Landscape roughness parameters for Sherwood Forest - Experimental results

The aim of this work is to present experimentally evaluated effective roughnesses (z(oe)) of a partly forested landscape. Although the ratio of boundary-layer height to obstacle size was only of the order of 50, there still seemed to exist a height range of 75-200 m where surface-layer similarity was approximately valid. Attempts were made to use conventional wind profile analysis to evaluate z(oe), but the small height range and the large number of variables initially led to unacceptable uncertainties. Fixing the displacement height z(d), rather than fitting it, reduced the data scatter to an acceptable level. The profile-derived roughness lengths z(op) obtained in this way were in good agreement with previous work, and with an alternative roughness length estimate z(of) for which flux-derived profile parameters u* and theta* were used. This implies that the profile-derived roughnesses were consistent with the measured surface-layer momentum flux. Comparison of both roughness estimates also yielded an improved estimate of the displacement height. Besides this, the authors tested a landscape roughness evaluation method which makes use of the gustiness parameter T-u = sigma(u)/U in the surface layer. The results obtained by this method were in fair agreement with the profile-derived data. In previous work, the gustiness method was advocated because it could be used at relatively low levels, perhaps even within the roughness sub-layer. At the present measuring Site, this was not the case as the gustiness method was only valid in an approximate way, and for a limited height range.