| Popis: |
The transport of windblown sand is controlled by many factors, including wind regime and sediment supply. Surface roughness at the sub-meter scale is also important because it influences both the threshold conditions for particle entrainment and the flux of sand once it is set into motion. In general, increases in surface roughness result in higher threshold speeds for particle movement and decreases in sand fluxes. Aerodynamic roughness (z 0) is the aeolian parameter related to surface roughness and is defined as the height above some mean level at which average wind speed is zero. Values of z 0 are derived from wind measurements through the boundary layer, but few z 0 values have been obtained over natural surfaces because of the expense and limitations of making such measurements. Rather, remote sensing using radar systems has the potential for addressing this problem. In this investigation, we derived z 0 values for a wide variety of surfaces in the southwestern United States and obtained radar data for these sites in P-band (wavelength = 68 cm), L-band (wavelength = 24 cm) and C-band (wavelength = 5.6 cm). We show that there are good correlations among z 0, the RMS height of the surface, and the radar backscatter coefficient, σ0, with the best correlation for L-band HV polarized radar data. This study shows the potential for mapping large regions with radar in order to derive aerodynamic roughness values, which in turn can be used in predictive models of sand transport. |
