Surface and aerodynamic roughness in arid and semiarid areas and their relation to radar backscatter coefficient

Abstract

Surface roughness is a key parameter for surface‐atmosphere exchanges of mass and energy. Only a few field measurements have been performed in arid or semiarid areas where it is an important control of the aeolian erosion threshold. An intensive field campaign was performed in southern Tunisia to measure the lateral cover, Lc, and the aerodynamic roughness length, Z0, over 10 sites with different surface roughnesses. Lc was determined by combining field measurements of the geometry of the roughness elements and simple assumptions on their shapes. Z0 was experimentally determined from high‐precision wind velocity and air temperature profiles. The resulting data were found to be in good agreement with the existing relationships linking the geometric and the aerodynamic roughness. This suggests that for natural surfaces, Z0 can be estimated on the basis of the geometric characteristics of the roughness elements. This data set was then used to investigate the capabilities of radar backscatter coefficients, σ0, to retrieve Lc and/or Z0. Significant relationships were found between σ0 and both Lc and Z0. The SAR/ERS data set is in agreement with the SIR‐C SLR data set from Greeley et al. (1997). On the basis of these two data sets including data from different arid and semiarid areas (North Africa, South Africa, North America), we propose an empirical relationship to retrieve Z0 using radar observations in the C band from operational sensors.