Model UKMO (HadCM2): Elaborations
Model UKMO (HadCM2) is an entry in both the CMIP1 and CMIP2
and is providing an optional extended set of output data in compliance
Starting from Levitus (1982) climatological
September values of oceanic potential temperature and salinity, the
spinup was implemented mostly in a fully coupled mode, alternating
relaxation boundary conditions on SST and SSS . Reference SST and SSS
for the relaxation phases were based on Levitus
(1982) monthly-mean surface conditions (but with some adjustments
in SSS near Antarctica during Austral winter). Flux adjustments for
and fresh water, which were calibrated from 5-day mean differences
the modeled and reference SST and SSS averaged over a 25-year run in
preceding relaxation phase, also were applied as prescribed,
terms. Cf. Johns et al. 1997
Land Surface Processes
- A vegetation canopy model (cf. Warrilow
et al. 1986 and Shuttleworth 1988)
includes effects of moisture condensation, precipitation interception,
direct evaporation from wet leaves and from surface ponding, and
via root uptake of soil moisture. The fractional coverage and
capacity of the canopy vary spatially by vegetation type, with a small
storage added to represent surface ponding. The canopy intercepts a
of the precipitation, which is exponentially distributed over each grid
box. Throughfall of canopy condensate and intercepted precipitation
in proportion to the degree of fullness of the canopy; evaporation from
the wet canopy occurs in the same proportion, as a fraction of the
potential evaporation. Cf. Gregory
Smith (1990) for further details.
- Soil moisture is predicted from a single-layer model with
water-holding capacity; it is augmented by snowmelt, precipitation, and
the throughfall of canopy condensate. This moisture infiltrates the
at a rate depending on saturated soil hydraulic conductivity enhanced
effects of root systems that vary spatially by vegetation type. The
moisture is treated as surface runoff. Subsurface runoff from
drainage is also parameterized as a function of spatially varying
soil hydraulic conductivity and of the ratio of soil moisture to its
value (cf. Eagleson 1978). Surface and
runoff are converted into river outflow using predefined geographical
over land and associated coastal outflow points defined relative to the
model grid. Because river transport is not modeled explicitly, the
is distributed instantaneously to these outflow points.
- Soil moisture is depleted by evaporation at a fraction beta
local potential rate. The value of beta depends on the ratio of
soil moisture to a spatially varying critical value, and on the ratio
the stomatal resistance to aerodynamic resistance (cf. Monteith
1965). The critical soil moisture and stomatal resistance vary
soil/vegetation type. Cf. Gregory
Smith (1990) and Smith (1990b) for
- Soil temperature is predicted after Warrilow
et al. (1986) from heat conduction in four layers. The depth of the
topmost soil layer is given by the penetration of the diurnal wave,
depends on the spatially varying soil heat conductivity/capacity. The
soil layers are, respectively, about 3.9, 14.1, and 44.7 times the
of this top layer. The top boundary condition for heat conduction is
net downward surface energy balance, including the latent heat of
for snowmelt; the bottom boundary condition is zero heat flux. Heat
by snow is modeled by reducing the thermal conductivity between the top
two soil layers; however, subsurface moisture does not affect the
properties of the soil. Cf. Smith (1990b)
- Sea ice is assumed to have a constant salinity of 0.6
salinity is affected by sea ice formation and melt, sublimation,
and snow melt, where the latter freshwater fluxes are assumed to pass
through ice leads to the ocean below. The weight of snow accumulating
sea ice may force the ice-snow interface below the water line, thereby
increasing the ice depth by the formation of "white ice" (cf. Ledley
1985). Snow cover does not affect the surface albedo of the ice,
- Sea ice thermodynamics are based on the zero-layer model of Semtner
(1976), with the inclusion of an ice concentration parameter A
after Hibler (1979). (A has
of 0.995 in the Arctic and 0.980 in the Antarctic, since completely
ice cover is rarely observed.) Internal brine pockets are not
Atmospheric surface fluxes and temperatures are calculated separately
the ice and the leads, with a linear temperature profile assumed in the
ice interior. Shortwave radiation does not penetrate the ice
but does warm the ocean where there are leads, just as it would at open
ocean grid squares. All other surface heat fluxes into the leads are
so that a fraction equal to the ice concentration is used to form/melt
ice, with the remainder applied to the ocean model. The heat flux
from the ocean below the ice is proportional to the difference between
the temperature of the topmost ocean layer and that of the base of the
ice (assumed to be -1.8 deg C).
- Sea ice dynamics follow the simple free-drift parameterization of
(1969b), for which the ice internal pressure gradient is
The current in the topmost ocean layer advects (by an upstream scheme)
the ice thickness, concentration and snow depth. A rough
of rheology prevents convergence of ice once its depth exceeds 4
Mechanical energy due to wind mixing, applied under the ice, is
using the drag coefficient appropriate for leads; it is weighted by the
leads fraction of each grid square. See Johns
et al. (1996) for further details.
The atmospheric component of coupled model HadCM2 differs from features
of the AMIP model UKMO
HadAM1 (2.5x3.75 L19) 1993 in the following respects:
Chief Differences from Closest
Errors in horizontal interpolation of the ozone reference climatology
were present in the AMIP
model now are corrected. (The effect of these corrections is
to be minor.)
The calculation of forced detrainment in the AMIP
model is modified to ensure internal consistency with the cloud top
and with the choice of reference level for the initial mass flux when a
parcel becomes unsaturated due to the addition of latent heat.
The parameterization of stratiform cloud formation is modified as
In the AMIP
model, the cloud condensate depends on the spatial fluctuations of
temperature within the grid box, which are assumed to take the form of
a "top-hat" probability distribution. In coupled model HadCM2, the
condensate depends instead on the spatial fluctuations of liquid
temperature, and these are assumed to follow a triangular
The range and temperature-dependence of the albedo of sea ice differ
those of the AMIP
model. In coupled model HadCM2, the sea ice albedo varies from 0.80
for ice surface temperatures at or below -10 deg C to 0.50 at 0 deg C
et al. 1997).
Bryan, K., (1969b) Climate and ocean
III The ocean model. Mon. Wea. Rev., 97: 806-827.
Eagleson, P.S., 1978: Climate, soil and
vegetation. Water Resources Res., 14, 705-776.
Gregory, D., and R.N.B. Smith,
1990: Canopy, surface, and soil hydrology. Unified Model Documentation
Paper No. 25, United Kingdom Meteorological Office, Bracknell,
RG12 2SZ, UK.
Hibler, 1979: A dynamic-thermodynamic sea
ice model. J. Phys. Oceanogr., 9: 817-846.
Johns. T.C., 1996: A description of
the Second Hadley Centre Coupled Model (HadCM2). Climate Research
Note 71, Hadley Centre, United Kingdom Meteorological
Office, Bracknell Berkshire RG12 2SY, United Kingdom, 19 pp.
Johns, T.C., R.E. Carnell, J.F.
J.M. Gregory, J.F.B. Mitchell, C.A. Senior, S.F.B. Tett, and R.A. Wood,
1997: The second Hadley Centre coupled ocean-atmosphere GCM: Model
spinup and validation. Climate Dyn., 13, 103-134.
Ledley, T.S., (1985) Sea ice:
cycles and white ice. J. Geophy. Res., 90, 5676-5686.
Levitus, S., 1982: Climatological atlas
the world's oceans. NOAA Professional Paper 13, 173 pp.
Monteith, J.L., 1965: Evaporation and
Soc. Exptl. Biol., 19, 205-234.
Semtner, A.J., 1976: A model for the
growth of sea ice in numerical investigations of climate. J.
Oceanogr., 6, 379-389.
Shuttleworth, W.J., 1988:
The new challenge for process hydrology. J. Hydrol., 100,
Smith, R.N.B., 1990b: Subsurface, surface,
and boundary layer processes. Unified Model Documentation Paper No. 24,
United Kingdom Meteorological Office, Bracknell, Berkshire RG12 2SZ,
Warrilow, D.A., A.B. Sangster,
A. Slingo, 1986: Modelling of land surface processes and their
on European climate. DCTN 38, Dynamical Climatology Branch, United
Meteorological Office, Bracknell, Berkshire RG12 2SZ, UK.
CMIP Documentation Directory
Last update 15 May, 2002. This page is maintained by Tom