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Focus Area on Land Surface Temperature & Emissivity Product Validation


Frank Göttsche, Karlsruhe Institute of Technology, Germany
Glynn Hulley, NASA JPL, USA

 


Land Surface Temperature Definition

Land surface temperature (LST) is a kinetic quantity, independent of wavelength, that represents the thermodynamic temperature of the skin layer of a given surface, i.e. it is a measure of how hot or cold the surface of the Earth would feel to the touch. For ground-based, airborne, and space borne remote sensing instruments LST is the aggregated radiometric surface temperature based on a measure of radiance. Therefore, in the literature, LST is also referred to as (directional) radiometric temperature or skin temperature. When derived from radiometric measurements of remote sensing instruments, LST represents the aggregated radiometric surface temperature of the ensemble of components within the sensor's field of view (Norman and Becker, 1995). This definition has been adopted by various international groups, e.g. CEOS WGCV, GCOS, ESA GlobTemperature, and ILSTE-WG.

Units: The unit of LST is Kelvin [K]. Degree Celsius [°C] is also commonly used.

Norman, G., and Becker, F. (1995). Terminology in thermal infrared remote sensing of natural surfaces.   Agricultural and Forest Meteorology, Volume: 77, Issue: 3-4, Pages: 153-166,
  DOI: 10.1016/0168-1923(95)02259-Z


Land Surface Emissivity Definition

Emissivity a wavelength-dependent quantity defined as the ratio of the radiance actually emitted by an isothermal,homogeneous body and the radiance emitted by a black body at the same thermodynamic temperature (Norman and Becker 1995).

Units: Dimensionless.


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

Validation stage 1 (LPV validation stage hierarchy) - The highest LPV validation stage reached for satellite-derived land surface temperature and emissivity products. For reaching validation stage 3 and higher, an increased number of global validation sites, covering all surface types, with extended temporal coverage, as well as intercomparisons of LST products are needed.


Validation Methods and Good Practice

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 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 1-2K.

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.), Best Practice 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:

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


LPV Focus Areas

 

Meetings

EGU General Assembly 2019: 'Taking the temperature of the Earth: observing surface temperature across all domains in a changing climate', Session: CL5.13/AS4.30/CR1.8/OS4.28 (Climate: Past, Present, Future), 7-12 April 2019, Vienna, Austria.

The 5th Sentinel-3 Validation Team Meeting, 7-9 May 2019 - ESA/ESRIN, Frascati, Italy.

ESA Living Planet Symposium, 13-17 May 2019, at the Convention Center, Milan, Italy.

2019 Joint Satellite Conference, 28 Sep - 4 Oct 2019, Westin Boston Waterfront Hotel, Boston, MA, USA; submission deadline: 1 Mar 2019.

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