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Two-Wavelength Method of Measuring Path-Averaged Turbulent Surface Heat Fluxes

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

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Abstract

Because few geophysical surfaces are horizontally homogeneous, point measurements of the turbulent surface fluxes can be unrepresentative. Path-averaging techniques are therefore desirable. This paper presents a method that yields path-averaged measurements of the sensible and latent heat fluxes with a potential accuracy as good as that for eddy-correlation measurements. The method relies on electro-optical measurements of the refractive index structure parameter Cn2 at two wavelengths: one in the visible-to-mid-infrared region, where Cn2 depends largely on turbulent temperature fluctuations, and a second in the near-millimeter-to-radio region, where Cn2 depends more strongly on humidity fluctuations. A sensitivity analysis, the cornerstone of the study, provides quantitative guidelines for selecting wavelength pairs to use for the measurements. The sensitivity analysis also shows that the method is not uniformly accurate for all meteorological conditions; for limited ranges of the Bowen ratio, the sensitivity becomes so large that accurately measuring one or both heat fluxes is impossible.

Abstract

Because few geophysical surfaces are horizontally homogeneous, point measurements of the turbulent surface fluxes can be unrepresentative. Path-averaging techniques are therefore desirable. This paper presents a method that yields path-averaged measurements of the sensible and latent heat fluxes with a potential accuracy as good as that for eddy-correlation measurements. The method relies on electro-optical measurements of the refractive index structure parameter Cn2 at two wavelengths: one in the visible-to-mid-infrared region, where Cn2 depends largely on turbulent temperature fluctuations, and a second in the near-millimeter-to-radio region, where Cn2 depends more strongly on humidity fluctuations. A sensitivity analysis, the cornerstone of the study, provides quantitative guidelines for selecting wavelength pairs to use for the measurements. The sensitivity analysis also shows that the method is not uniformly accurate for all meteorological conditions; for limited ranges of the Bowen ratio, the sensitivity becomes so large that accurately measuring one or both heat fluxes is impossible.

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