O the Dynamics of Coastal Winds.
The transition of an offshore flow, from land to ocean, is treated both observationally and theoretically. Observations were taken over several years near the Malaspina Glacier on the south coast of Alaska using research ships, buoys, and on one occasion an instrumented aircraft. At the beach, winds blow offshore almost continuously in the winter, and at night in the summer. Their direction is seemingly unperturbed by synoptic variations. However, within approximately 50 km of the beach, the wind is unpredictable by synoptic analysis. Upper-air soundings in this region reveal a complicated process whereby coastal air flows under the marine boundary layer and subsequently is absorbed into it by a process of entrainment and warming. A layer averaged, numerical model is developed to help discern the dominant physics. Entrainment at the top of the layer is parameterized with contributions from buoyancy flux, surface shear, and interfacial shear. Fluxes of momentum, heat, and vapor at the surface are parameterized with bulk transfer coefficients, corrected for layer stability and surface roughness. When a two-dimensional version of the model is applied to the case of offshore flow, rapid transition is easily confirmed. All three components of entrainment can be important, thus the effect is not strongly dependent on sea-air temperature difference. Various formulations of drag coefficient only mildly affect the salient features of the process. Finally, baroclinity in the layer, a result of offshore warming, contributes strongly to the dynamic balance and often creates a super-geostrophic surface wind for long distances offshore. The model is applied to a katabatic wind and shows how radiational cooling and entrainment combine to determine layer depth and wind. In well developed katabatic flows, dynamic balance is predominantly between buoyancy, drag, and baroclinity.
- Pub Date:
- Physics: Atmospheric Science; Geological Survey