A case study on microphysical and radiative properties of power-plant-originated clouds Andreas Keil * , Manfred Wendisch, Jost Heintzenberg Institute for Tropospheric Research e.V., Permoser-Str. 15, 04318 Leipzig, Germany Received 19 December 2001; received in revised form 10 April 2002; accepted 16 April 2002 Abstract Cloud microphysical characteristics and solar cloud albedo of a stratiform cloud field downwind a coal-fired power plant and of naturally formed stratiform continental clouds are compared. In situ measurements of aerosol, drop microphysical, and solar radiative parameters of both cloud types are complemented by one- and three-dimensional radiative transfer calculations. The aerosol particle concentrations in and around the power-plant-originated clouds were significantly enhanced and the cloud drop concentrations were nearly doubled in comparison with the naturally formed clouds, whereas the effective cloud drop radius remained roughly unchanged. The radiation measurements as well as the modeling revealed a higher solar albedo of the power-plant-originated cloud compared to the surrounding natural clouds. Even when high amounts of absorbing material were assumed in the drops of the power plant cloud, the calculations showed a higher albedo of this cloud. The possibility that the power plant’s water vapor emissions significantly alter the cloud microphysics downwind the power plant is discussed. D 2002 Published by Elsevier Science B.V. Keywords: Polluted clouds; Aerosol particles; Power plant; Twomey effect 1. Introduction One of the largest uncertainties in the impact of anthropogenic aerosol particles on the Earth’s radiation budget is associated with aerosol – cloud – radiation interactions and the related ‘indirect effects’ (Haywood and Boucher, 2000; IPCC, 2001). The first indirect effect, the so-called ‘Twomey effect’ (Twomey, 1977), describes an increase in solar cloud 0169-8095/02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII:S0169-8095(02)00040-6 * Corresponding author. Met Office, Cody Technology Park, Ively Road, Farnborough, Hampshire GU14 0LX, England, UK. Tel.: +44-1252-395402; fax: +44-1252-376588. E-mail address: andreas.keil@metoffice.com (A. Keil). www.elsevier.com/locate/atmos Atmospheric Research 63 (2002) 291 – 301