Irreversible processes in nature and numerical models

The pressure drop across a convective circulation such as the Hadley-Walker cell depends solely on the thermodynamics of its convective heat engine (Renn\'o et al. 1998; Renn\'o 1999). Indeed, the near-surface pressure drop across one of these circulations is a function of the thermodynamic efficiency of the circulation's heat engine and also a function of the difference in surface air temperature and water vapor content between the ascending and descending branches of the circulation. Thus, given the pressure drop and surface air temperature and moisture content, we can estimate the thermodynamic efficiency of the circulation. Indeed, by comparing this efficiency with that of a reversible heat engine operating under the same conditions, we can compute the degree of irreversibility of the circulation. We use the above framework to estimate the thermodynamic efficiency of the Hadley circulation in nature and numerical models. Our results show that, to a first order approximation, the heat engine of nature's Hadley circulation is reversible ($\eta \approx \eta_{rev} \approx 0.1$). Moreover, it shows that the heat engine of numerical models are highly irreversible when compared with nature ($\eta \lep 0.05$).