Observations of Mesoscale Effects on Frontal Movement Through an Urban Area

Data obtained from the extensive mesoscale anemometer network established during the New York University Urban Air Polution Dynamics Program were used to study the effects of New York City on frontal movement. Frontal position and movement were determined using sequential hourly streamline and isotach analyses of the flow through the New York City metropolitan area. Results showed that frontal movement during nonheat-island periods was retarded significantly over the entire central urban area. The retardation was probably due to the increased surface frictional drag exerted on the front by the increased surface roughness of the city as compared to that of its surrounding environs. During periods with well-developed urban heat islands, the results showed a retardation in frontal speed over the upwind half of the city, followed by a significant acceleration of the front over its downwind half.