The impact of an urban dry island on the summer heat wave and sultry weather in Beijing City

This paper studies interdecadal variations of heat waves and sultry weather (HW&;SW) in Beijing City by using climatological data gathered from 1940–2000. In general, HW&;SW decreased during the 1940s–1970s and increased during the 1970s–1990s. Although variation trends in the number of HW&;SW days coincide with air temperature variations, the greatest number of HW&;SW days occurred during the 1940s rather than during the much hotter 1990s (which had an average 1.0°C higher air temperature than during the 1940s). The synoptic situations of HW&;SW and the correlation analysis between HW&;SW occurrences and the Western Pacific Subtropical High (WPSH) index seem to indicate that HW&;SW days during the 1990s should exceed those during the 1940s; however, that is not the case. Post-1980s interdecadal variations of the Urban Dry Island (UDI) effect in Beijing City were analyzed in order to explain this inconsistent phenomenon. Our results show that, while the probability of HW&;SW occurrences in Beijing City should have been greater due to global warming and increasing summer Urban Heat Island (UHI) intensity, the UDI effect is magnified by rapid urbanization. In addition, the physical comfort index is more sensitive to relative humidity under high temperature conditions. Therefore, the occurrence of HW&;SW days during the 1990s is partially offset by the enhanced UDI in Beijing City.