Simulating an Evolving Cloud-topped Boundary Layer during a Cold-Air Outbreak over the North Atlantic with SHOC (Simplified Higher-Order Closure)

The first Grey Zone intercomparison case is based on observations of a cold-air outbreak over the North Atlantic Ocean during the CONSTRAIN field campaign. Cold-air outbreaks typically produce an evolving cloud-topped boundary layer whose structure is influenced by strong surface fluxes of sensible and latent heat, mixed-phase microphysics, cloud-top radiative cooling, and cloud-top entrainment. The purpose of the intercomparison is two-fold: Firstly, through systematically varying the horizontal resolution, the intercomparison will aim to systematically assess the relative contributions of the resolved and the subgrid-scale (SGS) cloud and convective processes in the Grey Zone for the present CONSTRAIN case. Secondly, the intercomparison is interesting in its own right as it is the first time that a comprehensive mid-latitude cold-air outbreak case has been organized. We are using a cloud-resolving model, SAM (System for Atmospheric Modeling) to examine the sensitivity of our simulations of this case to the representations of the SGS turbulence and cloudiness and of the mixed-phase microphysics. Our version of SAM includes SHOC (Simplified Higher-Order Closure, Bogenschutz and Krueger 2013) which combines several existing components: A prognostic SGS turbulence kinetic energy (TKE) equation, an assumed double-Gaussian PDF following Golaz et al. (2002), the diagnostic second-moment closure of Redelsperger and Sommeria (1986), the diagnostic closure for the third moment of vertical velocity by Canuto et al. (2001), and a turbulence length scale related to the SGS TKE (Teixeira and Cheinet 2004) and to eddy length scales. The image shows the cloud water path from a large-eddy simulation of the CONSTRAIN case. The domain size is 64 km by 64 km. Such simulations are used as benchmarks for coarse-grid simulations that use SHOC.