Effects of Ice Microphysics on Tropical Radiative–Convective–Oceanic Quasi-Equilibrium States

Abstract

The effects of ice microphysics on the mean state of tropical atmosphere and ocean are quantified using a coupled cloud?ocean model. The cloud-resolving model (CRM) treats explicitly the cloud-scale dynamics instead of using parameterization as is necessary in a general circulation model (GCM). The ocean model is a one-dimensional (1D) mixed layer model with a nonlocal K-profile parameterization to represent the vertical mixing in the oceanic surface boundary layer. Two sets of 40-day simulations attain radiative?convective?oceanic quasi-equilibrium states, one is a coupled simulation, the other has a fixed sea surface temperature (SST). Each set consists of two simulations, with a larger and smaller ice fall speed, respectively. The two coupled simulations (T0C and M2C) yield dramatically different radiative?convective?oceanic quasi-equilibrium states demonstrating the profound impact of ice microphysics on the water vapor, cloud, and radiation fields. The mean SST and mixed layer depth in M2C is 0.67 K colder and 33 m deeper than those in T0C, and the net surface solar radiation in M2C is 88 W m?2 smaller than that in T0C. The simulation associated with the larger ice fall speed achieves a quasi-equilibrium state characterized by a colder and drier atmosphere, less cloudiness, stronger convection and precipitation, and warmer SST. On the other hand, a quasi-equilibrium state associated with the smaller ice fall speed has a warmer and moister atmosphere, more cloudiness, weaker convection and precipitation, and colder SST. The key mechanism is cloud?radiation interaction: the more (less) cloudiness the ice microphysics produces, the weaker (stronger) the radiative cooling. The upper-ocean mixing and entrainment of the oceanic deep water play an important role in establishing the quasi-equilibrium SST. The comparison between two coupled and two fixed-SST simulations illustrates that the water vapor variation induced by the change of ice microphysics is reduced by the SST feedback, while the cloud and radiation variation is enhanced in the coupled simulation.