Changes in urban heat island intensity with background temperature and humidity and their associations with near-surface thermodynamic processes

This study investigates changes in urban heat island (UHI) intensity with background temperature and humidity and their associations with physical processes. For this, we conducted idealized ensemble simulations with different initial potential temperature and water vapor mixing ratio profiles using the Weather Research and Forecasting (WRF) model. The daytime and nighttime UHI intensities increase with increasing background temperature at rates of 0.03 °C °C −1 and 0.20 °C °C −1 , respectively. The daytime and nighttime UHI intensities decrease with increasing background humidity at rates of −0.01 °C (g kg −1 ) −1 and −0.28 °C (g kg −1 ) −1 , respectively. The increase in background temperature increases the radiative cooling of rural air, enhancing the evening rural 2-m temperature decline. This also decreases the radiative heating of urban air and increases urban advective cooling, but decreases urban turbulent mixing. Consequently, the evening urban 2-m temperature decline is less enhanced, increasing the nighttime UHI intensity. The increase in background humidity decreases the radiative cooling of rural air, weakening the evening rural 2-m temperature decline. This also increases the radiative heating of urban air and decreases urban advective cooling, but increases urban turbulent mixing. Consequently, the evening urban 2-m temperature decline is less weakened, decreasing the nighttime UHI intensity. • Changes in urban heat island (UHI) intensity with background state are examined. • The UHI intensity increases with increasing background temperature. • The UHI intensity decreases with increasing background humidity. • Changes in UHI intensity are greater during the nighttime than during the daytime. • The changes are associated with those in near-surface thermodynamic processes.