How the maximum power limit constrains precipitation dynamics and their responses to global climate change

Precipitation is the consequence of condensation within the atmosphere, which is intimately connected to the release of latent heat within the air.  Latent heating creates buoyancy, that is, it accelerates air, while precipitation removes moisture from the atmosphere, that is, it dehumidifies air.  Both of these basic aspects of precipitation involve physical work.  Here, we use a thermodynamic systems approach to constrain this work and thereby precipitation and its response to global climate change.  The central starting point is to view the release of latent heat as the fuel to drive a moist heat engine that generates the work to accelerate and dehumidify air.  We then maximise the fraction of that work that goes into the generation of motion, consistent with previous, successful applications of the maximum power limit to the surface energy balance and poleward heat transport.  This yields another constraint on the dynamics, which then provides temporal and spatial scales associated with precipitation, such as convective rainfall events and the Hadley circulation.  We then show that this relatively simple, yet physical formulation can directly be used to understand precipitation changes found in observations and models, such as the intensification and shortening of convective rainfall events, decreases in cloud cover, deviations from Clausius-Clapeyron scaling in the scaling of extreme rainfall events, as well as the “wet-gets-wetter” hypothesis.  These phenomena are directly consequences of the dynamics driven by condensational heating, and these become more powerful due to the simple fact that warmer air can hold more moisture, but also that moisture serves as the fuel for these dynamics.  In addition to providing a simple, physical picture of these hydrological responses within the atmosphere, the approach also suggests potential shortcomings in climate models with respect to resolving these dynamics.