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Journal article

Stratocumulus cloud thickening beneath layers of absorbing smoke aerosol

Wilcox E ...see all

Atmospheric Chemistry and Physics, vol. 10, issue 23 (2010) pp. 11769-11777

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Marine stratocumulus cloud properties, and the free-tropospheric
environment above them, are examined in NASA A-Train satellite data for
cases where smoke from seasonal burning of the West African savannah
overlay the persistent southeast Atlantic stratocumulus cloud deck.
CALIPSO space-borne lidar observations show that features identified as
layers of aerosol occur predominantly between 2 km and 4 km. Layers
identified as cloud features occur predominantly below 1.5 km altitude
and beneath the layer of elevated smoke aerosol. The diurnal mean
shortwave heating rates attributable to the absorption of solar energy
in the aerosol layer is nearly 1.5Kd(-1) for an aerosol optical
thickness value of 1, and increases to 1.8Kd(-1) when the smoke resides
above clouds owing to the additional component of upward solar radiation
reflected by the cloud. As a consequence of this heating, the 700 hPa
air temperature above the cloud deck is warmer by approximately 1 K on
average for cases where smoke is present above the cloud compared to
cases without smoke above cloud. The warmer conditions in the
free-troposphere above the cloud during smoke events coincide with cloud
liquid water path values that are greater by 20 gm(-2) and cloud tops
that are lower for overcast conditions compared to periods with low
amounts of smoke. The observed thickening and subsidence of the cloud
layer are consistent with published results of large-eddy simulations
showing that solar absorption by smoke above stratocumulus clouds
increases the buoyancy of free-tropospheric air above the temperature
inversion capping the boundary layer. Increased buoyancy inhibits the
entrainment of dry air through the cloud-top, thereby helping to
preserve humidity and cloud cover in the boundary layer. The direct
radiative effect of absorbing aerosols residing over a bright cloud deck
is a positive radiative forcing (warming) at the top of the atmosphere.
However, the greater liquid water path for cases of smoke overlaying
cloud contributes an additional negative semi-direct radiative forcing
(cooling) of climate in locations such as the southeast Atlantic Ocean
owing to the enhanced albedo of the thicker cloud.

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  • E. M. Wilcox

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