Strong Buffering of Aerosol–Cloud Interactions by Cloud Dynamics in Climate Models (Invited Presentation)

Studies with large-eddy simulations have suggested the possibility of strong
buffering of aerosol-induced increases in liquid water path by cloud
dynamics. In particular, changes in sedimentation near cloud top and
increased evaporative cooling associated with entrainment can reduce water
paths, opposing aerosol-related increases in water path due to changes in the
precipitation process.

Understanding the implications of these buffering processes for climate
forcing due to aerosol-cloud interactions has been a challenge. The scales
on which these processes occur are smaller than those resolved by climate
models. A goal for cloud and aerosol parameterizations in climate models is
to realistically represent these small-scale interactions among aerosols,
microphysics, and dynamics.

Reductions in water path as aerosols increase occur in some synoptic settings
in a version of the GFDL global atmospheric model AM3 in which boundary-layer clouds are parameterized
using the assumed-distribution higher-order closure CLUBB. The behavior
occurs when the air overlying the clouds has low humidity, consistent with
the explanation the buffering involves entrainment and associated
evaporation.