Tempospatial Pattern of Surface Wind Speed and the "Urban Stilling Island" in Beijing City

An hourly-resolution dataset from observations at the automatic weather stations (AWSs) is developed and applied to study the characteristics of weakening surface wind in the urban areas of Beijing City in 2008-2017. The "Urban Stilling Island (USI)" is defined and quantified to depict the surface wind speed (WS) differences between rural and urban regions. The urban (rural) sites are represented by 45 (6) stations within (outside) the 6th Ring Road (RR). The results demonstrate remarkable smaller annual and seasonal average WS values in urban areas than in rural areas, indicating significant USI, especially in the central urban areas (within the 4th RR) in spring and winter. Further analysis reveals that the surface roughness effect dominates and enhances the USI intensity (USII) under the stronger large-scale background wind in spring and winter, whereas the Urban Heat Island (UHI) effect may dominate and decrease the USII under weaker large-scale wind in summer and autumn. The diurnal USII variations are characterized by a steady low-value phase from 1900 to 0800 Beijing Time (BT) and a high-value phase from 1100 to 1500 BT, with rapid shifts of USII in between. Long-term variation of hourly USII shows large mean USII in 2008-2012 but decreased USII in 2013-2017, possibly attributed to the change of urbanization level around the rural observation sites.