There are many non-satellite measurement systems that, in principle, could be used for the purposes of satellite characterisation on a sustained basis. Such measurements are metrologically well characterised and understood. They often measure variables, which are measured or measurable from space. However, many of the measurement systems are discontinuous (discrete) in time and their measurement scheduling is typically made with no regard to satellite-overpass times.

As a result of fractured governance along with historical funding decisions, the geographical spread of observation systems, which may, in principle, be synergistic, are not presently sufficiently optimised in order to realise the potential benefits for numerous research applications, including, but not limited to, satellite cal/val. For example, a twice-daily radiosonde program may currently be undertaken 100km from a facility with lidars and an FTIR.

Current governance of high-quality measurement programs is highly fractured. Numerous networks exist at national, regional, and global levels that have been set up and funded under a variety of governance models. This fractured management of observational capabilities can lead to, amongst others: redundancies, spatiotemporal gaps, varied data policies and formats, varied data processing choices, and fractured provision of data.

Copernicus Services, including the Climate Change Service (C3S), will provide information in close to real time using global and regional reanalysis outputs, as well as satellite L2 products. These outputs are not always consistent with their own climatology, because input data are not produced with the same quality at real-time as they are in elaborated climate data records.

Presently, the evaluation of the quality of Fundamental Climate Data Records (FCDR) (observations at radiance level that serve as key inputs for model-based reanalyses and retrievals of GCOS ECVs) is based mainly on isolated activities by individual research groups. Given the importance of FCDRs for all downstream data records, there is an important and evolving requirement to improve the assessment of FCDRs by utilising non-satellite reference measurements and model fields, among other means, for validation.

Recently established quality assurance and validation guidelines and systems are not sufficiently well recognised or understood in the global community, where validation purposes, methodologies, and results can differ significantly from one report to another.

Climate research and services have an increasing need to consider a large amount of observational data and model outputs simultaneously in applications. Because the data volumes provided by satellite observations and ensemble model runs have increased to levels that prevent easy download to local compute environments, there is an enhanced need for tools that provide functionality for data extraction, analysis, and visualisation at source or on cloud compute resources.

Presently, access to high-quality reference network data and satellite data is obtained through a variety of portals, using a broad range of access protocols, and the data files are available in an array of native data formats that lack interoperability (see Gap 1.06). There also exists a broad range of data policies from open access through delayed mode restricted access.

For the retrieval of information about the vertical distribution of target species from FTIR spectra, it is important to know the FTIR instrument line shape (ILS). Therefore, regular cell measurements are carried out to characterize the ILS of the FTIR spectrometers. However, these cell measurements have their own uncertainties since these are obtained using optimal estimation: an ILS retrieval comes along with an uncertainty and an averaging kernel. In particular the averaging kernel for an ILS retrieval is often not adequately considered (Hase, 2012).

The GCOS Reference Upper Air Network (GRUAN) provides reference in-situ data for temperature and humidity with traceable estimates of uncertainty. This network can be used to validate NWP short-range forecasts for temperature and humidity to reference standards (see gap G4.01). The NWP temperature and humidity forecasts can then be used to perform satellite Cal/Val of new instruments, with improved knowledge of the associated uncertainties. However, there are very few GRUAN data above 40 hPa and none above 5hPa.