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Model Report

Hadley Coupled Atmosphere-Ocean General Circulation Model

Last Revision Date: 11/16/2009 View as PDF
General Information Back to Top
Model Abbreviated Name:

Model Extended Name:

Hadley Coupled Atmosphere-Ocean General Circulation Model
Model Overview/Abstract:
HadCM3 is a coupled atmosphere-ocean general circulation model (AOGCM) developed at the Hadley Centre and described by Gordon et al (2000) and Pope et al (2000). Unlike earlier AOGCMs at the Hadley Centre and elsewhere (including HadCM2), HadCM3 does not need flux adjustment (additional "artificial" heat and freshwater fluxes at the ocean surface) to produce a good simulation. The higher ocean resolution of HadCM3 is a major factor in this. HadCM3 has been run for over a thousand years, showing little drift in its surface climate.
For additional information, see the publications listed at http://www.metoffice.com/research/hadleycentre/pubs/index.html, and those highlighted at http://www.metoffice.com/research/hadleycentre/models/HadCM3.html
Model Technical Contact Information:
EPA contact: Brooke L. Hemming, Ph.D.
Tel: 919-541-5668
E-mail: hemming.brooke@epa.gov

Other: The Hadley Centre for Climate Prediction and Research http://www.metoffice.com/research/hadleycentre/index.html hadley@metoffice.gov.uk
Model Homepage: http://www.metoffice.gov.uk/research/hadleycentre/ Exiting the EPA Site

User Information Back to Top
Technical Requirements
Computer Hardware
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Compatible Operating Systems
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Other Software Required to Run the Model
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Download Information
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Using the Model
Basic Model Inputs
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Basic Model Outputs
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User Support
User's Guide Available?
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Model Science Back to Top
Problem Identification
As with all GCMs, the fundamental scientific basis of the HadCM3 model is the numerical solution on a discretized domain of the basic fluid mechanical equations governing atmospheric and oceanic flow, along with representations of physical/thermodynamic processes such as radiative transfer, turbulence, convection and the formation of clouds and precipitation, plant physiology, soil heat and water dynamics, etc. “Coupled atmosphere-ocean general circulation models (AOGCMs) These are the most complex models in use, consisting of an AGCM coupled to an OGCM. Some recent models include the biosphere, carbon cycle and atmospheric chemistry as well. AOGCMs can be used for the prediction and rate of change of future climate. They are also used to study the variability and physical processes of the coupled climate system. Global climate models typically have a resolution of a few hundred kilometres. Climate projections from the Hadley Centre make use of the HadCM2 AOGCM, developed in 1994, and its successor HadCM3 AOGCM, developed in 1998. Greenhouse-gas experiments with AOGCMs have usually been driven by specifying atmospheric concentrations of the gases, but if a carbon cycle model is included, the AOGCM can predict changes in carbon dioxide concentration, given the emissions of carbon dioxide into the atmosphere. At the Hadley Centre, this was first done in 1999. Similarly, an AOGCM coupled to an atmospheric chemistry model is able to predict the changes in concentration of other atmospheric constituents in response to climate change and to the changing emissions of various gases. Further information is available on: some aspects of ocean simulation in HadCM3 (thermohaline circulation, ventilation, vertical mixing), decadal variability in the ocean of HadCM3.”
Summary of Model Structure and Methods
The atmospheric component of HadCM3 has 19 levels with a horizontal resolution of 2.5° of latitude by 3.75° of longitude, which produces a global grid of 96 x 73 grid cells. This is equivalent to a surface resolution of about 417 km x 278 km at the Equator, reducing to 295 km x 278 km at 45° of latitude. The radiative effects of CO2, water vapour, ozone, minor greenhouse gases, and background aerosols are explicitly represented. A penetrative convective scheme is used, modified to include an explicit down-draught, and the direct impact of convection on momentum. Recently updated parametrizations of orographic and gravity wave drag are also included. The large-scale precipitation and cloud scheme is formulated in terms of an explicit cloud water variable. The atmospheric component of the model also optionally allows the emission, transport, oxidation and deposition of sulphur compounds (dimethylsulphide, sulphur dioxide and ammonium sulphate) to be simulated interactively. This permits the direct and indirect forcing effects of sulphate aerosols to be modelled given scenarios for sulphur emissions and oxidants.

More Information Below

Key Limitations to Model Scope

The oceanic component of HadCM3 has 20 levels with a horizontal resolution of 1.25 x 1.25°. Horizontal mixing of tracers uses a version of the Gent and McWilliams adiabatic diffusion scheme with a variable thickness diffusivity. There is no explicit horizontal diffusion of tracers. The along-isopycnal diffusivity of tracers is 1000 m2 s-1 and horizontal momentum viscosity varies with latitude between 3000 and 6000 m2 s-1 at the poles and equator respectively. Near-surface vertical mixing is parametrized partly by a Kraus-Turner mixed layer scheme for tracers, and a K-theory scheme for momentum. Convective adjustment is modified in the region of the Denmark Straits and Iceland-Scotland ridge better to represent down-slope mixing of the overflow water. Mediterranean water is partially mixed with Atlantic water across the Strait of Gibraltar as a simple representation of water mass exchange since the channel is not resolved in the model. The model is initialized directly from an observed ocean state at rest, with a suitable atmospheric and sea ice state. The atmosphere and ocean exchange information once per day. Heat and water fluxes are conserved exactly in the transfer between their different grids.
Case Studies

The sea ice model uses a simple thermodynamic scheme including leads and snow-cover. Ice is advected by the surface ocean current, with convergence prevented when the depth exceeds 4 m. There is no explicit representation of iceberg calving, so a prescribed water flux is returned to the ocean at a rate calibrated to balance the net snowfall accumulation on the ice sheets, geographically distributed within regions where icebergs are found. In order to avoid a global average salinity drift, surface water fluxes are converted to surface salinity fluxes using a constant reference salinity of 35 PSU.
A new land surface scheme includes a representation of the freezing and melting of soil moisture, as well as surface runoff and soil drainage; the formulation of evaporation includes the dependence of stomatal resistance on temperature, vapour pressure and CO2 concentration. The surface albedo is a function of snow depth, vegetation type and also of temperature over snow and ice.

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