Cloud resolving modeling of tropical cloud systems during phase III of GATE. Part III: effects of cloud microphysics

摘要

Large-scale conditions during the 7-day period of Phase III of the Global Atmospheric Research Program Atlantic Tropical Experiment are used to study effects of cloud microphysics on the convecting tropical atmosphere. Two-dimensional numerical experiments evaluate the effects of extreme changes to the cloud microphysics in the cloud resolving model. The main conclusions are the following. (a) Extreme changes in cloud microphysics affect the temperature and moisture profiles in a way that approximately retains relative humidity profiles in all experiments. (b) With prescribed radiative tendencies, effects of cloud microphysics on surface processes are paramount. Extreme changes in warm rain microphysics indirectly affect the temperature and moisture profiles by modifying surface sensible and latent heat fluxes. For 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 anvil clouds. Higher condensate mixing ratios inside anvil clouds consisting of small ice particles and greater upper-tropospheric cloud cover due to longer residence time of these particles result in the less negative temperature tendency in the upper troposphere. This change in the radiative flux divergence extends the modifications in the free-tropospheric temperature profiles associated with small cloud and precipitation particles into the upper troposphere. Changes in warm rain processes (e.g., in the rate of conversion of cloud water into rain) have some effect on the lower-tropospheric radiative flux divergence as well. d) Particle sizes applied in the radiation transfer model exaggerate this effect because smaller effective sizes of cloud and precipitation particles lead to less negative radiative tendencies, which, in turn, affect the temperature and moisture profiles.