HadCM3

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HadCM3 (Hadley Centre Coupled Model, version 3) is a coupled atmosphere-ocean general circulation model (AOGCM) developed at the Hadley Centre in the United Kingdom^[1]^[2]^[3]. 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).

1 Atmosphere model (HadAM3)
2 Ocean model (HadOM3)
3 Coupling
4 Slab model (HadSM3)
5 See also
6 External links
7 References

[edit] Atmosphere model (HadAM3)

Zonal mean temperatures in JJA (top) and DJF (bottom)

HadAM3 is a grid point model and has a horizontal resolution of 2.5x3.75 degrees in latitude x longitude. This gives 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.

The timestep is 30 minutes (with three sub-timesteps per timestep in the dynamics). Near the poles, fields are fourier-filtered to prevent instabilities due to the CFL criterion.

[edit] Ocean model (HadOM3)

Ocean temperatures

The ocean model has a resolution of 1.25x1.25 degrees and a timestep of 1 hour. 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.

[edit] 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.

[edit] Slab model (HadSM3)

The atmospheric model may be run coupled to a simpler "slab ocean" rather than the full dynamic ocean. This is faster (and requires less memory) than the full model, but lacks dynamical feedbacks from the ocean, which are incorporated into the full coupled-ocean-atmosphere models used to make projections of climate change out to 2100. To do this the model needs a calibration phase in which the ocean temperatures are held to climatology while it calculates the "flux correction"; extra ocean-atmosphere fluxes that would be needed to keep the model ocean in balance (the model ocean does not include currents; these fluxes to some extent replace the heat that would be transported by the missing currents). Then the model may be run in climate mode.

[edit] See also

[edit] External links

[edit] References

^ Gordon, C.; Cooper, C., Senior, C.A., Banks, H., Gregory, J.M., Johns, T.C., Mitchell, J.F.B., and Wood, R.A. (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.
^ Pope, V.D.; Gallani, M.L., Rowntree, P.R., and Stratton, R.A. (2000). "The impact of new physical parameterizations in the Hadley Centre climate model — HadAM3". Climate Dynamics 16: 123-146.
^ Collins, M.; Tett, S.F.B., and Cooper, C. (2001). "The internal climate variability of HadCM3, a version of the Hadley Centre coupled model without flux adjustments". Climate Dynamics 17: 61-81.