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tropical_cyclones - Exceptional Weather Tropical Cyclones...

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This document last updated on 26-Oct-2011 EENS 2040 Natural Disasters Tulane University Prof. Stephen A. Nelson Tropical Cyclones We have already discussed the factors involved in the circulation of the atmosphere and oceans. Knowledge of the general atmospheric circulation patterns, the Coriolis effect, and circulation around high and low atmospheric pressure areas, and the principles concerning the importance of water in transferring heat between the atmosphere and oceans from the previous lecture is essential for understanding the material in this section. Hurricanes (Tropical Cyclones) Tropical Cyclones are massive tropical cyclonic storm systems with winds exceeding 119 km/hr (74 miles/hour). The same phenomena is given different names in different parts of the world. In the Atlantic Ocean and eastern Pacific ocean they are called hurricanes. In the western Pacific they are called typhoons , and in the southern hemisphere they are called cyclones . But, no matter where they occur they represent the same process. Tropical cyclones are dangerous because of their high winds, the storm surge produced as they approach a coast, and the severe thunderstorms associated with them. Although death due to hurricanes has decreased in recent years due to better methods of forecasting and establishment of early warning systems, the economic damage from hurricanes has increased as more and more development takes place along coastlines. It should be noted that coastal areas are not the only areas subject to hurricane damage. Although hurricanes loose strength as they move over land, they still carry vast amounts of moisture onto the land causing thunderstorms with associated flash floods and mass-wasting hazards. Origin of Tropical Cyclones (Hurricanes) z When a cold air mass is located above an organized cluster of tropical thunderstorms, an unstable atmosphere results. (This is called a tropical wave ). This instability increases the likelihood of convection, which leads to strong updrafts that lift the air and moisture upwards, creating an environment favorable for the development of high, towering clouds. A tropical disturbance is born when this moving mass of thunderstorms maintains its identity for a period of 24 hours or more. This is the first stage of a developing hurricane. Exceptional Weather - Tropical Cyclones 10/26/2011 Page 1 of 14
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z Surface convergence (indicated by the small horizontal arrows in the diagram below) causes rising motion around a surface cyclone (labeled as "L"). The air cools as it rises (vertical arrows) and condensation occurs. The condensation of water vapor to liquid water releases the latent heat of condensation into the atmosphere. This heating causes the air to expand, forcing the air to diverge at the upper levels (horizontal arrows at cloud tops).
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