Investigating the Impact of Changing The ASOS Wind Gust Averaging Period From Five to Three Seconds

The National Weather Service's Automated Surface Observing System (ASOS) uses a cup and vane type anemometer to measure wind. A gust is subjectively defined as a "sudden brief increase in the speed of the wind." The ASOS algorithm reports a gust when the two minute average wind speed is greater than or equal to nine knots and the greatest five-second average wind speed (during the past minute) exceeds the current two-minute average by five knots or more. Two issues arise using the ASOS method of gust determination. One issue with the ASOS reporting scheme is that it uses a five-second average for computing gusts. The WMO currently recommends using a three-second average. The other issue occurs when different types of anemometers are used to measure gust speed and number of gusts.

To obtain data in a gusty, high wind environment, a number of cup and vane and sonic anemometers were installed near the summit of Mt. Washington, NH. This paper will summarize the findings of the intercomparison. More specifically, it will investigate the effect of using different averaging times to compute the gust speed for each anemometer. This paper will also present the impact of using different anemometer technologies on the number and speed of gusts.