New York City Impacts on a Regional Heat Wave

Heat waves are projected to increase in magnitude and frequency throughout this century because of increasing global temperatures, making it critically important to acquire improved understanding of their genesis and interactions with large cities. This study presents an application of the method of factor separation to assess combined impacts of a synoptic-scale heat wave, urban land cover, and urban energy and momentum fluxes on temperatures and winds over New York City, New York, via use of high-resolution simulations (1-km grid spacing) with an urbanized version of the Weather Research and Forecasting (WRF) Model. Results showed that factors behaved different throughout the day, with synoptic conditions dominating afternoon temperature contributions (>7 degrees C). At night, combined urban surface factors contributed over 5 degrees C during the heat wave and up to 1.5 degrees C on non-heat-wave days. Positive interactions among all factors during morning and nighttime indicate an amplification of the urban heat island of up to 4 degrees C during the heat wave. Midtown Manhattan vertical cross sections, where urban canopies are most dense, showed a change in the sign (from positive to negative) of the contribution of the urban fluxes between night and day below 500 m, possibly as a result of decreased radiative cooling from trapping by buildings and increased thermal storage by buildings as well as frictional effects that oppose the incoming warm air.