Diurnal variation of surface wind over central eastern China

Hourly wind observations from 452 meteorological stations are used to document the diurnal cycle of the surface wind over the central eastern China (100 degrees-122 degrees E, 20 degrees-42.5 degrees N). Both the surface wind speed and the wind direction show large diurnal variation with pronounced topographic effects. At most stations, the surface wind speed reaches the maximum in the afternoon and the minimum in early-morning. This diurnal phase shows small seasonal variation, whereas the diurnal amplitude varies significantly in different seasons. The diurnal amplitude of the surface wind speed reaches maximum in spring over the northern and southwestern China and in summer over the southern China. The diurnal cycle of the wind direction is more complicated. Over the coastal (mountain) regions, the diurnal wind direction is greatly influenced by the land-sea (mountain-valley) breezes with large (small) seasonal variation. Over the northern plain region, the wind direction exhibits small diurnal variation but with remarkable seasonal rotation. The surface wind over the stations located on the top of mountains shows distinct diurnal variation, which represents the diurnal cycle of the tropospheric low-level wind. The wind speed over these stations is highest in pre-dawn and lowest in the afternoon. The wind anomaly rotates clockwise from late night to late afternoon, and shows significant seasonal variation as influenced by the annual cycle of the monsoon system. The contribution of the diurnal surface wind to the diurnal feature of precipitation is briefly discussed.