Difference between IBTrACS and ATCF B-deck

[Vincent Maarek]

Hi,

I was wondering how the IBTrACS data differ from the ATCF B-deck data (https://ftp.nhc.noaa.gov/atcf/btk/ and https://ftp.nhc.noaa.gov/atcf/archive/) ?

Especially :

- Within a few days after a storm (0-20 days), which data source is considered to be the most "official" one or the most accurate ?

- How both datasets interact ?

- How often both are updated, and are they maintained by the same people ?

- When final data is released, should we expect the exact same data in both sets ? (I don't think so, as IBTrACS seems more granular compared to B-deck)

Many thanks for your help.

Kind regards,

Vincent

[Ken Knapp - NOAA Federal]

Hi Vincent-

These are good questions. From a U.S. perspective, the datasets can appear similar to identical. But internationally is where the difference lies. 

The best way to learn about the similarities and differences between B-deck and IBTrACS is by using the IBTrACS Documentation

https://www.ncei.noaa.gov/sites/g/files/anmtlf171/files/2024-07/IBTrACS_version4r01_Technical_Details.pdf

If you still have questions after that, please let us know.

Thanks-

-Ken

[Vincent Maarek]

Dear Ken,

Many thanks for this. I've read through the official documentation, but I am a bit unclear on several additional aspects.

• I understand that there are two kinds of Best Tracks : the Working Best Tracks and the "real" Best Tracks that are processed post TC season. For the Atlantic basin specifically, does the Working Best Track correspond to the B-deck data released right at the moment of TC occurrence ? Does the "real" Best Tracks correspond to the B-deck data revised post TC season ?
• Can the B-deck data, change for the next 0-20 days after a TC event (for instance, previous points locations updated, windspeed updated, etc.) ? Or is there a one-off data published through the B-deck, and updated only after at the end of the TC season ? If the above applies, does it mean that the associated IBTrACS data is updated ? 
• I understand that the IBTrACS data is smoothed with a 3h stamp, using a spline for location points, and a linear regression for e.g. windspeed/central pressure. Does the spline modify the location points of the "officially recorded 6h points" of the Best Track data of the B-deck ? Additionally, what is the regressor of the windspeed (is it the distance between the points) ? If so, does it mean that by construction, the IBTrACS synoptic points will never match the officially recorded synoptic points of the underlying best track data ?
• One additional "new question" : the Public Advisory data is usually published every 3h. How this data is "sampled" to produce the working B-deck data, which is only every 6h ? How accurate is the Public Advisory, and how close should we expect this data compared to the B-deck ? Are the estimations of the Public Advisory considered "human-dependent" to make sure that local people take much consideration of the potential danger ?

Many thanks for your help.

Kind regards,

Vincent

[Ken Knapp - NOAA Federal]

Hi

Thanks for reviewing the document. 

* N. Atlantic working best track corresponds to the B Decks as they are updated during the storm (and are available immediately after the storm). It is the information that NHC has for the storm at that time.

* After the season, NHC reanalyzes the storms. When complete, they update B Decks and also release the new HURDAT2 file.

* After a storm (0-20 days), NHC can change the storm data in B Decks, but I think that would be very rare (Their primary focus after that is forecasting the next storm). (In short: Can they? Yes. Will they change it? I really doubt it.)

* IBTrACS is interpolated to 3 hourly (not smoothed ... the latter implies some averaging).

* The spline does NOT change the 6h positions.

* Regressor for wind speed interpolation? Well, they are interpolated between the 6-hr (and other obs as well) points. So the information from NHC (6 hourly as well as any off synoptic points) is recorded in IBTrACS. IBTrACS simply provides interpolated locations at 3 hourly (interpolating between the 6-hr points) for temporal continuity with other international BT data.

* To be clear, you state "the IBTrACS synoptic points will never match the officially recorded synoptic points of the underlying best track data" and that is incorrect. The IBTrACS data is a reproduction of the Best Track data with interpolated points added. 

* Your questions on the Public Advisories is a question for NHC. Best track data is historically a 6-hr product from NHC and I am not familiar with all of their products.

I hope this helps.

-Ken

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Ken Knapp,

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[Jonathan Vigh]

Hi Ken and Vincent,

I wanted to mention that one situation in which the working best track could be changed during or soon after a storm event is when the warning agency realizes that they were behind the curve on rapid intensification. In that case, the intensity values in the working best track may be adjusted after the fact to "catch up" to what the forecasters thought the true intensity may have been (looking back with the benefit of more data). Since it is called the "working best track", this type of change is entirely normal and expected. I can't say how frequently it happens though, as I haven't analyzed this myself. The way to find out if such a change has occurred is to compare the real-time analyses for the parameter of interest (which are provided in the CARQ lines of the a-decks), with the corresponding parameter values of the b-decks.

Best regards,

Jonathan 

Jonathan Vigh						
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Research Applications Laboratory (RAL)
NSF National Center for Atmospheric Research (NSF NCAR)
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[Vincent Maarek]

Ken, Jonathan,

Many thanks for all your detailed answers, this has proven to be extremely helpful.

Kind regards,

Vincent