Articles | Volume 9, issue 22
Atmos. Chem. Phys., 9, 8841–8856, 2009
Atmos. Chem. Phys., 9, 8841–8856, 2009

  23 Nov 2009

23 Nov 2009

Effect of biomass burning on marine stratocumulus clouds off the California coast

J. Brioude1,2, O. R. Cooper1,2, G. Feingold2, M. Trainer2, S. R. Freitas3, D. Kowal4, J.K. Ayers5, E. Prins6, P. Minnis7, S. A. McKeen1,2, G. J. Frost1,2, and E.-Y. Hsie1,2 J. Brioude et al.
  • 1Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA
  • 2Chemical Sciences Division, Earth System Research Lab., NOAA, Boulder, Colorado, USA
  • 3Center for Weather Forecasting and Climate Studies, INPE, Cachoeira Paulista, Brazil
  • 4National Geophysical Data Center, NESDIS, NOAA, USA
  • 5Science Systems and Applications Incorporated, Hampton Virginia, USA
  • 6UW-Madison SSEC/CIMSS – Consultant, Grass Valley, CA, USA
  • 7NASA Langley Research Center, Hampton, Virginia, USA

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.

Final-revised paper