Cloud Responses to Abrupt Solar and CO2 Forcing Part II: Adjustment to
Forcing in Coupled Models
Abstract
In this paper we examine differences in cloud adjustments (often called
rapid adjustments) that occur as a direct result of abruptly increasing
the solar constant by 4% or abruptly quadrupling of atmospheric CO2. In
doing so, we devised a novel method for calculating the cloud
adjustments for the abrupt solar forcing experiment that uses
differences between coupled model simulations with abrupt solar and CO2
forcing, in combination with uncoupled, atmosphere-only, abrupt CO2
forced experiments that have prescribed sea-surface temperature. Our
main findings are that 1) there are substantial differences in the
response of stratocumulus and cumulus clouds to solar and CO2 forcing,
which follow the differences in the direct radiative effect that solar
and CO2 forcing have at cloud top, and 2) there are differences in the
adjustment of the average optical depth of high clouds to solar and CO2
forcing that we speculate are driven by the differences in the vertical
profile of radiative heating, and differences in the pattern of
sea-surface temperature change (for a fixed global mean temperature).
Such adjustments do contribute significantly to the total net cloud
radiative effect, even after 150 years of simulation.