Long-Path Scintillometry over Complex Terrain to Determine Areal-Averaged Sensible and Latent Heat Fluxes

摘要

Landscape scale measurements (1 - 10 km2) of sensible and latent heatudfluxes over heterogeneous areas are required for hydrological andudmeteorological modelling. Evaporation is strongly dependent on landudsurface properties, thus aggregating field-scale measurements hasudmuch uncertainty because of the need for detailed land-cover maps andudthe possible disproportionate contribution of vegetation transition zones;udin any case, such a collection of field-scale instruments is a highlyudresource intensive approach. A potentially better alternative is reportedudhere: the long-path (large aperture) scintillometer (LAS) has been usedudover topographically complex chalk downland with mixed vegetation, toudmeasure the sensible heat fluxes at the landscape scale, using a 2.4 kmudpathlength. These sensible heat fluxes agreed well with aggregatedudeddy covariance measurements made at the field scale for differentudvegetation types – the contrasting range of sensible heat fluxes in theudlate summer over various agricultural fields in southern England isudreported.udThe LAS in combination with a new custom-built millimetre-wave (94udGHz) scintillometer (MWS) was trialled to measure large-scale areaudaveraged latent and sensible heat fluxes, using the two-wavelengthudmethod of scintillometry, over the same complex terrain. The LAS-MWSudfluxes were found to close the energy balance well, except duringudperiods of either high windspeed or very low cross-wind conditions. Theudlatter conditions may lead to inappropriate filtering or reach audfundamental limitation of the method. The over-estimation of flux at highudwindspeeds may be due to limitations of the Monin-Obukhov similarityudtheory which was developed for homogeneous surfaces.udThe difference in the effective heights of the two scintillometers for theudmeasurement of their respective fluxes is recognised, and the formulaudfor these heights is derived. Application over certain complex topographyudshows that an increased measurement height would lead to bettermatchedudsource areas for the LAS and MWS.