Abstract
Abstract. Aerosol-cloud interactions are considered to be one of the most important and least known forcings in the climate system. Biomass burning aerosols are of special interest due to their radiative impact (direct and indirect effect) and their potential to increase in the future due to climate change. Combining data from Geostationary Operational Environmental Satellite (GOES) and MODerate-resolution Imaging Spectroradiometer (MODIS) with passive tracers from the FLEXPART Lagrangian Particle Dispersion Model, the impact of biomass burning aerosols on marine stratocumulus clouds has been examined in June and July of 2006–2008 off the California coast. Using a continental tracer, the indirect effect of biomass burning aerosols has been isolated by comparing the average cloud fraction and cloud albedo for different meteorological situations, and for clean versus polluted (in terms of biomass burning) continental air masses at 14:00 local time. Within a 500 km-wide band along the coast of California, biomass burning aerosols, which tend to reside above the marine boundary layer, increased the cloud fraction by 0.143, and the cloud albedo by 0.038. Absorbing aerosols located above the marine boundary layer lead to an increase of the lower tropospheric stability and a reduction in the vertical entrainment of dry air from above, leading to increased cloud formation. The combined effect was an indirect radiative forcing of −7.5% ±1.7% (cooling effect) of the outgoing radiative flux at the top of the atmosphere on average, with a bias due to meteorology of +0.9%. Further away from the coast, the biomass burning aerosols, which were located within the boundary layer, reduced the cloud fraction by 0.023 and the cloud albedo by 0.006, resulting in an indirect radiative forcing of +1.3% ±0.3% (warming effect) with a bias of +0.5%. These results underscore the dual role that absorbing aerosols play in cloud radiative forcing.
Highlights
Aerosol-cloud interactions are considered to be one of the most important and least known forcings in the climate system (IPCC, 2007)
The results show that modeled biomass burning (BB) aerosol concentrations can be applied to study the overall impact of BB aerosols on marine stratocumulus clouds
The multivariate regression is applied to the average me- fraction and cloud albedo due to the meteorologteorological values found in the presence or absence of BB ical parameters and non-BB aerosol burden
Summary
Aerosol-cloud interactions are considered to be one of the most important and least known forcings in the climate system (IPCC, 2007). Mauger et al (2007) have shown that both AOD and cloud fraction are correlated with static stability Another weakness is due to the fact that the aerosol vertical distribution is usually unknown, while it is a key component of the aerosol indirect effect (Johnson et al, 2004). Previous studies (Schwartz et al, 2002; Chameides et al, 2002) have used column-integrated anthropogenic aerosol burden from chemical transport models with satellite derived cloud products to study anthropogenic aerosol indirect effects. This is very important because the weather conditions that transport continental air across the eastern Pacific affect cloud formation differently from marine air masses To our knowledge, this is the first time that such a continental tracer is used to study the aerosol indirect effect by comparing cloud properties for conditions with and without BB aerosols, but when the study region is influenced by continental air masses.
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