The Global Environmental Multiscale Model (GEM), often known as the CMC model in North America, is an integrated forecasting and
data assimilation
Data assimilation is a mathematical discipline that seeks to optimally combine theory (usually in the form of a numerical model) with observations. There may be a number of different goals sought – for example, to determine the optimal state es ...
system developed in the
Recherche en Prévision Numérique
The Recherche en Prévision Numérique center (RPN) is responsible for the research and development of the modelling component of the Numerical Weather Prediction (NWP) System for the Canadian Meteorological Centre (CMC) and the Regional Meteorolo ...
(RPN),
Meteorological Research Branch (MRB), and the
Canadian Meteorological Centre (CMC). Along with the
NWS's
Global Forecast System
The Global Forecast System (GFS) is a global numerical weather prediction system containing a global computer model and variational analysis run by the United States' National Weather Service (NWS).
Operation
The mathematical model is run f ...
(GFS), which runs out to 16 days, the
ECMWF
The European Centre for Medium-Range Weather Forecasts (ECMWF) is an independent intergovernmental organisation supported by most of the nations of Europe. It is based at three sites: Shinfield Park, Reading, United Kingdom; Bologna, Italy; an ...
's
Integrated Forecast System The Integrated Forecasting System (IFS) is a global numerical weather prediction system jointly developed and maintained by the European Centre for Medium-Range Weather Forecasts (ECMWF) based in Reading, England, and Météo-France based in Toulous ...
(IFS), which runs out 10 days, the Naval Research Laboratory
Navy Global Environmental Model
The Navy Global Environmental Model (NAVGEM) is a global numerical weather prediction computer simulation run by the United States Navy's Fleet Numerical Meteorology and Oceanography Center. This mathematical model is run four times a day and pro ...
(NAVGEM), which runs out eight days, and the UK
Met Office
The Meteorological Office, abbreviated as the Met Office, is the United Kingdom's national weather service. It is an executive agency and trading fund of the Department for Business, Energy and Industrial Strategy and is led by CEO Penelope E ...
's
Unified Model The Unified Model is a numerical weather prediction and climate modeling software suite originally developed by the United Kingdom Met Office, and now both used and further developed by many weather-forecasting agencies around the world. The Unifi ...
, which runs out to six days, it is one of the five predominant synoptic scale medium-range models in general use.
Operation
Deterministic model
The GEM's operational model, known as the global 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 names ''regional deterministic prediction system'' or RDPS for the coarser mesh, available for all of North America and ''high-resolution deterministic prediction system'' or HRDPS for the finer mesh, available in Canada only) are produced overnight and are available to the operational forecasters. A growing number of
meteorological
Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did not ...
applications 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 European
Integrated Forecast System The Integrated Forecasting System (IFS) is a global numerical weather prediction system jointly developed and maintained by the European Centre for Medium-Range Weather Forecasts (ECMWF) based in Reading, England, and Météo-France based in Toulous ...
.
Ensemble model
The ensemble variant of the GEM is known as the Global Ensemble Prediction System (GEPS). It has 20 members (plus
control
Control may refer to:
Basic meanings Economics and business
* Control (management), an element of management
* Control, an element of management accounting
* Comptroller (or controller), a senior financial officer in an organization
* Controllin ...
) and runs out 16 days, the same range as the American
global forecast system
The Global Forecast System (GFS) is a global numerical weather prediction system containing a global computer model and variational analysis run by the United States' National Weather Service (NWS).
Operation
The mathematical model is run f ...
. The GEPS runs alongside the GFS ensemble to form the
North American Ensemble Forecast System The North American Ensemble Forecast System (NAEFS) is a joint project involving the Meteorological Service of Canada (MSC) in Canada, the National Weather Service (NWS) in the United States, and the National Meteorological Service of Mexico (NMSM) ...
. A regional ensemble prediction system (REPS), covering North America and also having 20 members plus control, runs out 72 hours.
Development
The GEM model has been developed to meet the operational
weather forecasting
Weather forecasting is the application of science and technology forecasting, to predict the conditions of the Earth's atmosphere, atmosphere for a given location and time. People have attempted to predict the weather informally for millennia a ...
needs of
Canada
Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...
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.
Mechanics
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
In applied mathematics, discretization is the process of transferring continuous functions, models, variables, and equations into discrete counterparts. This process is usually carried out as a first step toward making them suitable for numerical ...
is an implicit two-time-level
semi-Lagrangian scheme The Semi-Lagrangian scheme (SLS) is a numerical method that is widely used in numerical weather prediction models for the integration of the equations governing atmospheric motion. A Lagrangian description of a system (such as the atmosphere) focus ...
. The spatial
discretization
In applied mathematics, discretization is the process of transferring continuous functions, models, variables, and equations into discrete counterparts. This process is usually carried out as a first step toward making them suitable for numerical ...
is a
Galerkin grid-point formulation on an
Arakawa C-grid
The Arakawa grid system depicts different ways to represent and compute orthogonal physical quantities (especially velocity- and mass-related quantities) on rectangular grids used for Earth system models for meteorology and oceanography
Oceano ...
in the horizontal (lat-lon) and an unstaggered vertical
discretization
In applied mathematics, discretization is the process of transferring continuous functions, models, variables, and equations into discrete counterparts. This process is usually carried out as a first step toward making them suitable for numerical ...
. 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 vapor
(99.9839 °C)
, -
, Boiling point
,
, -
, specific gas constant
, 461.5 J/( kg·K)
, -
, Heat of vaporization
, 2.27 MJ/kg
, -
, Heat capacity
, 1.864 kJ/(kg·K)
Water vapor, water vapour or aqueous vapor is the gaseous pha ...
,
carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
,
ozone
Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...
and
clouds
In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may com ...
,
* prediction of
surface temperature over land with the force-restore method,
*
turbulence
In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...
in the
planetary boundary layer
In meteorology, the planetary boundary layer (PBL), also known as the atmospheric boundary layer (ABL) or peplosphere, is the lowest part of the atmosphere and its behaviour is directly influenced by its contact with a planetary surface. On Ear ...
through vertical diffusion, diffusion coefficients based on stability and
turbulent kinetic energy,
* surface layer based on
Monin–Obukhov 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 wave
In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media when the force of gravity or buoyancy tries to restore equilibrium. An example of such an interface is that between the atmosphere ...
drag.
Future
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
Interaction is action that occurs between two or more objects, with broad use in philosophy and the sciences. It may refer to:
Science
* Interaction hypothesis, a theory of second language acquisition
* Interaction (statistics)
* Interactions o ...
) and CLASS (
Canadian Land Surface Scheme The Canadian Land Surface Scheme (CLASS) 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, sno ...
) 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.
Usage
Output from Canadian forecast models such as the GEM is under Canadian
crown copyright but is issued under a
free license if properly attributed to Environment Canada.
Various unofficial sites thus redistribute GEM data, including the GDPS and GEPS.
See also
*
Global climate model
A general circulation model (GCM) is a type of climate model. It employs a mathematical model of the general circulation of a planetary atmosphere or ocean. It uses the Navier–Stokes equations on a rotating sphere with thermodynamic terms f ...
References
External links
*
Introduction to GEMDM, distributed memory version of the GEMGEM - Weather Forecast Visualisation* Model output from the
University of Quebec at Montreal
A university () is an institution of higher (or tertiary) education and research which awards academic degrees in several academic disciplines. Universities typically offer both undergraduate and postgraduate programs. In the United States, the ...
:
*
GDPS*
RDPS*
HRDPS*
REPS individual ensemble members*
GEPS individual ensemble members, out 10 daysGEPS mean output, out 16 daysfrom Tropical Tidbits
from
George Mason University
George Mason University (George Mason, Mason, or GMU) is a public research university in Fairfax County, Virginia with an independent City of Fairfax, Virginia postal address in the Washington, D.C. Metropolitan Area. The university was origin ...
's SubX program (top right)
{{Atmospheric, Oceanographic and Climate Models
Numerical climate and weather models
Meteorological Service of Canada