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Obtaining Surface Momentum and Sensible Heat Fluxes from Crosswind Scintillometers

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  • 1 U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire
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Abstract

A crosswind scintillometer measures the average wind speed perpendicular to its propagation path, U, and the refractive index structure parameter C2n. While C2n is a turbulence statistic, the worth of U for turbulence research is not immediately obvious. This paper demonstrates, however, that, with a measurement of wind direction, U converts to Uz, the mean wind speed at path height z. Furthermore, if an estimate of the roughness length for momentum, z0, is available, the surface layer similarity equations for Uz and C2n can be solved iteratively to yield two prime turbulence variables: the friction velocity u∗ and the surface flux of sensible heat Hs. A sensitivity analysis suggests that, in optimum conditions, a crosswind scintillometer should be capable of providing u∗ and Hs with uncertainties of ±10%–15% and ±20%–30%, respectively. The crosswind scintillometer, thus, can be an important tool for measuring path-averaged values of u∗ and Hs.

Corresponding author address: Dr. Edgar L Andreas, U.S. Army Cold Regions Research and Engineering Laboratory, 72 Lyme Road, Hanover, NH 03755-1290.

Email: eandreas@crrel.usace.army.mil

Abstract

A crosswind scintillometer measures the average wind speed perpendicular to its propagation path, U, and the refractive index structure parameter C2n. While C2n is a turbulence statistic, the worth of U for turbulence research is not immediately obvious. This paper demonstrates, however, that, with a measurement of wind direction, U converts to Uz, the mean wind speed at path height z. Furthermore, if an estimate of the roughness length for momentum, z0, is available, the surface layer similarity equations for Uz and C2n can be solved iteratively to yield two prime turbulence variables: the friction velocity u∗ and the surface flux of sensible heat Hs. A sensitivity analysis suggests that, in optimum conditions, a crosswind scintillometer should be capable of providing u∗ and Hs with uncertainties of ±10%–15% and ±20%–30%, respectively. The crosswind scintillometer, thus, can be an important tool for measuring path-averaged values of u∗ and Hs.

Corresponding author address: Dr. Edgar L Andreas, U.S. Army Cold Regions Research and Engineering Laboratory, 72 Lyme Road, Hanover, NH 03755-1290.

Email: eandreas@crrel.usace.army.mil

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