WRF Modeling of Hurricane Norbert Record Rainfall with Monsoon Flooding in Arizona

In early September 2014, remnants of Hurricane Norbert brought record-setting rainfall that swept across the Southwest U.S. Flash flooding in Phoenix caused major damage to infrastructure, roadways, and many human casualties including two fatalities. The Phoenix flash flood of September 7-8th, 2014 resulting from the hurricane Norbert is investigated in this Weather Research and Forecasting (WRF) modeling study. Our goal is to simulate the general features of the boundary layer in Arizona prior and during the flash flood events and to study the related hazardous weather patterns. The unusual vigor of Norbert as it entered Arizona allowed remnant storm bands to cross the core Phoenix metropolitan area, and a record 5.51 inches of rain to fall over the Chandler area in just under seven hours. This was the highest recorded rainfall amount since 1895 for the Chandler area. Warmer than normal sea surface temperatures in the Gulf of California (GC), helped to maintain the intensity of the storm. The rainfall total in these 7-8 hours was greater than the total precipitation during the months of June, July, and August 2014 (NWS 2014). According to NOAA, this was a once in 200 year period event.

       Our WRF modeling study supports the hypothesis that higher than usual for early September Sea Surface Temperatures (SSTs) in GC significantly enhanced the intensity of Norbert and influenced the rainfall rates and the intensity of the flash flood. In particular, the onset of the heavy rainfall occurs after the SSTs exceeded 29^oC. Here we explore this idea in a modeling context using the WRF to simulate the 2014 Arizona flood. To test this hypothesis, we investigate boundary layer and the atmospheric circulation in Arizona before and during the heavy rain events. Both the boundary layer water content and CAPE over Maricopa County, and the atmospheric circulation over Arizona changed dramatically over the course of the numerical simulations. WRF ARW (Advanced Research WRF model) successfully simulated the boundary layer properties and CAPE during the flood. The simulated Norbert moisture movement triggers strong winds, damaging rain, and thunderstorms for several days across Arizona. These are serious life and aviation hazards.

       After analyzing the predicted evolution of the monsoonal boundary layer properties, and the impact of Hurricane Norbert in numerous WRF simulations, a new understanding emerges as to how the lower atmosphere over the GC interacts with the remnants of hurricane moisture to enhance dramatically the monsoon rainfall. A boundary layer parameterization having high vertical resolution with an accurate treatment of physical processes appears essential for capturing accurately the flood event.