Comparison of satellite-based instantaneous radiative forcing of tropospheric ozone to four global chemistry climate models

Radiative forcing is the perturbation to the Earth's radiation balance from changes in an atmospheric constituent. Ozone is a radiatively significant atmospheric species through its absorption of both shortwave and longwave radiation. Driven primarily by longwave absorption in the troposphere, ozone exerts the third largest positive radiative forcing, following carbon dioxide and methane, with an estimate of 0.35 W/m2 (0.25 - 0.65 W/m2) according to the fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC AR4), Climate Change 2007. We use the spectrally resolved outgoing longwave radiation sensitivity to tropospheric ozone from the Tropospheric Emissions Spectrometer (TES) measurements to calculate the three-dimensional instantaneous radiative forcing (IRF) kernel under all-sky conditions in August 2006. These kernels were applied to ozone concentration calculated by four different state-of-the-art global chemistry-climate models, which are AM2-Chem, CAM-Chem, ECHAM5-MOZ and G-PUCCINI, to provide an observation-based estimate of model IRF. Primary differences in IRF are due to varying estimate of sub-tropical and upper tropospheric ozone concentration, leading to model tropospheric IRF bias of between -0.4 to +0.7 W/m2. We show that the multi-model ensemble average IRF values are closest to TES estimate.