Aerodynamic roughness length (z_om) 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 z_om. This study analyzes the seasonal, diurnal and wind-direction-dependent variations in z_om 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 z_om are obvious and similar at a Changbai Mountain (CBS) site located in northeast China and a Qianyanzhou (QYZ) site located in southeast China. z_om is much higher at night than during the day. The diurnal variation in z_om is due to a change in atmospheric stratification. The seasonality of z_om differs at two sites. z_om has sizable seasonal variation and is lower in the leaf-off season than in the growing season at CBS. At QYZ, z_om 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 z_om at CBS is related to the great seasonal change in the leaf area index. The noticeable short-term wind-direction-dependent variations in z_om at QYZ are attributed to the heterogeneous terrain there.