On the use of IASI spectrally resolved radiances to test the EC-Earth climate model (v3.3.3) in clear-sky conditions

Abstract. The long-term comparison between simulated and observed spectrally resolved radiances can represent a stringent test for the direct verification and improvement of General Circulation Models (GCMs). From the mid of 2000s, stable hyperspectral observations of the Mid-Infrared region (667 to 2750 cm-1) the Earth emission spectrum have been provided by different sensors (e.g., AIRS, IASI and CrIS). In addition, the FORUM mission, selected to be the ninth ESA Earth Explorer mission, will measure, starting from 2027, the terrestrial radiation emitted to space at the top of the atmosphere (TOA) from 100 to 1600 cm-1 filling the observational gap in the far-infrared (FIR) region, from 100 to 667 cm-1. In this work, in anticipation of FORUM measurements, we compare existing IASI observations to radiances simulated on the basis of the atmospheric fields predicted by the EC-Earth GCM (version 3.3.3) in clear-sky conditions. In order to simulate spectra based on the atmospheric and surface state provided by the climate model, the radiative transfer model σ-IASI has been implemented in the Cloud Feedback Model Intercomparison Project (COSP) package. Therefore, on-line simulations provided by EC-Earth model equipped with the new COSP + σ-IASI module have been performed in clear-sky conditions with prescribed sea surface temperature and sea-ice cover, every 6 hours, over a timeframe consistent with the availability of IASI data. Systematic comparisons between observed IASI MetOp-A L1C data and model outputs have been performed in 10 cm-1 spectral intervals, on global and regional scales, by distinguishing the surface type (land, sea). The long term analysis shows a warm bias of the climate model in the roto-vibrational water vapour bands and in the CO2 absorption band. These biases represent a strong evidence of a temperature bias of the model in the upper-troposphere and in the stratosphere, while a cold bias occurs over land.