On the structure of instantaneous radiative forcing kernels for greenhouse gases

The SOCRATES offline radiative transfer code is used to investigate the magnitude and structure of the instantaneous radiative forcing kernels (IRFKs) for five major greenhouse gases (GHGs; CO2, CH4, N2O, CFC-11, and O3). All gases produce IRFKs that peak in the tropical upper troposphere. In addition to differences in spectroscopic intensities and the position of absorption features relative to the peak of the Planck function for Earth’s temperature, the variation in current background concentration of gases substantially affects the IRFK magnitudes. When the background concentration of CO2 is reduced from parts per million to parts per trillion levels, the peak magnitude of the IRFK increases by a factor of 642. When all gases are set to parts per trillion concentrations in the troposphere, the peak IRFK magnitudes are 1.0, 3.0, 3.1, 58 and 75 Wm−2 ppmv−1 100 hPa−1 for CH4, CO2, N2O, O3 and CFC-11, respectively. The altitude of the IRFK maximum also differs, with the maximum for CFC-11 and water vapour occurring above 100 hPa while the other gases peak near 150-200 hPa. Overlap with water vapour absorption decreases the magnitude of the IRFKs for all the GHGs, particularly in the low-to-mid troposphere, but it does not strongly affect the peak IRFK altitude. Cloud radiative effects reduce the magnitude of the IRFK for CO2 by around 10-20% in the upper troposphere. The use of IRFKs to estimate IRF is found to be accurate for small amplitude perturbations, but becomes inaccurate for large amplitude changes (e.g. a doubling) for gases with a higher atmospheric optical depth.