The great atmospheric dehumidifer: A thermodynamic perspective on the general circulation.

Most of the Earth's atmosphere is unsaturated for condensation, and cloudy areas account for a small fraction of its total mass. This is noteworthy because the oceans provide an ample supply of moisture, so that the atmosphere would have to be saturated if the climate system were ever to reach thermodynamic equilibrium. The fact that the atmosphere remains unsaturated is due to its active hydrological cycles. Air parcels gain water vapor through evaporation at the Earth's surface. The atmospheric circulation subsequently lifts them, and their temperature drops, resulting in condensation and the removal of condensed water through precipitation. The air parcels then slowly subside back to the air surface at a much lower specific humidity than during their ascent. This atmospheric dehumidification occurs through a broad range of scales and phenomena - from clouds and convection to planetary circulation - but their combined effect is to maintain large portions of the atmosphere away from saturation.

This talk examines the atmospheric dehumidification and its connection to the general circulation. First, we will demonstrate how dehumidification limits the ability of the atmosphere to act as a heat engine in idealized thermodynamic cycles. Second, we will present a general framework to analyze thermodynamic processes in state-of-the-art numerical simulations and use it to quantify how the dehumidification reduces the mechanical output of various atmospheric flows. Finally, we will investigate the impacts of dehumidification on the behavior of midlatitudes baroclinic eddies.