low levels, thus larger cold pools, than the Morrison scheme. The Thompson simulation produces a mesoscale and synoptic organization that more closely matches observations than does the Morrison simulation. This reflects differing relationships of vertical rain structure and the environmental wind shear between the two simulations, which we conclude is an important relationship for the successful simulation of trade-wind cumulus cold pools.