Question on cyclone closest to equator

[Jesse Ferrell]

In the Northern Hemi, Wikipedia (yeah, I know) says Vamei 2001 was closest "storm" to equator at 1.4 lat, and the NOAA Coastal tracks site seems to bear that out. 

However, there was Unnamed 1973 which seems to have formed as a Depression farther south, at 0.5 lat. It later became a storm, and ibtracs says that happened at 0.7 lat at 29 knots, but that record is only from the CMA. If that were true, it would beat Vamei as a TS. The NOAA Coastal tracks site, however, says it wasn't a TS until much later, 10.4 lat. 

My question is, on the disagreement for where/when it became a TS, is the NOAA Coastal tracks site correcting ibtracks because of the 1-minute vs. 10-minute sustained rule difference between the basins?  Which is really correct?

Jesse Ferrell, Meteorologist @ AccuWeather

[Jennifer Gahtan - NOAA Federal]

Hi Jesse,

The confusion related to the classification of the storm Unnamed 1973 is due to differences in how CMA grades storm intensities.

As provided in the ibtracs column documentation:

CMA_CAT 

Intensity category according to the Chinese National Standard for Grade of Tropical Cyclones (which has been used since 15 June 2006): 

0 –– Weaker than Tropical Depression or unknown intensity; 

1 –– Tropical Depression (TD: 10.8–17.1 m/s); 

2 –– Tropical Storm (TS:17.2–24.4 m/s); 

3 –– Severe Tropical Storm (STS: 24.5–32.6 m/s); 

4 –– Typhoon (TY: 32.7–41.4 m/s); 

5 –– Severe Typhoon (STY: 41.5–50.9 m/s); 

6 –– Super Typhoon (SuperTY: ≥51.0 m/s); 

9 –– Extratropical Cyclone (ET) stage.

Thus the '1' would only indicate a TD which fits with the max wind speed of 29 knots from CMA and TD category given on the NOAA Coastal tracks site.

Note, IBTrACS does not correct wind speeds from the 2-min wind averaging period provided by CMA to the 1-min averaging period used by USA agencies, but does convert units from m/s to knots.

-Jennifer

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Jennifer Gahtan, Ph.D.

Physical Scientist
CSSD
NOAA's National Centers for Environmental Information (NCEI)
151 Patton Ave. Asheville, NC 28801

[Jason Rees]

Hi Jesse,

 

Your email to the IBTRACS Group about the 1973 depression, possibly holding the record for forming closest to the equator, caught my eye. As a result of being generally curious, a fan of yours, nosey and a Wikipedia editor, I decided to take a deeper look at the data from the China Meteorological Administration's Shanghai Typhoon Institute (https://tcdata.typhoon.org.cn/en/index.html). When I look at the BT Data published by the STI for 1973 (via: https://tcdata.typhoon.org.cn/en/zjljsjj.html (Download required)) or (https://web.archive.org/web/20190118175519/http://tcdata.typhoon.org.cn/CMABSTdata/CH1973BST.txt (Web Version)), I notice that they are saying that it was a Type 0 at 0.5N (ie: Unknown intensity/Below TD), Type 1 at 0.7 (Tropical Depression),  Type 0 at 3.7N, Type 1 at 8.5N, Type 2 (Trop Storm) at 10.4N etc. I also took at the data presented for 1973 from PAGASA, the JMA, JTWC and the HKO, which are all semi-official for the WPAC and noticed that none of which show a TC impacting the Philippines in December 1973.

 

When i turn my attention to the data that IBTrACS publishes here (https://ncics.org/ibtracs/index.php?name=v04r01-1973355N01152) for the same system, I notice that they classify the 1973 depression as a TS based on the data from the CMA saying that the system had winds of 15 m/s (30kts) at 0.7N. IBTRACS does this to every single system (See Katrina: https://ncics.org/ibtracs/index.php?name=v04r01-2005236N23285) and I can only presume that they do this so that they do not end up with complaints that a system with 35 kts on a 1-minute scale is classified as a Tropical Depression rather than a Tropical Storm when it only has 30 kt winds per the official RSMC who uses a 10-minute scale. It is worth noting at this junction though that IBTRACS are not a warning center and does not classify storms themselves, but is a collection of records containing BT Data from various agencies, including NHC, CMA, Fiji Met Service, Australia's Bureau of Meteorology etc

I also saw the article which you published on Accuweather entitled: A hurricane has never crossed the equator. Here's why. (https://www.accuweather.com/en/hurricane/a-hurricane-has-never-crossed-the-equator-heres-why/1783017) In which you talk about both systems, claim that NHC monitored the 1973 Depression - which they didn't since it was well out of their AOR. You also mention that "at the equator, however, the Coriolis force is essentially zero, making it impossible for a tropical system to cross over from one hemisphere to another" However, the precursor system to Agni 2004 dipped below the equator and was classified as an anticyclonic circulation by RSMC La Reunion. Then there is the matter of twin tropical cyclones (Eg: Cyclone Raja and Typhoon Noris 1986/7)  .... 

 

I will conclude by saying that while for now Wikipedia will continue to show Vamei, as the TC that formed closest to the equator - it probably isn't 100% correct for several reasons. This includes the fact that a global reanalysis for TC's is urgently needed (not just in the Atlantic) as it will probably find more cases like Vamei 2001 or Agni 2004, which either formed near or crossed the equator as well as several mistakes in how intense some of these TC's are. 

Kind Regards,

Jason Rees

Wikipedia Editor 

https://en.wikipedia.org/wiki/User:Jason_Rees

[Jennifer Gahtan - NOAA Federal]

Hi Jason and Jesse,

The IBTrACS variable NATURE isn't a strength classification and shouldn't be used as such, but denotes whether the nature of the storm is tropical, extratropical, etc. Tropical depressions do end up getting grouped with TS-Tropical here.

NATURE

Combined storm type. 

This is assigned based on all available storm types. 

They include: DS - Disturbance, TS - Tropical, ET - Extratropical, SS - Subtropical, NR - Not reported, MX - Mixture (contradicting nature reports from different agencies)

Sorry for any confusion, and it's possible we should look into how these are grouped and explained again.

Thanks,

Jennifer