A 15-Year Climatology of North Atlantic Tropical Cyclones. Part I: Size Parameters

The extended best-track (EBT) dataset combines the information contained in the tropical cyclone best-track dataset with measurements of tropical cyclone “size parameters.” These parameters include the radii of the eye (REYE), maximum winds (RMW), gale-force winds (or size; 17.5 m s−1; R17), damaging-force winds (25.7 m s−1; R26), hurricane-force winds (32.9 m s−1; R33), and the outermost closed isobar (ROCI). The latest update of this dataset, to be used in this study for a size parameter climatology, contains the size parameters for North Atlantic tropical cyclones from 1988 to 2002. Such a climatology has not yet been established in this basin. Most of the results of this North Atlantic study agree with documented tropical cyclone theory and results from similar studies of northwest Pacific tropical cyclones. This provides confidence that the observations of the size parameters in the dataset are reliable. Furthermore, data west and east of 55°W (the boundary beyond which no aircraft observations are made) are compared. Some differences occur in some of the size parameters, but the sample west of 55°W is significantly larger and displays a greater spread. This provides confidence that the total dataset may not be affected by the nonaircraft data east of 55°W. The spatial and temporal distribution of the size parameters is investigated. The radii of gale-force (R17), damaging-force (R26), and hurricane-force (R33) winds tend to increase as storms move poleward and westward. North of 40°N, R33 and R26 decrease, while R17 increases. This is a reflection of storm weakening after recurvature. Gulf of Mexico storms have larger ROCIs but smaller eyes, R33s, R26s, and R17s than North Atlantic storms between 50° and 80°W. Gulf systems tend to form in the gulf instead of moving into this area from the Atlantic. Gulf incipient systems are likely to be tropical upper-tropospheric trough (TUTT) cells or monsoon trough features from the eastern Pacific instead of easterly waves from Africa. Early-season storms tend to be small; late-season storms are larger; and storm size peaks in September. Weakening storms tend to have smaller eyes than intensifying storms; most weakening storms are intense systems that have reached the end of their intensification and eyewall contraction process. These highly organized systems take a long time to spin down. Weak systems with large eyes take a long time to get organized and require a long time to intensify. Knowledge of the areal extent of damaging winds will provide forecasters and emergency managers with additional information to assess the damage potential of approaching storms.