|
Definition:
|
1. Most widely used in mountain meteorology to denote a downslope flow driven by cooling at the slope surface during periods of light larger-scale winds; the nocturnal component of the along-slope wind systems. The surface cools a vertical column of the atmosphere starting at the slope surface and reaching perhaps 10-100 m deep. This column is colder than the column at equivalent levels over the valley or plain, resulting in a hydrostatic pressure excess over the slope relative to over the valley or plain. The horizontal pressure gradient, maximized at the slope surface, drives an acceleration directed away from the slope, or downslope. Although the pressure-gradient forcing is at its maximum at the slope, surface friction causes the peak in the katabatic wind speeds to occur above the surface, usually by a few meters to a few tens of meters. The depth of the downslope flow layer on simple slopes has been found to be 0.05 times the vertical drop from the top of the slope. Surface-wind speeds in mountain-valley katabatic flows are often 3-4 m/s, but on long slopes, they have been found to exceed 8 m/s. Slopes occur on many scales, and consequently katabatic flows also occur on many scales. At local scales katabatic winds are a component of mountain-valley wind systems. At scales ranging from the slopes of individual hills and mountains to the slopes of mountain ranges and massifs, katabatic flows represent the nocturnal component of mountain-plains wind systems. Besides diurnal-cycle effects, surface cooling can also result from cold surfaces such as ice and snow cover. Katabatic flows over such surfaces have been studied as glacier winds in valleys and as large-scale slope flows in Antarctica and Greenland. The large- scale katabatic wind blowing down the ice dome of the Antarctic continent has sometimes reached 50 m/s on the periphery of the continent. The persistence of the surface forcing and the great extent of the slopes on these great landmasses means that the flows are subject to Coriolis deflection, and thus they are not pure katabatic flows. 2. Occasionally used in a more general sense to describe cold air flowing down a slope or incline on any of a variety of scales, including phenomena such as the bora, in addition to thermally forced flows as described above. From its etymology, the term means simply "going down" or "descending," and thus could refer to any descending flow; some authors have further generalized it to include downslope flows such as the foehn or chinook even though they do not represent a flow of cold air. This concept has given rise to the expression katafront, which indicates flow down a sloped cold-frontal surface.
|