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Ground-based remote sensing of surface fluxes and crosswind using scintillometry over complex terrain

Ward, Helen; Evans, Jonathan; De Bruin, Henk; Grimmond, Sue. 2010 Ground-based remote sensing of surface fluxes and crosswind using scintillometry over complex terrain. [Poster] In: Remote Sensing & Hydrology 2010 Symposium, Jackson Hole, Wyoming, 27-30 September 2010.

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Abstract/Summary

Scintillometry provides the opportunity to obtain spatially-averaged surface heat fluxes at a scale of hundreds of metres to kilometres. Measurements of both sensible and latent heat fluxes, as well as the wind speed perpendicular to the path can be made, providing information along the length of the beam without requiring a network of in situ instrumentation. Thus it has great potential as a ground-based remote-sensing tool for validation of weather models and satellite images. Offering the advantage over eddy covariance of measuring a larger area makes scintillometry an increasingly used technique, particularly over heterogeneous land surfaces. At the mesoscale level of weather models and satellite pixels, obtaining representative fluxes can be difficult, especially when complicated by varying topography and surface heterogeneity such as within urban areas. Due to the mixture of land uses, building heights and locally varying terrain, the urban land surface is complicated and very different from the idealised locations where scintillometry has been well-tested. Therefore it is necessary to evaluate whether the theory can still be applied or whether adjustments are required; and to identify any warning signs in the data which might be indicative of particularly challenging conditions. The accuracy of the refractive index structure parameter (Cn2), from which the heat fluxes are derived, and the quality of crosswind speed data need to be verified. In this study, the effect of a non-uniform wind field along the path of a scintillometer was investigated. Theoretical spectra were calculated for a range of scenarios and the variance of the amplitude fluctuations at the scintillometer receiver was estimated. It was found that the refractive index structure parameter relation with the scintillometer signal remained valid and invariant for spatially varying crosswinds but the spectral shape may change significantly, preventing accurate estimation of the crosswind speed from the frequency spectrum. On comparison with experimental data, it appears that non-uniform crosswind conditions could be responsible for previously unexplained features in the observed spectra. The results presented here demonstrate that, with caution, scintillometry can be appropriately used under conditions of varying crosswind speed and therefore can provide reliable surface fluxes at scales suitable for comparison with satellite data.

Item Type: Publication - Conference Item (Poster)
Programmes: CEH Topics & Objectives 2009 - 2012 > Unallocated
CEH Sections: Harding (to July 2011)
NORA Subject Terms: Ecology and Environment
Meteorology and Climatology
Date made live: 11 Jun 2013 11:17 +0 (UTC)
URI: http://nora.nerc.ac.uk/id/eprint/502167

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