Code Table 4.201: changing entry 0 "reserved" to "no precipitation/none"

[sebvi]

Branch

https://github.com/wmo-im/GRIB2/tree/iss110

Summary and purpose

Update code Table 4.201 entry 0 meaning from "reserved" to "no precipitation"

Action proposed

The team is invited to review the proposal and validate it to be included in the next Fast Track.

Discussions

The code Table 4.201 contains the precipitation type used by the parameter "precipitation type" (discipline 0, category 1, entry 19). When using the precipitation type parameter to report the type of precipitation in each grid box, represented by an integer defined in code table 4.201, there is obviously several grid boxes where no precipitation is occurring. What is the integer then used to indicate "no precipitation"? Since code table 4.201 do not contain an entry for "no precipitation", I am assuming that the only way to encode the information is the set those grid boxes to "missing/not applicable/not a value" and use a bitmap in the bitmap section. This seems to me unpractical. I am wondering why entry 0 in code Table 4.201 does not mean "no precipitation/none" but is instead "reserved"! Looking at similar tables 4.XXX, the entry 0 always seems to indicate "none/not present/etc.". For instance, 4.203 (cloud type) -> clear (which means no clouds), 4.204 (thunderstorm coverage) -> none, 4.205 (aerosol) -> not present, 4.206 (volcanic ash ) -> not present, 4.207 (icing) -> none, 4.208 (turbulence) -> none(smooth), etc. I have also discovered that, at ECMWF, we use entry 0 to denote "no precipitation", probably to have it consistent with cloud type (4.203). If this does not cause any issues (and provided that the change is accepted), I would be grateful if this could be added to the current Fast track.

Detailed proposal

Update Code Table 4.201	Code	Meaning
0	Reserved None (or no precipitation)

[sebvi]

branch updated

[jitsukoh]

@sebvi @amilan17 I confirm that the branch is updated and the issue was moved to the validated status.