Towards operational validation systems for global satellite-derived terrestrial essential climate variables

Authors: BAYAT, B. - Julich Research Center; CAMACHO, F. - Universidad De Valencia; NICKESON, J. - Goddard Space Flight Center; Cosh, Michael; BOLTEN, J. - Goddard Space Flight Center; VEREECKEN, H. - Julich Research Center; MONTZKA, C. - Julich Research Center

Submitted to: International Journal of Applied Earth Observation and Geoinformation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/21/2020
Publication Date: 3/1/2020
Citation: Bayat, B., Camacho, F., Nickeson, J., Cosh, M.H., Bolten, J., Vereecken, H., Montzka, C. 2020. Towards operational validation systems for global satellite-derived terrestrial essential climate variables. International Journal of Applied Earth Observation and Geoinformation. 95:102240. https://doi.org/10.1016/j.jag.2020.102240.
DOI: https://doi.org/10.1016/j.jag.2020.102240

Interpretive Summary: Essential Climate Variables are a collection of scientific parameters which have been declared by the scientific community as critical to monitor for the proper understanding and modeling of climate and weather. These variables can be estimated from a variety of instruments and satellite platforms. This presents a challenge to calibrate and validate those monitoring methods against common known values or ground truth. A review was conducted to determine the readiness status of the essential climate variables that are considered land products. Primary hinderances of these validation stages are adequate systems for monitoring, and the complex nature of the land surface. Conclusions include the need for more monitoring systems and operational networks with sufficient ancillary information.

Technical Abstract: Essential Climate Variables in the terrestrial domain, called terrestrial ECVs, are key sources of information for both application-oriented and scientific research. A large number of global terrestrial ECV products have been derived from satellite observations, and more are forthcoming. However, to ensure their utility, end-users need to know the general error levels for such terrestrial ECV products. Diverse validation practices have been employed to assess the quality of such products resulting in reduced comparability of validated products at the global scale, even for the same terrestrial ECV. Addressing this challenge in validation practices requires the use of unified and standard, publicly available and objective validation procedures that are operational for all products of a specific terrestrial ECV, and preferably also applicable for all ECVs at the global scale. This can allow end-users to perform comparative assessments. To this end, the current study aims to investigate the readiness status of operational validation systems for a select group of eight satellite-derived terrestrial ECVs at the global scale and with a long-term perspective. Selected variables are Leaf Area Index (LAI), Land Surface Temperature (LST), Evapotranspiration (ET), Soil Moisture (SM), albedo (a), the fraction of Absorbed Photosynthetically Active Radiation (fAPAR), photosynthesis [Gross Primary Production (GPP)] and Land Cover (LC). For each of these terrestrial ECVs, we reviewed key prerequisites and primary tools [notably, long term satellite-derived global product availability, global in situ measurement accessibility, a validation protocol, and an online validation platform] required for adopting an operational validation system. With respect to the “readiness level”, the investigation results demonstrate that LAI, SM, and LC are in the highest level of readiness for moving towards a full operational validation procedure at the global scale. However, ET and GPP are in the lowest level of readiness, mainly due to the lack of standard validation protocols and online validation platforms. The remainder of the selected terrestrial ECVs is identified at mid-level of readiness mainly because of either validation platform (i.e., LST, and a) or standard protocol (i.e., fAPAR) still need development. This contribution can pave the way for moving toward open-access, traceable, transparent, and operational validation procedures of satellite-derived global terrestrial ECVs.