WRF simulations of urban heat island under hot-weather synoptic conditions: The case study of Hangzhou City, China

The urban heat island (UHI) effect over Hangzhou, east China during a long-lasting heat wave was simulated by a weather research and forecasting (WRF) model coupled with an urban canopy model (UCM) at a horizontal resolution of 1 km. Based on satellite-measured nighttime light data and the normalized difference vegetation index, a human settlement index was used to represent the current urban land cover and define three urban land subcategories in the UCM. Three numerical simulations representing different urbanization scenarios and an idealized simulation with all the urban surface replaced with cropland were performed. Using up-to-date urban land use data, the coupled WRF/UCM model reasonably reproduced the majority of the observed spatial and temporal characteristics of the 2-m temperature field over the simulation period in Hangzhou. Strong UHI effects that can cause intensification and expansion of the areas experiencing extreme heat stress were observed in both actual measurements and simulations. In the simulation, an average temperature increase of 0.74 °C in the city center was observed under high urbanization conditions. The UHI peak reached a maximum value of 1.6 °C at 1900 LST around sunset. Analysis of the surface energy balance showed that the UHI is mainly caused by a greater heat storage in the urban fabric during the day and the release of this heat in the evening. Comparisons among the results of four sensitivity runs showed that urban land use, classification of three urban land subcategories, and consideration of anthropogenic heat release respectively contributed 56.8% (0.42 °C), 13.5% (0.10 °C), and 29.7% (0.22 °C) to the simulated UHI effects.