HadCM3 is a coupled atmosphere-ocean GCM developed at the Hadley Centre and described by Gordon et al. (1999). It has a stable control climatology and does not use flux adjustment. The atmospheric component of the model has 19 levels with a horizontal resolution of 2.5 degrees of latitude by 3.75 degrees 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 degrees of latitude (comparable to a spectral resolution of T42). A new radiation scheme is included with 6 and 8 spectral bands in the shortwave and longwave. The radiative effects of minor greenhouse gases as well as CO2, water vapour and ozone are explicitly represented (Edwards and Slingo, 1996). A simple parametrization of background aerosol (Cusack et al. 1998) is also included. A new land surface scheme (Cox et al. 1999) 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 to 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. A penetrative convective scheme (Gregory and Rowntree, 1990) is used, modified to include an explicit down-draught, and the direct impact of convection on momentum (Gregory et al. 1997). Parametrizations of orographic and gravity wave drag have been revised to model the effects of aniostropic orography, high drag states, flow blocking and trapped lee waves (Milton and Wilson 1996; Gregory et al. 1998). The large-scale precipitation and cloud scheme is formulated in terms of an explicit cloud water variable following Smith (1990). The effective radius of cloud droplets is a function of cloud water content and droplet number concentration (Martin et al. 1994). The atmosphere component of the model also optionally allows the transport, oxidation and removal by physical deposition and rain out of anthropogenic sulphur emissions to be included interactively. This permits the direct and indirect forcing effects of sulphate aerosols to be modelled given scenarios for sulphur emissions and oxidants. The oceanic component of the model has 20 levels with a horizontal resolution of 1.25 x 1.25 degrees. At this resolution it is possible to represent important details in oceanic current structures. Horizontal mixing of tracers uses a version of the Gent and McWilliams (1990) adiabatic diffusion scheme with a variable thickness diffusion parametrization (Wright 1997; Visbeck et al. 1997) is used. There is no explicit horizontal diffusion of tracers. The along-isopycnal diffusivity of tracers is 1000 m**2/s and horizontal momentum viscosity varies with latitude between 3000 and 6000 m**2/s at the poles and equator respectively. Near-surface vertical mixing is parametrized partly by a Kraus-Turner mixed layer scheme for tracers (Kraus and Turner 1967), and a K-theory scheme (Pacanowski and Philander 1981) for momentum. Below the upper layers the vertical diffusivity is an increasing function of depth only. 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, which is allowed to find its proper level of neutral buoyancy rather than mixing vertically with surrounding water masses. The scheme is based on Roether et al. (1994). 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 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 (Cattle and Crossley 1995). 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. The model is initialized directly from the Levitus (1994) 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. Cattle, H. and J. Crossley, 1995: Modelling Arctic climate change. 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Inness, 1997; Parametrization of momentum transport by convection. II: tests in single column and general circulation models. QJR Meteor. Soc. 123: 1153-1183. Gregory, D., G.J. Shutts and J.R. Mitchell, 1998; A new gravity wave drag scheme incorporating anisotropic orography and low level wave breaking: Impact upon the climate of the UK Meteorological Office Unified Model. QJR Meteor. Soc. 124: 463-493. Kraus, E.B. and J.S. Turner, 1967; A one dimensional model of the seasonal thermocline. Part II. Tellus, 19: 98-105. Levitus, S. and T.P. Boyer, 1994; World Ocean Atlas 1994, Volume 4: Temperature. NOAA/NESDIS E/OC21, US Department of Commerce, Washington, DC, 117pp. Martin, G.M., D.W. Johnson and A. Spice, 1994; The measurement and parametrization of effective radius of droplets in warm stratocumulus clouds. J. Atmos. Sci. 51: 1823-1842. Milton, S.F. and C.A.Wilson, 1996: The impact of parametrized sub-grid scale orographic forcing on systematic errors in a global NWP model. Mon. 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