The Impact of Urbanized Areas on the Spatial Characteristics of Deep Convection Initiation in the Central United States

Abstract Growing urban populations make understanding the impact of cities on their local climates ever more important. One of the larger and more well-studied impacts is the urban heat island (UHI), but others include roughness length changes and increased aerosols in the atmosphere around urbanized areas. This study examines the effect of urbanized areas on deep convection initiation (DCI) spatial patterns near 11 cities across the central United States between 2005 and 2015. The Thunderstorm Observation by Radar (ThOR) algorithm is used to identify DCI points, and Rapid Update Cycle (RUC)/Rapid Refresh (RAP) model data analyses are used to identify the boundary layer winds associated with each DCI point to place each into a wind relative frame of reference. The analysis was then conducted using DCI density directly downwind of urban centers (including both over and beyond the downwind half of the urban-developed area) compared to density in peripherally downwind zones (regions outside of the zone directly downwind of the city centroid). Direct downwind perturbations would be positive if direct downwind density was larger than peripheral downwind density. Direct downwind perturbations were also compared to direct upwind perturbations. Statistically significant positive direct downwind perturbations are found across all cities, and these perturbations are statistically significantly larger than direct upwind perturbations. The summer season and times when boundary layer winds were greater than 5 kts (1 kt ≈ 0.51 m s −1 ) were most likely to have positive direct downwind perturbations and direct downwind perturbations larger than direct upwind perturbations. Results are largely insensitive to the diurnal cycle and the presence of a UHI.