Analysis of surface wind and roughness length evolution with fetch using a combination of airborne lidar and radar measurements

Abstract

A combination of surface wind speed (SWS) and sea state variables, derived from quasi‐simultaneous airborne lidar and radar measurements, made in the framework of the Flux, État de mer et Télédétection en Condition de fetcH variable (FETCH) experiment, is used to analyze the evolution of surface roughness length, neutral drag coefficient, and friction velocity coefficient with fetch in the first hundred kilometers offshore over the Gulf of Lion, Western Mediterranean. The study focuses on the Tramontane/Mistral event documented in the afternoon of 24 March 1998. Particular attention is given to SWS derived from nadir lidar measurements. The SWS retrieval methodology developed and validated for open ocean conditions byFlamant et al.[1998]has been modified to account for the specificity of the coastal Mediterranean environment (complex mixture of continental and maritime aerosol; turbid, productive waters). The lidar‐derived SWS evolution with fetch observed on 24 March 1998 in the afternoon was validated against in situ and remote sensing measurements made from a buoy, a ship, as well as from the spaceborne altimeter TOPEX. The spatial variability in SWS observed with the airborne lidar was controlled by the structure of the wake regions downstream of the Massif Central and the Maritime Alps, delimiting the longitudinal extension of the Mistral, and was influenced by swell resulting from the action of a steady northeasterly flow coming from the Ligurian Sea in connection with intense Alpine lee cyclogenesis. These findings were supported by the other measurements. It is further shown that, based on a formulation of the dimensionless roughness dependance with wave age, airborne lidar and radar measurements can be combined to provide insight into the evolution with fetch of roughness length, neutral drag coefficient, and friction velocity. Four distinct sea state regimes over a distance of 100 km could be identified from the remotely sensed variables obtained with this novel approach in connection with atmospheric forcing. The dependance of lidar/radar derived drag coefficient with lidar‐derived SWS for the four regimes was found to be remarkably consistent with the relationship derived from the buoy measurements. Finally, lidar/radar derived friction velocities were found in good agreement with the buoy and in situ aircraft measurements.