HadCM3

HadCM3 (Hadley Centre Coupled Model, version 3) 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). It was one of the major models used in the IPCC TAR in 2001.

Unlike earlier AOGCMs at the Hadley Centre and elsewhere (including its predecessor 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; other factors include a good match between the atmospheric and oceanic components; and an improved ocean mixing scheme (Gent and McWilliams). HadCM3 has been run for over a thousand years, showing little drift in its surface climate.

HadCM3 is composed of two components: the atmospheric model HadAM3 and the ocean model (which includes a sea ice model).

Atmosphere model (HadAM3)

HadAM3 is a grid point model and has a horizontal resolution of 2.5x3.75 degrees in latitude x longitude, giving 96x73 grid points (on the scalar (pressure, temperature and moisture) grid; the vector (wind velocity) grid is offset by 1/2 a grid box) [1]. This gives a resolution of approximately 300 km, roughly equal to T42 in a spectral model. There are 19 levels in the vertical.

Ocean model

The ocean model has a resolution of 1.25x1.25 degrees. Thus there are 6 ocean grid points for every atmospheric one. For ease of coupling the two models the grids are aligned and the ocean coastline is forced to be aligned to the atmospheric grid.

Coupling

The atmospheric model is run for a day, and the fluxes (of heat, moisture and momentum) at the atmos-ocean interface are accumulated. Then the ocean model is run for a day, with the reverse fluxes accumulated. This then repeats through the length of the run. Unlike its predecessor HadCM2 there is no need for flux correction – the model climate remains stable and does not significantly drift. The lack of flux correction is cited by the IPCC as one of the advances in modelling since the SAR [2].

The ocean model incorporates a thermodynamic-dynamic seaice model with a primitive (ocean drift) dynamics.

External links

• http://www.met-office.gov.uk/research/hadleycentre/models/HadCM3.html
• http://cera-www.dkrz.de/IPCC_DDC/IS92a/HadleyCM3/hccpruns.html

References

• Gordon, C., C. Cooper, C.A. Senior, H. Banks, J.M. Gregory, T.C. Johns, J.F.B. Mitchell and R.A. Wood, 2000: The simulation of SST, sea ice extents and ocean heat transports in a version of the Hadley Centre coupled model without flux adjustments. Climate Dynamics 16: 147–168 [3].
• Pope, V. D., M. L. Gallani, P. R. Rowntree and R. A. Stratton, 2000: The impact of new physical parametrizations in the Hadley Centre climate model — HadAM3. Climate Dynamics, 16: 123–146 [4].