Appendix A. Proposal for Radiative
Forcing Calculations (attached to a letter from the IPCC cochairs)
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 10year 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
oneyear 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.
