The GFDL Longwave Radiation Algorithm: Improvements and Prospects

Abstract:

The GFDL longwave radiation algorithm has been extensively modified during the last two years. The most significant changes have been the incorporation of the 10 μm CO2 band; the use of the mt_ckd 2.5 continuum for water vapor; and the adoption of the HITRAN 2012 catalogue for all line and absorption coefficients. Further changes include inclusion of the CO2 4.3 μm band and extension of the calculation range to 3000 cm-1. The effect of the CO2 10 μm band is a ~0.4 W/m2 change in the surface longwave forcing, which leads to a ~10% change in CO2 radiative forcing. The adoption of the HITRAN 2012 catalog generally leads to an increase in downward flux at the surface. The mt_ckd 2.5 continuum replaces the previously used CKD 2.1 continuum; the effects are a decrease in downward flux in the upper troposphere (due to the foreign-broadened continuum in the 16-29 μm region) and an increase in upward flux in the lower troposphere (in most atmospheres).

The GFDL line-by-line algorithm calculation has correspondingly been modified to employ the mt_ckd 2.5 continuum for all species; further, the self-broadening of lines and line shifting effects are now included.

Using CIRC profiles, a comparison of the current results with the previous parameterization, with the revised GFDL line-by-line algorithm and with LBLRTM indicate significant reduction of flux and heating rate errors in most cases.

Future changes include inclusion of a nonzero surface emissivity, especially in the 500-1200 cm-1 range. It is contemplated that further developments may include use of a band model, exponential sum-fit or correlated-k method (as in the RRTM-g algorithm) for computation of water vapor.