A rain evaporation and downdraft parameterization to complement a cumulus updraft scheme and its evaluation using GATE data

Abstract

The scheme invokes: 1) a diagnostic determination of the commencement level of rain-evaporation-induced downdrafts, 2) a method for calculating downdraft mass fluxes emanating from different levels of the atmosphere, and 3) an explicitly prescribed overall fraction of rain evaporation within the downdraft. The parameterization was tested with the GATE Atlantic Tropical Experiment phase III data in a fully prognostic mode and with the entire atmospheric and surface forcings prescribed with data. The near-surface downdraft cooling largely mitigates the observed surface sensible heating. In the absence of this cooling, the boundary layer must get rid of 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 vertical temperature profiles are somewhat larger for simulations without downdrafts and are much larger for simulations without both downdrafts and rain evaporation. The downdrafts are found to produce a useful correction in the simulated near-surface temperature and humidity in GCM simulations. -from Authors