Climate evolution in the spectral signatures of simulated and observed radiances

Since the mid-2000s, stable hyperspectral observations of the mid-infrared (MIR) region (667 to 2750 cm-1) of the Earth's emitted radiance have been provided by different space-based sensors (IASI, AIRS, etc.) producing a long-term dataset that has proven to be crucial for climatological studies. In addition, the FORUM mission, whose launch will occur in 2027, will provide unique measurements of the far-infrared (FIR) region (100 to 667 cm-1), filling the missing portion of the Earth's emission spectrum seen from space. Since these measurements contain the spectral signatures of temperature, water vapour, clouds and gases concentration, they can be exploited to strictly test General Circulation Models (GCMs) and constrain the parametrizations of sub-grid-process. In perspective, the long-term sequence of observations from IASI and FORUM will allow the study of spectral climate feedbacks on both inter-annual and decadal timescales, adding further constraints for climate model evaluation. In this work, 9 years (2008-2016) of IASI Metop-A measurements are compared to simulated spectral radiances provided by the EC-Earth GCM (ECE, version 3.3.3) based on the atmospheric and surface fields predicted in all-sky conditions by the model. An innovative strategy is adopted to take into account the cloud variability within the large model grid cell (roughly 80-km grid spacing near the equator) and to optimally compare the climate model outputs with the higher spatial resolution (about 15 km of diameter) observations performed by the instrument. The spectral radiances are simulated online every 3 hours by the ?-IASI radiative transfer model, that has been previously embedded in the climate model through the Cloud Feedback Model Intercomparison Project (COSP) module. The comparison is performed on both low-resolution bands, between 190 to 2500 cm-1, and on selected high-resolution channels, that mimic IASI and FORUM observation in the MIR and in the FIR regions. The comparison between simulated and observed spectral trends in different channels allows for tracking the evolution of key climate variables and to study the driving mechanisms both in the real world and in the simulated climate