Code Table 4.2: Subgrid-scale lightning potential index

[SibylleK]

Branch

https://github.com/wmo-im/GRIB2/tree/iss108

Summary and purpose

Request of a new lightning potential index parameter.

Action proposed

The team is kindly asked to review and approve the content for inclusion within the next update to the WMO Manual on Codes.

Discussions

The existing entry in code table 4.2 discipline 0, category 17:
1 Lightning potential index (LPI) (J kg-1)
stands for the LPI derived from grid scale (large scale) model quantities in convection permitting models, if we follow the definition after Lynn et al. 2010.
DWD requests a new LPI parameter derived from sub-grid scale (convective) model quantities.

Detailed proposal

Add in code table 4.2 discipline 0, category 17

Number	Parameter	Units
5	Subgrid-scale lightning potential index (see Note 3)	J kg-1

Note 3: The lightning potential index (LPI, Number 1), as defined by Lynn et al. 2010, is derived from grid scale (resolved) model information in convection permitting models. In contrast, the subgrid-scale lightning potential index is derived from subgrid-scale information (from parameterized convection) for models with coarser resolution.

[sebvi]

Dear @SibylleK ,

thank you for this proposal. I have forwarded it to our LPI expert so that he can eventually comment.

[sebvi]

Dear @SibylleK ,

please see below the answer from our LPI expert:

If I understand DWD's request correctly, they would like to add a new parameter for the Lightning Potential Index (LPI) when the latter is computed from a model that parametrizes convection (i.e. run at resolutions coarser than say 2 km). This new parameter would come in addition to the original LPI as computed from a model that resolves convection explicitly (typ. at resolutions finer than 2 km).

However, the proposed naming for the new parameter could be misleading. I would suggest using 'resolved' and 'subgrid-scale' LPI, respectively. The proposed qualifier 'convective' sounds inappropriate here, since by nature, lightning always requires the presence of convection. The goal here is simply to identify the way the LPI was computed.

More generally, do we really expect both LPI parameters to be non-zero at the same time? I would expect the subgrid-scale LPI to be zero when using a convection-resolving model, and the resolved LPI to be zero when using a model with parameterized convection. If this turned out to be the case, defining two LPI parameters might be irrelevant.

[SibylleK]

Dear @sebvi ,

thank you very much for your support and the comments of your colleague.
I have received the following comments from my colleagues:

We agree that "subrid-scale LPI" is a more precise name for the new parameter.

However, we also believe that having two separate parameters is beneficial because the two parameters are not computed the same way. Their structural behavior is different. For a forecaster, for example, it would be helpful to know what kind of LPI he is dealing with, to avoid confusion. One can compare this to "precipitation" where we also distinguish between precip coming from the sub-grid scale and resolved precip.

[SibylleK]

Dear @jitsukoh , dear @amilan17 , et.al.,

this requirement and proposal came up only very recently. I would like to ask, if it is still possible to include it in the current fast-track?

[sebvi]

Dear @SibylleK ,

I tend to disagree regarding the analogy with precipitation if you mean by that large-scale precipitation vs convective precipitation. In the case of precipitation, it is not the same parameter computed by 2 different methods, it is 2 distinct components of the precipitation coming from 2 different physical processes and they could both be non zero in the same grid box.
In the case of your proposal, it is the same parameter but computed differently, and more it is the same parameter but computed by 2 different types of models where the short scale process is resolved or parameterized. As pointed out by our expert at ECMWF, the existing LPI and the one you propose can't coexist in a model output, you have either one or the other.
That said, I agree that it is important for the forecaster to have this information but I believe it is part of the documentation of the model producing the parameter.
If I take the example of 2m temperature, it is not evaluated the same by all the models (global model vs limited area model vs land surface model etc.) but we don't have multiple 2m temperature.

I think comments from others in @wmo-im/tt-tdcf would be valuable for this discussion.

[amilan17]

@SibylleK remove convective from the title and update issue summary with finalized proposal
@amilan17 create branch

[SibylleK]

I have updated the proposal. A "Subgrid-scale lightning potential index" is proposed now and the note is changed a little bit for a better understandig, hopefully.

[jitsukoh]

@SibylleK thank you for the updated proposal. Looks good to me.
@sebvi could you comment, if any?

[SibylleK]

branch is updated

[jitsukoh]

@SibylleK @amilan17 I confirm that the branch is updated and I move this issue to the validated status.

[amilan17]

@jitsukoh @SibylleK Editorial question about "Subgrid-scale...", because I found different versions in the manual. Can you please verify the correct version?  

1. subgrid-scale (not found in manual, only in this proposal)
2. sub-grid scale (orthography and tiling in 4.2, discipline 0, category 3)
3. sub-gridscale (orography in 4.2, discipline 0, category 3)
4. subgrid (liquid and ice in 4.2, discipline 0, category 1)

[sebvi]

maybe we need the point of view of a native english speaker for this. @jbathegit ?

In any case, once we agree on 1 term, all others should be corrected consistently . My preference goes with option 3.