How Does Climate Change Increase the Severity of a Flood-Producing Rain Event in the U.S. Southeast?

Flash floods are a major cause of death, injury, and damage to property. It is, therefore, crucial not only to understand the mechanisms underlying these storms, but to project how these systems will change under a warming climate. This has proven to be difficult as many flash floods are associated with heavy, warm-season, convective rainfall, and global climate models cannot resolve convection. Convection-resolving global simulations of climate change are still largely out of reach. The present study circumvents these difficulties by employing a regional model in a pseudo global warming (PGW) context.

The PGW downscaling approach and a Weather Research and Forecasting (WRF) model ensemble are used to examine how a modern-day, flood-producing, heavy rain event could change when it is placed in the context of a warmer climate. We use the July 16, 2016 Crabtree Creek heavy rain event in Raleigh, North Carolina as a case study. This was a “generic” summer storm system, with unexceptional mesoscale organization, that formed in an environment of weak synoptic forcing. It produced up to 4 inches of rain over a few hours, resulting in significant damage, most notably when Crabtree Creek rose more than 13 feet and flooded a busy shopping mall. Present-day simulations were prepared with North American Mesoscale Forecast System (NAM) input and were nested from a 12 km grid-spacing to a convection-permitting 4 km. Through multiple sensitivity tests and analyses, we demonstrate that WRF adequately reproduces the salient features of the event – its intensity, location, and organization – and can thus serve as the basis for a PGW case study.

PGW simulations were conducted in the same manner as the present-day simulations, but the model input was modified such that it also incorporated temperature changes obtained from Coupled Model Intercomparison Project Phase 5 (CMIP5) projections under the Representative Concentration Pathway 8.5 (RCP8.5) scenario. Sensitivity tests are conducted to ensure that the PGW results are robust. We hypothesize that introducing the effects of climate change will cause more rainfall through the intensification of storm dynamics and a slowing of storm propagation. Our results indicate that future-climate simulations show increased rainfall. Thus, we present analyses that compare this increase with Clausius-Clapeyron scaling (7% increase per degree C warming) and explore possible changes in convective dynamics in a warmer climate.