Influence of Orographic and Canopy Conditions on Friction Velocities Observed during Frontal Events Using Doppler Sodar Observations

Abstract

Sodar friction velocities, obtained during frontal events traversing areas characterized by different orgraphic and canopy conditions (flat, bare ground, small hills and valleys with agricultural crops and trees, agricultural crops and forest on a flat ground, bare ground on the side of a mountain), are compared in order to identity the influence of topography on this parameter. For some case studies, sounding and sodar data are combined in order to provide a relation between the friction velocity and the low-level jet presence. For the cases analyzed in this paper, the following results are obtained: the frontal passage is associated with a decrease of the horizontal wind speed (about 50% in magnitude) in the surface layer, and an increase of the friction velocity before the frontal passage followed by a decrease just at the time of the frontal passage or with a little delay. Friction velocity is more intense in the cold side of the low-level jet and its maximum represents 2% of the low-level jet maximum magnitude. As it concerns the influence of the terrain conditions on friction velocity, mountain effects yield to more intense friction-velocity values and to a superposition of an oscillating behavior on the time variation of friction velocity, while forests effects induce a shift of the frontal signature an the time variation of friction velocity at higher height levels.