Seasonal Variations of the Urban Heat Island at the Surface and the Near-Surface and Reductions due to Urban Vegetation in Mexico City

Abstract

he contrast of vegetation cover in urban and surrounding areas modulates the magnitude of the urban heat island (UHI). This paper examines the seasonal variations of the UHI using the Moderate Resolution Imaging Spectroradiometer (MODIS), surface meteorological observations, and the Weather Research and Forecasting (WRF) model. A distinction is made between the land surface UHI observed by satellite and the near-surface UHI observed by measuring the air temperature. The land surface UHI is found to be high at night throughout the year but drops during the wet season. The daytime UHI is low or even exhibits an urban cool island throughout the year but increases during the wet season. The near-surface air temperature UHI trend is similar to the land surface temperature UHI at night. By day, however, the air temperature UHI remains constant throughout the year. Regression analysis showed that the daytime land surface UHI correlates with the difference in vegetation fraction between the urban and surrounding areas, and, to a lesser extent, with daytime insolation. At night, the UHI correlates with nighttime atmospheric stability and only weakly with differences in vegetation cover and daytime insolation. WRF simulations with the single-layer Urban Canopy Model were initialized with MODIS data, especially for the urban fraction parameter. The simulations correctly represented the distinct seasonal variations of both types of UHIs. The model was used to test the impact of changes in vegetation fraction in the urban area, indicating that increased vegetation would reduce both the land surface UHI and the air temperature UHI at night, as well as the land surface UHI during the daytime.