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. The characterisation of errors and uncertainties in numericalweather prediction (NWP) model fields is a major challenge that is addressedas part of the Horizon 2020 Gap Analysis for Integrated Atmospheric ECVCLImate Monitoring (GAIA-CLIM) project. In that regard, observations from theGCOS (Global Climate Observing System) Reference Upper-Air Network (GRUAN)radiosondes are being used at the Met Office and European Centre forMedium-Range Weather Forecasts (ECMWF) to assess errors and uncertaintiesassociated with model data. The software introduced in this study and referred to as the GRUAN processorhas been developed to collocate GRUAN radiosonde profiles and NWP modelfields, simulate top-of-atmosphere brightness temperature at frequenciesused by space-borne instruments, and propagate GRUAN uncertainties in thatsimulation. A mathematical framework used to estimate and assess theuncertainty budget of the comparison of simulated brightness temperature isalso proposed. A total of 1 year of GRUAN radiosondes and matching NWP fields from the Met Office andECMWF have been processed and analysed for the purposes of demonstration ofcapability. We present preliminary results confirming the presence of knownbiases in the temperature and humidity profiles of both NWP centres. Thenight-time difference between GRUAN and Met Office (ECMWF) simulatedbrightness temperature at microwave frequencies predominantly sensitive totemperature is on average smaller than 0.1 K (0.4 K). Similarly, thisdifference is on average smaller than 0.5 K (0.4 K) at microwavefrequencies predominantly sensitive to humidity. The uncertainty estimated for the Met Office–GRUAN difference ranges from0.08 to 0.13 K for temperature-sensitive frequencies and from 1.6 to 2.5 Kfor humidity-sensitive frequencies. From the analysed sampling, 90 % ofthe comparisons are found to be in statistical agreement. This initial study has the potential to be extended to a larger collectionof GRUAN profiles, covering multiple sites and years, with the aim ofproviding a robust estimation of both errors and uncertainties of NWP modelfields in radiance space for a selection of key microwave and infraredfrequencies.

Using reference radiosondes to characterise NWP model uncertainty for improved satellite calibration and validation