Aerodynamic roughness over an urban area and over two farmlands in a populated area as determined by wind profiles and surface energy flux measurements

Aerodynamic roughness lengths (z0) were determined for an urban area and two rice paddies mixed with buildings over a complex terrain in central Taiwan. At each of the three sites, surface net radiation, ground heat flux and vertical profiles of wind speed, temperature and humidity within the atmospheric surface layer (ASL) were measured. The similarity theory of the ASL in conjunction with the energy budget equation of land surface under unstable atmospheric conditions was used to determine the roughness values. Over the urban area, the roughness was determined to be 1.2 m with a zero-plane displacement height (d0) of 11 m. Over the two mixed farmlands, the roughness values were determined to be 0.2-0.5 m, 20 times higher than the values (0.008-0.02 m) for homogeneous rice paddies reported in the literature. A preliminary relationship for estimating roughness value as a function of residential fraction, farmland fraction and building height (or population density) is derived. The observations show that during the daytime, the mean height range of the ASL was 30 (±6) ≤ (z − d0)/z0 ≤ 106 (±38) and the fingerprint areas extended 1.5 km upwind from the three profile sites.