Offshore-Directed Winds in the Vicinity of Prince William Sound, Alaska

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  • 1 Pacific Marine Environmental Laboratory/N0AA, Seattle, Washington
  • | 2 Pacific Marine Environmental Laboratory/N0AA, and Department of atmospheric Sciences, University of Washington, Seattle, Washington
  • | 3 Pacific Marine Environmental Laboratory/N0AA, Seattle, Washington
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Abstract

The thermal contrast between cold air over continental Alaska and relatively warm marine air over the Gulf of Alaska causes frequent, low-level, offshore-directed winds over the south-central Alaskan coast during the cold season. Coastal mountains affect these winds by inhibiting low-level mixing of continental and marine air masses near the coast, by providing channels that focus and accelerate drainage winds, and by exciting mountain-lee waves.

Offshore-directed winds were observed twice with a research aircraft. The strongest winds were measured at the mouth of the Copper River and over and downwind of Resurrection Bay. The synoptic weather pattern and its orientation to local topographic features influenced wind magnitude. With the pressure gradient perpendicular to the coast, offshore-directed flow was light except at the Copper River and Resurrection Bay drainages. When the pressure gradient was aligned with the shore, regional surface winds were stronger and more uniform, although the Copper River and Resurrection Bay drainages wore still discernible.

Evaluation of local force balances showed the largest ageostrophy at coastal locations downwind of bays and river valleys. Seaward from the coast, ageostrophic accelerations and cross-isobaric wind components were smaller, indicating a transition toward geotriptic equilibrium. This spatial adjustment pattern is consistent with the hypothesis that equilibrium is achieved within a distance similar to the regional Rossby radius of deformation.

Abstract

The thermal contrast between cold air over continental Alaska and relatively warm marine air over the Gulf of Alaska causes frequent, low-level, offshore-directed winds over the south-central Alaskan coast during the cold season. Coastal mountains affect these winds by inhibiting low-level mixing of continental and marine air masses near the coast, by providing channels that focus and accelerate drainage winds, and by exciting mountain-lee waves.

Offshore-directed winds were observed twice with a research aircraft. The strongest winds were measured at the mouth of the Copper River and over and downwind of Resurrection Bay. The synoptic weather pattern and its orientation to local topographic features influenced wind magnitude. With the pressure gradient perpendicular to the coast, offshore-directed flow was light except at the Copper River and Resurrection Bay drainages. When the pressure gradient was aligned with the shore, regional surface winds were stronger and more uniform, although the Copper River and Resurrection Bay drainages wore still discernible.

Evaluation of local force balances showed the largest ageostrophy at coastal locations downwind of bays and river valleys. Seaward from the coast, ageostrophic accelerations and cross-isobaric wind components were smaller, indicating a transition toward geotriptic equilibrium. This spatial adjustment pattern is consistent with the hypothesis that equilibrium is achieved within a distance similar to the regional Rossby radius of deformation.

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