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More than 400 tropical cyclones that occurred worldwide from 1999 through 2005 were examined. Changes in tropical cyclone behavior were observed using geostationary satellite imagery and archival data from the major tropical centers. The upper air environment was observed on satellite data, with emphasis on 6.7 micrometer water vapor imagery, and forecast model winds and temperatures between 500mb and 300mb. Seven categories of tropical cyclone behavior, such as turns and intensity changes were defined; and, 361 "events" were identified and analyzed. Likewise, 6 categories of changes in the upper air environment were defined; and, 376 "events" were identified from the tropical cyclone cases.

Specific types of changes in the upper air environment were found to be related to certain changes in tropical cyclone behavior. Two specific types of tropical cyclone cloud patterns were observed with weakening storms. Middle tropospheric dry air that arrived at the cold cloud shield boundaries of tropical cyclones at small angles and was ingested into the storms, was correlated with spiral shaped "intrusions" in the storm cloud pattern and weakening. Eye replacement cycles were also likely with this type of environmental change. Opening of adjacent upper air systems, that brought flow to the tropical cyclone cold cloud shields at large angles was correlated with cloud pattern deforming and weakening. Four types of environmental changes were well correlated with storm formations and intensification. Although the 153 right turns and 79 left turns were well related to specific categories of upper air changes, the relationships did not provide the quantitative information necessary for accurate track forecasting. However, specific categories of environmental changes related to turns were highly correlated with storm intensity changes during or after the turns. Relationships found in the study are likely to be useful in choosing model results, when various model forecasts diverge. Overall, changes in the adjacent upper air ridge and anticyclones made the greatest contributions to changes in tropical cyclone behavior. The eastward passages of short wave ridges in the westerlies, on the poleward side of storms, was found to be a particularly important type of environmental change affecting tropical cyclone behavior.