G4.02     Lack of traceable uncertainty estimates for NWP and reanalysis fields & equivalent TOA radiances - relating to humidity

Gap detailed description

The gap is closely related to that described above relating to temperature. The text above is therefore repeated here, for completeness.

Numerical Weather Prediction (NWP) models are already routinely used in the validation and characterisation of EO data, but a lack of robust uncertainties associated with NWP model fields and related TOA radiances prevent the use of these data for a complete and comprehensive validation of satellite EO data, including an assessment of absolute radiometric errors in new satellite instruments. Agencies and instrument teams, as well as key climate users, are sometimes slow (or reluctant) to react to the findings of NWP-based analyses of satellite data, due to the current lack of traceable uncertainties. The aim is to assess uncertainties in NWP fields through systematic monitoring, using GRUAN data, as part of WP4.

Activities within GAIA-CLIM related to this gap

GAIA-CLIM WP4 aims to advance current state-of-the-art (in using NWP models to validate satellite data) in two respects:  firstly by further demonstrating the value of NWP in the validation of microwave temperature sounding instruments (including F-19 SSMIS and JPSS-1  ATMS),  then evaluating and developing the method for the validation of microwave humidity sounders (e.g. FY-3C MWHS-2) and microwave imagers (GCOM-W AMSR-2);  and secondly  by estimating the uncertainties in NWP model fields through comparison with reference radiosonde data from the GRUAN network.  These latter comparisons will be conducted both in geophysical variables (temperature, and humidity) and in top-of-atmosphere brightness temperatures.

It is estimated that significant progress can be made on both elements of this plan during the GAIA-CLIM project. (Timescale and cost estimate: GAIA-CLIM 2015-2018: 48 man/months of effort).

Gap remedy

Remedy #1

Specific remedy proposed

Developing a ‘GRUAN processor’ as a software deliverable from GAIA-CLIM WP4.  The software will be open-source and will enable users (albeit fairly expert users) to compare NWP fields from both ECMWF and Met Office (in the first instance) models with GRUAN data.  This will include a comparison of temperature and humidity, as well as TOA brightness temperatures for all sensors supported by the (publically available) RTTOV radiative transfer model.

Measurable outcome of success

·         Statistics available on the comparison, for all GRUAN sites, wrt ECMWF and Met Office NWP fields.

·         A web page displaying these statistics.

·         An open-source GRUAN processor available to the wider community.

·         Integration of the GRUAN processor into the GAIA-CLIM Virtual Observatory.

Achievable outcomes

Technological / organizational viability: High

Indicative cost estimate:  low (<1 million Euros)


The solution proposed here is fully aligned with the requirement (to establish traceable uncertainties for NWP fields and radiances calculated from them).


The remedy proposed here is a key focus, and deliverable, of GAIA-CLIM WP4, due for delivery (D4.4) in Month 24.

Gap risks to non-resolution


Identified future risk / impact

Probability of occurrence if gap not remedied

Downstream impacts on ability to deliver high quality services to science / industry / society

NWP-based validation results /conclusions not taken into account by space agencies developing new instruments – as a result of a lack of uncertainties.


(‘Medium’ for established agencies with some experience of this type of validation, ‘High’ for newer agencies developing new capability in this area.)

Failure to rectify sub-optimalities in instrument design and/or processing chains, resulting in sub-optimal data being used in downstream applications (climate studies & operational NWP, for example).

Work package: