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Global Environmental Multiscale Model

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TheGlobal Environmental Multiscale Model(GEM), often known as the CMC model in North America, is an integrated forecasting anddata assimilationsystem developed in theRecherche en Prévision Numérique(RPN),Meteorological Research Branch(MRB), and theCanadian Meteorological Centre(CMC). Along with theNWS'sGlobal Forecast System(GFS), which runs out to 16 days, theECMWF'sIntegrated Forecast System(IFS), which runs out 10 days, the Naval Research LaboratoryNavy Global Environmental Model(NAVGEM), which runs out eight days, the UKMet Office'sUnified Model,which runs out to seven days, andDeutscher Wetterdienst's ICON (ICOsahedral Nonhydrostatic), which runs out to 7.5 days, it is one of the global medium-range models in general use.[citation needed]

Operation

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Deterministic model

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The GEM's operational model, known as theGlobal Deterministic Prediction System(GDPS), is currently operational for the global data assimilation cycle and medium-range forecasting, the regional data assimilation spin-up cycle and short-range forecasting. Mesoscale forecasts (distributed under the namesregional deterministic prediction systemor RDPS for the coarser mesh, available for all of North America andhigh-resolution deterministic prediction systemor HRDPS for the finer mesh, available in Canada only) are produced overnight and are available to the operational forecasters. A growing number ofmeteorologicalapplications are now either based on or use the GEM model. Output from the GEM goes out to 10 days, on par with the public output of the EuropeanIntegrated Forecast System.

Ensemble model

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The ensemble variant of the GEM is known as theGlobal Ensemble Prediction System(GEPS). It has 20 members (pluscontrol) and runs out 16 days, the same range as the Americanglobal forecast system.The GEPS runs alongside the GFS ensemble to form theNorth American Ensemble Forecast System.A regional ensemble prediction system (REPS), covering North America and also having 20 members plus control, runs out 72 hours.

Development

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The GEM model has been developed to meet the operationalweather forecastingneeds ofCanadafor the coming years. These presently include short-range regional forecasting, medium-range global forecasting, and data assimilation. In the future they will include nowcasting at the meso-scales, and dynamic extended-range forecasting on monthly to seasonal timescales. The essence of the approach is to develop a single highly efficient model that can be reconfigured at run time to either run globally at uniform-resolution (with possibly degraded resolution in the "other" hemisphere), or to run with variable resolution over a global domain such that high resolution is focused over an area of interest.

Mechanics

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The operational GEM model dynamics is formulated in terms of thehydrostatic primitive equationswith a terrain followingpressure vertical coordinate(h). The timediscretizationis an implicit two-time-levelsemi-Lagrangian scheme.The spatialdiscretizationis aGalerkin grid-point formulationon anArakawa C-gridin the horizontal (lat-lon) and an unstaggered verticaldiscretization.The horizontal mesh can be of uniform or variable resolution, and furthermore can be arbitrarily rotated, the vertical mesh is also variable. The explicit horizontal diffusion is -2 on all prognostic variables.

The operational GEM model is interfaced with a full complement of physical parametrizations, these currently include:

Future

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The next stage of development of the GEM model is to evaluate the non-hydrostatic version for mesoscale applications where the hydrostatic assumption breaks down. The limited-area (open-boundary) version is scheduled to follow. The distributed memory version of GEM is almost completed,[when?]it is a major recoding effort that is based upon a locally developed communication interface currently usingMessage Passing Interface.Research on the performance of different land surface schemes such as ISBA (Interaction Soil-Biosphere-Atmosphere) and CLASS (Canadian Land Surface Scheme) is making progress.[when?]

The strategy is progressing towards a unified data assimilation and forecast system, at the heart of which lies a single multipurpose and multiscale numerical model.

Usage

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Output from Canadian forecast models such as the GEM is under Canadiancrown copyrightbut is issued under afree license if properly attributedto Environment Canada.[1]Various unofficial sites thus redistribute GEM data, including the GDPS and GEPS.

See also

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References

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  1. ^"Environment and Climate Change Canada Data Server End-use Licence".Environment and Climate Change Canada.September 2017.RetrievedJune 19,2021.
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