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

Surface roughness is a key parameter for surface-atmosphere exchanges of mass andenergy. Only a few field measurements have been performed in arid or semiarid areaswhere it is an important control of the aeolian erosion threshold. An intensive fieldcampaign was performed in southern Tunisia to measure the lateral cover, Lc, and theaerodynamic roughness length, Z0, over 10 sites with different surface roughnesses. Lcwas determined by combining field measurements of the geometry of the roughnesselements and simple assumptions on their shapes. Z0was experimentally determined fromhigh-precision wind velocity and air temperature profiles. The resulting data were found tobe in good agreement with the existing relationships linking the geometric and theaerodynamic roughness. This suggests that for natural surfaces, Z0can be estimated onthe basis of the geometric characteristics of the roughness elements. This data set wasthen used to investigate the capabilities of radar backscatter coefficients, s0, to retrieve Lcand/or Z0. Significant relationships were found between s0and both Lcand Z0. TheSAR/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 toretrieve Z0using radar observations in the C band from operational sensors.