Irene winds at points vs gust factor

Larry Brown at our office put together some interesting statistics from Irene here is his post.

While not comprehensive, I plotted hourly wind speed, wind gust speed, and peak gust wind speed (in this case for the previous hour), and then derived the gust factor.  This was done at RIC, NGU (land sites), at a marine site, RPLV2 (Rappahannock Light), and a land site right next to the water CHYV2 (Cape Henry).

For more specifics and detail see the attached graphs, but as an overall summary what verified was a gust factor of about 1.4 to 1.65 over land sites (with little change in the gust factor for varying wind directions at RIC….to a much more significant change in gust factor with wind direction at NGU.  At NGU…the gust factor was generally 1.3 to 1.4 when winds were NE…then 1.6 to 2.0 when winds shifted to the WSW.  At the true marine site RPLV2, there was basically no change in gust factor with wind direction, generally remaining a rather low factor of 1.15 to 1.20 throughout the event.  At CHYV2 the gust factor was very dependent upon wind direction…a significantly higher factor (1.5-2.0) (although not necessarily higher wind speeds) when the wind direction was off land late in the event (NW to SW)…and significantly lower factor (1.2-1.35) when the wind direction was off the water (NE).Note that most of our land sites, outside of perhaps ECG and points twds the coast in NC, sustained tropical storm force winds/40 mph+ were almost non-existent (or only lasted for a few hrs) which would suggest TCM wind grids (even those issued during the day Sat) required about a 40-50% reduction factor over land to verify (in some cases even more). However,  a high gust factor was required since peak gusts at most places reached 60 mph or higher.

I have not shown graphs from Isabel here but overall conditions were not that much different with respect to gust factor (a bit lower but still fairly high over land).  The track was different and therefore winds never really shifted to a westerly direction during the event.  Once again though, sustained winds over land were barely into the tropical storm forec range…while peak gusts were to 75 mph or higher across most of our inland CWA.

Larry

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3 Responses to Irene winds at points vs gust factor

  1. ryanelliswforah says:

    Hi Larry (John),

    Nice work with the gust factors in Irene. I suspect boundary layer turbulence and surface roughness length may be part of what is behind the varying numbers. The numbers make perfect sense in that over land the sustained wind speeds will decrease due to increasing friction (roughness length) but the variability will be higher because of different things (trees, buildings, varying land use) are creating more opportunities for higher turbulence which would then increase the gust factor tremendously. Over the more laminar surface of the ocean greater sustained winds are possible but there are less opportunities for turbulence to increase the gust factor. The fetch of where the wind is coming from makes all the difference as well as how long that fetch is. For instance with RIC it doesnt matter what direction the wind is coming from because the approaching wind is encountering land of about the same roughness length in all directions. Near the ocean the direction is crutial because it may determine an over ocean or over land fetch. I do suspect that if RIC were closer to the mountains, the direction may once again become a factor as the roughness length of the land would increase significantly. Thanks for the numbers and the graphs!

    -Ryan Ellis
    WFO Raleigh

  2. Jonathan Blaes @ WFO RAH says:

    Thanks for putting this together and sharing Larry. Your efforts here have inspired us to have a student put together some factors for some locations in NC. We have messed around with this a little bit but haven’t done this before. In your methodology, you used the regular hourly METAR to create the gust factor correct just grabbed the peak wind for the prvious hour correct? We hope to include hourly precipitation in our chart and perhaps the T/Td spread as a surrogate for surface stability.

  3. Pingback: CSTAR Update – notes from the quarterly conference call on 02 December | CIMMSE

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