An assessment of the NASA/GEWEX SRB longwave release-3.0

The NASA/GEWEX Surface Radiation Budget project has produced an updated version of the longwave algorithm that includes improvements to the infrared radiative parameterization of ice clouds and also an updated version of the water vapor continuum. An error found in ozone profile values was corrected. Smaller changes include an updated surface vegetation type map and a monthly varying CO₂ concentration. In addition, a special version of the new algorithm that uses only the GEOS-4 values for surface skin temperature instead of the blended GEOS-4 and ISCCP values was developed for producing fluxes for a 10-year period (January 1998 - December 2007). This version was developed to assess the effect of changes in operational TOVS algorithm on October 1, 2001 that affected the surface skin temperature from the ISCCP project used in the standard version.

In this study, we compare monthly averages of surface and TOA fluxes for the 10-year period from both versions to CERES AVG, CERES SRB-AVG, ISCCP FD, and a previous version of the longwave algorithm. The new standard version has reduced the global average upward TOA flux by about 2 Wm^-2 from the previous version of the algorithm. The downward surface flux has increased by less than 1 Wm^-2. These changes are assessed relative to mean cloud and surface properties implied by the changes in the algorithm. The effect of using only GEOS-4 surface skin temperature (special version) has decreased the global average of upward surface flux by almost 1 Wm^-2 from the standard version. This decrease is larger in the tropics by almost 5 Wm^-2. The upward TOA and surface downward fluxes have smaller differences. This provides an initial assessment of the effect of TOVS algorithm change on fluxes. This assessment will have important repercussions for not only the interpretation of this Release of the data set but also for future Releases.