Abstract:
The fine features of near surface winds in winter over four typical areas of Beijing (the mountain area, the transitional area between mountain and plain, the plain area, and the urban area), as well as the alpine area of Yanqing, are investigated based on two sets of hourly observations of 10 m wind collected at conventional Automatic Weather Stations (AWS) network over Beijing area and special AWS observations in Yanqing alpine area. Results are as follows:(1) In the winter, the average near-surface wind characteristics over Beijing area reflect the joint effects of the topography in western and northern Beijing, the roughness of its urban underlying surface and cold-air injection. The wind speed is distributed along the topographic gradient with higher speed in western/northwestern mountain area, lower speed in the plain area, and the lowest speed in the urban area; prevailing winds in the northwest and north directions (relevant to cold air) bypass the urban area, and local dominant wind directions (non-northwest and north directions) exist in some mountain and transitional areas. (2) In the four typical areas, the wind speed exhibits similar diurnal variation feature, i.e., the wind speed is higher in the daytime than in the nighttime, and the highest single-peak occurs around noon time. Compared to the other three areas, the above feature is most stable in urban area suggested by the low data variance. (3) Weak wind (< 1 m/s) frequencies in the four typical areas are between 31%-42%, with the highest frequency in urban area and the lowest frequency in mountain area. Strong wind (> 10.8 m/s) that mostly comes from the west to north directions and is closely linked to cold-air injection shows an opposite pattern. Southerly wind is the least frequent in urban, plain and transitional areas, indicating that the well-known regional mountain-plain wind system of Beijing exists implicitly and is hardly detectable in winter. (4) The altitude around 1500 m (the general top of the Planetary Boundary Layer (PBL)) is the turning point of the diurnal variation feature of near surface wind speed at Yanqing alpine area. Two AWSs below that altitude reflect the typical diurnal variation feature mentioned above, while the two AWSs above that altitude show the opposite characters. This is related to the diurnal variation of turbulence in the PBL and the downward momentum transfer from the lower free atmosphere to the PBL. (5) The average daily wind speed at the two higher-altitude (about 2000 m) stations in the Yanqing alpine area is several times larger than that at the two lower-altitude (about 1000 m) stations. From the daytime to midnight, westerly winds prevail at the two higher-altitude stations, while wind speed could vary within a wide range. On the contrary, for the two lower-altitude stations, the wind speed is relatively stable, but directional change is more frequent.