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frontogenesis
Frontogenesis is a meteorological process of tightening of horizontal temperature gradients to produce fronts. In the end, two types of fronts form: cold fronts and warm fronts. A cold front is a narrow line where temperature decreases rapidly. A warm front is a narrow line of warmer temperatures and essentially where much of the precipitation occurs. Frontogenesis occurs as a result of a developing baroclinic wave. According to Hoskins & Bretherton (1972, p. 11), there are eight mechanisms that influence temperature gradients: horizontal deformation, horizontal shearing, vertical deformation, differential vertical motion, latent heat release, surface friction, turbulence and mixing, and radiation. Semigeostrophic frontogenesis theory focuses on the role of horizontal deformation and shear.
==Kinematics==
Horizontal deformation in mid-latitude cyclones concentrates temperature gradients—cold air from the north and warm air from the south. Horizontal shear has two effects on an air parcel; it tends to rotate the parcel (think of placing a wheel at a point in space and as the wind blows, the wheel rotates) and deform the parcel through stretching and shrinking. In the end, this can also tighten temperature gradient, but most importantly, this rotates a concentrated temperature gradient for example, from the x-axis to the y direction. Within a mid-latitude cyclone, these two key features play an essential role in frontogenesis. On the west side of a typical mid-latitude cyclone, there are northerly winds (associated with cold air) and east of the cyclone, southerly winds (associated with warm air); resulting in horizontal shear deformation. In the end, this results to concentrate a cyclonic shear along a line of maximum shear (which in this case is the birth of a cold front). On the eastern side of a cyclone, horizontal deformation is seen which turns into confluence (a result of translation + deformation). Horizontal deformation at low levels is an important mechanism for the development of both cold and warm fronts (Holton, 2004).
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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