Investigating the intensity of urban heat island and the impacts of local climate using verified WRF data: A case study of Rasht, Northern Iran

Abstract

Due to the complicated and uncertain dynamics of the urban heat island (UHI) in humid subtropical areas, the diurnal, seasonal and long-term variations of the UHI were investigated in Rasht, Iran. Besides, the study utilizes the heat and moisture transfer concepts to analyze the impacts of daytime rainfall duration on UHI. Meteorological data at five virtual stations was acquired by verified Weather Research and Forecasting (WRF) model. Overall, the nighttime UHI peaks in summer (3.4 °C) and is lowest in fall (1.4 °C) over 2017–2019. Moreover, the maximum and minimum daytime UHI occurred in winter (1.2 °C) and summer (0.6 °C), respectively. In hot seasons, an increase in daytime precipitation duration initially leads to an increase in daytime UHI, followed by a subsequent decrease. Conversely, in cold seasons, daytime UHI shows a decreasing trend regardless of rain conditions. Additionally, extended daytime precipitation attenuates nocturnal UHI in hot seasons, while no significant influence is observed in cold seasons. An increase in midday cloud cover during hot seasons reduces the nighttime UHI. The nighttime UHI is inversely proportional to the third root of wind speed, in low cloudiness conditions of hot seasons. Furthermore, the Ladsat-5/7/8 imageries revealed an increasing trend in the surface UHI (SUHI) during hot seasons over the past two decades, while SUHI in cold seasons remains insignificant. The article reports the most influential parameters on daytime, nighttime and seasonal UHI and the best mitigation policies for the humid subtropical climate, particularly in hot seasons.