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Abstract. The top-of-atmosphere (TOA) outgoing longwave flux over the 9.6  µm ozone band is a fundamental quantity for understanding chemistry–climatecoupling. However, observed TOA fluxes are hard to estimate as they exhibitconsiderable variability in space and time that depend on the distributionsof clouds, ozone ( O3 ), water vapor ( H2O ), air temperature( Ta ), and surface temperature ( Ts ). Benchmarking present-dayfluxes and quantifying the relative influence of their drivers is the firststep for estimating climate feedbacks from ozone radiative forcing andpredicting radiative forcing evolution. To that end, we constructed observational instantaneous radiative kernels(IRKs) under clear-sky conditions, representing the sensitivities of the TOAflux in the 9.6  µm ozone band to the vertical distribution ofgeophysical variables, including O3 , H2O , Ta , and Ts based upon the Aura Tropospheric Emission Spectrometer (TES) measurements.Applying these kernels to present-day simulations from the Chemistry-ClimateModel Initiative (CCMI) project as compared to a 2006 reanalysisassimilating satellite observations, we show that the models have largedifferences in TOA flux, attributable to different geophysical variables. Inparticular, model simulations continue to diverge from observations in thetropics, as reported in previous studies of the Atmospheric ChemistryClimate Model Intercomparison Project (ACCMIP) simulations. The principalculprits are tropical middle and upper tropospheric ozone followed by tropicallower tropospheric H2O . Five models out of the eight studied here haveTOA flux biases exceeding 100 mW m −2 attributable to tropospheric ozonebias. Another set of five models have flux biases over 50 mW m −2 dueto H2O . On the other hand, Ta radiative bias is negligible in allmodels (no more than 30 mW m −2 ). We found that the atmospheric component (AM3) of the Geophysical Fluid Dynamics Laboratory (GFDL) general circulation model and Canadian Middle Atmosphere Model (CMAM) have thelowest TOA flux biases globally but are a result of cancellation of oppositebiases due to different processes. Overall, the multi-model ensemble meanbias is - 133 ± 98  mW m −2 , indicating that they are tooatmospherically opaque due to trapping too much radiation in the atmosphereby overestimated tropical tropospheric O3 and H2O . Having too much O3 and H2O in the troposphere would have different impacts on thesensitivity of TOA flux to O3 and these competing effects add moreuncertainties on the ozone radiative forcing. We find that the inter-modelTOA outgoing longwave radiation (OLR) difference is well anti-correlatedwith their ozone band flux bias. This suggests that there is significantradiative compensation in the calculation of model outgoing longwaveradiation.

Attribution of Chemistry-Climate Model Initiative (CCMI) ozone radiative flux bias from satellites