The use of very high resolution orography, as in the GDPS, when implemented in ECCC’s 10-km continental-scale regional deterministic prediction system, is found to result in severe under-prediction of freezing rain accumulations in the valleys around the Canadian Rockies. Further investigation reveals a pattern of spurious winter-time warming in these valleys, which is also evident in the GDPS simulations to some extent. In order to better comprehend the underlying issues, theoretical test models with the characteristic numerical details identical to GEM, i.e. terrain-following vertical coordinate with semi-Lagrangian treatment for advection, were developed to simulate one-dimensional and two-dimensional flows in the presence of mountains and valleys of specified resolutions.
With the help of one dimensional shallow water problem in the presence of orography, the effective resolution, i.e., the smallest scales that are resolved by a model with sufficient accuracy, has been found to be larger than 6Δ for the GEM numerics. The one-dimensional problem shows that orography scales below the effective resolution can lead to considerable distortions in the solution. Super-critical flow over two-dimensional valleys with statically stable conditions reconfirms these findings. Furthermore, the two-dimensional model demonstrates that error in the flow representation at scales below a model’s effective resolution can lead to secondary erroneous signals similar to valley warming in the full three-dimensional case. The results from this study highlight the importance of appropriately filtering orography scales in accordance with a model’s effective resolution in order to obtain acceptable forecast guidance in the presence of complex terrain. The different test problems and the associated findings will be presented at the conference.