On the formation of AMEX tropical cyclones Irma and Jason

The discussion focuses on the synoptic to cyclone scale changes during formation. Time sections of various kinematic variables, together with an index of deep convection obtained from digital satellite cloud imagery, are used to trace the development. Both systems developed during active phases of the monsoon and initially were of maximum intensity in the middle troposphere. Low level spinup occurred in three stages. The first stage was associated with the establishment of a favorable large-scale environment and the development of a closed, low-level circulation. The second stage was marked by a strengthening in the low-level outer circulation and the development of a deep vortex. The final stage was the transformation of the tropical depressions into tropical cyclones, and was indicated by a large increase in low-level convergence, a burst in inner core convection, and intensification of the low-level inner circulation. -from Authors