Boundary layer parametrization in Environment Canada's NWP models: lessons learned from three GASS experiments

In the atmospheric models currently used by Environment Canada for numerical weather prediction (NWP), vertical transport in the planetary boundary layer (PBL) due to subgrid-scale turbulence is parametrized in the form of vertical diffusion. The parametrization is based on a turbulent kinetic energy (TKE) closure of order 1.5. Two alternative treatments of subgrid-cloud effects and two optional formulations of mixing length are available, and different combinations thereof are used in the various operational NWP systems, e.g. global versus regional, deterministic and ensemble forecasting systems of Environment Canada. The effect of turbulent hysteresis were recently implemented in the PBL scheme, leading to improved forecasts overall. In this presentation, we discuss the results produced for and the lessons learned from the Canadian contribution to three numerical model experiments, coordinated by the Global Atmospheric System (GASS) panel and related to PBL parametrization issues: (1) the DIurnal land/atmosphere coupling experiment (DICE), which aims to assess the impact of land/atmosphere feedbacks; (2) the Surface Drag project, whose primary goal is to compare various components of surface stress in NWP and climate models; and (3) the Arctic Air experiment, designed to understand how models represent the Arctic winter boundary layer.