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Tropical cyclone rainfall forecasting
Tropical cyclone rainfall forecasting involves using scientific models and other tools to predict the precipitation expected in tropical cyclones such as hurricanes and typhoons. Knowledge of tropical cyclone rainfall climatology is helpful in the determination of a tropical cyclone rainfall forecast. More rainfall falls in advance of the center of the cyclone than in its wake. The heaviest rainfall falls within its central dense overcast and eyewall. Slow moving tropical cyclones, like Hurricane Danny and Hurricane Wilma, can lead to the highest rainfall amounts due to prolonged heavy rains over a specific location. However, vertical wind shear leads to decreased rainfall amounts, as rainfall is favored downshear and slightly left of the center and the upshear side is left devoid of rainfall. The presence of hills or mountains near the coast, as is the case across much of Mexico, Haiti, the Dominican Republic, much of Central America, Madagascar, Réunion, China, and Japan act to magnify amounts on their windward side due to forced ascent causing heavy rainfall in the mountains. A strong system moving through the mid latitudes, such as a cold front, can lead to high amounts from tropical systems, occurring well in advance of its center. Movement of a tropical cyclone over cool water will also limit its rainfall potential. A combination of factors can lead to exceptionally high rainfall amounts, as was seen during Hurricane Mitch in Central America.〔Federal Emergency Management Agency. (Are You Ready? ) Retrieved on 2006-04-05.〕
Use of forecast models can help determine the magnitude and pattern of the rainfall expected. Climatology and persistence models, such as r-CLIPER, can create a baseline for tropical cyclone rainfall forecast skill. Simplified forecast models, such as the Kraft technique and the eight and sixteen-inch rules, can create quick and simple rainfall forecasts, but come with a variety of assumptions which may not be true, such as assuming average forward motion, average storm size, and a knowledge of the rainfall observing network the tropical cyclone is moving towards. The forecast method of TRaP assumes that the rainfall structure the tropical cyclone currently has changes little over the next 24 hours. The global forecast model which shows the most skill in forecasting tropical cyclone-related rainfall in the United States is the GFS (Global Forecasting System).
==Rainfall distribution around a tropical cyclone==
A larger proportion of rainfall falls in advance of the center (or eye) than after the center's passage, with the highest percentage falling in the right-front quadrant. A tropical cyclone's highest rainfall rates can lie in the right rear quadrant within a training (non-moving) inflow band.〔Ivan Ray Tannehill. Hurricanes. Princeton University Press: Princeton, 1942. Pages 70-76.〕 Rainfall is found to be strongest in their inner core, within a degree of latitude of the center, with lesser amounts farther away from the center. Most of the rainfall in hurricanes is concentrated within its radius of gale-force winds.〔Corene J. Matyas. (Relating Tropical Cyclone Rainfall Patterns to Storm Size. ) Retrieved on 2007-02-14.〕 Larger tropical cyclones have larger rain shields, which can lead to higher rainfall amounts farther from the cyclone's center.〔 Storms which have moved slowly, or loop, lead to the highest rainfall amounts. Riehl calculated that of rainfall per day can be expected within one-half degree, or , of the center of a mature tropical cyclone.〔Herbert Riehl. Tropical Meteorology. McGraw-Hill Book Company, Inc.: New York, 1954. Pages 293-297.〕 Many tropical cyclones progress at a forward motion of 10 knots, which would limit the duration of this excessive rainfall to around one-quarter of a day, which would yield about of rainfall. This would be true over water, within of the coastline,〔Russell Pfost. (Tropical Cyclone Quantitative Precipitation Forecasting. ) Retrieved on 2007-02-25.〕 and outside topographic features. As a cyclone moves farther inland and is cut off from its supply of warmth and moisture (the ocean), rainfall amounts from tropical cyclones and their remains decrease quickly.
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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