Aerodynamic roughness parameters for semi-arid natural shrub communities of Wyoming, USA

Estimates of aerodynamic roughness length (z(0)) were calculated at nine sites for natural sagebrush( Artemisia tridentata spp.), saltbush( Atriplex nuttallii) and greasewood (Sarcobatus vermiculatus) plant communities in two semi-arid basins in Wyoming, USA. Estimates were based on wind and temperature profiles measured above the plant canopies during summer (August) of 1994 and fall (September and October) of 1995. Values of z(0) were estimated for periods of near-neutral stability (fully forced convection) for four scenarios of displacement height (d) at each site, and for a fifth scenario based on using the low level drag coefficient for constraining friction velocity (u*). Vegetation canopy height, fractional cover, shrub density, average canopy area (per shrub), and average biomass/area were measured for all sites. Additionally, leaf and plant area indices (LAI, PAI) were measured at three of the sites, and average shrub height was measured at the six sites. Roughness lengths calculated using an iterative method with unconstrained u* averaged 0.01 m for the saltbush sites, 0.02 m for the sagebrush sites, and 0.07 m for the greasewood site. z(0)/canopy height ratios (z(0)/h(c)) averaged 0.04, 0.04 and 0.13, respectively, for the three shrub types, although the average is misleading for saltbush, which showed considerable variation between sites. Roughness length appears to be related to shrub structure, as expressed by the dominant species, and by shrub density at the sites, although differences were large depending on which calculation method was used. When u* was constrained, calculated z(0) and z(0)/h(c) were smaller and less consistent in terms of relationships to vegetation structure, suggesting that further constraints on the iterations may be necessary. The results highlight the importance of improving aerodynamic roughness parameterization of natural vegetation communities. (C) 1997 Elsevier Science B.V.