Intra-seasonal and diurnal variations of summer surface thermal environment in the Yangtze River Delta

Based on MODIS-based land surface temperature（LST） data, land use and land cover change (LUCC) data and normalized difference vegetation index (NDVI) data, the temporal and spatial characteristics and the associated driving factors of summer surface thermal environment in the Yangtze River Delta（YRD） megalopolis from 2013 to 2017 were investigated. The results show that: ①The summer surface-high-temperature risk zones (i.e.,high LST zone and sub-high LST zone) were mainly distributed in the day time on the farmland covering areas in the north of Anhui, urban architectural covering areas of the YRD and some urban buildings and some farmland in southern Zhejiang, while the surface-high-temperature risk zones transferred to the water-covered areas and urban densely populated areas along the Yangtze River at night. ②The intra-seasonal variations show that from June to August, the surface-high-temperature risk zones gradually shifted from the north of Anhui and the north of Jiangsu to the YRD megalopolis. The study area presents two opposite trends of intra-seasonal variations: the surface-high-temperature risk zones in the northern part of the YRD are gradually mitigating, while the surface-high-temperature risk zones in the YRD Delta megalopolis are gradually increasing. ③The spatial distribution of the surface thermal environment is highly related to the spatial distribution of the underlying surface coverage type, and the surface-high-temperature risk zones of urban construction land areas are significantly higher than other underlying surface coverage types. Summer crop rotations in the northern part of YRD have changed the vegetation coverage of the surface, resulting in the difference between