Accuracy assessment of land surface temperature retrievals from MSG2-SEVIRI data
Research highlights
► The accuracy of the EUMETSAT LSA SAF LST product was assessed (T- & R-based methods). ► Both validation methods showed significant biases for the LSA SAF LST product. ► The biases can be due to the land surface emissivities used as input data. ► An alternative LST algorithm is proposed and checked, showing better accuracies.
Introduction
Land Surface Temperature (LST) is defined as the temperature of the interface between the Earth's surface and its atmosphere and thus it is a critical variable to understand land–atmosphere interactions and a key parameter in a wide range of environmental applications, such as meteorological, climatological and hydrological studies, which involve energy and water fluxes (Kustas and Norman, 1999, Sánchez et al., 2009), irrigation need estimates (Brasa-Ramos et al., 1998), forest fire risk forecasting, detection and monitoring (Briess et al., 2003), air temperature and humidity determination (Florio et al., 2004, Vogt et al., 1997), leaf wetness evaluation (Deshpande et al., 1995) and desertification, deforestation and climate change monitoring (Allen et al., 1994, Lambin and Ehrlich, 1997). Additionally, a frequent and accurate determination of LST could help to improve the forecasting of natural hazards and the analysis of thermal diurnal cycles.
Thermal infrared (TIR) remote sensing is used to obtain LST measurements at different spatial and temporal resolutions. Examples are the LST products generated from the Advanced Along-Track Scanning Radiometer (AATSR) (Llewellyn-Jones et al., 2001) onboard the European ENVISAT, as well as the Moderate Resolution Imaging Spectroradiometer (MODIS) (Wan, 2008) and the Advanced Spaceborne Thermal Emission and Reflection radiometer (ASTER) (Gillespie et al., 1998) on EOS platforms. The Spinning Enhanced Visible and Infrared Imager (SEVIRI) on board the geostationary METEOSAT Second Generation (MSG) platform allows a semi-global coverage with a higher temporal resolution (one image every 15 min) and thus offers the possibility to provide more frequent LST determinations and to study the diurnal LST cycle. An operational MSG-SEVIRI LST product is generated by the Land Surface Analysis of the Satellite Application Facility (LSA SAF) at the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT).
The aim of this paper is to evaluate the accuracy of LSTs retrieved with predefined MSG algorithms before making use of the high temporal resolution of MSG-SEVIRI data. The paper first describes the operational LSA SAF procedure to generate MSG LST images. Then, the paper proposes an alternative algorithm with a physical theoretical background that corrects atmospheric effects by using angular-dependent coefficients. Both the LSA SAF and the proposed algorithms use land surface emissivity (LSE) as input data, and previous accurate LSE maps are thus required since emissivity uncertainties can involve important errors in terms of LST. The sensitivity of the proposed algorithm to different uncertainty sources is analyzed in Section 3. The accuracy of the proposed algorithm and the LSA SAF product are then checked in Section 4 by using concurrent in situ data at two dedicated experimental sites in Eastern Spain, and also by means of a new radiance-based validation method (Wan & Li, 2008) that does not require ground LST measurements and thus avoids possible problems of spatial scale dissimilarity between ground and satellite data. Finally, the main conclusions are summarized in Section 5.
Section snippets
The operational MSG-SEVIRI LST algorithm
The LSA SAF LST product is generated by using a Generalized Split-Window (GSW) algorithm (Madeira, 2002, Trigo et al., 2008a) that follows the formulation first proposed by Wan and Dozier (1996) for data from the Advanced Very High Resolution Radiometer (AVHRR) on the National Oceanic and Atmospheric Administration (NOAA) platforms and EOS-MODIS. The LST is a function of the cloud-free top-of-atmosphere brightness temperatures measured by MSG-SEVIRI channels 9 and 10, centered at 10.8 μm and 12
Sensitivity analysis
The uncertainty of the algorithm proposed in Eq. (4) was evaluated with a sensitivity analysis, similarly as in Freitas et al. (2010). LST uncertainty was estimated as the combination of the model error, δ(LST)M, and the LST uncertainties associated with the input uncertainties, δ(LST)P:where we assumed that both errors are independent. δ(LST)M is defined as:and δ(LST)P follows:σAC being the fitting error of the
Validation
Trigo et al., 2008a, Kabsch et al., 2008 quantified LSA SAF LST accuracy by using ground data collected from a validation station in a Quercus woodland plain near Evora, Portugal (38.54°N, 8.00°W). Accuracies from ± 1.0 K to ± 4.9 K were obtained depending on the season and the selection of daytime/night-time data, and the existence of significant biases between satellite-retrieved and in situ LSTs was pointed out. These inaccuracies could be a consequence of the mentioned spatial-scale
Conclusions
The operational LST product generated every 15 min by the EUMETSAT LSA SAF from MSG-SEVIRI data was first tested at two dedicated, partly and fully vegetated ground-truth sites in Eastern Spain, and LST uncertainties larger than ± 1.5 K were observed as a consequence of the existence of significant biases. Eq. (4), which is a quadratic split-window equation with angular-dependent coefficients, was proposed as an alternative algorithm. This equation follows the theoretical split-window model first
Acknowledgements
The research described in this paper was supported by the Spanish Ministerio de Ciencia e Innovación (projects CGL2007-65774/CLI, CGL2008-04550, CGL2010-16364, and CONSOLIDER-INGENIO 2010 CSD2007-00067; and Dr. Niclòs' Research Contract funded by the “Subprograma Ramón y Cajal" (MICINN-RYC)), the Conselleria d'Educació of the Generalitat Valenciana (PROMETEO/2009/086), and the European Union (Project CIRCE No. 036961). Instituto Universitario CEAM-UMH is supported by the Generalitat Valenciana
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