Satellite Observation and Radiative Modeling of the Influence of Aerosol Particles from Biomass Burning on Cloud Microphysiscal Properties

Various types of aerosols affect cloud microphysical properties and cloud lifetime through the so-called “indirect effect”. The decrease of particles size when the cloud is polluted by small aerosol particles serving as additional cloud condensation nuclei and the reduction of the efficiency of the precipitation forming processes are phenomena described in numerous recent studies (Bréon et al. 2002; Rosenfeld 1999, 2000). Biomass burning aerosol is considered highly responsible of cloud property modifications, due to the high frequency of extended forest and cropland fires all over the planet. In case of lack of wet scavenging biomass aerosol can reside in the atmosphere for time scales of days to weeks and may therefore be transported to considerable distances in an elevated (2-4 km) polluted layer (Keil and Haywood 2003). Cloudy scenarios are thus possible where the cloud layer is located below the aerosol layer and separated from it. The presence of the aerosol layer above the cloud deck can significantly affect the satellite retrieval of cloud properties (effective radius and optical thickness) and lead to an erroneous detection of the “indirect effect” (Haywood 2003). Radiative transfer simulations in the VIS, NIR and IR for a scenario characterized by a stratocumulus cloud deck with and without an overlying biomass burning aerosol layer are carried out. The idea is to evaluate the variations of the radiance data within the satellite channels due to the aerosol presence, and understand the eventual effects on the retrieval of the cloud particle effective radius and optical thickness. The vertical structure of the atmosphere in terms of cloud and aerosol layer top and bottom heights and the optical properties of the aerosol and other parameters useful for the description of the simulated scenario are those of the Southern African Regional Science Initiative (SAFARI 2000) (Swap et al. 2003), a multi-national measurement campaign that took place in Southern Africa in August and September 2000 during the fire season.