Mountain-valley wind systems

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mountain–valley wind systems

The diurnal cycle of local winds in a mountain valley during clear or mostly clear periods of weak synoptic flow.

The traditional components of the cycle are upslope (anabatic) winds, the daytime upvalley wind, downslope (katabatic) winds, and the nighttime downvalley wind (Defant,1951). In this traditional view, each component has corresponding compensatory currents aloft, presumably to form a closed circulation. For example, the downvalley wind would lie beneath a wind aloft directed up the valley. Observationally, these compensatory currents have been verified in some cases but not found in others. The classic model has the upvalley wind continuing until after sunset, but in many semiarid regions (or during dry periods), when the Bowen ratio is large and the surface heat flux is strong, boundary layer convection interrupts this model by mixing ridgetop winds down to the surface for much of the mid- to late afternoon. Where the valley opens onto a plain, a valley outflow jet, which represents a continuation of the downvalley wind, often extends many kilometers over the plain at night. Variations from this basic scheme arise from valley orientation, especially affecting the transition periods. In a north–south valley, for example, sunset occurs first on the east-facing slopes, and katabatic flows begin earlier there, which can result in cross-valley winds connecting with anabatic flow components on the west-facing sidewalls still in the sun. Similarly, at sunrise the east-facing slope is exposed to sunshine earlier than the rest of the valley, and therefore anabatic flows begin earlier there.

Defant, F. 1951. Compendium of Meteorology. 663–665.

Banta, R. M., and W. R. Cotton 1981. An analysis of local wind systems in a broad mountain basin. J. Climate Appl. Meteor.. 20. 1255–1266.

Whiteman. C. D. 1990. Observations of thermally developed wind systems in mountainous terrain. Meteorol. Monographs. 45. 5–42.

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