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Effects of coastal forcing on turbulence and boundary-layer structure
Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences.
1999 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Coastal mountains of significant elevation impose constraints for the surrounding flow. The aim of this study is to describe the modifications of the marine atmospheric boundary layer that occur offshore of the west coast of the United States. Aircraft measurements, up to 1000 km off the coast from two experiments, are used.

This boundary layer is capped by a subsidence inversion, which slopes down toward the coast and produces large thermal winds. Low-level wind maxima (i.e. jets) are typical for these conditions, commonly a 40-50% increase relative to the 30 m wind speed. The effects of coastal forcing on low-level winds cancel in average when no regard is taken for position relative a cape or point. The variability of the low-level wind speed increases nevertheless significantly toward the coast, the standard deviation is ±40% of the offshore value. The scale of the adjustment downstream of a cape or point is specifically addressed. Some measurements support a formulation of the coastal extent based on an inviscid shallow-water concept; mean variables (i.e. 30 m wind speed and boundary-layer depth) and turbulent parameters (i.e. dissipation and shear production of turbulent kinetic energy) vary in a uniform, predicted manner.

The effects of coastal forcing on winds result in cold sea surface temperatures at the coast, due to upwelling. Stability becomes a function of offshore distance. Surface-layer turbulence statistics and spectra (and cospectra) of turbulence variables are presented. Across- and along-wind sampled spectra (and cospectra) show that large wind shear and shallow boundary layer affect the scales of the turbulence eddies. The relation between the standard deviations of wind components are affected. The turbulence appears to be non-local in some aspects, entrainment fluxes are proposed to be important due to a shallow boundary layer with a sharp, sloping inversion and a low-level jet.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 1999. , 40 p.
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 478
Keyword [en]
Earth sciences
Keyword [sv]
National Category
Earth and Related Environmental Sciences
Research subject
URN: urn:nbn:se:uu:diva-1002ISBN: 91-554-4557-8OAI: oai:DiVA.org:uu-1002DiVA: diva2:160536
Public defence
1999-10-29, Axel Hamberg-salen, Geocentrum, Villavägen 16, Uppsala, Uppsala, 10:00
Available from: 1999-10-08 Created: 1999-10-08Bibliographically approved

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