Abstract
An autonomous system for field land surface temperature (LST) measurements taken at different observation angles was developed to be deployed easily at any conventional meteorological tower station. The system permits ground-truth data to be acquired on a continuous basis, and angularly scans land and sky hemispheres with a single thermal-infrared (TIR) radiometer. This paper describes the autonomous angular system and the methodology to assess ground-truth LST and relative-to-nadir emissivity data from system measurements. Ground-truth LSTs were used to validate satellite-retrieved LST products at two experimental sites (rice crop and shrubland areas). The relative-to-nadir emissivity values were used to analyze the anisotropy of surface emissive properties over thermally-homogeneous covers. The EOS-MODIS MOD11_L2/MYD11_L2 LST product was evaluated and shown to work within expected uncertainties (<2.0 K) when tested against the system data. A slight underestimation of around −0.15 K was observed, which became greater for the off-nadir observation angles at the shrubland site. The system took angular measurements for the different seasonal homogeneous covers at the rice crop site. These measurements showed emissivity angular anisotropies, which were in good agreement with previously published data. The dual-view ENVISAT-AATSR data reproduced them, and revealed that the system data collected for thermally-homogeneous surfaces could be used to test future satellite TIR sensors with multi-angular or bi-angular capabilities, like the forthcoming SLSTR on board Copernicus Sentinel-3A.
Highlights
Background and ObjectivesLand surface temperature (LST) and thermal-infrared emissivity are defined at the interface between the Earth’s surface and its atmosphere, and are critical variables for understanding land-atmosphere interactions
Cloud-free data were selected by filtering cloudy data with the quality control bits of the Moderate Resolution Imaging Spectroradiometer (MODIS) products and the concurrent sky measurements collected by the angular system
The single pixel that covered the permanent station location was considered for the comparison, and the mean and standard deviation values of the 3 × 3 pixels of the MODIS products centered at this location
Summary
Land surface temperature (LST) and thermal-infrared emissivity are defined at the interface between the Earth’s surface and its atmosphere, and are critical variables for understanding land-atmosphere interactions. LST and emissivity are key parameters in a wide range of environmental applications, such as meteorological, climatological, and hydrological studies, which involve energy and water fluxes [1] They can be used for other applications, like detecting changes in land cover and use [2]. We are unaware of the existence of previous similar systems and our design was not inspired in previously published devices This device performs complete angular scans to sample land and sky temperatures with a single TIR radiometer, rather than using radiometers fixed at specific viewing angles.
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