Beijing 325m Meteorological Tower Observations Statistical Analysis

With the tremendous urbanization of Beijing City, the dimension of the city has largely expanded during the past 20 years, so many areas between the north third ring road and the fourth ring road have greatly changed from outskirts to inner city. And the Beijing 325m meteorological tower which was built for measuring the suburb boundary layer is now in a prosperous city zone completely, so the meteorological tower now becomes the best observation station to study the Urban Boundary Layer (UBL). We analyse the data by using the data measured by the tower from the summer of 1994 to 2003, plus the whole 2002a, in order to study the structure of the UBL of Beijing City. The results include: (1) Based on the statistic analysis of the wind speed from the summer of 1994 to 2003, we have analyzed the influence on the boundary wind structure dominated by the urbanization of Beijing. We found that: firstly, the direction of the wind became more and more disordered on the surface layer which is strongly impacted by the underlying surface. Nowadays the wind field cannot reflect the characteristics of inhomogeneous underlying surface of suburb but of the inner city. Secondly, its summer mean speed is decreasing yearly below 47m. And the nearer of the earth's surface, the more remarkable decreasing trend. So we have drawn a conclusion that the data of the tower below 47m reflect the characteristics of Urban Canopy Layer (UCL), and the height of the UCL may reach 47m now. Finally, we have not seen the yearly decreasing trend of the gust based on the statistic analysis, so we got the result that the influence on the gust is not obvious, though the UCL has risen. (2) Using the data of the whole year of 2002, we study the structure of the UBL seasonal variation of Beijing. The results show that: It is dominated by the northwest wind in the winter but southwest wind in the summer above 80m height. But the wind direction is very disordered below UCL except that there is no southwest airflow. The monthly mean wind speed above 100m height has remarkable diurnal variation, and it has great difference among different months. But below the UCL the monthly mean wind speed has much less diurnal variation, it is a typical structure of half sine wave. The monthly mean temperature has much less seasonal difference. There are several inversion layers in evenfall of January and August, but the isotherm is much more smooth in April and October and the inversion layer is not obvious. (3) We discussed some characteristics of atmospheric boundary layer structure and turbulence transfer during the strong dust storm weather from Mar.18 to 22,2002. The results show that, a) Strong inversion layers appeared in the upper ABL during the initial stages of the synoptic process. With the dust storm breaking, both the mean wind speed and the turbulence speed are increasing sharply, the temperature is decreasing continuously, also the gust coefficient and the turbulence intensity are decreasing sharply and kept very little during the synoptic process. b) There is systematic upward flow at the 280m height in the ABL, but the 47m and 120m heights present systematic downward flow during the synoptic process. The momentum transport at 47m height is upward as a whole, but at 120m and 280m height, the downward momentum is approximately as much as the upward. For the sensible heat transport, it is upward as a whole.