Surface energy imbalance revisited in the light of nonequilibrium thermodynamics

Land surface energy imbalance is a long outstanding challenge in that the amount of dissipated turbulent (sensible and latent) heat does not match that of the available energy (net radiation minus ground heat storage). The myth of the “missing energy” has been attributed to multiple factors associated with measurement and numerical techniques, including surface heterogeneity, mismatch of source areas, omission of atmospheric heat storage, phase lags, to name a few. Yet it remains puzzling why the energy imbalance is usually found largest over bare soils, where most of the aforementioned factors have apparently the least influence. In this study, we shed a new light into the old myth of surface energy imbalance by resort to the principles of nonequilibrium thermodynamics. We construct an irreversible Carnot cycle between the temperature gradient of land surface and the air, and estimate the available kinetic energy that can be withdrawn from this cycle under different conditions. The theoretical results are then compared with field measurements to estimate the practical bounds of the energy residual that is dissipated via entropy generation. Not only the new method capable of explaining large portion of the missing energy, it also explains well why the seemingly innocent homogeneous bare soil surface plays the biggest sink of available energy.