David Glenn from WFO MHX examined the sustained winds, wind gusts, and gust factors for Hurricane Irene across marine locations just off the coast of Virginia, North Carolina and South Carolina. Hourly observations of winds and wind gusts from seven marine platforms impacted by the wind field associated with Hurricane Irene were examined from 25 August through 29 August, 2011. The locations contained in the analysis include buoys 41001, 41004, 41013, 41036, 41037, 44009, and 44014. Numerous other marine platforms were examined but were excluded for various reasons including missing gust data, observational outage, and platform height.
A map of Irene’s track and the location of the buoys used in the analysis is shown to the upper-right. A table of the platform used along with pertinent details is shown below. For each observation, the hourly wind gust factor was computed. The gust factor is defined as the ratio between the wind gust of a specific duration to the mean (sustained) wind speed for a period of time. A total of 334 gust factors were computed for Irene. A dynamic map of Hurricane Irene’s intensity and track is available from the NOAA Coastal Service Center (link). The NHC summary report is also available (link).
Also note that a previous blog post, “A Wind Gust Factor Database from 10 Tropical Cyclones for Use with GFE Tool Development”, highlighted the examination of 14,938 gust factors at locations over land for ten tropical cyclones across the Carolinas and Virginia.
The chart to the right is a scatter plot of the sustained winds in MPH versus gust factors for the 334 observations included in the study along with a best fit regression curve for a tight range of gust factors. These values are clustered very tightly but show a very slight upward trend with increasing sustained wind speed.
On the left side of the chart below, the same scatter plot as above is shown but with a larger vertical axis. On the right, the 2,247 observations for land locations during Irene along with a best fit curve is shown. Note the tight clustering of marine gust factors in the chart on the left and the very slight upward trend in gust factors with increasing wind speed compared to the much wider distribution of gust factors with the land locations and the noticeable decrease in gust factors over land with increasing wind speed.
A histogram of the frequency of marine gust factors for Irene is shown to the right. Note the very few occasions of gust factors greater than 1.3 with the overwhelming majority, around 87%, of gust factors occurring between 1.10 and 1.29. The mean gust factor was 1.24. A total of 95 observations or more than 28% of all gust factors occurred within +/- 0.02 of the mean while 215 observations or more than 64% of all gust factors occurred within +/- 0.05 of the mean. The density of gust factors is encouraging for GFE tool development work and is consistent with other studies of gust factors in marine environments.
The chart to the right shows a plot of marine sustained wind speeds and wind gusts for Irene. The tight clustering of gust factors noted above is also shown in the chart below. The red line is the best fit trend line The correlation coefficient (R2) was found to be .983 or 98 percent of the variance explained, an excellent result.
We are currently examining several other tropical cyclones to affect the Southeast including Fran and Ernesto. Preliminary results appear to be consistent with the Irene results and other studies. Final results will be shared in a subsequent blog post.