Using micrometeorological observations and the physical structure of the vegetation during the HAPEX-Sahel experiment it was possible to evaluate some methods to determine the roughness length (z0) and the zero-plane displacement (d). It is also proposed an alternative to estimate such parameters for the atmospheric flow over sparse vegetation in a semi-arid region. It combines the wind logarithmic profile with z0 = lambda (h - d), where, h is the vegetation height, and lambda is a coefficient determined by the physical structure of the vegetation. Here, lambda values were determined for the site and also taken from the literature. The average vegetation height in the site was 2.06 (± 0.47) m. The wind logarithmic profiles were defined by measurements at four level above the surface (at 3.0, 4.1, 5.3 and 8.5m), and the friction velocity (u*) determined by eddy correlations at 9 m. The proposed method estimated well the surface roughness with lambda = 0.188 and 0.190, both determined by the structure of the vegetation in the site, but failed with lambda = 0.166, the average value of literature. The roughness parameters determined by methods that used u* in the solution of the wind logarithmic law better described the surface roughness. The aerodynamic roughness of the site was described well by d = 0.95 m = 0.46 h and z0 = 0.204 m = 0.1 h.
HAPEX-Sahel; zero-plane displacement; roughness length; momentum transfer
