Atmospheric Chemistry and Physics Discussions
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10.5194/acpd-10-8341-2010
http://www.atmos-chem-phys-discuss.net/10/8341/2010/
http://www.atmos-chem-phys-discuss.net/10/8341/2010/acpd-10-8341-2010.html
http://www.atmos-chem-phys-discuss.net/10/8341/2010/acpd-10-8341-2010.pdf
8341
8378
2010-03-31
Precipitation and cloud cellular structures in marine stratocumulus over the southeast pacific: model simulations
H. Wang
hailong.wang@pnl.gov
G. Feingold
R. Wood
J. Kazil
Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, Colorado, USA
NOAA Earth System Research Laboratory (ESRL), Boulder, Colorado, USA
Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA
now at: Pacific Northwest National Laboratory (PNNL), Richland, Washington, USA
Microphysical and meteorological controls on the formation of open and
closed cellular structures in the Southeast Pacific are explored using model
simulations based on aircraft observations during the VAMOS
Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx). The
effectiveness of factors such as boundary-layer moisture and temperature
perturbations, surface heat and moisture fluxes, large-scale vertical motion
and solar heating in promoting drizzle and open cell formation for
prescribed aerosol number concentrations is explored. For the case
considered, drizzle and subsequent open cell formation over a broad region
are more sensitive to the observed boundary-layer moisture and temperature
perturbations (=0.9 g kg<sup>−1</sup>; −1 K) than to a five-fold decrease in
aerosol number concentrations (150 vs. 30 mg<sup>−1</sup>). When embedding the
perturbations in closed cells, local drizzle and pockets of open cells
(POCs) formation respond faster to the aerosol reduction than to the
moisture increase, but the latter generate stronger and more persistent
drizzle. The local negative perturbation in temperature drives a mesoscale
circulation that prevents local drizzle formation but promotes it in a
remote area where lower-level horizontal transport of moisture is blocked
and converges to enhance liquid water path. This represents a potential
mechanism for POC formation in the Southeast Pacific stratocumulus region
whereby the circulation is triggered by strong precipitation in adjacent
broad regions of open cells. A simulation that attempts to mimic the
influence of a coastally induced upsidence wave results in an increase in cloud
water but this alone is insufficient to initiate drizzle. An increase of
surface sensible heat flux is also effective in triggering local drizzle and
POC formation.
Both open and closed cells simulated with observed initial conditions
exhibit distinct diurnal variations in cloud properties. A stratocumulus
deck that breaks up due solely to solar heating can recover at night.
Precipitation in the open-cell cases depletes the aerosol to the extent that
cloud formation is significantly suppressed within one diurnal cycle. A
replenishment rate of cloud condensation nuclei of 0.72 mg<sup>−1</sup> h<sup>−1</sup>
is sufficient to maintain clouds and prevent the boundary layer from
collapsing the following day, suggesting that some local and/or remote
aerosol sources are necessary for POCs to be able to last for days.
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