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

Landscape scale measurements (1 - 10 km2) of sensible and latent heat fluxes over heterogeneous areas are required for hydrological and meteorological modelling. Evaporation is strongly dependent on land surface properties, thus aggregating field-scale measurements has much uncertainty because of the need for detailed land-cover maps and the possible disproportionate contribution of vegetation transition zones; in any case, such a collection of field-scale instruments is a highly resource intensive approach. A potentially better alternative is reported here: the long-path (large aperture) scintillometer (LAS) has been used over topographically complex chalk downland with mixed vegetation, to measure the sensible heat fluxes at the landscape scale, using a 2.4 km pathlength. These sensible heat fluxes agreed well with aggregated eddy covariance measurements made at the field scale for different vegetation types – the contrasting range of sensible heat fluxes in the late summer over various agricultural fields in southern England is reported. The LAS in combination with a new custom-built millimetre-wave (94 GHz) scintillometer (MWS) was trialled to measure large-scale area averaged latent and sensible heat fluxes, using the two-wavelength method of scintillometry, over the same complex terrain. The LAS-MWS fluxes were found to close the energy balance well, except during periods of either high windspeed or very low cross-wind conditions. The latter conditions may lead to inappropriate filtering or reach a fundamental limitation of the method. The over-estimation of flux at high windspeeds may be due to limitations of the Monin-Obukhov similarity theory which was developed for homogeneous surfaces. The difference in the effective heights of the two scintillometers for the measurement of their respective fluxes is recognised, and the formula for these heights is derived. Application over certain complex topography shows that an increased measurement height would lead to bettermatched source areas for the LAS and MWS.