add new entries in Code Table 4.2, product discipline 0, category 1 (moisture)

[SibylleK]

Branch

https://github.com/wmo-im/GRIB2/tree/issue-59

Summary and purpose

Addition of a new entry in Table 4.2, discipline 0 (Meteorological products), category 1 (Moisture)

Action proposed

The team is requested to approve the content of this proposal for inclusion with the next update of the WMO Manual on Codes.

Discussions

A new entry for precipitation of cloud ice analog to snow precipitation rate is requested to be able to distinguish between snow and cloud ice particle deposition.

Detailed proposal

ADD in Table 4.2, discipline 0, category 1:

Title_en	SubTitle_en	CodeFlag	Value	MeaningParameterDescription_en	Note_en	UnitComments_en	Status
Parameter number by product discipline and parameter category	"Product discipline 0 - Meteorological products, parameter category 1: moisture"	146		Cloud ice precipitation rate		kg m-2 s-1

[sebvi]

Following my comment during the meeting, I would like to have additional information on what is "cloud ice" precipitation rate and how it differs from or complements the existing similar entries in Code Table 4.2, discipline 0, category 1. For instance:
67 Freezing rain precipitation rate
68 Ice pellets precipitation rate
73 Hail precipitation rate
75 Graupel (snow pellets) precipitation rate
95 Freezing or frozen precipitation rate

[SebastianB2]

We consulted our experts on this issue, and here's their answer:

The precipitation categories you mentioned all refer to large-size particles commonly considered as precipitation.
"Cloud ice" is the small particle mode of a bi-modal ice particle distribution, which is often observed in the atmosphere. The large mode is usually called "snow". The small ice particle mode consist primarly of primary ice crystals (monomers), but may include some small aggregates. Particle shape is dominated by irregular crystals, plates, columns or dendrites, but in some models this category may also include small frozen drops and ice pellets.
Cloud ice has a slow, but but still non-negligible sedimentation speed. Not all microphysics schemes take into account cloud ice sedimentation, but the new table entry is supposed to allow distinguishing between the precipitation rates originating from cloud ice and snow if desired.
In general "ice pellets" are not the same as cloud ice (see, e.g., the definition on: https://glossary.ametsoc.org/wiki/Ice_pellets ).

The entry:
0 Cloud ice (kg m-2)
does already exist in discipline 0 and category 6, but there is no corresponding "precipitation rate" yet.

In order to avoid misinterpretations, would it be of help, if we add a note to the new entry?

146 Cloud ice precipitation rate****

**** Assuming a cloud containing a bi-modal ice particle distribution, “cloud ice” refers to the small particle mode,
whereas the large mode is usually called “snow”. (“Ice pellets”, in contrast, may refer to the precipitation
of sleet, formed from freezing raindrops or refreezing (partially) melted snow flakes, or the precipitation of small hail.)

[SibylleK]

The branch issue-59 has been updated with the new entry Cloud ice precipitation rate.