National Aeronautics and Space Administration

Focus Area on Land Surface Temperature & Emissivity Product Validation

Glynn Hulley, NASA JPL, USA
Lluís Pérez-Planells, Karlsruhe Institute of Technology, Germany

 

Land Surface Temperature Definition

Land Surface Emissivity Definition

Highest Validation Stage Currently Reached for Satellite-Derived Land Surface Temperature and Emissivity Products

Validation Good Practices

The purpose of validation is to characterize product uncertainties and to evaluate the performance of retrieval algorithms. Four categories of validation methods (Schneider et al., 2012) are commonly used to assess a satellite LST product's compliance with its specifications:

• Ground-based validation refers to comparisons with LST obtained from in-situ measurements: if the measurements are performed with well maintained, high quality instrumentation and are representative for the satellite sensor footprint, it represents the reference validation method.
• Radiance-based Validation is a comparison against the surface temperature that minimises the difference between the radiance measured by the satellite and that obtained with a radiative transfer model. The method requires accurate surface emissivities as well as atmospheric temperature and water vapour profiles coincident with the satellite overpass.
• Satellite Product Inter-Comparison with another - generally already validated - LST product. The method is particularly valuable for identifying disagreements between LST products over large areas and different land cover types.
• Time Series Inter-Comparison is used to detect instrument problems, e.g. calibration drift, or outliers due to undetected clouds. The approach requires a relatively long time series of observations over temporally stable targets, e.g. inland water bodies or deserts.

The above categories complement each other and provide different information about the quality of an LST product. Due to a lack of global reference datasets, methods from all four categories are required to achieve Stage-3 validation status. Over homogeneous sites state-of-the-art satellite LST and ground-based LST are generally expected to agree with each to within 1-2 K.

Current methods and best practises for land surface temperature and emissivity product validation are described in the following documents:

Guillevic, P., Göttsche, F., Nickeson, J., Hulley, G., Ghent, D., Yu, Y., Trigo, I., Hook, S., Sobrino, J.A., Remedios, J.,   Román, M. & Camacho, F. (2018). Land Surface Temperature Product Validation Best Practice Protocol. Version   1.0. In P. Guillevic, F. Göttsche, J. Nickeson & M. Román (Eds.), Good Practices for Satellite-Derived Land Product   Validation (p. 58): Land Product Validation Subgroup (WGCV/CEOS), doi:10.5067/doc/ceoswgcv/lpv/lst.001

Göttsche, F., Olesen, F.S., Høyer, J.L., Wimmer, W., and Nightingale, T. (2017). Fiducial Reference Measurements for   Validation of Surface Temperature from Satellites (FRM4STS).Technical Report 3 - A Framework to Verify the Field   Performance of TIR FRM. Internal Publication, OFE- D120-V1-Iss-3-Ver-1-ISSUED, Issue 3 Revision 0, page 1-75.

Schneider, P., Ghent, D., Corlett, G., Prata, F., and Remedios, J., 2012. AATSR validation: LST validation protocol.   Internal publication, UL-NILU-ESA-LST-LVP Issue 1 Revision 0, page 1-39.

Land Surface Temperature and Emissivity Validation Reference Data Sets

Currently available reference data for satellite-derived LST and emissivity validation are often campaign-based. For a few sites longer time series are available:

• The JPL network: Calval, Lake Tahoe, Salton Sea, Russell Ranch, Emissivity
• The Surface Radiation (SURFRAD) network
• Karlsruhe Institute of Technology (KIT) stations
• The stations of the Copernicus LAW project
• The Global Change Unit (GCU)
• The ASTER Spectral Library

A more complete and detailed list of current in-situ LST validation sites is provided here.