Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 9 1 2009 10.5194/acpd-9-3207-2009 http://www.atmos-chem-phys-discuss.net/9/3207/2009/ http://www.atmos-chem-phys-discuss.net/9/3207/2009/acpd-9-3207-2009.html http://www.atmos-chem-phys-discuss.net/9/3207/2009/acpd-9-3207-2009.pdf 3207 3241 2009-01-30 The relationship between aerosol and cloud drop number concentrations in a global aerosol microphysics model K. J. Pringle pringle@mpch-mainz.mpg.de K. S. Carslaw D. V. Spracklen G. M. Mann M. P. Chipperfield Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, UK now at: Max-Planck-Institute for Chemistry, Mainz, Germany Empirical relationships that link cloud droplet number (CDN) to aerosol number or mass are commonly used to calculate global fields of CDN for climate forcing assessments. In this work we use a sectional global model of sulfate and sea-salt aerosol coupled to a mechanistic aerosol activation scheme to explore the limitations of this approach. We find that a given aerosol number concentration produces a wide range of CDN concentrations due to variations in the shape of the aerosol size distribution. On a global scale, the dependence of CDN on the size distribution results in regional biases in predicted CDN (for a given aerosol number). Empirical relationships between aerosol number and CDN are often derived from regional data but applied to the entire globe. In an analogous process, we derive regional "correlation-relations" between aerosol number and CDN and apply these regional relations to calculations of CDN on the global scale. The global mean percentage error in CDN caused by using regionally derived CDN-aerosol relations is 20 to 26%, which is about half the global mean percentage change in CDN caused by doubling the updraft velocity. 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