A RAIN EVAPORATION AND DOWNDRAFT PARAMETERIZATION TO COMPLEMENT A CUMULUS UPDRAFT SCHEME AND ITS EVALUATION USING GATE DATA

A rain evaporation and downdraft parameterization is designed to compl ement the cumulus convection scheme of the Goddard Laboratory for Atmo spheres General Circulation Model (GLA GCM). The scheme invokes (i) a diagnostic determination of the commencement level of rain-evaporation -induced downdrafts, (ii) a method for calculating downdraft mass flux es emanating from different levels of the atmosphere, and (iii) an exp licitly prescribed overall fraction of rain evaporation within the dow ndraft. The parameterization was tested with the GATE [GARP (Global At mospheric Research Program) Atlantic Tropical Experiment] phase III da ta in a fully prognostic mode and with the entire atmospheric and surf ace forcings prescribed with data. It was found that the near-surface downdraft cooling largely mitigates the observed surface sensible heat ing. In the absence of this cooling, the boundary layer must get rid o f the surface heat flux by spurious turbulent transport, which becomes significant in simulations that ignore both the rain evaporation and downdrafts. The time mean as well as root-mean-square errors in the ve rtical temperature profiles are somewhat larger for simulations withou t downdrafts and are much larger for simulations without both downdraf ts and rain evaporation. The downdrafts are found to produce a useful correction in the simulated near-surface temperature and humidity in G CM simulations, and in that way, the parameterization improves the sim ulation of tropospheric temperature and humidity. In a one-year compar ison of GLA GCM simulations with and without downdrafts, the former pr oduced better precipitation climatology and surface temperatures.