Regional Roughness of the Landes Forest and Surface Shear Stress under Neutral Conditions

Mean wind velocity profiles were measured by means of radio-windsondes over the Landes region in southwestern France, which consists primarily of pine forests with scattered villages and clearings with various crops. Analysis of neutral profiles indicated the existence of a logarithmic layer between approximately .z 4 = 67(*18)z, and 128(*32)z, (z is the height above the ground, z0 the surface roughness and do the displacement height). The upper limit can also be given as z do = 0.33 (kO.l8)h, where h is the height of the bottom of the inversion. The profiles showed that the surface roughness of this terrain is around 1.2 m and the displacement height 6.0 m. Shear stresses derived from the profiles were in good agreement with those obtained just above the forest canopy at a nearby location with the eddy correlation method by a team from the Institute of Hydrology (Wallingford, England).