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The use of the ENSEMBLES s2d data requires additional information on the way some variables are computed by the different models. You can find below a list of the specific definitions for those variables.
- Atmosphere
This is an instantaneous value computed as a vertical accumulation. Since the values of soil moisture in the individual soil layers are volumetric, the integrated volumetric soil moisture is calculated as a weighted vertical sum. The weight for each layer is the depth of that layer divided by the total soil depth.
The depth of the layers varies depending on the atmospheric model. ECMWF (IFS) levels are 7 cm (top layer), 21 cm, 72 cm, and 189 cm (bottom layer), with a total depth of 289 cm. Met Office levels for HadAM3 are 10cm, 25cm, 65cm and 200cm, giving a total soil depth of 300cm.
Note that GRIB cannot describe the depth of those layers with upper and/or lower boundaries at depths >255cm. Therefore, the characteristic of each model will be described in the MARS documentation.
- Soil level 1 temperature:
This variable contains the temperature of the first soil layer over land, an average of the surface water temperature and the layer 1 sea-ice temperature, weighted by the sea-ice fraction over open waters and the layer 1 sea-ice temperature over ice. Note that neither the soil nor the sea-ice temperatures are skin temperatures. Although this is a variable automatically postprocessed by IFS, other models, such as the Met Office one, need to create the data for this variable from the sea surface, ice and first soil layer temperature fields.
- Ocean
- Sea level elevation:
- CERFACS (OPA): it is a prognostic variable.
- ECMWF (HOPE-C): sea level is a prognostic variable.
- IfM (HOPE-E): the ocean model is formulated with a free surface elevation (i.e., sea level
in the model is a prognostic variable, calculated from the horizontal
divergence of ocean currents).
- INGV (OPA8.2): although the ocean model is not rigid lid, the sea level
elevation is not prognostic, the computation being
based on the kinematic
surface boundary condition for a not impermeable upper boundary,
allowing the penetration
of water through the sea surface. Mass fluxes through the air-sea
interface determine the
tendency for the free surface.
- Met Office: it produces the rigid-lid pressure scaled to have the units
of height.
- Mixed layer depth:
- CERFACS (OPA): it is based on a turbulent kinetic energy criterion.
- ECMWF (HOPE-C): this depth is given by z_mld = min(z) for SST-T(z) > 0.5 K, that is the
depth where the temperature difference to the surface exceeds 0.5 K for
the first time.
- IfM (HOPE-E): it is defined in terms of the density profile, and is computed as
the depth at which the density change from the surface density exceeds 0.125
psu.
- INGV (OPA8.2): it is diagnosed using a
criterium based on
vertical density differences. Specifically, the MLD is diagnosed as the
last vertical (w) level
(starting from the bottom) where the density exceeds the surface
density plus a threshold
value set equal to 0.01 Kg/m^3.
- Met Office: the mixed layer depth diagnostic is a diagnostic of the mixing process, rather than of the resulting
vertical profile and is calculated as a product
of the Kraus-Turner mixed layer model, where the balance of
energy (wind mixing vs introduction of buoyancy) available for mixing
the water column is calculated and then the water is mixed from the
surface down to a level where the energy runs out.
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