Shallow-to-Deep Transition of Continental Moist Convection: Cold Pools, Surface Fluxes, and Mesoscale Organization

Rigorous evaluation of cold pool effects driven by rain evaporation for surface-driven shallow-to-deep convection transition over land is performed with large-eddy simulation (LES) model. Three small simulation ensembles are contrasted. The first ensemble eliminates cold pools by removing a cloud- scale variability of temperature and moisture tendencies due to rain evaporation and thus mimicking the single-column model approach. The second and third ensembles compliment the first one by including cold pools and applying either interactive or prescribed surface fluxes. The interactive fluxes formulation is based on a diagnosed evolution of the land-surface temperature and moisture. Without cold pools, the flow preserves key features of the buoyancy-driven quasi-steady cellular convection associated with a field of non-interacting convective plumes as assumed in a typical convection parameterization. With cold pools, a significant enhancement of surface heat and moisture fluxes and about an hour delay of their daily maxima is simulated. The near-surface standard deviations of temperature and moisture are significantly larger when cold pools are present. Interactive surface fluxes introduce a damping mechanism that reduces the standard deviations. The critical impact of cold pools is on the cloud field depth during the deep convection phase. As a result, the 10-hour accumulated surface precipitation is two to three times larger when cold pools are present. Prescribed surface fluxes feature the highest upper-tropospheric cloud fractions and the largest surface rain accumulations. These results are discussed from the perspective of including cold pool effects into existing single-column parameterizations of convection.