Estimation of area‐average sensible heat flux using a large‐aperture scintillometer during the Semi‐Arid Land‐Surface‐Atmosphere (SALSA) Experiment

Abstract

Abstract. The use of a large-aperture scintillometer to estimate sensible heat flux has been successfully tested by several investigators. Most of these investigations, however, have been confined to homogeneous or to sparse with single vegetation-type surfaces. The use of the scintillometer over surfaces made up of contrasting vegetation types is problematic because it requires estimates of effective roughness length and effective displacement height in order to derive area-average sensible heat from measurements of the refractive index. In this study an approach based on a combination of scintillometer measurements and an aggregation scheme has been used to derive area-average sensible heat flux over a transect spanning two adjacent and contrasting vegetation patches: grass and mesquite. The performance of this approach has been assessed using data collected during the 1997 Semi-Arid Land-Surface-Atmosphere field campaign. The results show that the combined approach performed remarkably well, and the correlation coefficient between measured and simulated area-average sensible heat flux was -0.95. This is of interest because this approach offers a reliable means for validating remotely sensed estimates of surface fluxes at comparable spatial scales. IntroductionThe turbulent heat fluxes near the ground surface are strongly affected by the ability of the surface to redistribute the radiative energy absorbed from the Sun and the atmosphere into sensible and latent heat. These fluxes play a key role in regulating the energy balance of the atmosphere, which in turn drives atmospheric circulation. For this reason, recent efforts have concentrated on improving the parameterization of landsurface processes in atmospheric models by taking into account the effect of surface heterogeneities on the exchanges processes [Avissat, 1995]. The problem, however, is the difficulty in validating model simulations at regional and certainly at the global circulation model (GCM) scale. On the other hand, it is necessary to validate GCM output because unless these models can reliably simulate the observed water and energy cycles in the present climate, future predictions of climate change are The objective of this study is to use a large-aperture scintillometer (LAS) to estimate areally averaged sensible heat flux over a transect made up of two adjacent patches (a grasscovered patch and a primarily mesquite-covered patch) with contrasting water status and roughness length. Scintillometerbased sensible heat flux is compared to a weighted average of those measured over each individual patch using two independent three-dimensional eddy correlation systems. The experiment took place in the San Pedro Basin within the context of 2505

Comparison of Large Aperture Scintillometer and Eddy Covariance Measurements: Can Thermal Infrared Data Be Used to Capture Footprint-Induced Differences?

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