The impact of urbanization on wind speed and surface aerodynamic characteristics in Beijing during 1991–2011

Abstract

Urbanization has a significant influence on climate and meteorological conditions through altering surface aerodynamic characteristics. Based on observational data collected at 15 levels on a 325 m meteorological tower in Beijing during 1991–2011, changes in wind speed, vertical profile, aerodynamic roughness length ($z_{0}$), and zero-plane displacement height ($z_{\text{d}}$) were analyzed. Decreasing trends were observed predominantly during this period, especially for levels between 65 and 140 m where the largest decreasing rates often occur. The annual and seasonal (spring, summer, autumn, and winter) mean wind speeds at 15 levels all present decreasing trends with average rates of 0.029, 0.024, 0.023, 0.040, and 0.019 m s⁻¹ a⁻¹, respectively. The decreases in strong wind categories contribute most to the reduction of mean wind speed. Furthermore, in 2005–2011, the diurnal maximum wind speeds at lower levels tend to appear earlier as compared to those in 1991–1997, while the patterns of diurnal cycle between different levels become more similar in these periods. Besides, the phenomena of “kink” in wind profiles are visible in various atmospheric stabilities, and the average height of a kink has increased from about 40 m to nearly 80 m associated with urbanization during 1991–2011. In addition, the results of $z_{0}$ and $z_{\text{d}}$ calculated using the wind profile method vary with wind directions due to surface heterogeneity and that larger values often occur along with southerly winds. Both $z_{0}$ and $z_{\text{d}}$ show increasing trends in different sectors during 1991–2011, and the annual mean $z_{0}$ and $z_{\text{d}}$ have increased from less than 1 m to greater than 2 m, and from less than 10 m to greater than 20 m, respectively.