The roles of dry convection, cloud-radiation feedback processes and the influence of recent improvements in the parameterization of convection in the GLA GCM

Abstract The sensitivity of the simulated July circulation to modifications in the parameterization of dry and moist convection, evaporation from failing raindrops, and cloud-radiation interaction is examined with the GLA (Goddard Laboratory for Atmospheres) GCM (general circulation model). Interferences are based on several 47-day summer integrations using the same prescribed boundary forcings. The Arakawa-Schubert cumulus parameterization, together with a more realistic dry convective mixing calculation that allowed moisture, heat and momentum to mix uniformly, yielded a far better intertropical convergence zone (ITCZ) over North Africa than did the previous convection scheme. It also produced a rain-free Sahara desert, which is well-known feature in observations, but is poorly simulated by a number of GCMs. The physical mechanism for the improvement was identified to be the upward mixing of planetary boundary layer (PBL) moisture by vigorous dry convective mixing, which prevented the buildup of moist c...