Precipitation efficiency constraint on climate change

Precipitation efficiency (PE) relates cloud condensation to precipitation and intrinsically binds atmospheric circulation to the hydrological cycle. Due to PE’s inherent microphysical dependencies, definitions and estimates vary immensely. Consequently, PE’s sensitivity to greenhouse warming and implications for climate change are poorly understood. Here, we quantify PE’s role in climate change by defining a simple index ϵ as the ratio of surface precipitation to condensed water path. This macroscopic metric is reconcilable with microphysical PE measures and higher ϵ is associated with stronger mean Walker circulation. We further find that state-of-the-art climate models disagree on the sign and magnitude of future ϵ changes. This sign disagreement originates from models’ convective parameterizations. Critically, models with increasing ϵ under greenhouse warming, in line with cloud-resolving simulations, show greater slowdown of the large-scale Hadley and Walker circulations and a two-fold greater increase in extreme rainfall than models with decreasing ϵ. Falling raindrops play an essential but as-yet unquantified role in planetary climate change. Here the authors use the concept of precipitation efficiency to establish that raindrops play a critical role in predicting future tropical atmospheric circulation and extreme precipitation.