A numerical model of the cloud‐topped marine boundary layer with explicit treatment of supersaturation–number concentration correlations

Abstract

SUMMARYA numerical model of the cloud-topped marine boundary layer is presented. The model, based on ensembleaveraged conservation equations and containing detailed aerosol microphysics, differs from previous models in one major respect: the mean supersaturation-cloud drop number concentration correlation term, generally neglected in previous numerical mean cloud models, is parametrized using a traditional boundary-layer uxgradient relationship. The present work highlights the importance of this correlation term and shows how a simple representation of this correlation can lead to dramatically different, and improved, simulations of cloud behaviour. Simulations are presented for a well-studied marine boundary-layer stratocumulus case. Inclusion of the supersaturation-cloud drop number concentration correlation leads to more physically realistic cloud drop spectra and cloud drop concentrations throughout the height of the cloud. Although the parametrization presented must be viewed as a rst approximation, the crudeness of the approach should be seen as a justi cation and starting point for future work.

Eulerian and Lagrangian approaches to multidimensional condensation and collection

“…The particles are cloud droplets in the meteorological context and dust grains in astrophysics. A possible approach to treat collection is to solve the Smoluchowski equation (also known as the stochastic collection equation in the meteorological context) [ Ogura and Takahash , ; Svensson and Seinfeld , ; Bec et al ., ], which couples the spatiotemporal evolution equations of the particle distribution function for different particle sizes. The particle motion can be treated using a fluid description for each particle size, which we refer to as the particle fluid.…”