Journal article

Modelling microphysical and meteorological controls on precipitation and cloud cellular structures in Southeast Pacific stratocumulus

Wang H, Feingold G, Wood R, Kazil J ...see all

Atmospheric Chemistry and Physics, vol. 10, issue 13 (2010) pp. 6347-6362

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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(-1); -1 K) than to a five-fold
decrease in aerosol number concentration (150 vs. 30 mg(-1)). When
embedding the perturbations in closed cells, local drizzle and pockets
of open cell (POC) formation respond faster to the aerosol reduction
than to the moisture increase, but the latter generates stronger and
more persistent drizzle. A 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 order 1
mg(-1) h(-1) is sufficient to maintain clouds and prevent the boundary
layer from collapsing the following day, suggesting that some local
and/or remote aerosol sources is necessary for POCs to be able to last
for days.

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  • H. Wang

  • G. Feingold

  • R. Wood

  • J. Kazil

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