Recent changes to the PBL scheme in GEM

Several changes to the planetary boundary layer (PBL) parameterization scheme used in the Canadian GEM (Global Environmental Multiscale) model have been done recently. The introduction of vertical staggering of the basic variables in the dynamics of GEM prompted some of those changes. In particular, an unphysical gradual deepening of the PBL was noted during GEM integrations as a result of a progressive erosion of the capping inversion due to too much vertical mixing. This problem was found to be enhanced in regimes of stratocumulus-capped PBL leading to too warm and too dry PBLs and to significant reduction of cloud covers over the oceans. Another important problem in GEM is related to “warm episodes” during strong low-level advection conditions in the warm frontal sector of winter continental cyclones. Large temperature errors (approaching 10 K in 6-h forecasts) often appear in regions of precipitation due to over-mixing in the PBL, making freezing rain forecasts difficult. Various modifications to the PBL scheme and to the representation of PBL clouds have been examined to reduce these problems. The inclusion of a turbulent hysteresis process (during transitions between turbulent and laminar flow regimes) was found to lead to overall improvements during the warm episodes. A modified formulation of the stable PBL regime, based on the total turbulent energy closure, is also currently being examined. Several examples and results of these changes to the PBL scheme, in both single column mode (SCM) and in full 3D GEM model simulations, will be presented at the Symposium.