Determination of Vapor and Momentum Roughness Lengths above an Undulating Soil Surface Based on PIV-measured Velocity Profiles

Abstract:

Accurate characterization of soil-atmosphere exchange processes based on Monin-Obukhov similarity theory (MOST) relies on the proper prediction of the vapor roughness length (z_0v) and momentum roughness length (z_0m). These two crucial parameters, characterizing the mass and momentum transfer respectively, are heavily related to the boundary layer in the vicinity of the soil surface. The assumption of z_0v approximating z_0m when applying MOST to bare soil evaporation has been widely adopted, without considering the disruption of soil undulations on the near-surface velocity field. The relationship between z_0v and z_0m, which further determines the evaporative flux, remains unclear in the presence of an undulating soil surface. Thus, this study aims to fill this gap through direct measurements of the velocity field developed closed to the undulating soil surface. To achieve this goal, four wind tunnel – soil tank experiments were conducted considering different wind speed and surface configurations. Particle Imaging Velocimetry (PIV) was employed to collect the velocity field information. The experimental data were applied to formulize relationship between z_0v and z_0m. Results demonstrate that z_0v is averagely smaller than z_0m by 3 to 7 orders of magnitude, owing to the undulating soil surface. Combined with the PIV-measured velocity field, the effects of surface configurations and wind speed on the ratio of z_0v to z_0m are demonstrated closely related to the distinct mechanisms of mass and momentum transfer. The newly proposed formulation was then used to evaluate the evaporative flux for laboratory and field experiments, demonstrating the efficacy of the approach.