Urban induced convergence zones and their effects on convective thunderstorms and air quality

Urban areas affect prevailing mesoscale and synoptic flow patterns due to a variety of physical processes, including urban heat island (UHI) induced accelerations, surface roughness induced decelerations, and building barrier effects. These influences produce either flow-speed accelerations or decelerations, as well as either confluent or difluent directional changes. Resulting convergence/divergence fields can alter thunderstorm movement and initiation patterns, as well as pollutant transport patterns. This paper summarizes results from a number of urban field studies that show urban induced convergence zone influences on thunderstorms and/or air pollutant concentrations.

Recent analysis of the NASA sponsored project ATLANTA data shows well developed daytime and nighttime UHIs that produce well defined convergence zones that induce formation of local convective thunderstorm precipitation over the city. Blockage of insolation by the clouds then reduces UHI intensity, and the precipitation creates moist atmospheric areas. These results are consistent with previous results obtained in New York City (NYC) that show it capable of initiating thunderstorm cells during otherwise near-calm conditions.

Air pollutant concentrations peak in urban induced confluence zones or in regional confluence zones intensified by urban induced confluence. Data from a number of cities (i.e., Nashville, Atlanta, NYC, Athens, and Los Angeles) will be presented to illustrate this effect during periods with regional, sea breeze, and/or pure urban induced confluence zone influences. Some of these observational results will be compared to concurrent simulated wind flow and/or concentration values.