The Tropical Water and Energy Cycles in a Cumulus Ensemble Model. Part I: Equilibrium Climate

Abstract

A cumulus ensemble model is used to study the tropical water and energy cycles and their role in the climate system. The model includes cloud dynamics, radiative processes, and microphysics that incorporate all important production and conversion processes among water vapor and five species of hydrometeors. Radiative transfer in clouds is parameterized based on cloud contents and size distributions of each bulk hydrometeor. Several model integrations have been carried out under a variety of imposed boundary and large-scale conditions. In Part I of this paper, the primary focus is on the water and heat budgets of the control experiment, which is designed to simulate the convective?radiative equilibrium response of the model to an imposed vertical velocity and a fixed sea surface temperature at 28°C. The simulated atmosphere is conditionally unstable below the freezing level and close to neutral above the freezing level. The equilibrium water budget shows that the total moisture source, Ms, which is contributed by surface evaporation (0.24 Ms) and the large-scale advection (0.76 Ms), all converts to mean surface precipitation Ps. Most of Ms is transported vertically in convective regions where much of the condensate is generated and falls to surface (0.68 Ps). The remaining condensate detrains at a rate of 0.48 Ps and constitutes 65% of the source for stratiform clouds above the melting level. The upper-level stratiform cloud dissipates into clear environment at a rate of 0.14 Ps, which is a significant moisture source comparable to the detrained water vapor (0.15 Ps) to the upper troposphere from convective clouds. In the lower troposphere, stratiform clouds evaporate at a rate of 0.41 Ps, which is a more dominant moisture source than surface evaporation (0.22 Ps). The precipitation falling to the surface in the stratiform region is about 0.32 Ps. The associated latent heating in the water cycle is the dominant source in the heat budget that generates a net upward motion in convective regions, upper stratiform regions (above the freezing level), and a downward motion in the lower stratiform regions. The budgets reveal a cycle of water and energy resulted from radiation?dynamic?convection interactions that maintain the equilibrium of the atmosphere.