Code Table 4.2-0-7: add new codes for Storm Severity Index (SSI)

[edouardsandrine]

Initial request

ECCC is proposing a new thermodynamic stability index, the Storm Severity Index (SSI), to detect the potential of severe weather.

The Storm Severity Index is an index developed by Environment Canada's Quebec Region (1983) and inspired by the publications of E.N. Rasmussen and R.B. Wilhelmson (1983). Its name in the original publication was Storm Severity Index, hence the acronym SSI.

As its name suggests, this index is used to predict the development of severe thunderstorms, i.e. the presence of one or more of the following phenomena :

• Hail over 2 cm in diameter
• Torrential rain of more than 50 mm
• Violent winds of over 90 km/h
• Tornadoes

Amendment details

The SSI (no unit) depends on the convective potential energy available between the free convection level (FCL) and the equilibrium level, and the average vertical shear (SH) between the surface and 4000m.

SSI = 100.0 * (2.0 + (0.276 * ln(SH)) + (2.011x10-4 * BE))

Where:

• SH is the mean vertical wind shear from the surface to 4000m in units of 10-3 s-1
• BE is the buoyant energy (CAPE) in J.kg-1

Interpretation:

SSI index Value	Interpretation
>= 100	Severe thunderstorms to be feared
95 < SSI < 100	Severe thunderstorms possible
< 95	No severe thunderstorm

SMC forecasters use it to quickly detect the potential for severe weather (hail, high winds, tornadoes, etc.).

In a context of increasingly frequent severe weather episodes with climate change, we believe this index could be useful to the community at large for various applications related to severe weather detection.

Consequently, we propose to add this new parameter in official WMO codes.

Comments

No response

Requestor(s)

Sandrine Edouard, @edouardsandrine , CCMEP, ECCC

Stakeholder(s)

Enter list of stakeholder(s).

Publication(s)

Manual on Codes (WMO-No. 306), Volume I.2, GRIB code table 4.2, discipline 0, category 7

Expected impact of change

None

Collaborators

No response

References

• Turcotte, V., and D. Vigneux, 1987: Severe thunderstorm and hail forecasting using derived parameters from standard RAOB data. Preprints, Second Workshop on Operational Meteor., Halifax, Nova Scotia, Canada, Atmos. Env. Serv./Canadian Meteor. And Oceanogr. Soc., 142-153.
• https://cmosarchives.ca/Congress_P_A/program_abstracts1988.pdf (p30)
• https://journals.ametsoc.org/view/journals/wefo/13/4/1520-0434_1998_013_1078_oposte_2_0_co_2.xml
• https://journals.ametsoc.org/view/journals/wefo/21/3/waf921_1.xml

Validation

No response

[sebvi]

Dear @edouardsandrine ,

thank you for this interesting proposal. The proposed category looks sensible to me, we can use the next available entry.

I will run the proposal to expert colleagues at ECMWF to see if they have further comments.

[edouardsandrine]

Excellent. If you have any question, do not hesitate!

[amilan17]

https://github.com/wmo-im/tt-tdcf/wiki/2024.11.13.tt.tdcf notes:

• Sandrine introduced the proposal
• @edouardsandrine please add reference to SSI article
• @edouardsandrine update branch (once ready)

[edouardsandrine]

Please find attached the initial technical note (1987) :

NoteTechnique_87N-001_DenisVigneux.pdf

In this paper the indice was called IN it has been changed for SSI later...

[edouardsandrine]

@sebvi, did you receive any comments from your colleagues ? Thanks a lot!

[amilan17]

https://github.com/wmo-im/tt-tdcf/wiki/2024.11.12.tt.tdcf notes:
@edouardsandrine branch is ready for updates;

[amilan17]

https://github.com/wmo-im/et-data/wiki/2025.01.14.et.data notes:
@edouardsandrine will update the branch

[edouardsandrine]

Branch updated

[amilan17]

SubTitle_en	CodeFlag	Value	MeaningParameterDescription_en	Note_en	noteIDs	UnitComments_en
Product discipline 0 - Meteorological products, parameter category 7: thermodynamic stability indices	21		Storm severity index			Numeric

@edouardsandrine Is this consistent with the proposal? To add one code for SSI?

[edouardsandrine]

Yes, I confirm. Thanks !

[amilan17]

Thank you. The branch has accidental revisions in Table 4.5. Can you update the branch to only include the proposal? Thx.

[edouardsandrine]

Done. Let me know if everything is OK.

Thanks!