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 (which includes a sea ice model). Simulations often use a 360-day calendar, where each month is 30 days
The HadCM3 atmospheric model (HadAM3) is a grid point model and has a horizontal resolution of 3.75×2.5 degrees in longitude × latitude. This gives 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. This gives a resolution of approximately 300 km, roughly equal to T42 in a spectral model. There are 19 levels in the vertical. Its ocean model (HadOM3) has a resolution of 1.25×1.25 degrees, 20 levels, 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.