Wind speed definition at sea

[Dennis Wagenaar]

Dear ibtracs team,

We have a question about the ibtracs definition for maximum sustained wind speed. What roughness do you assume at sea? I understand from the documentation that you assume an unobstructed situation but there is quite a big difference between the roughness at sea and for an open situation on land. Does the roughness change abruptly when the track makes landfall and how are timesteps handled when the wind is both over land and over sea?

Thanks in advance!

Kind regards,

Dennis

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[Howard Diamond]

Hello Dennis,
Thanks for your inquiry.  Frankly, IBTrACS is a compilation of best track information from the global Regional Specialized Meteorological Centers (RSMCs) tasked with developing best tracks for their ocean basin.  So, we in the IBTrACS program do not make any assumptions with respect to calculating winds speeds; we depend on the best track data that are provided to us and we do not apply any corrections other than basic quality control. 

The various RSMCs use the WMO standard for TC forecasting and this is what they use in the final determination of their best tracks data.  On page 89 of the WMO standard, with respect to roughness, it notes the following ,
"Sustained winds are greatly influenced by exposure and surface roughness characteristics. A better measure of tropical cyclone sustained wind is frequently obtained by using the observed gust and then converting it to an associated sustained wind as determined from a general relationship that better pertains to winds observed in tropical storms and hurricanes (e.g., Krayer and Marshall 1992; Harper et. al. 2010)". 

The guidelines by Harper et al (2010) go into great detail with respect to roughness, and in particular, I would focus your attention on Table 1.1 and the link to that publication is just below in the References section of this message.  As for your question on timesteps, I am frankly note really sure, but perhaps someone on the google groups address can chime in there.  As a side note, the IBTrACS team (except for me) is located at NOAA/NCEI in Asheville, North Carolina, and as you have no doubt heard, the recent passage of Hurricane Helene on 27-Sep has essentially paralyzed the entire region of western North Carolina, and so with power, water, and communications outages, coupled with inaccessible roads from washouts, mud, and debris (although I am told that the building housing NCEI is intact), I am not certain when others on the IBTrACS Team might be able to comment, and so I have done the best I could here; although the specific questions you have posed here related to TC dynamics and winds, as well as how those timesteps are accounted are not my area of expertise.

References:
Harper, B.A.,, Kepert, J.D. and Ginger, J.D., 2010: Guidelines for converting between various wind averaging periods in tropical cyclone conditions. World Meteorological Organization, TCP Sub-Project Report, WMO/TD-No. 1555.
Krayer, W.R., and Marshall R.D., 1992: Gust factors applied to hurricane winds, Bull. Am. Meteorol. Soc.73(5), 613-617 (and attached).

Hope that helps a bit, but if I you need me to look into this (or other aspects of IBTrACS) in further detail, please let me know and I will do my best to assist you here.

Regards.

Howard Diamond, PhD
Member IBTrACS Team

[Dennis Wagenaar]

Hi Howard,

So sorry to hear that the Ibtracs office was affected by hurricane Helene. I hope everyone is alright. 

Thank you for the explanation of how the sustained wind speeds are estimated, that is very interesting. Whenever your colleagues are available again it would be nice if they can comment on how the sea land transition is included with respected terrain roughness. My understanding is that the roughness length at sea is about ~0.0002 versus ~0.03 on open and unobstructed terrain on land and I wonder how and whether that is typically reflected in the sustained wind speed estimates in ibtracs.

Thanks again!

Kind regards,

Dennis

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From: Howard Diamond <howard....@noaa.gov>
Sent: Tuesday, October 1, 2024 7:21 PM
To: Dennis Wagenaar <dennis....@ntu.edu.sg>
Cc: IBTrAC...@noaa.gov <IBTrAC...@noaa.gov>; ibtra...@googlegroups.com <ibtra...@googlegroups.com>
Subject: Re: Wind speed definition at sea

 

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[Howard Diamond]

Hi Dennis,
Okay, thanks for your understanding of the situation in Asheville, but again, we depend on the data provided to us from the various RSMCs and the processes and procedures they use to develop the best tracks data from their center, but someone else on the team might have better insights on this particular issue you have raised, and so I will defer to them on this issue.

Regards.

Howard