Aircraft observations and high resolution NWP simulations of Northern Hemisphere cold-air outbreaks

Several recent studies have shown marked deficiencies in the Met Office Unified Models representation of clouds in cold-air outbreak conditions. In particular the model has difficulty in capturing the overcast stratiform boundary layer cloud. The lack of cloud cover in the global model and it's bias in the radiative properties of the cloud when present (due to a lack of sufficient liquid water) have been proposed to play a key role in a persistent bias in the downwelling shortwave radiation at the surface in the southern Ocean (Bodas-Salcedo et al. 2012). At convection permitting resolutions of the Unified Model the same issues arise, with a lack of cloud cover and liquid water in simulations performed to the north of the United Kingdom (Field et al. 2013). There are however a distinct lack of in-situ observations available to fully evaluate these model simulations.

A series of research flights on the Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 research aircraft have therefore been undertaken to provide a comprehensive set of measurements of the mixed-phase stratocumulus topped boundary layer in cold-air outbreak scenarios. We use these observations to evaluate convection permitting simulations with the Unified Model, with a focus on the models' representation of the boundary layer structure, turbulent fluxes and cloud properties.