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The Taku Wind of Southeast Alaska: Its Identification and Prediction

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  • 1 NOSS/ERL/FSL, Analysis and Prediction Program, Boulder, Colorado
  • | 2 National Weather Service Forecast Office, Juneau, Alaska
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Abstract

The purpose of this study is to investigate the occurrence of severe winds in southeast Alaska (locally known as Taku winds) based on recent theoretical advances in the understanding of severe downslope windstorms. We found that the Taku wind is a manifestation of an amplified mountain wave. A complicating factor in understanding the Taku is the coincident occurrence of gap flow. Analysis of a number of historical events, in addition to a unique set of wind records from a nearby ridge, shows the separate identity of these concurrent phenomena. A set of criteria is identified that must be fulfilled in order for the downslope winds to occur, which is much more restrictive than the conditions necessary for gap flow. The three necessary criteria are 1) an inversion at or just above ridgetop, somewhere between 1500 and 2000 m MSL, 2) strong cross-barrier flow near ridgetop, typically 15–20 m s−1 in geostrophic wind speed, and 3) cross-barrier flow decreasing with height to a critical level somewhere between 3000 and 5500 m MSL. The similarities to other local downslope windstorms are also discussed.

Abstract

The purpose of this study is to investigate the occurrence of severe winds in southeast Alaska (locally known as Taku winds) based on recent theoretical advances in the understanding of severe downslope windstorms. We found that the Taku wind is a manifestation of an amplified mountain wave. A complicating factor in understanding the Taku is the coincident occurrence of gap flow. Analysis of a number of historical events, in addition to a unique set of wind records from a nearby ridge, shows the separate identity of these concurrent phenomena. A set of criteria is identified that must be fulfilled in order for the downslope winds to occur, which is much more restrictive than the conditions necessary for gap flow. The three necessary criteria are 1) an inversion at or just above ridgetop, somewhere between 1500 and 2000 m MSL, 2) strong cross-barrier flow near ridgetop, typically 15–20 m s−1 in geostrophic wind speed, and 3) cross-barrier flow decreasing with height to a critical level somewhere between 3000 and 5500 m MSL. The similarities to other local downslope windstorms are also discussed.

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