Gust factors in hurricane and non-hurricane conditions

The question of whether gust factors observed in hurricanes differ from those observed in non-hurricane conditions has proven to be a rather contentious one, with a number of authors arguing that they are indeed higher in hurricanes, when compared to those observed for the equivalent terrain conditions in non-hurricane conditions, while others have argued that there is no difference between the two sets of values. A significant part of the problem with comparing gust factors observed in hurricane and non-hurricane conditions is that in many cases a number of a priori assumptions about the behaviour of the atmospheric boundary layer with height above ground under these conditions are required in order to establish some form of equivalency between the observed gust factors. Further complications arise in that a consistent methodology has not generally been used to determine the gust factors in both hurricane and non-hurricane conditions.

In this paper, rather than trying to compare gust factors observed under differing terrain and measurement conditions in both hurricane and non-hurricane conditions, we simply pose the question are gust factors observed at a particular location during the passage of a hurricane significantly different from those observed under non-hurricane conditions at the same location?

We consider data recorded at a number of ASOS and C-MAN stations located along the Gulf and Atlantic coastlines of the United States. The data records examined cover an 8-year time period extending from 1998 through to 2005, during which time a total of 21 landfalling hurricanes that affected one or more stations occurred. For each station we first establish the observed distribution of gust factors by both direction and wind speed for non-hurricane conditions, before comparing these distributions with the gust factors observed during the passage of all the hurricanes that have affected the station. By means of statistical testing we then consider whether the gust factors observed during hurricane conditions form a separate distinct population, or whether they are indistinguishable from the larger population of gust factors observed under non-hurricane conditions.