Cloud resolving modeling of tropical cloud systems during Phase III of GATE. Part III: Effects of cloud microphysics

Large-scale conditions during the 7-day period of Phase III of the Global A tmospheric Research Program Atlantic Tropical Experiment are used to study effects of cloud microphysics on the convecting tropical atmosphere. Two-di mensional numerical experiments evaluate the effects of extreme changes to the cloud microphysics in the cloud resolving model. The main conclusions a re the following. (a) Extreme changes in cloud microphysics affect the temp erature and moisture profiles in a way that approximately retains relative humidity profiles in all experiments. (b) With prescribed radiative tendenc ies, effects of cloud microphysics on surface processes are paramount. Extr eme changes in warm rain microphysics indirectly affect the temperature and moisture profiles by modifying surface sensible and latent heat fluxes. Fo r instance, smaller raindrops, and to a lesser degree slower conversion of cloud water into rain, result in enhanced updraft and downdraft cloud mass fluxes, a colder and drier boundary layer, larger surface fluxes, a warmer and more humid free atmosphere, and a lower convective available potential energy. c) With fully interactive radiation, the above picture is modified mostly through the effect of cloud microphysics on the upper-tropospheric a nvil clouds. Higher condensate mixing ratios inside anvil clouds consisting of small ice particles and greater upper-tropospheric cloud cover due to l onger residence time of these particles result in the less negative tempera ture tendency in the upper troposphere. This change in the radiative flux d ivergence extends the modifications in the free-tropospheric temperature pr ofiles associated with small cloud and precipitation particles into the upp er troposphere. Changes in warm rain processes (e.g., in the rate of conver sion of cloud water into rain) have some effect on the lower-tropospheric r adiative flux divergence as well. d) Particle sizes applied in the radiatio n transfer model exaggerate this effect because smaller effective sizes of cloud and precipitation particles lead to less negative radiative tendencie s, which, in turn, affect the temperature and moisture profiles.