A LES-based analysis of assumed PDF methods for diagnosing lead-generated boundary layer cloud fractions in the Arctic

Abstract:

We use output from a large eddy simulation (LES) to study the effectiveness of assumed PDF methods for diagnosing cloud fractions within the boundary layer of the Arctic atmosphere. We focus on the boundary layer over regions where cracks in the sea ice (leads) produce and dissipate low-level clouds via modulation of turbulent fluxes of sensible and latent heat, and we consider the effects of varying lead orientation (releative to wind direction) and width. This is a step in evaluating the validity of this type of boundary layer cloud parameterization scheme for use by global climate models in the Arctic. We address the thermodynamic aspects of cloud formation, the partitioning of condensate between liquid and ice phases, and study how the associated cloud radiative effects impact surface energy balance. Results are motivated by and analyzed in conjunction with observed relationships between boundary layer clouds and Arctic leads.