United Kingdom Meteorological Office: Model UKMO HadAM1 (2.5x3.75 L19) 1993

AMIP Representative(s)

Dr. Vicky Pope, Hadley Centre for Climate Prediction and Research, United Kingdom Meteorological Office, London Road, Bracknell, Berkshire RG12 2SY, United Kingdom; Phone: +44-1344-854655; Fax: +44-1344-854898; e-mail: vdpope@meto.gov.uk; World Wide Web URL: http://www.met-office.gov.uk

Model Designation

UKMO HadAM1 (2.5x3.75 L19) 1993

Model Lineage

The UKMO HadAM1 model is the first of a line of Unified Models (UM) intended to provide a common framework for forecasting and climate applications (cf. Cullen 1993 [1]). The dynamical formulations are those described by Bell and Dickinson (1987) [13]; the physical parameterizations are substantially modified from those of an earlier UKMO model documented by Slingo (1985) [2].

Model Documentation

Cullen (1993) [1] gives an overview of the UKMO Unified Model. Key documentation of different model features is provided by Cullen (1991) [3], Cullen et al. (1991) [4], Ingram (1993) [5], Gregory (1990) [6], Gregory and Smith (1990) [7], Smith (1990a [27] , b [16]), Smith(1993) [8], Smith and Gregory (1990) [9], Wilson (1989) [10], and Wilson and Swinbank (1989) [11].

Numerical/Computational Properties

Horizontal Representation

Fourth-order finite differences on a B-grid (cf. Arakawa and Lamb 1977 [12], Bell and Dickinson 1987 [13]) in spherical polar coordinates. Mass-weighted linear quantities are conserved, and second moments of advected quantities are conserved under nondivergent flow.

Horizontal Resolution

2.5 x 3.75-degree latitude-longitude grid.

Vertical Domain

Surface to about 5 hPa; for a surface pressure of 1000 hPa, the lowest atmospheric level is at about 997 hPa.

Vertical Representation

Finite differences in hybrid sigma-pressure coordinates after Simmons and Strüfing (1981) [14]. Mass and mass-weighted potential temperature and moisture are conserved. See also Horizontal Representation.

Vertical Resolution

There are 19 unevenly spaced hybrid levels. For a surface pressure of 1000 hPa, 4 levels are below 800 hPa and 7 levels are above 200 hPa.

Computer/Operating System

The AMIP simulation was run on a Cray Y/MP computer using two processors in a UNICOS environment.

Computational Performance

For the AMIP experiment, about 4.7 minutes Cray Y/MP computation time per simulated day (about half this time being associated with output postprocessing).


For the AMIP experiment, the model atmosphere, soil moisture, and snow cover/depth were initialized for 1 December 1978 from a previous model simulation. Snow mass for areas of permanent land ice was initially set to 5 x 10^4 kg/(m^2). The model was then integrated forward to the nominal AMIP start date of 1 January 1979.

Time Integration Scheme(s)

Time integration proceeds mainly by a split-explicit scheme, where the solution procedure is split into "adjustment" and "advection" phases. In the adjustment phase, a forward-backward scheme that is second-order accurate in space and time is applied. The pressure, temperature, and wind fields are updated using the pressure gradient, the main part of the Coriolis terms, and the vertical advection of potential temperature. In the advective phase, a two-step Heun scheme is applied. A time step of 30 minutes (including a 10-minute adjustment step) is used for integration of dynamics and physics, except for full calculation of shortwave/longwave radiation once every 3 hours. In addition, an implicit scheme is used to compute turbulent vertical fluxes of momentum, heat, and moisture in the planetary boundary layer (PBL). Cf. Cullen et al. (1991) [4]for further details. See also Diffusion, Planetary Boundary Layer, and Surface Fluxes.


To prevent numerical instability, the orography is smoothed in high latitudes (see Orography), and Fourier filtering is applied to mass-weighted velocity and to increments of potential temperature and total moisture. Negative values of atmospheric moisture are removed by summing the mass-weighted positive values in each horizontal layer, and rescaling them to ensure global moisture conservation after the negative values are reset to zero.

Sampling Frequency

For the AMIP simulation, the model history is written once every 6 hours. (All average quantities in the AMIP monthly-mean standard output data are computed from samples taken at every 30-minute time step.)

Dynamical/Physical Properties

Atmospheric Dynamics

Primitive-equation dynamics, formulated to ensure approximate energy conservation, are expressed in terms of u and v winds, liquid/ice water potential temperature, total water, and surface pressure (cf. White and Bromley 1988 [15]).


Gravity-wave Drag

The parameterization of orographic gravity-wave drag follows a modified Palmer et al. (1986) [17] scheme, as described by Wilson and Swinbank (1989) [11]. The drag is given by the vertical divergence of the wave stress. Near the surface, the stress is equal to the product of a representative mountain wave number, the square of the wave amplitude (taken to be the subgrid-scale orographic variance--see Orography), and the density, wind, and Brunt-Vaisalla frequency evaluated in near-surface layers. At higher levels, the stress is given by this surface value weighted by the projection of the local wind on the surface wind. If this projection goes to zero, the stress is also zero; otherwise, if the minimum Richardson number falls below 0.25, the gravity wave is assumed to break. Above this critical level, the wave is maintained at marginal stability, and a corresponding saturation amplitude is used to compute the stress.

Solar Constant/Cycles

The solar constant is the AMIP-prescribed value of 1365 W/(m^2). Both seasonal and diurnal cycles in solar forcing are simulated. The seasonal cycle of solar insolation is based on a 360-day year (each month 30 days in length), with the date of perihelion adjusted to minimize discrepancies (cf. Ingram 1993[5]).


The carbon dioxide concentration is the AMIP-prescribed value of 345 ppm. Zonally averaged monthly ozone profiles are specified from the climatology of Keating et al. (1987) [18] above hybrid level 0.0225 (see Vertical Representation), and from satellite data of McPeters et al. (1984) [19] below this level. Radiative effects of water vapor and clouds, but not those of aerosol, are also included (see Radiation).



Cloud Formation


Planetary Boundary Layer

Conditions within the PBL are typically represented by the first 5 levels above the surface (centered at about 997, 975, 930, 869, and 787 hPa for a surface pressure of 1000 hPa), where turbulent diffusion of momentum and cloud-conserved thermodynamic and moisture variables may occur (see Cloud Formation and Diffusion). The PBL top is defined either by the highest of these layers, or by the layer in which a modified bulk Richardson number (that incorporates buoyancy parameters for the cloud-conserved variables) exceeds a critical value of unity. Nonlocal mixing terms are included for heat and moisture. See also Surface Characteristics, Surface Fluxes, and Land Surface Processes.


Orography obtained from the U.S. Navy 10-minute resolution dataset (cf. Joseph 1980 [35]) is grid-box averaged, and is further smoothed with a 1-2-1 filter at latitudes poleward of 60 degrees. The orographic variances required by the gravity-wave drag parameterization are obtained from the same dataset (see Gravity-wave Drag).


AMIP monthly sea surface temperature fields are prescribed, with daily values determined by linear interpolation.

Sea Ice

AMIP monthly sea ice extents are prescribed. The ice may occupy only a fraction of a grid box, and the effects of the remaining ice leads are accounted for in the surface roughness length, shortwave albedo and longwave emission, and turbulent eddy fluxes (see Surface Characteristics and Surface Fluxes). The spatially variable sea ice thickness is prescribed from climatological data. Snow falling on sea ice affects the surface albedo (see Surface Characteristics), but not the ice thickness or thermodynamic properties. Ice temperature is prognostically determined from a surface energy balance (see Surface Fluxes) that includes a conduction heat flux from the ocean below. Following Semtner (1976) [36], the conduction flux is proportional to the difference between the surface temperature of the ice and the subsurface ocean temperature (assumed to be fixed at the melting temperature of sea ice, or -1.8 degrees C), and the conduction flux is inversely proportional to the prescribed ice thickness.

Snow Cover

Surface snowfall is determined from the rate of frozen large-scale and convective precipitation in the lowest vertical layer. (Snowfall, like surface rainfall, is assumed to be distributed exponentially over each land grid box--see Precipitation.) On land only, prognostic snow mass is determined from a budget equation that accounts for accumulation, melting, and sublimation. Snow cover affects the roughness and heat conduction of the land surface, and it also alters the albedo of both land and sea ice (see Surface Characteristics). Snow melts when the temperature of the top soil/snow layer is > 0 degrees C, the snowmelt being limited by the total heat content of this layer. Snowmelt augments soil moisture, and sublimation of snow contributes to the surface evaporative flux over land. See also Surface Fluxes and Land Surface Processes.

Surface Characteristics

Surface Fluxes

Land Surface Processes

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Last update May 27, 1998. For further information, contact: Tom Phillips ( phillips@tworks.llnl.gov)

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