Effect of Urbanization on the Hydroclimate and Deep Convection in the Southern Great Plain and Northeastern US

Deep convection is associated with a wide range of hazards including hail, wind gusts, tornadoes, lightning strikes, intense precipitation, and flooding which pose a profound threat to urban areas where population density and asset values are high. Feedback from urban land surfaces also has the potential to influence the occurrence/intensity of deep convection via their unique surface energy balance, atmospheric stability, and flow fields plus aerosol and aerosol precursor emissions. Here we investigate the response of deep convection to different urban morphologies and areal extent using the Weather Research and Forecast (WRF) model version 4.3.3 and focus on the roles played by the surface energy balance and surface roughness. We explore the role of the local environmental context by considering two very different urban settings: the relatively isolated Dallas-Fort Worth (DFW) metropolis in the Southern Great Plains (SGP) and the mega-metropolitan aggregate in the Northeastern (NE) US that extends from Washington D.C., through New York City, to Boston. The NE urban corridor has a comparable total warm season (March to September) precipitation amount to DFW, but the hydroclimates exhibit important differences in terms of the frequency of deep convection and, for example, hail. Storm and hail occurrences in the NE corridor are 3.99 year^-1km^-2 and 0.47 year^-1km^-2. Both are considerably smaller than comparable statistics for DFW of 4.48 year^-1km^-2 and 0.97 year^-1km^-2.

In the SGP region, we focus on two representative deep convective systems that moved over DFW. They had different large-scale environments. One was associated with a low-pressure system and strong synoptic forcing as is typical of springtime mesoscale convective systems. The other is associated with a high-pressure ridge and is more locally forced. We create an ensemble of WRF simulations for each case using lateral boundary conditions from the High Resolution Rapid Refresh (HRRR) and a grid spacing of 1 km. The ensemble samples across five different microphysical schemes; Milbrandt-Yau, Morrison, Thompson aerosol aware, WRF double moment 7 categories, and NSSL. The simulations are evaluated against the measurements from several National Weather Service WSR-88D dual-polarization Doppler RADAR in terms of intensity and spatial coverage of rain rate, composite radar reflectivity, and hail. The optimal model configurations are then used in a sensitivity experiment where the size of the DFW metroplex is varied from the replacement of DFW by the dominant surrounding land surface (non-urban) to an eight-fold increase in the areal extent of DFW with the morphology of the urban area maintained. The perturbation experiments are thus: complete removal of DFW, doubling, quadrupling, and applying an eight-fold increase to DFW. To investigate the urban effect only on the deep convection, we applied a mesoscale convective system (MCS) tracking algorithm to the model output and evaluated the responses of MCS-scale average properties. Results from these simulations over DFW indicate that WRF captures the details of the synoptically forced deep convection case better for all microphysics schemes. The land use perturbation experiments yield impacts on rainfall rates, hail probability, and up- and down-draft intensity within the MCS that scale with the magnitude of the change in urban areal extent but importantly exhibit different signs in simulations with different microphysics schemes. The fractional impact of the land use perturbation on the deep convection is considerably small for the more locally forced deep convection case.

For the Northeastern corridor, 20 convection cases have been simulated with the two microphysics schemes that exhibited the highest fidelity in the DFW simulations: Milbrandt-Yau and Morrison. Perturbation experiments are now being conducted that mirror those used in the DFW case to examine if a similarly strong dependence on the microphysics scheme to land use perturbations is also observed in this environment.