Aerodynamic roughness length (zom) is an important parameter for reliably simulating surface fluxes. The parameter varies with wind speed, atmospheric stratification, terrain and other factors; however, variations of this parameter are not properly considered in most models, which may result in uncertainties in simulating surface latent heat and sensible heat flux. There have been few studies of the diurnal and wind-direction dependent variations in zom. This study analyzes the seasonal, diurnal and wind-direction-dependent variations in zom calculated from the profile of meteorological data for two forest systems of China, and explores the mechanism underlying these variations.
Annually averaged and monthly averaged diurnal variations in zom are obvious and similar at a Changbai Mountain (CBS) site located in northeast China and a Qianyanzhou (QYZ) site located in southeast China. zom is much higher at night than during the day. The diurnal variation in zom is due to a change in atmospheric stratification. The seasonality of zom differs at two sites. zom has sizable seasonal variation and is lower in the leaf-off season than in the growing season at CBS. At QYZ, zom does not have an obvious seasonal pattern. However, at QYZ, the short-term fluctuation is greater than that at CBS. The obvious seasonal variation in zom at CBS is related to the great seasonal change in the leaf area index. The noticeable short-term wind-direction-dependent variations in zom at QYZ are attributed to the heterogeneous terrain there.