Atmos. Chem. Phys. Discuss., 10, 18635-18659, 2010
© Author(s) 2010. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Stratocumulus cloud thickening beneath layers of absorbing smoke aerosol
E. M. Wilcox
1Desert Research Institute, Reno, Nevada, USA

Abstract. 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 altitude with double the frequency of occurrence of aerosol features in the boundary layer. 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 1.5 K d−1 for an aerosol optical thickness value of 1, and increases to 1.8 K d−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 g m−2 and cloud tops that are lower by approximately 50 m for overcast conditions compared to smoke-free periods. 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 greater liquid water path for cases of smoke overlaying cloud implies a negative semi-direct radiative forcing of regional climate in locations such as the southeast Atlantic Ocean where absorbing aerosol layers frequently exist above persistent stratus cloud decks.

Citation: Wilcox, E. M.: Stratocumulus cloud thickening beneath layers of absorbing smoke aerosol, Atmos. Chem. Phys. Discuss., 10, 18635-18659, doi:10.5194/acpd-10-18635-2010, 2010.
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