A critical revision of the estimation of the latent heat flux from two-wavelength scintillometry

Ward, H.C.; Evans, J.G. ORCID: https://orcid.org/0000-0003-4194-1416; Hartogensis, O.K.; Moene, A.F.; De Bruin, H.A.R.; Grimmond, C.S.B.. 2013 A critical revision of the estimation of the latent heat flux from two-wavelength scintillometry. Quarterly Journal of the Royal Meteorological Society, 139 (676). 1912-1922. 10.1002/qj.2076

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Abstract/Summary

Simultaneous scintillometer measurements at multiple wavelengths (pairing visible or infrared with millimetre or radio waves) have the potential to provide estimates of path-averaged surface fluxes of sensible and latent heat. Traditionally, the equations to deduce fluxes from measurements of the refractive index structure parameter at the two wavelengths have been formulated in terms of absolute humidity. Here, it is shown that formulation in terms of specific humidity has several advantages. Specific humidity satisfies the requirement for a conserved variable in similarity theory and inherently accounts for density effects misapportioned through the use of absolute humidity. The validity and interpretation of both formulations are assessed and the analogy with open-path infrared gas analyser density corrections is discussed. Original derivations using absolute humidity to represent the influence of water vapour are shown to misrepresent the latent heat flux. The errors in the flux, which depend on the Bowen ratio (larger for drier conditions), may be of the order of 10%. The sensible heat flux is shown to remain unchanged. It is also verified that use of a single scintillometer at optical wavelengths is essentially unaffected by these new formulations. Where it may not be possible to reprocess two-wavelength results, a density correction to the latent heat flux is proposed for scintillometry, which can be applied retrospectively to reduce the error.

Item Type:	Publication - Article
Digital Object Identifier (DOI):	10.1002/qj.2076
Programmes:	CEH Topics & Objectives 2009 - 2012 > Water
UKCEH and CEH Sections/Science Areas:	Reynard
ISSN:	0035-9009
Additional Keywords:	density effects, evaporation, humidity, millimetre-wave scintillometer, refractivity
NORA Subject Terms:	Hydrology Meteorology and Climatology
Date made live:	17 Jan 2013 12:34 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/21159

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