A Negative Lifetime Effect in Large-Eddy Simulations of Trade-Wind Cumulus Clouds

Abstract:

Large-eddy simulation is used to study trade-wind cumulus clouds and their
sensitivity to perturbations in cloud droplet number concentrations. We find
that the trade wind cumulus system approaches a radiative-convective
equilibrium state, modified by net warming and drying from imposed large-scale
advective forcing. The system requires several days to reach equilibrium when
cooling rates are specified but much less time, and with less sensitivity to
cloud droplet number density, when radiation depends realistically on the
vertical distribution of water vapor. The transient behavior and the
properties of the near-equilibrium cloud field depends on the microphysical
state and therefore on the cloud droplet number density, here taken as a proxy
for the ambient aerosol. The primary response of the cloud field to changes in
cloud droplet number density is a deepening of the cloud layer. This deepening
leads to a decrease in relative humidity in most of the trade wind boundary
layer. As a consequence small clouds and cloud remnants evaporate faster in an
environment with high aerosol load than in the undisturbed control case
constituting a negative lifetime effect. This negative lifetime effect is
confirmed by means of cloud tracking diagnostics in the LES. In the
near-equilibrium regime the decrease in cloud cover associated with the
negative lifetime effect compensates much of the Twomey effect, i.e., the
brightening of the clouds, and the overall aerosol effect on the albedo of the
organized precipitating cumulus cloud field is small. The figure summarizes
this conceptual model of the revised cloud lifetime effect.