Urban Impacts on New York City Weather During a Heat Wave

Extreme heat events are projected to increase in magnitude and frequency throughout the 21^st century due to a warmer global environment making it critically important for an improved understanding of their genesis and of the their interactions with our large cities. This study presents an application of the factor separation method to assess the contributions of the impervious land cover, the representation of the urban environment, and large scale heat wave conditions to temperatures over New York City. Results of a simulation study found that while the synoptic heat wave conditions had the largest contribution to the temperature field, the surface factors could match it in magnitude in locations where wind flows encounter the urban barrier, such as along eastern Manhattan and along the southern coasts of the city where the tallest buildings are located. The overall effect of the land cover and urban representation held a higher relative contribution at nighttime and early morning, when calmer land breeze conditions result in a marked urban heat island effect. The contribution of the interaction between all three factors is positive during morning and night, which results in a magnification of the urban heat island during a heat wave. A vertical cross-section across part of the city where the urban canopy is densest show effects near the surface that follow the patterns observed in the 2 m air temperature field. The effects of the urban land cover and its interaction with the city during the heat wave are negative during the day, hinting at an enhancement of the wind flow through channeling between buildings.