Uncertainty in a Path-averaged Measurement of the Friction Velocity u*

Edgar L. Andreas U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire

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Abstract

Several electro-optical methods exist for measuring a path-averaged value of the inner scale of turbulence l0. By virtue of Monin–Obukhov similarity, in the atmospheric surface layer such l0 measurements are related to the friction velocity u* or to the surface stress τ = &rhou*2, where ρ is the air density. Because l0 is a path-averaged quantity, u* is too. Here the question of how precisely u* can be measured is investigated by combining these inner-scale measurements with two-wavelength scintillation measurements that yield the sensible and latent heat fluxes and, thereby, facilitate stability collections. The analysis suggests that current path-averaging instruments can generally measure u* to ± 20%–30%.

Abstract

Several electro-optical methods exist for measuring a path-averaged value of the inner scale of turbulence l0. By virtue of Monin–Obukhov similarity, in the atmospheric surface layer such l0 measurements are related to the friction velocity u* or to the surface stress τ = &rhou*2, where ρ is the air density. Because l0 is a path-averaged quantity, u* is too. Here the question of how precisely u* can be measured is investigated by combining these inner-scale measurements with two-wavelength scintillation measurements that yield the sensible and latent heat fluxes and, thereby, facilitate stability collections. The analysis suggests that current path-averaging instruments can generally measure u* to ± 20%–30%.

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