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The meteorological history of Hurricane Ivan, the longest tracked tropical cyclone of the 2004 Atlantic hurricane season, lasted from late August through late September. The hurricane developed from a tropical wave that moved off the coast of Africa on August 31. Tracking westward due to a ridge, favorable conditions allowed it to develop into Tropical Depression Nine on September 2 in the deep tropical Atlantic Ocean. The cyclone gradually intensified until September 5, when it underwent rapid deepening and reached Category 4 status on the Saffir-Simpson Hurricane Scale; at the time Ivan was the southernmost major North Atlantic hurricane on record. Ivan quickly weakened due to dry air, but it gradually reorganized, passing just south of Grenada as a major hurricane on September 7. The hurricane attained Category 5 status in the central Caribbean Sea. Over the subsequent days its intensity fluctuated largely due to eyewall replacement cycles, and Ivan passed just south of Jamaica, the Cayman Islands, and western Cuba with winds at or slightly below Category 5 status. Turning northward and encountering unfavorable conditions, Ivan gradually weakened before making landfall just west of Gulf Shores, Alabama on September 16 with winds of 120 mph (195 km/h). The cyclone quickly weakened to tropical depression status as it turned to the northeast, and Ivan transitioned into an extratropical cyclone on September 18. The remnant low of Ivan turned to the south and southwest, and after crossing Florida on September 21 it began to reacquire tropical characteristics. It became a tropical depression again on September 22 to the southeast of Louisiana, and Ivan reached winds of 60 mph (95 km/h) before weakening and moving ashore along southwestern Louisiana as a tropical depression; the circulation of Ivan dissipated after crossing into Texas on September 25. The cyclone broke several intensity records, and its duration was the tenth longest on record for an Atlantic hurricane. == Formation and intensification == On August 31, a large tropical wave moved off the west coast of Africa. A tropical system along the wave axis contained a low pressure area as well as an impressive outflow pattern, though initially its convection was disorganized and limited. By September 1 a cyclonic circulation with a diameter of 690 mile (1115 km) was evident on satellite imagery, well to the southeast of the Cape Verde Islands, and several hurricane forecast models anticipated development and strengthening. As it tracked quickly westward, the convection organized and developed into rainbands – bands of showers and thunderstorms that spiral cyclonically toward the storm center – and late on September 1 meteorologists began tracking the system using the Dvorak technique. Low amounts of wind shear and favorable outflow allowed vigorous deep convection to develop and persist near the center, and by 1800 UTC on September 2 the system developed into Tropical Depression Nine about 450 miles (730 km) southwest of Praia, Cape Verde.〔 Upon being classified as a tropical cyclone, the depression was embedded within a deep easterly steering current provided by a ridge to its north. Expected to track across sea surface temperatures greater than 82 °F (28 °C), the cyclone was forecast to gradually strengthen and within four days attain hurricane status; the Geophysical Fluid Dynamics Laboratory predicted the depression to reach Category 4 status on the Saffir-Simpson Hurricane Scale within three days. Increased northeasterly wind shear shifted the center to the northeastern edge of the deep convection, and despite the shear and its relatively low latitude of 9.7° N, the depression strengthened to attain tropical storm status early on September 3; upon reaching the intensity, the National Hurricane Center classified the system as Tropical Storm ''Ivan''.〔 Tropical Storm Ivan gradually became better organized as wind shear decreased, and its outflow expanded in all quadrants. Satellite imagery late on September 3 depicted a well-defined curved band wrapping around much of the circulation. The next day, the deep convection temporarily became ill-defined, before reorganizing and developing an eye feature. Convection strengthened further as the feature transitioned into an eye, and Ivan became a hurricane at around 0600 UTC on September 5. After reaching hurricane status, Ivan began to rapidly intensify with continued favorable conditions, and in an 18‑hour period the pressure dropped 39 mbar (1.15 inHg) as the winds increased by 60 mph (95 km/h); early on September 6 Ivan attained an initial peak intensity of 135 mph (215 km/h) while located about 825 miles (1330 km) east of the island of Tobago in the southern Lesser Antilles.〔 While at major hurricane status, Ivan maintained very strong convection in its core with a well-defined eye. Operationally, the probability for further rapid strengthening was considered nearly nine times the average of a typical hurricane. Accordingly, Ivan was forecast to pass near Barbados with winds of about 150 mph (240 km/h). Shortly after attaining Category 4 status, the outer convection of the hurricane became ragged. Hurricane Hunters found a Saharan Air Layer in the northern portion of the eye, leaving the eyewall eroded which caused a marked decrease in winds; by late on September 6 Ivan weakened to winds of 105 mph (165 km/h).〔 The inner eyewall dissipated as a 23 mile (37 km) outer eyewall became dominant, and concurrently the overall organization of the hurricane improved. Ivan again attained major hurricane status as it approached the Lesser Antilles, and at 2130 UTC on September 7 the cyclone passed 7 miles (11 km) south-southwest of the southern tip of Grenada, its closest approach to the island. At the time, the hurricane maintained an eye diameter of 12 miles (19 km), with the northern portion of the eyewall producing strong winds on the island.〔 The hurricane brought strong winds to Grenada, and tropical storm force winds were reported as far north as Dominica.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Meteorological history of Hurricane Ivan」の詳細全文を読む スポンサード リンク
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