Understanding Regional Projections of Extreme Precipitation

Under global warming, coupled climate models project that there will be increased extreme precipitation over most of the globe. This projection generally results from the thermodynamic increase of atmospheric moisture (i.e., Clausius-Clapeyron scaling) cancelled in part by the reduction of the moist adiabatic lapse rate (O'Gorman and Schneider, 2009). In particular regions, however, extreme precipitation projections require understanding additional physical processes, such as changes in extreme updrafts. This paper explores these additional physical processes using output from large ensembles of the Community Earth System Model version 1 (CESM1) and the Canadian Earth System Model version 2 (CanESM2) simulating the historical and RCP8.5 climate change scenarios. Despite their different spatial resolutions and convection schemes, CESM1 and CanESM2 produce large scale patterns of extreme precipitation change that have much in common. For example, both models project decreases in the 99.9 percentile of daily mean precipitation throughout the subtropical dry zones. Scalings that assume saturated conditions (as in earlier studies) do not accurately explain extreme precipitation changes in drier regions, and we remedy this by accounting for precipitation events with saturated conditions that last much shorter than a day. Over ocean, we show that subtropical decreases of extreme precipitation are associated with less frequent extreme updrafts resulting from less frequent occurrence of convective available potential energy (CAPE). Over land, however, changes in CAPE properties have surprisingly little effect on extreme precipitation, and boundary layer height increase plays a key role. This boundary layer effect is strong enough to produce decreases in extreme precipitation over many land regions during boreal summer. These physical processes are general enough that we expect them to be at work in the real world and in climate models with different resolutions and convection schemes.