The forcing of 25-knot winds at Hickam and Andersen AFB
Smith, Scott G.
Nuss, Wendell A.
Harr, Patrick A.
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This study investigates synoptic scale regimes in the forcing of 25 knot winds at Hickam and Andersen AFB. Ten years of data from January 1996 through December 2005, as well as case studies from June, July and August of 2006 were considered for this study. Days were grouped together to isolate the events of trade wind flow only and to alleviate days where trade wind flow was interrupted by synoptic scale weather events or local weather phenomena. Of the approximately 3,650 days of observations, Hickam AFB had 258 days in which the winds gusted to or above 25 knots and 1,077 days in which the winds remained between 15 and 24 knots. Similarly, Andersen AFB had 99 days where the winds gusted to or above 25 knots and 448 days where the winds remained between 15 and 25 knots. These days were then combined in their respective lists and were compiled to create composite sea level pressure surface analyses, winds, temperature, dew point, and geopotential height for each list of days. Upon examination of the compiled charts, identifiable climatological regimes became evident for days in which the winds gust to or above 25 knots and when the winds remained between 15 and 24 knots. The climatological average for Hickam AFB for winds gusting to or above 25 knots consisted of the subtropical high located 894 miles almost due north of Hawaii and the strength of the high at 1024 mb. The subsequent gradient across Hawaii produced an average geostrophic flow of 15 m s-1 across. North to South cross sections of potential temperature and winds across Hawaii indicated low static stability and analyzed winds of 7-9 m s-1. The climatological average when the winds remained between 15 and 24 knots consisted of the subtropical high located much further east and with a strength of 1022 mb. This reduced the gradient across Hawaii and produced an average geostrophic flow of 11 m s-1. North to South cross sections indicated stronger static stability and analyzed winds of 5-7 m s-1. Similarly for Andersen AFB, the climatological average for winds gusting to or above 25 knots consisted of a protrusion of the Siberian High due north of Guam, approximately 1050 miles away. The strength of the high was 1018 mb causing a gradient that produced a geostrophic flow of 15 m s-1 across Guam. North to South cross sections again revealed lower static stability and higher wind speeds associated with this regime. For the climatological average when the winds remained between 15 and 24 knots, the Siberian High had retreated to the west allowing the gradient across Guam to decrease causing the geostrophic flow to drop to 10 m s-1.
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