Impact of cropland degradation in the rural-urban fringe on urban heat island and heat stress during summer heat waves in the Yangtze River Delta

China has experienced rapid urbanization since the 1980s, which has significantly reduced croplands around cities. Apart from transition to urban land, croplands have also experienced degradation into cropland/natural vegetation mosaic in the rural-urban fringe. However, scant attention has been paid to the climatic effect of such land cover change so far. This study thus investigated the potential impacts of the conversion from irrigated cropland into the mosaic on regional hot extremes and heat stress by taking the Yangtze River Delta (YRD) as an example. Four heatwave events in the summer of 2013 and 2017 were simulated using the Weather Research and Forecasting Model (WRF). Three group experiments with various representative land cover scenarios, i.e., a default setting experiment with outdated land cover (Exp USGS), a control experiment with updated land cover (CTL), and a sensitivity experiment with modified mosaic land cover (Exp MOS), were performed for comparative analyses. Results show that CTL using the updated land cover data from China (ChinaLC) reproduced the spatial variation of observed 2-m air temperature and relative humidity better than Exp USGS. After the irrigated cropland was converted into the mosaic, the 2-m temperatures in most areas of the YRD became significantly higher than those in Exp MOS due to the lower albedo and leaf area index (LAI) in the newly emerged mosaic areas. The land cover change produced a negative contribution to the urban heat island (UHI) intensity, while it had a positive effect on extreme high temperature under heat wave conditions. This suggests a more cautious usage of the traditional definition of the UHI index is required when investigating the heat island effect, because the rural warming around an urban area may expand the heat island and enhance its heat effect. During the heatwave periods, the high temperature areas in CTL were larger than those in MOS, and the hot day areas increased by an average of 25.9%. Increase in air temperature further enhanced regional heat stress. Those results imply that the effects of land cover change in the rural-urban fringe may increase the risks of extreme hot events and heat stress for urban residents under global warming.