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Claude Klapisz and Alain Weill

Abstract

We have studied a set of detailed mean horizontal wind profiles obtained with a three-component Doppler sodar. For inversion-capped, convective boundary layer conditions, empirical expressions for the mean horizontal wind and the wind shear in the first few hundred meters of the atmosphere are presented. The observations are parameterized with the height of the lowest inversion zi, the Monin-Obukhov length L and the friction velocity u*. From experimental values and empirical expressions, values for the turbulent heat flux are then deduced.

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Christine Mazaudier and Alain Weill

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.

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Laurence Eymard and Alain Weill

Abstract

Radar observations of the convective boundary layer wore made during the “COPT81” experiment in May–June 1981 over western Africa. The lower atmosphere was characterized by the interaction between the southerly monsoon and the African easterly jet above it. The mean wind shear between both Bows is found to organize convective elements along preferred directions and to transfer energy to the perturbed field (velocity variances, momentum fluxes). However, the convective organization is also the cause of increasing horizontal variance and in some cases of a negative shear stress production. The global behavior of the convective boundary layer in the four observed situations appeared similar to those observed in middle latitudes, but the strong wind shear is an important feature of the layer.

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