Influence of aerosol particles from biomass burning on cloud microphysical properties and radiative forcing

Abstract

Aerosol from biomass burning has been shown to strongly modify cloud microphysical properties and cloud lifetime through the so-called “indirect effect.” However, in the case of a lack of wet scavenging, it stays suspended for days to weeks and can be transported to considerable distances within an elevated layer above low-level cloud tops with minimal aerosol–cloud interactions. The observations carried out during the Southern African Regional Science Initiative (SAFARI) 2000 dry season field campaign often revealed the presence of an elevated biomass-burning aerosol layer above a semi-permanent stratiform cloud deck off the southern African coasts. MODerate-resolution Imaging Spectroradiometer (MODIS) cloud products were used to investigate the existence of an aerosol indirect effect on convective clouds. Results are presented documenting cloud effective radius and cloud radiative forcing variations due to the presence of the aerosol during the development of convective clouds. Radiative transfer simulations in the visible (0.8 μm, VIS) and near-infrared (1.6, 2.1 and 3.7 μm, NIR) wavelengths were instrumental in establishing the extent of the influence of a biomass-burning aerosol layer overlying a water cloud sheet on the MODIS satellite retrieval of cloud parameters, in particular the effective radius and the optical thickness. The radiative transfer simulations suggest that the presence of the aerosol induces a significant underestimation of the cloud optical thickness, whereas an underestimation of the retrieved effective radius is more pronounced in the retrieval that makes use of the 1.6 μm waveband than the 2.1 and 3.7 μm wavebands. The MODIS cloud products of 3 days of the SAFARI 2000 campaign were analyzed to determine whether the aerosol induced biases evidenced by the simulations also affect the operational cloud property retrieval. Cloud parameters, in particular the effective radius, are usually employed as indicators of the occurrence of aerosol–cloud interaction according to the “indirect effect.” However, these results highlight some of the difficulties associated with satellite retrievals of cloud properties and show the importance of an accurate sighting of the cloud and aerosol layer top and bottom heights in order to prevent erroneous detections of indirect effects.