| The entropy budget of an atmosphere in radiative-convective equilibrium is analyzed here. The differential heating of the atmosphere, resulting from surface heat fluxes and tropospheric radiative cooling, results into a net entropy sink. In statistical equilibrium, this entropy sink is balanced by the entropy production due to various irreversible processes such as frictional dissipation, diffusion of heat, diffusion of water vapor and irreversible phase changes. Determining the relative contribution of each individual irreversible process to the entropy budget provides important information on the behavior of convection. In particular, following Rennó and Ingersoll (1996) and Emanuel and Bister (1996), it is shown that the entropy budget can be used to derive vertical velocity, Convective Available Potential Energy (CAPE) and intermittency of convective systems.; The differences between dry and moist convection are investigated by combining theoretical arguments with numerical simulations with a high resolution Cloud Ensemble Model. In these numerical simulations, the dominant irreversible entropy source for dry convection is due to frictional dissipation. In contrast, for moist convection, the irreversible entropy production is dominated by irreversible phase changes and diffusion of water vapor. In addition, a large fraction of the frictional dissipation in moist convection results from falling precipitation. This specific behavior indicates that moist convection acts more like an atmospheric dryer than a heat engine. These findings differ significantly from the assumptions made in Rennó and Ingersoll (1996) and Emanuel and Bister (1996). It is also shown that the irreversible entropy production by phase change and diffusion of water vapor is directly related to the convective transport of latent heat. The behavior of convective systems thus depends on how the heat transport is divided between sensible and latent heat.; Finally, the study of the entropy budget is extended to include the effects of the large-scale circulation. The energy, entropy and water vapor transports by a large-scale flow can reinforce convective activity in regions of large-scale ascent while reducing it in regions of subsidence. |
