Developing versus non-developing disturbances for tropical cyclone formations. Part I: The Atlantic

The genesis of tropical cyclones depends on atmospheric circulations with a wide range of scales both in time and in space. While cyclonic disturbances exist all the time in the tropics, only a small percentage of them became tropical cyclones. Due to recent improvements of numerical models, data assimilation techniques and applications of remotely-sensed data, global analysis can provides us much more reliable representation of the atmosphere than before. In this study, the daily analysis from the U. S. Navy Operation Global Atmosphere Prediction System (NOGAPS) and TRMM TMI data are analyzed to understand why some disturbances formed tropical cyclones while others did not. The data cover the period of 2003 to 2005 and a time filtering technique is applied to separate circulations with different time/spatial scale.

Gray (1979) identified environmental conditions suitable or necessary for the development of tropical cyclones. Among the atmospheric parameters we examined, the mid-level moisture content of the disturbances and the vertical wind shear stand out as important factors that determine the formation of TCs in the Atlantic. A second paper focusing on TCs in the western North Pacific will be presented in conjunction with this one. Different characteristics in the Atlantic and the western North Pacific are identified that suggests different mechanisms behind TC formation in different basins.