Effective aerodynamic roughness estimated from airborne laser altimeter measurements of surface features

Aerodynamic roughness length (z(0)) and displacement height (d(0)) are important surface parameters for estimating surface fluxes in numerical models. These parameters are generally determined from wind flow characteristics using logarithmic wind profiles measured at a meteorological tower or by balloon release. It would be an advantage to use measurements of land surface characteristics instead of wind flow characteristics to estimate the z(0), and d(0) for large areas. Important land surface characteristics are the size and distribution of roughness elements (obstacles). This research evaluates the use of high resolution laser altimeter data to obtain these land surface characteristics. Data were collected at the US Department of Agriculture, Agricultural Research Service (USDA-ARS), Jornada Experimental Range in southern New Mexico, USA over a coppice dune dominated area. These dunes are covered by honey mesquite (Prosopis glandulosa Torr.) with flat and mostly bare interdunal areas. For this analysis, three 450 m laser transects with a 2 cm measurement interval were used. The distribution and size of dunes were calculated from these laser transects and used to compute z(0). Analysis gave an average z(0) = 4.3 cm and d(0) = 70 cm for the three laser transects, which compares to z(0) = 7 +/- 4 cm and d(0) = 98 +/- 48 cm calculated from wind profile data measured at a 10 m tower near the laser transects. These results show that the estimation of z(0) and d(0) for a complex terrain is possible using simple land surface features computed from high resolution laser altimeter data.