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Evidence for Gap Flows in the Birch Creek Valley, Idaho

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  • 1 Field Research Division, NOAA/Air Resources Laboratory, Idaho Falls, Idaho
  • | 2 Fire Sciences Laboratory, U.S. Forest Service, Missoula, Montana
  • | 3 Field Research Division, NOAA/Air Resources Laboratory, Idaho Falls, Idaho
  • | 4 Laboratory for Atmospheric Research, Washington State University, Pullman, Washington
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Abstract

A field study was conducted of flows in the Birch Creek Valley in eastern Idaho. There is a distinct topographic constriction in the Birch Creek Valley that creates two subbasins: an upper and lower valley. The data were classified into one of three groups based on synoptic influence (weak/absent, high wind speeds, and other evidence of synoptic influence). Gap flows commonly developed downwind of the constriction in association with the weak/absent group but also occurred in association with the two synoptic groups suggesting the potential for more diverse origins. In general, the frequency and strength of gap flows appeared to be linked to the development of the requisite thermal regime and minimization of any synoptically driven southerly winds that would suppress outflows. Gap flows were characterized by high wind speeds with jetlike vertical profiles along the axis of the lower valley. For all three groups the morning transition in the upper valley and western sidewall usually proceeded slightly ahead of the lower valley, consistent with the principles of the topographic amplification factor. The persistence of southerly winds in the lower valley past evening transition inhibited the development of gap flows, promoted strong nighttime inversions, and delayed the onset of morning transition relative to the upper valley. Nocturnal temperature inversions in the lower valley were largely eliminated with the onset of strong gap flows resulting in earlier morning transitions there. The form for a method of predicting gap flow wind speeds is proposed.

Corresponding author address: D. Finn, NOAA/Air Resources Laboratory, Field Research Division, 1750 Foote Dr., Idaho Falls, ID 83402. E-mail: dennis.finn@noaa.gov

Abstract

A field study was conducted of flows in the Birch Creek Valley in eastern Idaho. There is a distinct topographic constriction in the Birch Creek Valley that creates two subbasins: an upper and lower valley. The data were classified into one of three groups based on synoptic influence (weak/absent, high wind speeds, and other evidence of synoptic influence). Gap flows commonly developed downwind of the constriction in association with the weak/absent group but also occurred in association with the two synoptic groups suggesting the potential for more diverse origins. In general, the frequency and strength of gap flows appeared to be linked to the development of the requisite thermal regime and minimization of any synoptically driven southerly winds that would suppress outflows. Gap flows were characterized by high wind speeds with jetlike vertical profiles along the axis of the lower valley. For all three groups the morning transition in the upper valley and western sidewall usually proceeded slightly ahead of the lower valley, consistent with the principles of the topographic amplification factor. The persistence of southerly winds in the lower valley past evening transition inhibited the development of gap flows, promoted strong nighttime inversions, and delayed the onset of morning transition relative to the upper valley. Nocturnal temperature inversions in the lower valley were largely eliminated with the onset of strong gap flows resulting in earlier morning transitions there. The form for a method of predicting gap flow wind speeds is proposed.

Corresponding author address: D. Finn, NOAA/Air Resources Laboratory, Field Research Division, 1750 Foote Dr., Idaho Falls, ID 83402. E-mail: dennis.finn@noaa.gov
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