incorrect units of "Potential evaporation rate" in Code Table 4.2

[jbathegit]

Branch

https://github.com/wmo-im/GRIB2/tree/issue-15

Summary and purpose

This document proposes a solution for the problem of incorrect units of "Potential evaporation rate" in Code Table 4.2, Discipline 0, Category 1, Parameter 41

Action proposed

As part of the November 2020 fast-track updates, the existing parameter 41 should be retained in the tables but marked with a note, and a new parameter with the correct units should be added to the same discipline and category within Code Table 4.2. A similar approach was used previously for "Evapotranspiration" in Discipline 2, Category 0, Parameter 6.

Discussions

Discipline 0, category 1, parameter 41 is defined as "Potential evaporation rate" with units of W m-2. Given that W = J s-1 = kg m2 s-3, then the existing units of W m-2 equate to kg s-3, which seems odd since W m-2 are units normally used for flux parameters, not for an evaporation rate. Instead we believe that the correct units should be kg m-2 s-1 to conform with other rate parameters in this table, and also because the preceding parameter 40 ("Potential evaporation") is defined with units of kg m-2.

As far as we can tell, this parameter 41 has been in the GRIB tables going all the way back to 2010 (version 5), so this may be another example of an incorrect parameter that nobody is likely using but which has been carried on for years through the manuals as a holdover from the old GRIB1 tables (like "Evapotranspiration", which we dealt with last year at IPET-CM-3 in Marrakech) In fact, during that meeting last year, Atsushi noted that there may be many such questionable parameters lurking in the tables and that it would be beneficial (for any folks who had some spare time and were so inclined ;-) to do a thorough review of Code Table 4.2 to flag any and all such parameters, if for no other reason than to make sure they don't get propagated forward yet again into a future GRIB3.

Regardless of whether a larger cleanup effort is undertaken in the future, we believe it would make sense to go ahead now and correct the problem in the current Code Table 4.2 as follows:

Detailed proposal

Within Code Table 4.2, Discipline 0, Category 1:

• Add new Note (1) to existing parameter 41: "The listed units for this parameter appear not to be appropriate for potential evaporation rate. Instead, it is recommended to use parameter 122."
• Add new parameter 122 as "Potential evaporation rate" with units of kg m–2 s–1

[sebvi]

I fully agree that the units are totally wrong and should be kg m-2 s-1.

I also agree that a huge cleaning is needed in Code Table 4.2 for future GRIB3, possibly reorganizing the categories and creating new disciplines because discipline "meteorology" has because a "fit all" discipline. (but it is a discussion for the GRIB3 repo)

[chenxiaoxia2019]

@jbathegit Hi, Jeff, a new branch for this issue has been created. Could you please check it? Thanks.

[chenxiaoxia2019]

@jbathegit Hi, Jeff, I found that the new parameter 122 as "Potential evaporation rate" with units of kg m–2 s–1 clashed with issue #17. So I change from 122 to 143. Is it okay? FYI,
#17
122-127
#15
122 changed to 143
#20
123 changed to 128
#24
124 changed to 144
125 changed to 145

[jbathegit]

Hi @chenxiaoxia2019, and yes it's fine with me to switch our parameter from 122 to 143. It's just a number as far as I'm concerned, and this would allow @tomkralidis to keep all of his proposed entries together within #17.

[jbathegit]

Hi (again ;-) @chenxiaoxia2019 I just checked the issue-15 branch, and everything looks correct.

[efucile]

@chenxiaoxia2019 we need to remember to add the following note
Add new Note (1) to existing parameter 41: "The listed units for this parameter appear not to be appropriate for potential evaporation rate. Instead, it is recommended to use parameter 143."

[amilan17]

SUMMARY: Add entry 143 with correct units to Code Table 4.2, Discipline 0, Category 1 and add note to entry 41.

[amilan17]

Approved by FT-2020-2.