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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 commnunitycommunity. 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.
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