On the Formation of AMEX Tropical Cyclones Irma and Jason

Abstract

The AMEX observational dataset, with its high temporal and spatial resolution, has been used to study the formation and structure of Tropical Cyclones Irma and Jason. These systems developed and evolved entirely within the experiment's special observing network. The study is mostly based upon six hourly numerical analyses of the mass and wind fields, on 11 vertical levels over a 1.25 lat/long grid. The systems are traced from prior to the formation of a resolvable closed surface circulation to when they were operationally classified as tropical cyclones. 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. The evolution of the flow during development agrees well with the results of earlier tropical cyclogenesis studies. Large scale spinup appears to be at least partly associated with downstream Rossby-wave dispersion leading to increases in low-level horizontal wind shear and eventually the formation and strengthening of the low-level outer circulation. For the final transformation to cyclone status we suggest that the establishment of favorable patterns of vertical wind shear and inward propagation of eddy angular momentum flux convergence in the upper troposphere were important for intensification. Thermodynamic structure changes suggest that maintenance, rather than triggering of core convection, was dependent on surface evaporation. The role of the observed structure changes, together with the processes operating during each phase of development are documented and discussed.