Attribution of Chemistry-Climate Model Initiative (CCMI) ozone radiative flux bias from satellites
Authors: Kuai Le , Bowman Kevin W. , Miyazaki Kazuyuki , Deushi Makoto , Revell Laura , Rozanov Eugene , Paulot Fabien , Strode Sarah , Conley Andrew , Lamarque Jean-François , Jöckel Patrick , Plummer David A. , Oman Luke D. , Worden Helen , Kulawik Susan , Paynter David , Stenke Andrea , Kunze Markus 
Published: January 2020

Abstract. The top-of-atmosphere (TOA) outgoing longwave flux over the 9.6 µm
ozone band is a fundamental quantity for understanding chemistry–climate
coupling. However, observed TOA fluxes are hard to estimate as they exhibit
considerable variability in space and time that depend on the distributions
of clouds, ozone (O3), water vapor (H2O), air temperature
(Ta), and surface temperature (Ts). Benchmarking present-day
fluxes and quantifying the relative influence of their drivers is the first
step for estimating climate feedbacks from ozone radiative forcing and
predicting radiative forcing evolution. To that end, we constructed observational instantaneous radiative kernels
(IRKs) under clear-sky conditions, representing the sensitivities of the TOA
flux in the 9.6 µm ozone band to the vertical distribution of
geophysical 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-Climate
Model Initiative (CCMI) project as compared to a 2006 reanalysis
assimilating satellite observations, we show that the models have large
differences in TOA flux, attributable to different geophysical variables. In
particular, model simulations continue to diverge from observations in the
tropics, as reported in previous studies of the Atmospheric Chemistry
Climate Model Intercomparison Project (ACCMIP) simulations. The principal
culprits are tropical middle and upper tropospheric ozone followed by tropical
lower tropospheric H2O. Five models out of the eight studied here have
TOA flux biases exceeding 100 mW m−2 attributable to tropospheric ozone
bias. Another set of five models have flux biases over 50 mW m−2 due
to H2O. On the other hand, Ta radiative bias is negligible in all
models (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 the
lowest TOA flux biases globally but are a result of cancellation of opposite
biases due to different processes. Overall, the multi-model ensemble mean
bias is -133±98 mW m−2, indicating that they are too
atmospherically opaque due to trapping too much radiation in the atmosphere
by overestimated tropical tropospheric O3 and H2O. Having too much
O3 and H2O in the troposphere would have different impacts on the
sensitivity of TOA flux to O3 and these competing effects add more
uncertainties on the ozone radiative forcing. We find that the inter-model
TOA outgoing longwave radiation (OLR) difference is well anti-correlated
with their ozone band flux bias. This suggests that there is significant
radiative compensation in the calculation of model outgoing longwave
radiation.