Cloud microphysics and the tropical climate: Cloud-resolving model perspective

This paper discusses the role cloud microphysics play in the tropical climate. It is argued that this problem can best be studied within the context of a cloud-resolving model because of complicated interactions among cloud dynamics, cloud microphysics, radiative processes, and surface processes. Two sets of highly idealized cloud-resolving simulations are discussed. In each set, the simulations differ only in the parameters applied in the representation of cloud microphysics. The simulations are run until quasi-equilibrium between sinks and sources of moisture and energy is achieved. The comparisons between simulations applying dramatically different parameters exposes the effects of cloud microphysics on the mean state of the tropical atmosphere and on the tropical climate. The first set considers large-scale flow driven by sea surface temperature gradients and prescribed radiative cooling. The cloud microphysics appears to have minor effects on the large-scale flow as well as temperature and moisture profiles within ascending branches featuring deep convection. The second set considers tropical convection over a shallow isothermal layer of water ('swamp ocean') applying an interactive radiation transfer model. The sea surface temperature beneath each column of the atmospheric model changes according to the local budget of surface heat and radiative fluxes. For simplicity, equatorial equinox conditions, no diurnal cycle, and a zero zenith angle are assumed. Changes in parameters in cloud microphysics appear to have a dramatic impact on the quasi-equilibrium ocean temperature that ranges from about 37°-38°C in the simulation assuming large cloud and precipitation particles to 32°-35°C in the simulation assuming small particles. These highly idealized simulations suggest that the main effect of cloud microphysics is not on atmospheric processes and dynamics but rather on the ocean surface. If this suggestion is realistic, effects of cloud microphysics will be very difficult to quantify in the ocean-atmosphere system because of the much longer timescales associated with ocean dynamics.