Global Environmental Multiscale Model
Encyclopedia
The Global Environmental Multiscale Model (GEM) is an integrated forecasting and data assimilation
system developed in the Recherche en Prévision Numérique
(RPN), Meteorological Research Branch (MRB), and the Canadian Meteorological Centre (CMC).
The GEM model is currently operational for the global data assimilation cycle and medium-range forecasting, the regional data assimilation spin-up cycle and short-range forecasting. Furthermore mesoscale forecasts are produced overnight and are available to the operational forecasters. A growing number of meteorological
applications are now either based on or use the GEM model. Output from the GEM goes out to 10 days, on par with the Integrated Forecast System
and the effective range of the Global Forecast System
, the GEM's primary rivals from Europe and the United States respectively.
The GEM model has been developed to meet the operational weather forecasting
needs of Canada
for 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.
The operational GEM model dynamics is formulated in terms of the hydrostatic primitive equations with a terrain following pressure vertical coordinate (h). The time discretization
is an implicit two-time-level semi-Lagrangian scheme
. The spatial discretization
is a Galerkin grid-point formulation on an Arakawa C-grid in the horizontal (lat-lon) and an unstaggered vertical discretization
. 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:
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, it is a major recoding effort that is based upon a locally developed communication interface currently using Message 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.
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.
Data assimilation
Applications of data assimilation arise in many fields of geosciences, perhaps most importantly in weather forecasting and hydrology. Data assimilation proceeds by analysis cycles...
system developed in the Recherche en Prévision Numérique
Recherche en Prévision Numérique
The center Recherche en Prévision Numérique is responsible for the research and development of the modelling component of the Numerical Weather Prediction System for the Canadian Meteorological Centre and the Regional Meteorological Centers of the Meteorological Service of Canada , Environment...
(RPN), Meteorological Research Branch (MRB), and the Canadian Meteorological Centre (CMC).
The GEM model is currently operational for the global data assimilation cycle and medium-range forecasting, the regional data assimilation spin-up cycle and short-range forecasting. Furthermore mesoscale forecasts are produced overnight and are available to the operational forecasters. A growing number of meteorological
Meteorology
Meteorology is the interdisciplinary scientific study of the atmosphere. Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the 18th century. The 19th century saw breakthroughs occur after observing networks developed across several countries...
applications are now either based on or use the GEM model. Output from the GEM goes out to 10 days, on par with the Integrated Forecast System
Integrated Forecast System
The Integrated Forecast System is an operational global meteorological forecasting model. IFS is developed and maintained by the European Centre for Medium-Range Weather Forecasts based in Reading, England...
and the effective range of the Global Forecast System
Global Forecast System
The Global Forecast System is a global numerical weather prediction computer model run by NOAA. This mathematical model is run four times a day and produces forecasts up to 16 days in advance, but with decreasing spatial and temporal resolution over time...
, the GEM's primary rivals from Europe and the United States respectively.
The GEM model has been developed to meet the operational weather forecasting
Weather forecasting
Weather forecasting is the application of science and technology to predict the state of the atmosphere for a given location. Human beings have attempted to predict the weather informally for millennia, and formally since the nineteenth century...
needs of Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
for 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.
The operational GEM model dynamics is formulated in terms of the hydrostatic primitive equations with a terrain following pressure vertical coordinate (h). The time discretization
Discretization
In mathematics, discretization concerns the process of transferring continuous models and equations into discrete counterparts. This process is usually carried out as a first step toward making them suitable for numerical evaluation and implementation on digital computers...
is an implicit two-time-level semi-Lagrangian scheme
Semi-Lagrangian scheme
The Semi-Lagrangian scheme is a numerical method that is widely used in Numerical Weather Prediction models for the integration of the equations governing atmospheric motion...
. The spatial discretization
Discretization
In mathematics, discretization concerns the process of transferring continuous models and equations into discrete counterparts. This process is usually carried out as a first step toward making them suitable for numerical evaluation and implementation on digital computers...
is a Galerkin grid-point formulation on an Arakawa C-grid in the horizontal (lat-lon) and an unstaggered vertical discretization
Discretization
In mathematics, discretization concerns the process of transferring continuous models and equations into discrete counterparts. This process is usually carried out as a first step toward making them suitable for numerical evaluation and implementation on digital computers...
. 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:
- solar and infrared radiation interactive with water vaporWater vaporWater vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
, carbon dioxideCarbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, ozoneOzoneOzone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
and clouds, - prediction of surface temperature over land with the force-restore method,
- turbulenceTurbulenceIn fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
in the planetary boundary layerPlanetary boundary layerThe planetary boundary layer , also known as the atmospheric boundary layer , is the lowest part of the atmosphere and its behavior is directly influenced by its contact with a planetary surface. On Earth it usually responds to changes in surface forcing in an hour or less...
through vertical diffusion, diffusion coefficients based on stability and turbulent kinetic energyTurbulent Kinetic EnergyIn fluid dynamics, turbulence kinetic energy is the mean kinetic energy per unit mass associated with eddies in turbulent flow. Physically, the turbulence kinetic energy is characterised by measured root-mean-square velocity fluctuations....
, - surface layer based on Monin-ObukhovMonin-Obukhov LengthThe is used to describe the effects of buoyancy on turbulent flows, particularly in the lower tenth of the atmospheric boundary layer. It was first defined by Alexander Obukhov in 1946,...
similarity theory, - shallow convection scheme (non precipitating),
- Kuo-type deep convection scheme (global forecast system),
- Fritsch-Chappell type deep convection scheme (regional forecast system),
- Sundqvist condensation scheme for stratiform precipitation,
- gravity waveGravity waveIn fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media which has the restoring force of gravity or buoyancy....
drag.
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, it is a major recoding effort that is based upon a locally developed communication interface currently using Message Passing Interface
Message Passing Interface
Message Passing Interface is a standardized and portable message-passing system designed by a group of researchers from academia and industry to function on a wide variety of parallel computers...
. Research on the performance of different land surface schemes such as ISBA (Interaction Soil-Biosphere-Atmosphere
Interaction Soil-Biosphere-Atmosphere
Interaction Soil-Biosphere-Atmosphere is a land-surface parameterisation model scheme describing soil-vegetation-atmosphere interactions....
) and CLASS (Canadian Land Surface Scheme
Canadian Land Surface Scheme
The Canadian Land Surface Scheme is a land surface parametrization scheme for use in large scale climate models. It is a state-of-the-art model, using physically based equations to simulate the energy and water balances of vegetation, snow and soil. CLASS is being developed in a research project...
) is making progress.
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.