Abstract
Abstract. Observations from Earth observing satellites indicate that dark carbonaceous aerosols that absorb solar radiation are widespread in the tropics and subtropics. When these aerosols mix with clouds, there is generally a reduction of cloudiness owing to absorption of solar energy in the aerosol layer. Over the subtropical South Atlantic Ocean, where smoke from savannah burning in southern Africa resides above a persistent deck of marine stratocumulus clouds, radiative heating of the smoke layer leads to a thickening of the cloud layer. Here, satellite observations of the albedo of overcast scenes of 25 km2 size or larger are combined with additional satellite observations of clouds and aerosols to estimate the top-of-atmosphere direct radiative forcing attributable to presence of dark aerosol above bright cloud, and the negative semi-direct forcing attributable to the thickening of the cloud layer. The average positive direct radiative forcing by smoke over an overcast scene is 9.2±6.6 W m−2 for cases with an unambiguous signal of absorbing aerosol over cloud in passive ultraviolet remote sensing observations. However, cloud liquid water path is enhanced by 16.3±7.7 g m−2 across the range of values for sea surface temperature for cases of smoke over cloud. The negative radiative forcing associated with this semi-direct effect of smoke over clouds is estimated to be −5.9±3.5 W m−2. Therefore, the cooling associated with the semi-direct cloud thickening effect compensates for greater than 60 % of the direct radiative effect. Accounting for the frequency of occurrence of significant absorbing aerosol above overcast scenes leads to an estimate of the average direct forcing of 1.0±0.7 W m−2 contributed by these scenes averaged over the subtropical southeast Atlantic Ocean during austral winter. The regional average of the negative semi-direct forcing is −0.7±0.4 W m−2. Therefore, smoke aerosols overlaying the decks of overcast marine stratocumulus clouds considered here yield a small net positive radiative forcing, which results from the difference of two larger effects.
Highlights
Radiative forcing by aerosols owing to the scattering of shortwave solar radiation is presently offsetting a portion of the warming of climate attributable to the rise in atmospheric greenhouse gas concentrations (Forster et al, 2007)
The empirical relationship between cloud albedo and liquid water path (LWP) determined from satellite observations of overcast scenes is used in this study to estimate, separately, the direct radiative forcing of smoke above stratocumulus clouds assuming that the cloud layer is unchanged by the smoke, and the semidirect radiative forcing attributable to the cloud thickening reported in Wilcox (2010)
This cross-over point is near to the observed LWP value exhibiting the minimum difference between the albedo curves in Fig. 3 for different levels of Ozone Monitoring Instrument (OMI) aerosol index (AI), indicating that the satellite observations are reflecting the expected relationships between direct radiative forcing and LWP provided by the model calculations
Summary
Radiative forcing by aerosols owing to the scattering of shortwave solar radiation is presently offsetting a portion of the warming of climate attributable to the rise in atmospheric greenhouse gas concentrations (Forster et al, 2007). In cases where absorbing aerosols mix within a layer of low cloud, there is often a reduction in cloud cover yielding a positive radiative forcing of climate (Hansen et al, 1997; Ackerman et al, 2000; Kaufman and Koren, 2006) This so-called semi-direct forcing results from the heating of the mixed smoke-cloud layer owing to the absorption of solar radiation by the aerosols. The clouds beneath the smoke layer exhibit cloud liquid water path (LWP) amounts that are greater by 20 g m−2 and cloud tops that are lower compared to clouds without overlaying smoke These observations confirm the large-eddy simulation results obtained by Johnson et al (2004), where it is argued that warming by absorption of solar energy above the cloud increases the buoyancy of the air above the temperature inversion that caps the cloud-topped marine boundary layer.
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