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 convective instability in the PBL. A modified rain-evaporation parameterization which takes into account the raindrop size distribution, the atmospheric relative humidity, and a typical spatial rainfall intensity distribution for convective rain was developed and implemented. It evaporated about 50% of the convective rain while greatly reducing the evaporation of large-scale rain. As compared to the old scheme which produced no evaporation of convective rain and the maximum possible evaporation of large-scale rain, the new scheme led to some major improvements in the monthly mean vertical profiles of relative humidity and temperature, convective and large-scale cloudiness, rainfall distributions, and mean relative humidity in the PBL. When the convective cloud-radiation interaction was included by assuming that an entire sigma layer of a grid-box at the detrainment level(s) of Arakawa-Schubert clouds of appropriate optical thickness, some major changes in the surface, diabatic heating, and orientation of the ITCZ over equatorial America were simulated. The experiment suggests a strong potential for further improvement of the GCM simulations by including more realistic parameterization of cloud-radiation interaction.