HadCM3

HadCM3 (abbreviation for Hadley Centre Coupled Model, version 3) is a coupled atmosphere-ocean general circulation model (AOGCM) developed at the Hadley Centre in the United Kingdom. It was one of the major models used in the IPCC Third Assessment Report 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 to produce simulations for periods of 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 HadOM3 (which includes a sea ice model). Simulations use a 360-day calendar, where each month is 30 days.

HadAM3 is a grid point model that has a horizontal resolution of 3.75 × 2.5 degrees in longitude × latitude. This corresponds to a spacing between points of approximately 300 km and is roughly comparable to T42 truncation in a spectral model. There are 96 × 73 grid points on the scalar (pressure, temperature and moisture) grid; the vector (wind velocity) grid is offset by 1/2 a grid box (see Arakawa B-grid). There are 19 levels in the vertical using a hybrid (sigma and pressure) coordinate system.

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.

This is the model behind PRECIS (Providing Regional Climates for Impacts Studies) as well as being the atmosphere component of the distributed computing project Climateprediction.net.

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