WMO proposals for new UERRA parameters

Based on following documents:

• 2015-10-20 Meeting notes
• UERRA parameter list

1. New short-wave radiation parameters

Proposed parameters for Product discipline 0 - Meteorological products, parameter category 4: short-wave radiation

Question: What about "Direct short-wave radiation flux, clear sky" produced by IFS and archived in MARS under paramId 228022 (Clear-sky direct solar radiation at surface)

Justification: The proposed parameters are commonly computed by numerical weather prediction models and operationally archived. They are of the same type like ones already existing in WMO code tables (e.g. Downward short-wave radiation flux, Net short-wave radiation flux, clear sky). Additional information illustrating their meaning and handling can be checked in the attached document about radiation quantities in the IFS model developed by ECMWF.

2. New long-wave radiation parameter

 Proposed parameters for Product discipline 0 - Meteorological products, parameter category 5: long-wave radiation

Justification: The proposed parameters are commonly computed by numerical weather prediction models and operationally archived. They are of the same type like ones already existing in WMO code tables (e.g. Downward short-wave radiation flux, Net short-wave radiation flux, clear sky). Additional information illustrating their meaning and handling can be checked in the attached document about radiation quantities in the IFS model developed by ECMWF.

3. Soil heat flux

Proposed parameter for Product discipline 2 - Land surface products, parameter category 3: soil products

Justification: The heat flux is currently defined only under Product discipline 2 - Land surface products, parameter category 0: vegetation/biomass. The currently existing parameters in the same category related to soil are mostly deprecated and moved to the category 3: soil products where the soil heat flux is not defined at all.

4. Percolation 

Proposed parameter for   Product discipline 1 - Hydrological products, parameter category 0: hydrology basic products 

Justification: There is only "Water runoff and drainage" defined in the WMO tables which is not suitable for this newly proposed parameter. 

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

Newly proposed type of level soil level for Code table 4.5-Fixed surface types and units

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 community. 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.

How to represent soil levels.

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

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