An Analysis of Applying Three Statistical Methods for Calculating the Standard Deviation of the Wind Direction at the Tropical Region

Abstract

In this paper, a review about three classical methods for calculating the standard deviation of the horizontal wind direction (σ_θ) is presented. These methods described by Verral and Williams, 1982 (VW), Ackermann, 1983 (ACK), and Yamartino, 1984 (YM) are evaluated by intercomparison between them using wind data obtained from radiosonde measurements. These measurements were taken every six hours over a period of 11 days of September 2008 (dry season of the north of NE region of Brazil, with strong trade winds). Calculations indicated that YM method showed to be the most reliable, whereas the results of VW were very close to YM. Comparing the methods two by two, the maximum differences were 29 degrees between ACK and YM and 26.3 degrees between ACK and VW, while between YM and VW the maximum difference was only 2.6 degrees. According to the reviewed literature, YM is the most stable method over the range of σ_θ, with maximum error of 2%.

Keywords: standard deviation; vertical profile; radiosondes; trade winds