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Real-life example: The newly proposed percolation rate what be the parameter K3 in the SURFEX 3-layers scheme (right picture ISBA 3-L; it is called gravitational drainage there).
5. Soil levels
How to represent soil levels.
| Code Block | ||
|---|---|---|
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Discipline=2
parameter Category=3
parameter Number=18
First fixed surface=150 (Generalised vertical height coordinate)
scale Factor=0
scaled Value=0
Second fixed surface=150 (Generalised vertical height coordinate)
scale Factor=0
scaled Value=1 |
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| title | Height of the soil level |
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Newly proposed type of level soil level for Code table 4.5-Fixed surface types and units
| Type of Level | Units | Description |
|---|---|---|
Soil level | - | This level represents a soil model level (similar to hybrid model level above surface). The scaled value encoded together with this new type of level represents the level number. The aim of this type of the level is to encode a soil level that has not a constant depth across the model domain (each grid cell has soil levels of different depths). The non-constant depth is then encoded as a parameter on this type of level alongside the other parameters like temperature, soil moisture, etc. A "soil layer" (useful to encode volume integrated parameters) can be represented by defining 2 soil model levels using the usual GRIB2 mechanism ("first fixed surface/second fixed surface" where the deepest soil level is always encoded as the second fixed surface). |
| Parameter | Units | GRIB2 code | Description |
|---|---|---|---|
| Soil depth | m | discipline 2 category 3 number ?? | This parameter is used to encode the soil depth in meters (The depth is positive downward by convention). It is meant to be used together with the type of level "soil level" to encode the depth of the level at each grib point. |
Justification: In some models, the depth of the soil levels is not constant for all model grid points. The depths will vary depending on several factors like the type of land cover (grass, forests, crops, city, etc.), time of the year, etc. (see below for concrete examples). It is not possible to encode soil parameters like soil temperature for such systems using the actual WMO code tables: type of level 106 - "depth below Land surface" only allows encoding of constant depth for the whole domain, types of level 167 and 177 could be useful, but only grasp 2 very specific depths and do not solve the encoding of an n-levels soil model. The newly proposed type of level "Soil level" would allow a generic encoding of these levels similarly like hybrid levels do for model levels above surface. Soil levels would be dimensionless and the level number would be encoded using scale factor/scaled value mechanism. Once a soil level is defined, one can define a parameter on that level to encode the non constant depth in meters. This is why we would like to propose a new "soil depth" parameter in "Product discipline 2 – Land surface products, parameter category 3 – soil products" to be used in conjunction with the new type of level.
Real-life example: In some surface schemes like the SURFEX surface model, the depths of soil layers are not constant. SURFEX is part of Hamonie system and will be used in Arpege (part of IFS) and will also be used in the Aladin commnunity. The depths of soil layers are computed at the beginning (during Analysis) according to several factors: type of land cover, time of the year, etc. To illustrate this lets look at the soil scheme provided in SURFEX documentation: 3-layers scheme. The key point to understand is that the values of "ds", "d2" and "d3", which are the depths corresponding to the bottom of the surface layer, bottom of root zone layer and deep soil layer are NOT the same for each grid cell. This is because an oak tree does not have the same root length like a pine which have itself a different root length than grass, etc. The time of the year has also an influence on the depth of the bottom of the root zone: early in the Spring the crops are very small and are only starting to grow for the seeds disseminated on the fields, their roots are very small. During the summer (or the harvesting season) the roots goes much deeper hence the bottom of the root zone is deeper as well to reflect that. After the fields have been harvested the root zone does not exist any more (like during the winter for instance). In case of SURFEX the information about depths of each level for each grid point are extracted from the ECOCLIMAP II database (see particularly fig1). These depths can be written by the model in GRIB messages if the appropriate type of level and soil depth parameter are defined.Concrete examples of encoding using the new type of level "soil level":
How to represent soil levels.
| Code Block | ||
|---|---|---|
| ||
Discipline=2
parameter Category=3
parameter Number=18
First fixed surface=151 "Soil level"
scale Factor=0
scaled Value=0
Second fixed surface=151 "Soil level"
scale Factor=0
scaled Value=1 |
| Code Block | ||
|---|---|---|
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Discipline=2
parameter Category=3
parameter Number=27 "Soil depth"
First fixed surface=151 "Soil level"
scale Factor=0
scaled Value=1
Second fixed surface=missing
scale Factor=missing
scaled Value=missing |