Sensitivity of Extreme Rainfall to Atmospheric Moisture Content in the Arid/Semiarid Southwestern United States: Implications for Probable Maximum Precipitation Estimates

Abstract

Sensitivity of extreme rainfall to atmospheric moisture content for the 19 August 2014 storm in Arizona is investigated based on numerical experiments using the Weather Research and Forecasting model. Analyses are designed to develop an improved understanding of the roles of atmospheric moisture content and complex terrain in controlling spatial and temporal variability of extreme rainfall in the arid/semiarid southwestern United States. The control simulation identifies complex interactions of low‐level moisture transport and orographic lift as key elements in producing extreme rainfall for the first storm episode. Two sensitivity experiments are performed by increasing the relative humidity by 10% and 20% in both initial and boundary conditions used for the control simulation. Changes in atmospheric moisture content modify the storm structure and evolution, instability of the storm environment, and interactions of synoptic flow with complex terrain. The two storm episodes within this event exhibit contrasting responses to the increase of moisture content, with rainfall accumulation and maximum convective available potential energy increased for the second storm episode as opposed to a more complex relationship for the first storm episode. A moisture maximization simulation was also designed to mimic the framework of moisture maximization in probable maximum precipitation estimates. Our results highlight the nonlinear relationship between extreme rainfall and atmospheric moisture content in complex environments where small‐scale convection plays a dominant role. Limitations of probable maximum precipitation estimates based physical models are also discussed.

Urban Impacts on Extreme Monsoon Rainfall and Flooding in Complex Terrain

“…There is a higher potential temperature and water vapor mixing ratio over the city during the second storm episode in the CTRL simulations as compared to the NoURBAN simulations (Figure S8). One of our previous studies shows monotonic increases of rain rates with atmospheric moisture content for the rainfall in the second storm episode of the 19 August 2014 storm (Yang & Smith, ).…”

“…For CTRL and NoTOPO scenarios (two “urban” scenarios), we couple the single‐layer urban canopy model to accurately represent exchanges of heat, moisture and, momentum between the urban surfaces and lower atmosphere. An overview of WRF physics options adopted in all experiments is provided in Table S1 (also refer to Yang & Smith, , for details). Ensemble simulations are implemented through perturbations of initial conditions to increase the reliability of the results.…”

“…The 19 August 2014 storm consists of two consecutive storm episodes during a 20‐hr period. Extreme rainfall for the first storm episode (10–17 UTC 19 August) is associated with the southerly monsoon flow impinging on complex terrain and propagation of convective outflow boundaries away from higher terrain, while the second storm episode (18 UTC 19 August to 06 UTC 20 August) is driven by a fast‐moving cold front sweeping across Phoenix from the northwest (see Yang & Smith, , for details). Contrasting storm evolution during the two back‐to‐back storm episodes enables us to examine the flow regime dependence of urban impacts on extreme rainfall.…”

Effects of urbanization on extreme rainfall in an arid/semiarid region

“…For the other BL schemes, the RRTM for global (RRTMG) scheme for shortwave and long‐wave scheme (Iacono et al ., ) is used. These model physics schemes have already been used in this region for difference studies (e.g., Bright and Mullen, ; Castro et al ., ; Newman and Johnson, ; Mahalov et al, ; Yang and Smith, ; Yang et al ., and Yang et al, ; and many others). The Noah land surface scheme with a single‐layer urban canopy (Chen and Dudhia, ; Kusaka and Kimura, ; Chen, ) is used in all runs.…”

“…Investigating the effects of Phoenix urbanization on extreme storm rainfall is critically important to urban flood warnings, to urban water resources management, and to urban expansion planning, substantially more important than investigating climatological averages. Yang and Smith () have investigated the effects of atmospheric moisture transport on storms that occurred north of Greater Phoenix using WRF at 1‐km resolution and concluded that the responses of storms to moisture are event‐dependent (time‐dependent). In the January 2019 American Meteorological annual meeting, Castro et al .…”