Simulated effects of urban environments on the dynamics of a supercell thunderstorm

Abstract

The world's population is increasingly concentrated in large urban areas. Many observational and modeling studies have explored how large, population-dense cities modify local and mesoscale atmospheric phenomena. Urban modeling studies often use an explicit urban canopy model to parameterize urban surfaces. However, it is unclear whether this approach is appropriate for more suburban cities, such as those found in the Great Plains. To investigate this problem, the Weather Research and Forecasting model is used to simulated a week of conditions in and around the Oklahoma City, Oklahoma area, and results from these simulations are compared with observations. Overall, five simulations with varying urban land-surface parameterization configurations are examined. Three simulations use the Noah land surface model (LSM): one with all urban areas removed, one using the original Noam LSM, and the other with urban areas parameterized by a modified Noah LSM with three urban categories. Additional simulations utilize a single layer urban canopy model either with default urban fraction values or with urban fractions taken from the National Land Cover Database. In general, all simulations produce warmer, drier urban areas, with a stronger urban heat island at night. However, the prediction of near-surface winds is problematic in the two simulations that use the single layer urban canopy model as neither simulation correctly reproduces reduced wind speeds over the city. The modified Noah LSM provides the best overall agreement with observations and represents a reasonable option for simulating the urban effects of more suburban cities.

The effect of urban areas on weakly-forced precipitation systems has also been studied extensively. However, interactions between urban areas and synoptically-active convection, such as supercells, remain relatively unexamined. Simulations of a supercell thunderstorm, with an urban area parameterized using the modified Noah LSM scheme, are used to quantify the impacts of a large Plains urban area on the evolution and strength of a supercell thunderstorm. Simulations with urban areas are compared to an initial-condition ensemble of simulations without any urban areas, with hierarchical clustering analysis used to form statistically similar groups of simulations. In this analysis, the effects of the storm having various city-relative paths, as well as the storm life cycle stage during urban interactions, are investigated. The results suggest that, when the storm passes to the north of or directly over the city center late in its life cycle, low-and mid-level mesocyclone strength increases, and the mesocyclone tracks further south. In general, low-level storm characteristics are more sensitive to the location of the city than are mid-level storm properties.

To supplement this analysis, a factor separation approach is undertaken to determine the relative importance of the roughness and thermal characteristics of urban areas on storm modification. City locations near the beginning and end of the storm's life cycle are used to determine if the storm's maturity while interacting directly with the city modulates these effects. Results generally suggest that surface roughness and its interactions between thermodynamic properties are the dominant contributors to urban-induced effects on storm strength and evolution. Additionally, the amplitude of interactions between shear and thermodynamic modifications is often similar in magnitude to either effect individually.