Climate change effects on summertime precipitation organization in the Southeast United States

As the earth warms, it is unclear how the organization of precipitation will change, or how these changes will impact regional rainfall and the hydrological cycle. This study combines Weather Research and Forecasting (WRF) model simulations and the pseudo global warming downscaling approach with a precipitation feature identification algorithm to help improve our understanding of the effect of warming on precipitation organization. The WRF model was used to simulate precipitation during a six-day summer period in the southeastern United States under present and future warmer climate conditions. The domain averaged precipitation increased by 45% in the future climate simulation compared to current climate. Modeled precipitation features were classified into either mesoscale precipitation features (MPF) larger than 100 km in length or smaller isolated precipitation features (IPF). In terms of organization, future IPF precipitation fraction decreased while MPF precipitation fraction and feature sizes increased, especially over the ocean, indicating a general increase in mesoscale organization in the future warmer climate. Despite higher thermodynamic instability, future climate IPF precipitation was unchanged over land, possibly responding to stronger subsidence under a strengthened western ridge of the North Atlantic Subtropical High. Yet over the ocean IPF precipitation increased and the amplitude of the IPF diurnal cycle doubled. The most notable precipitation intensity increase, as measured by feature height and rainrate distributions, occurred in oceanic MPFs. These increases in oceanic IPF and MPF precipitation may highlight the role of surface water vapor fluxes in realizing increased precipitation in a warmer climate. This study demonstrates that the application of a simple precipitation feature identification algorithm to WRF simulations can give valuable insight into the effect of climate change upon precipitation organization and intensity. Nevertheless longer simulations are still needed to obtain a robust statistics of the changes of precipitation organization under a warming climate.