Seasonal forecasts based on numerical weather prediction models
- Introduction
- Forecast Methodology:
- References
Introduction
Since September 1995, the Canadian Meteorological Centre has been producing 0-3 month outlooks for Canada. The outlooks are predictions of the surface air temperature (here after called SAT) and the precipitation (PCPN) anomalies for a period of three months.
The seasonal forecast results from an ensemble of 12 model runs: 6 runs from a Global Environmental Multiscale model (GEM of Recherche en Prévision Numérique, see Côté et al. 1998) that has a horizontal resolution of 1.875 degrees with 50 vertical levels and 6 runs from a Climate model (GCM2 of the Canadian Centre for Climate Modelling and Analysis, see McFarlane et al. 1992) T32 with 10 vertical levels.PLEASE NOTE THAT ON FEBRUARY 1 2004 THE SEF MODEL WAS REPLACED BY THE GEM MODEL TO DO THE MONTHLY AND SEASONAL OUTLOOKS.
Both models use the same CMC atmospheric analyses. However, they differ in the way they use the analyzed surface fields. Surface forcing, crucial in controlling the seasonal atmospheric variability, has to be treated carefully. The numerical models used in the CMC system are forced by 3 fields: sea surface temperatures (SST), sea ice and snow cover. The treatment of these fields is done in the following way:
For the GEM model: Since no interactive ocean is used, the SST anomalies observed just prior (preceding 30 days) to the beginning of the forecast are fixed throughout the forecast period and they are added to the evolving climatology. The sea ice extent and snow cover analyses are relaxed toward the climatology during the first 15 days of the forecast period.
For the GCM2 model: The treatment of SST anomalies is done in the same way as for the GEM model. The sea ice cover is climatological all along the numerical integrations. The snow cover is a prognostic variable of the model and no special treatment is required.
The climatic drift in both models is removed using their known climatology (see below for more explanations). The models climatology comes from the 26 year hindcasts of the seasonal Historical Forecasting Project (HFP). The HFP was also essential to estimate the forecast skill of the models (see discussion in Derome et al., 2000; Kharin and Zwiers, 2001; Kharin et al., 2001; Plante and Gagnon, 2000).
Surface Air Temperature Forecast Methodology
The surface air temperature forecasts are made in doing first an average of the daily temperature as predicted by the models. The climatologies of the models are then subtracted from the mean forecast seasonal temperatures to derived the forecast anomalies of each model. The anomalies of the two models are then normalized and combined using an arithmetic average. The surface air temperature forecast anomalies are the anomalies of the mean daily temperature measured at the Stevenson screen height (2 metres). Finally the anomalies are divided in three categories (above, near and below the normal).
Precipitation Forecast Methodology
The precipitation forecasts are made using the total accumulated water precipitation over the season. The precipitation predicted by the models is the total liquid and includes all types: snow, rain, ice pellets, etc. The climatology of the models is subtracted from the total precipitation forecast to derive the anomalies. The anomalies of the two models are then combined using a simple normalized average as described for the surface air temperature. Finally the precipitation anomalies are divided in three categories (above, near and below the normal) as is done for the temperature anomaly forecast.
References
Côté, J., S. Gravel, A. Méthot, A. Patoine, M. Roch, and A. Staniforth, 1998: The operational CMC-MRB Global Environmental Multiscale (GEM) model: Part I - Design considerations and formulation, Mon. Wea. Rev., 126, 1373-1395. [paper]
Derome J., G. Brunet, A. Plante, N. Gagnon, G. J. Boer, F. W. Zwiers, S. J. Lambert, J. Sheng, et H. Ritchie, 2001: Seasonal Predictions Based on Two Dynamical Models.Atmos. Ocean., 39, 485-501. [paper]
Kharin, V. V. et F. W. Zwiers, 2001: Skill as a function of time scale in ensemble of seasonal hindcasts. Climate Dynamics, 17, 127-141. [abstract]
Kharin, V.V ., F. W. Zwiers et N. Gagnon, 2001: Skill of seasonal hindcasts as a function of the ensemble size. Climate Dynamics, 17, 835-843. [abstract]
McFarlane, N.A., G.J. Boer, J.-P. Blanchet et M. Lazare. 1992: The Canadian Climate Centre second-generation general circulation model and its equilibrium climate. J. Climate, 5, 1013-1044. [abstract]
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