A Method of Determination of Dynamic Influence of the Forest on the Boundary Layer Using Two Doppler Sodars

Abstract

Doppler sodar soundings of wind velocities were routinely performed during the intensive phase of the HAPEX?MOBILHY experiment over two different canopies: forested and bare ground. The purpose of the experiment was to observe the turbulent frictional effect induced by the forest canopy. Different effects were observed: 1) the frictional forest effect depends on the magnitude and direction of the wind system; and 2) the wind speed reduction over the forest is observed up to heights equal to or greater than 65 m. In the layer from the top of the trees (19 m) up to 50 m the speed is reduced by 30%?60%. Above 50 m, the wind speed is reduced by 10%?15%. These observations are consistent with an estimated forest surface layer thickness of 30 m. Finally this paper proposes a method to derive the differential ?forested/unforested? turbulent friction from data.