Toward operational validation systems for global satellite-based terrestrial essential climate variables

Terrestrial Essential Climate Variables, known as terrestrial ECVs, are key sources of information for both application- and scientific- oriented research. A large number of global terrestrial ECV products have been derived from satellite observations, and more are forthcoming. To unlock the full potential of these products, end-users need to know their uncertainties and error magnitudes. Due to the lack of conformity among validation strategies, a wide range of validation approaches have been employed to assess the quality of these products, and have resulted in reduced comparability even for the same terrestrial ECV. Addressing this challenge in validation practices requires the use of unified, standard, publicly available, traceable 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 a selected group of seven global long-term satellite-based terrestrial ECVs for operational validation. Selected variables are Leaf Area Index (LAI), Land Surface Temperature (LST), Evapotranspiration (ET), Soil Moisture (SM), Albedo, the fraction of Absorbed Photosynthetically Active Radiation (fAPAR), and Land Cover (LC). For each of these terrestrial ECVs, we reviewed key prerequisites and primary tools [notably, long term global product availability, globally distributed in situ measurement availability, a validation good practice protocol, and an online validation platform] required for developing an operational validation system. With respect to the readiness level, the investigation results demonstrate that LAI, SM, and LC are at the highest level of readiness for moving toward a full operational validation at the global scale. However, ET is at the lowest level of readiness, mainly due to the lack of standard validation good practice protocol and lack of a pilot online validation platform. The remainder of the selected terrestrial ECVs are identified to be at mid-level readiness, mainly because either a validation platform (i.e., LST and albedo) or good practice protocol (i.e., fAPAR) still needs to be developed. This review can pave the way for open-access, traceable, transparent, and operational validation procedures of satellite-based global terrestrial ECVs.

Automated summary

Terrestrial Essential Climate Variables are crucial for research, and understanding their uncertainties is important for end-users. A study investigated the readiness of seven global satellite-based terrestrial ECV products for operational validation, finding that LAI, SM, and LC are most prepared for full operational validation at a global scale.