Geometry of the hemispherical radiometric footprint over plant canopies

B Marcolla,A Cescatti

doi:10.1007/s00704-017-2326-z

Abstract

Radiometric measurements of hemispherical surface reflectance and long-wave irradiance are required to quantify the broadband albedo and the outgoing thermal radiation. These observations are typically integrated with eddy covariance measurements of sensible and latent heat fluxes to characterize the surface energy budget. While the aerodynamic footprint has been widely investigated, the geometry of the hemispherical radiometric footprint over plant canopies has been rarely tackled. In the present work, the size and shape of the hemispherical radiometric footprint are formalized for a bare surface and in presence of a vegetation cover. For this purpose, four idealized canopies are analyzed and the dependency of the radiometric footprint on leaf area index and canopy height is explored. Besides, the radiometric footprint is compared with the aerodynamic footprint in conditions of neutral stability. It was observed that almost 100% of the hemispherical radiometric signal originates within a distance of a few radiometer heights, while only about 50–80% of the cumulative aerodynamic signal is generated within a distance of about 20 sensor heights. In order to achieve comparable extensions of the footprint areas, hemispherical radiometric measurements should therefore be taken about 6–15 times higher than turbulent flux ones, depending on the vegetation type. The analysis also highlights that the size of the radiative footprint decreases at increasing leaf area index, whereas the aerodynamic footprint shows an opposite behavior. For the abovementioned reasons, this work may support the interpretation of energy flux measurements and the optimal design of eddy covariance stations located in heterogeneous sites.

Highlights

Radiometric measurements of broadband upwelling shortwave and long-wave radiation are fundamental in micrometeorology to quantify surface albedo, net radiation and the surface energy balance (Law et al 2002; Eklundh et al 2011; Cescatti et al 2012; Stoy et al 2013)
The measurements of hyper-spectral surface reflectance at eddy covariance sites is an important step for the spatial extrapolation of in situ measurements with remote sensing retrievals from satellite platforms (Balzarolo et al 2011) and to parameterize light use efficiency models for the up-scaling of gross primary productivity (GPP) (Sims et al 2006; Meroni et al 2009; Meroni et al 2011; Peñuelas et al 2011; Rossini et al 2012)
Given the typical inhomogeneity of the land surface in topography, soil properties and vegetation cover, an evaluation of the spatial footprint of the observations is required for the proper integration of hemispherical radiometric measurements with turbulent fluxes

Summary

Introduction

Radiometric measurements of broadband upwelling shortwave and long-wave radiation are fundamental in micrometeorology to quantify surface albedo, net radiation and the surface energy balance (Law et al 2002; Eklundh et al 2011; Cescatti et al 2012; Stoy et al 2013). In situ observations of spectral reflectance are increasingly applied to link surface measurements of energy and carbon fluxes with Earth observations (Gamon et al 2006). To fulfill the objectives listed above, radiometric measurements have to be integrated with observations of surface fluxes performed with the eddy covariance technique. This methodology is nowadays applied for the quantification of the turbulent exchange of carbon, water and energy between vegetation and the atmosphere at hundreds of experimental sites organized in continental networks (Baldocchi 2003). Given the typical inhomogeneity of the land surface in topography, soil properties and vegetation cover, an evaluation of the spatial footprint of the observations is required for the proper integration of hemispherical radiometric measurements with turbulent fluxes.

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Conclusion