Three-dimensional simulations of a spring time Arctic boundary layer
cloud observed during the FIRE/SHEBA 1998 spring IOP were conducted
to study the influence of entrainment of cloud condensation nuclei (CCN)
at cloud top on cloud microphysical and dynamical structure, radiative
properties, and cloud evolution. The model is a large-eddy version of
the Regional Atmospheric Modeling System (RAMS) with explicit
representation of the CCN spectrum and cloud droplet spectrum.
Results show that droplet concentrations increase about two-fold, effective
radii decrease by 25 - 30%, liquid water content increases about
21%, and no drizzle reaches the ground when the air containing higher CCN
concentration at the inversion is entrained into the cloudy boundary layer
in comparison with the simulation using a CCN profile of smaller constant
value. More vigorous eddies are attributed to the significant dynamical
response of the simulated cloud to the increased CCN concentration due to
entrainment. The response of the cloud optical properties to entrainment
occurs from the begining of the simulations. Cloud albedo shows a 12%
increase, while cloud optical depth has a 33% increase due to
the entrainment of higher CCN concentrations. These results are
consistent with both observations and modeling studies. Thus entrainment
alters a drizzling case to a nondrizzling case and helps sustain higher
liquid water path, more persistent, and more reflective clouds, provided the
entrainment is moderate.
