Analytical estimation of cloud droplet nucleation based on an underlying aerosol population

In this paper, we describe an analytical approach to the approximation of the nucleation rate of cloud condensation nuclei (CCN) in an updraught. This approach is developed as a subgrid parametrization for numerical models whose vertical grid structure prohibits the resolution of supersaturation and thence CCN activation. The method used is a simple extension to the upper bound approximation of Twomey (1959), and follows a similar procedure to that adopted by Cohard et al. (1998). However, unlike these approaches to the problem, we incorporate a dependence upon the physicochemical properties of an underlying dry aerosol population which is not constrained to be chosen a priori. The parametrization outlined here is also able to respond to a multimodal representation of the underlying dry aerosol spectrum. Each mode of this spectrum can further have different chemical characteristics and as such could be suitable for use in a coupled cloud-chemistry model whereby each mode of the aerosol distribution is free to evolve independently. The results of the parametrization calculated using a range of properties of the dry aerosol are validated against those from a more detailed parcel model. Crown Copyright (c) 2009 Published by Elsevier B.V. All rights reserved.