The Influence of Low Frequency and Synoptic Variability on the Timing, Magnitude, and Geographical Distribution of Extreme Wind Events
Jeffrey H. Yin, NCAR, Boulder, CO; and G. Branstator
Extreme wind events associated with synoptic scale extratropical storms can have substantial economic impacts on populated areas of the midlatitudes in winter. This study seeks to understand the dynamics that influence the timing, magnitude, and geographical distribution of extreme wind events, in order to provide a dynamical framework for understanding how the statistics of extreme wind events are affected by variability on different time scales. However, because extreme events occur very rarely, it is difficult to characterize their dynamics using the short observational record. In order to alleviate this problem, this study takes advantage of the large sample size provided by the 9-member ensemble of Climate of the 20th Century experiments performed with the NCAR Community Climate System Model.
Because extreme wind events tend to occur during strong synoptic scale storms, one may assume that extreme winds are primarily controlled by variability on synoptic time scales. However, synoptic variability is superimposed on variability that occurs at longer time scales. If synoptic and low frequency variability are defined by time scales shorter and longer than 10 days, respectively, we find that wind extremes depend more strongly on the low frequency variability than on synoptic variability in most locations. Moreover, low frequency variability influences extreme winds in two ways: in an additive sense, by changing the magnitude of the low frequency wind, and in a multiplicative sense, by organizing synoptic variability. This study focuses on the geographical variations of the additive and multiplicative effects of low frequency variability on extreme wind events.
At the downstream end of the Atlantic storm track, we find that the relationships between extreme wind events and low frequency variability depend largely upon the location relative to the climatological jet and storm track. All points near the downstream end of the Atlantic storm track exhibit strong additive effects due to westerly low pass anomalies. However, the positive multiplicative effect on the equatorward flank of the jet results in a strong relationship between westerly anomalies and extreme wind events, while the negative multiplicative effect on the poleward flank of the jet results in weaker relationships. The baroclinic growth region at the upstream end of the Pacific storm track has an unusually weak relationship between extreme wind events and low frequency variability, caused by the large negative multiplicative effect due to the negative correlation between synoptic variability and the strength of the jet in that region.
Extended Abstract (1.5M)
Session 7, Climate and Extreme Weather Events I
Thursday, 18 January 2007, 1:30 PM-5:30 PM, 214B
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