The effects of urban environments on the dynamics of a simulated supercell thunderstorm

Severe storms can be especially dangerous and costly when they encounter urban areas, causing significant damage and loss of life such as that seen with the 20 May 2013 Moore, OK tornado and the June 2012 DFW hail storm. These impacts will continue to grow in scope with increased urbanization throughout much of the world, hence understanding the effects that the urban environment can have on supercells is of paramount importance. Previous studies have shown that urban areas modify the structure of approaching thunderstorms due to land surface heterogeneity. The goal of this study is to begin to quantify the relative importance of changes in surface roughness and thermal capacity associated with urban regions to thunderstorm dynamics. An isolated supercell storm that forms to the west of the Oklahoma City urban area and then passes across the city was simulated as a control run using the Advanced Research Weather Research and Forecasting (ARW-WRF) model with a 2 km horizontal resolution in the larger domain, and a 500 m resolution for the smaller domain centered over the city. To test the sensitivity of this supercell to urban-type surface roughness values, multiple simulations will be run with identical starting conditions aside from the surface roughness length associated with the urban area. Typical values of surface roughness length associated with Oklahoma City, as well as values much higher and much lower (such as that associated with the native vegetation), will be tested. Similar sensitivity tests will be run for thermal capacity to simulate a greater or lesser urban heat island effect. In addition to testing the sensitivity of the dynamics of a supercell to the values of surface roughness and thermal capacity of a single city, different cities (e.g., Dallas, Chicago, Wichita, Minneapolis) will be moved into the path of the supercell allowing the aforementioned sensitivity tests to be applied to different urban areas. For each simulation, time series of vertical motion, vertical vorticity, and updraft helicity, as well as the evolution of storm-scale wind and temperature patterns throughout the lifetime of the storm will be analyzed. Because wind speed and direction, as well as near-surface temperature and wind patterns, affect storm structure and intensity, perturbation pressure and simulated radar reflectivity will also be examined and results reported.