Atmospheric convection as an irreversible heat engine

Pauluis et al. (1999) argue that frictional dissipation of energy around falling hydrometeors is an important entropy source in the tropical atmosphere. Their calculations suggest that the frictional dissipation around hydrometeors is about 1/3 of the work available from a reversible convective heat engine. Moreover, based on the residual of the energy budget of a numerical model, not shown in their note, the authors argue that irreversible entropy sources due to diffusion of water vapor and phase changes reduce the mechanical work available from the convective heat engine by about 2/3. Pauluis et al. (1999) conclude that only a tiny fraction of the maximum work potentially available from a convective heat engine is used to perform work.

Renno and Ingersoll (1996) show that frictional heating can be easily included in the heat engine framework via increases in the thermodynamic efficiency of a reversible heat engine. We show that the effect of any other irreversible process is merely to reduce the thermodynamic efficiency of a reversible convective heat engine. Thus, the framework proposed by Renno and Ingersoll is valid even when the heat engine is as irreversible as proposed by Pauluis et al. (1999). Since, irreversible entropy sources reduce the mechanical work available from the convective heat engine, the study of Pauluis et al (1999) implies that the bulk thermodynamic efficiency of the tropical atmosphere is only a tiny fraction of that predicted by the framework proposed by Renno and Ingersoll (1996). We show both theoretical and observational evidence that the calculations performed by Pauluis et al. (1999) grossly overestimate the irreversible entropy changes in the real tropical atmosphere. Moreover, we show evidence that numerical models are highly dissipative when compared with nature. Therefore, we do not agree with the interpretation of Pauluis et al. (1999) that the reversible heat engine framework grossly overestimates the rate at which work is performed by tropical convective systems.