Evaluation of stratocumulus cloud prediction in the Met Office forecast model during VOCALS-REx

The prevalence of low level stratocumulus clouds over the global oceans and their radiative effects have long been recognized as a key component of the Earth's climate system, yet many general-circulation models struggle to accurately simulate these cloudy boundary layers. The VOCALS-REx field experiment in October and November 2008 provided a wealth of in-situ and remote sensing observations of the stratocumulus topped boundary layer in the southeast Pacific. The observations span significant gradients in the cloud and boundary layer structure, aerosol composition and cloud microphysics, from the more polluted coastal region to a cleaner maritime air mass away from the coast. In this paper we evaluate the ability of NWP configurations of the Met Office Unified Model to forecast the boundary layer structure, cloud cover, LWP, drizzle and aerosol. A key focus of the paper is to assess the models representation of the daily variability and of the diurnal cycle of the cloud in both the coastal and more remote maritime regions. The impact of including aerosol-cloud interactions in the forecast model will also be presented.