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Carl F. Dierking

Abstract

A mountain wave windstorm in 1993 is documented for the Taku River Valley near Juneau, Alaska. The mountain wave produces a mesoscale pressure response that appears to enhance local gap flow and complicates efforts to separate the two forcing mechanisms. In addition, the Nested Grid Model is evaluated in terms of its ability to predict atmospheric structure favorable for the development of a mountain wave, since current model resolutions are not small enough to predict the event itself. In the 1993 case, numerical guidance is shown to have some value in the prediction of an atmospheric state conducive to the development of the windstorm.

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Bradley R. Colman and Carl F. Dierking

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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Nicholas A. Bond, Carl F. Dierking, and James D. Doyle

Abstract

The flow in Gastineau Channel near Juneau, Alaska, during the moderate Taku wind event of 18 October 2004 is examined using observations from the University of Wyoming’s King Air research aircraft, two wind profilers, and surface weather stations. These data sources reveal low-level winds directed down the central portion of Gastineau Channel, that is, gap flow. Farther down the channel, and above this gap flow, the winds were strongly cross channel in association with the downslope flow that characterizes Taku events. The transition region between these two flows included strong vertical wind shear and severe turbulence; measurements from the King Air indicate turbulent kinetic energy locally exceeding 50 m2 s−2. A high-resolution simulation of this case using the Naval Research Laboratory’s Coupled Ocean–Atmosphere Mesoscale Prediction System reproduced the observed character of the mean flow. This case illustrates the hazard to aviation posed by even a moderate Taku wind event and shows the value of a wind profiler for monitoring the vertical wind shear responsible for the hazard.

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