The Impact of Distributing the Drag due to Unresolved Small-Scale Orography on Simulations with the Global Environmental Multiscale (GEM) Model

The relatively new trend of separating the turbulent drag due to unresolved small-scale orography (SSO) from that caused by the vegetation cover and specifying its exponential decay with height in numerical weather prediction models is followed in this study. However, unlike the approaches that calculate the surface SSO stress directly by using information about the filtered small-scale orography, ours calculates it indirectly, by subtracting the turbulent stress due to the vegetative cover from the total surface stress. We obtain the latter by means of the effective roughness length, and rely on the wide acceptance of this method as being accurate. Results showing the impact of the distributed SSO drag on simulations with the GEM model (both in single-column and in full 3D mode) are presented, with emphasis on the sensitivity of the model over the Canadian Arctic.