Urban-rural wind velocity differences

Abstract Wind speeds along a streamflow line through New York City are found to be decreased below (increased above) those at sites outside of the city during periods with regional wind speeds above (below) about 4 m/s. The decrease is attributed to increased values of the surface roughness parameter in the city, as compared to values in nearby non-urban regions. The increase is associated with accelerations produced by a well-developed urban heat island. Cyclonic turning of the flow along the streamflow line through the city during the high wind speed periods results from decreases in the value of the Coriolis force associated with the increased urban frictional drag. During periods of acceleration over the city, either cyclonic or anticyclonic turning was found, due to a combination of two effects. The origin of the flow was found to influence the direction of the turning over the city during night-time heat island hours. For easterly flows the origin is a highly stable rural region, while for westerly flows it is most likely one of the less stable, but aerodynamically smooth, water bodies around New York City.