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mid-level convection on the budget of heat, water vapour and momentum
(Tiedtke, 1989). Cumulus clouds are represented by a bulk model including
the effect of entrainment and detrainment on the updraft and downdraft
convective mass fluxes. Mixing due to shallow stratocumulus convection is
considered as a vertical diffusion process with the eddy diffusion coefficients
depending on the cloud water content, cloud fraction and the gradient of
relative humidity at the top of the cloud. The closure for deep convection and organized entrainment in the original scheme has been modified
and is now based on buoyancy instead of the moisture budget, and organized detrainment is computed for a spectrum of clouds detraining at
different heights (Nordeng, 1994). Stratiform clouds are predicted per se
in accordance with a cloud water equation including sources and sinks due
to condensation/evaporation and precipitation formation both by coalescence of cloud droplets and sedimentation of ice crystals (Sundqvist, 1978;
Roeckner et aL, 1991). Sub-grid scale condensation and cloud formation
is taken into account by specifying appropriate thresholds of relative humidity depending on height and static stability. Convective cloud water
detrained in cumulus anvils as well as in shallow non-precipitating cumulus
clouds is used as a source term in the stratiform cloud water equation. The
land surface model considers the budget of heat and water in the soil, snow
over land and the heat budget of permanent land and sea ice (Diimenil and
Todini, 1992). The heat transfer equation is solved in a five-layer model
assuming vanishing heat flux at the bottom. Vegetation effects such as the
interception of rain and snow in the canopy and the stomatal control of
evapotranspiration are grossly simplified. The local run-off scheme is based
on catchment considerations and takes into account sub-grid scale variations of field capacity over inhomogeneous terrain. In the coupled model
the hydrological cycle is closed by a river routing scheme (Sausen et aL,
1994) which directs the local runoff into the oceans.
4.2
Ocean and sea ice
The oceanic model is based on the OPYC model (Oberhuber, 1993a,
1993b). It consists of three sub-models; the interior ocean, the surface
mixed layer and the sea-ice respectively. The governing equations are
solved on an Arakawa B-grid with no-slip horizontal boundary condition.
An implicit time stepping scheme is used and an alternating direction
implicit solution technique. Poleward of 36°, the horizontal resolution is
mid-level convection on the budget of heat, water vapour and momentum
(Tiedtke, 1989). Cumulus clouds are represented by a bulk model including
the effect of entrainment and detrainment on the updraft and downdraft
convective mass fluxes. Mixing due to shallow stratocumulus convection is
considered as a vertical diffusion process with the eddy diffusion coefficients
depending on the cloud water content, cloud fraction and the gradient of
relative humidity at the top of the cloud. The closure for deep convection and organized entrainment in the original scheme has been modified
and is now based on buoyancy instead of the moisture budget, and organized detrainment is computed for a spectrum of clouds detraining at
different heights (Nordeng, 1994). Stratiform clouds are predicted per se
in accordance with a cloud water equation including sources and sinks due
to condensation/evaporation and precipitation formation both by coalescence of cloud droplets and sedimentation of ice crystals (Sundqvist, 1978;
Roeckner et aL, 1991). Sub-grid scale condensation and cloud formation
is taken into account by specifying appropriate thresholds of relative humidity depending on height and static stability. Convective cloud water
detrained in cumulus anvils as well as in shallow non-precipitating cumulus
clouds is used as a source term in the stratiform cloud water equation. The
land surface model considers the budget of heat and water in the soil, snow
over land and the heat budget of permanent land and sea ice (Diimenil and
Todini, 1992). The heat transfer equation is solved in a five-layer model
assuming vanishing heat flux at the bottom. Vegetation effects such as the
interception of rain and snow in the canopy and the stomatal control of
evapotranspiration are grossly simplified. The local run-off scheme is based
on catchment considerations and takes into account sub-grid scale variations of field capacity over inhomogeneous terrain. In the coupled model
the hydrological cycle is closed by a river routing scheme (Sausen et aL,
1994) which directs the local runoff into the oceans.
4.2
Ocean and sea ice
The oceanic model is based on the OPYC model (Oberhuber, 1993a,
1993b). It consists of three sub-models; the interior ocean, the surface
mixed layer and the sea-ice respectively. The governing equations are
solved on an Arakawa B-grid with no-slip horizontal boundary condition.
An implicit time stepping scheme is used and an alternating direction
implicit solution technique. Poleward of 36°, the horizontal resolution is