As another follow up to Jonathan’s post, David Glenn from NWS Morehead City provided me with Weatherflow data for Sandy (2012). The data included five minute wind speed observations for sixteen sites in the Outer Banks. Per David, none of these sites reported any outages during the storm, providing us with wind speed observations higher than in some of our previous gust factor analyses. The five minute temporal resolution of the data provides us with a more comprehensive wind data set. As in previous analyses, gust factors were only calculated for sustained winds winds of 10 mph or greater. A total of 8,800 gust factors were computed for Sandy from this WeatherFlow data.

The chart below shows a scatter plot of five minute sustained wind speeds in mph versus gust factors for the 8,800 available observations. A logarithmic fit was also added to the plot. As in previous analyses, the chart demonstrates an inverse relationship between the wind speed and gust factor as well as a decrease in the variability in observations as wind speeds decrease.

A few interesting observations:

- Previous regressions indicated an R^2 value ranging from .0005-.4229. The analysis presented here suggested a higher R^2 value of .4595, indicative of a stronger correlation between sustained wind and gust factor. I suggest this can be partially explained based on the locations of the WeatherFlow sites. These sites are mostly located over Cape Hatteras and Ocracoke in the Outer Banks, where flow is primarily maritime with little surface land roughness. As a result, the wind direction and surface roughness contributions to explain gust factor variance are not likely as important in the case of these Outer Banks sites.
- Previous analyses showed very few occurrences of gust factors below 1.2. This current analysis shows gust factor values below 1.2, particularly for stronger wind speeds. Hence, the previously considered asymptotic decay to 1.2 may need to be revised taking into account this data set with stronger wind speeds.
- The mean gust factor was calculated as 1.31. This is in line with what has been suggested in recent studies for coastal stations by the CSTAR TC Winds group.
- The equation for the regression curve (y=-.4595ln(x) + 2.928) is similar to the other storms previously examined, but it tends to be on the higher side of the curves for lower end sustained winds. This is consistent with the results presented in Jonathan’s previous post for Sandy.

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Two factors that could decrease the correlation are instrumentation heights and sample periods for the wind gusts. Regarding the latter, ASOS’s utilize a 5 second sample period for wind gusts, which is a little longer than many other wind sysems utilize. As a result, ASOS reported wind gusts tend to run a little lower than instumentation that utilizes 1 or 2-second sample periods. I am not sure there is much you can do about this, as opposed to instrumentation height differences where there are documented techniques to normalize. Just pointing it out as an additional complicating factor.

Aside, compared to many meso-networks, I know Weatherflow has made a substantial effort to optimize the siting of their stations (exposure-wise) and make them as compatible as possible with other data sets that are considered higher quality.

Jeff

Bryce,

Thanks for putting this together and sharing it. I appreciate your analysis and thanks to David for tracking down the data. As many folks have stated, exposure and averaging periods have a profound effect on measured wind speeds and resulting gust factors.

In our previous entries on gust factors on the blog, we have included both ASOS and AWOS units with a preference for the ASOS when it is available. I cannot speak to the specifics for AWOS systems, but the algorithm used by ASOS is explained in detail in the ASOS User’s Guide (http://www.nws.noaa.gov/asos/pdfs/aum-toc.pdf) on page 23 in sections 3.2.2.2 Wind Character and 3.2.2.2a Gusts

…Once every 5 seconds the ASOS computes the current 2-minute average wind speed and compares it with the greatest 5-second average wind speed during the past minute. If the current 2-minute average wind speed is equal to or greater than 9 knots and the greatest 5-second average wind speed (during the past minute) exceeds the current 2-minute average speed by 5-knots or more, then the greatest 5-second average speed observed during the past minute is stored in memory as a gust for 10 minutes…

So, the minimum gust speed reported by ASOS is 14 knots. Wind speeds from 3 knots and 11 knots may be reported with gusts to 14 knots. Because of this limitation, ASOS will likely have a gust factor floor of around 1.2. The 2-minute average wind speed requirement of 9kts is why plots we have shown in previous blog posts have omitted any winds, gusts, gust factors associated with winds that are 9kts or less (~10.4 MPH).

The regression curve does provide a nice R squared value. Eyeballing it and noting the 1.2 gust factor flow for land sites suggests that the asymptotic decay well below 1.2 might be overdone. With fewer gust factor values less than 1.2, the mean of 1.31 may nudge upward as well.

Thanks again, JB

Hi JB,

Thanks for the insight. From what I can tell, there are no limitations in the WeatherFlow data as far as minimum sustained wind speeds in order to report a gust factor. As you might imagine, the data was very noisy for wind speeds below 10 mph (gust factors as high as 3.0+).

I understand your reasoning as far as no data availability below 9kts for the gust factor data. However, can you clarify this statement: “Wind speeds from 3 knots and 11 knots may be reported with gusts to 14 knots. Because of this limitation, ASOS will likely have a gust factor floor of around 1.2.” I am not understanding what you mean by this.

Thanks, JB.

Bryce

Hey Bryce,

Thanks for the note. I am not sure my comments were well written and I think I may have made a goof in my calculations. I was trying to refer to the fact that since wind gusts had to exceed the wind by at least 5kts for them to be reported, any observation with them would have a minimal predicable gust factor of at least wind/(wind+5kts).

So if we calculate each of the gust factors for the minimum 5kt exceedance of the wind gust over the sustained wind you get minimum gust factors for each of the sustained winds shown below…

sustained wind/minimum gust factor

9/1.56

20/1.25

30/1.17

40/1.13

50/1.10

JB