Appendix A.   Proposal for Radiative Forcing Calculations (attached to a letter from the IPCC co-chairs)

1. Online Approximation to Forcing:

The objective is to approximate radiative forcing for the major radiatively active species using online calculations during model integrations.  The models should be configured to save monthly mean net shortwave  and longwave fluxes at the surface, top of atmosphere (TOA), and 200mb. The net fluxes at 200mb will be used as surrogates for the net fluxes at the tropopause.  The archived fluxes should correspond to the radiative heating rates interacting with the remainder of the model physics.

At 10-year intervals during the integration, for a period of one year, a second set of diagnostic radiative calculations should be performed using the same state information as the interactive radiative calculations.  The differences between the diagnostic and interactive radiative fluxes will provide estimates of the radiative forcing.  The diagnostic calculations should include (1) a calculation with all radiatively active trace gases and sulfate set to the same values as the 19th century control integrations; and (2) separate calculations for each species at current concentrations with the other species held at 19th century values.

2. Reference Calculations:

A separate computation is necessary to quantify the differences between these approximate forcing estimates and the standard definition of forcing with the tropospheric state held fixed and the stratosphere adjusted using fixed dynamical heating.  An additional set of diagnostic calculations should be performed at the same decadal interval for one-year periods. In this second set, the tropospheric state parameters should be set to values from a monthly mean climatology derived from the 19th century control integration.  The stratosphere  should be adjusted via the fixed dynamical heating derived from the control integration.  Net fluxes should be archived at the surface, TOA, 200mb, and model tropopause for all species reset to control values and for each individual species set to current values.  In addition, the net fluxes at the model tropopause should be computed using current model state information and concentrations of radiatively active species.  These calculations can be performed either during the model integration or using offline RT codes with complete state information archived from the model.