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Stratocumulus cloud thickening beneath layers of absorbing smoke aerosol

by E. M. Wilcox
Atmospheric Chemistry and Physics ()
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Abstract

Marine stratocumulus cloud properties, and the free-tropospheric\nenvironment above them, are examined in NASA A-Train satellite data for\ncases where smoke from seasonal burning of the West African savannah\noverlay the persistent southeast Atlantic stratocumulus cloud deck.\nCALIPSO space-borne lidar observations show that features identified as\nlayers of aerosol occur predominantly between 2 km and 4 km. Layers\nidentified as cloud features occur predominantly below 1.5 km altitude\nand beneath the layer of elevated smoke aerosol. The diurnal mean\nshortwave heating rates attributable to the absorption of solar energy\nin the aerosol layer is nearly 1.5Kd(-1) for an aerosol optical\nthickness value of 1, and increases to 1.8Kd(-1) when the smoke resides\nabove clouds owing to the additional component of upward solar radiation\nreflected by the cloud. As a consequence of this heating, the 700 hPa\nair temperature above the cloud deck is warmer by approximately 1 K on\naverage for cases where smoke is present above the cloud compared to\ncases without smoke above cloud. The warmer conditions in the\nfree-troposphere above the cloud during smoke events coincide with cloud\nliquid water path values that are greater by 20 gm(-2) and cloud tops\nthat are lower for overcast conditions compared to periods with low\namounts of smoke. The observed thickening and subsidence of the cloud\nlayer are consistent with published results of large-eddy simulations\nshowing that solar absorption by smoke above stratocumulus clouds\nincreases the buoyancy of free-tropospheric air above the temperature\ninversion capping the boundary layer. Increased buoyancy inhibits the\nentrainment of dry air through the cloud-top, thereby helping to\npreserve humidity and cloud cover in the boundary layer. The direct\nradiative effect of absorbing aerosols residing over a bright cloud deck\nis a positive radiative forcing (warming) at the top of the atmosphere.\nHowever, the greater liquid water path for cases of smoke overlaying\ncloud contributes an additional negative semi-direct radiative forcing\n(cooling) of climate in locations such as the southeast Atlantic Ocean\nowing to the enhanced albedo of the thicker cloud.

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