10-extratropical_cyclones_v2010_v2

10-extratropical_cyclones_v2010_v2 - Chapter 10 Chapter...

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Chapter 10 Chapter 10 Mid-latitude Cyclones Mid-latitude Cyclones
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Extratropical or midlatitude Cyclones in IR satellite images
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The Polar Front Theory was postulated in the early part of the twentieth century to describe the formation, development, and dissipation of mid-latitude cyclones. Mid-latitude cyclones are large systems that travel great distances and often bring precipitation and sometimes severe weather to wide areas. Lasting a week or more and covering large portions of a continent, they are familiar as the systems that bring abrupt changes in wind, temperature, and sky conditions.
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In the real world we find a number of alternating semi-permanent cells of high and low pressure. Among the most prominent features in the Northern Hemisphere during winter are the Aleutian and Icelandic Lows over the Pacific and Atlantic Oceans, and the Siberian High over central Asia.
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In summer of the NH, the best-developed semi-permanent cells are the Hawaiian and Bermuda-Azores Highs of the Pacific and Atlantic Oceans and the Tibetan Low of southern Asia. ITCZ is the intertropical convergence Zone
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In the polar cells of the three-cell model, surface air moves from the polar highs to the subpolar lows . Compared to the poles, air at subpolar locations is slightly warmer, resulting in low surface pressure and rising air. Very cold conditions at the poles create high surface pressure and low-level motion toward the equator. In both hemispheres, the Coriolis force turns the air to form a zone of polar easterlies in the lower atmosphere.
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Cyclogenesis is the formation of a mid-latitude cyclone. Initially, the polar front separates the cold easterlies and the warmer westerlies. As cyclogenesis begins, a “kink” develops along the boundary. The cold air north of the front begins to push southward behind the cold front, and air behind the warm front advances northward, creating a counterclockwise rotation around a weak low-pressure system.
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With further intensification, low pressure deepens with distinct warm and cold fronts emerging. Convergence can lead to uplift and cloud formation, while linear bands of deeper cloud cover develop along the frontal boundaries. Cold air moves faster than warm air Occlusion represents the end of the cyclone’s life cycle and takes place as the center of the low pressure pulls back from the warm and cold fronts.
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The top figure depicts the typical structure of a mature cyclone and the processes causing uplift. Shaded areas represent cloud cover while the numbers represent an approximation of the precipitation probability. The bottom figure represents the isobar pattern of a mature cyclone
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Rossby waves are waves in the mid-latitude westerlies having wavelengths on the order of thousands of kilometers. Moving from points 1 to 3,
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This note was uploaded on 12/27/2011 for the course GEOG 110 taught by Professor Leila during the Fall '09 term at UCSB.

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10-extratropical_cyclones_v2010_v2 - Chapter 10 Chapter...

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