Surface roughness around a 325-m meteorological tower and its effect on urban turbulence

Based on slow- and fast-response measurements under neutral stratification conditions from a 325-m meteorological tower located in a built-up area of north-central Beijing as well as a descriptive survey of surface roughness elements (i.e., buildings and trees) around the tower site, urban roughness lengths, z(0), with zero-plane displacement height are estimated using logarithmic wind profile and morphometric methods in eight 45 degrees directional sectors. When comparing their results with each other, the slow-response method tends to give smaller z(0) values. At a given location, considerable directional variations in values are observed. The effect of surface roughness on urban turbulence characteristics in terms of non-dimensional standard deviations of three-component velocity, sigma(i)/u(*1) (where i = u, v, w and u(*1) is local friction velocity), is investigated.