Upper tropospheric flow transitions during rapid tropical cyclone intensification

Evidence is presented of upper tropospheric flow transitions during rapid tropical cyclone (TC) intensification. Transitions occur when a mid-latitude upper trough, with a wind maximum on its eastern flank, located well to the west of a storm, relaxes as anticyclogenesis occurs near to, and east of its apex. During these episodes, the flow is characterised by weak inertial stability. This allows extremely rapid and large scale changes to occur in the upper level environment of the storm. The new environment provides favourable conditions of reduced wind shear and sustained outflow for intensification.

A global, single layer shallow water model, initialised with objective analyses at the 200hPa level, is used to study the phenomenon, and the interaction between the environmental flow and local sources of mass and momentum. The sources are used to represent the effects of inner core deep convection in the outflow layer of an intensifying TC. It is shown that flow transitions are defined entirely by the large scale flow and independent of the intensification. During these events, the upper level ridge rapidly relocates equatorward to over the storm, while equatorial and/or poleward outflow channels develop. The environmental flow modulates the divergence in the region of the mass source, but largely controls the vorticity associated with the momentum source. Shortterm, local enhancement of divergence over the TC occurs during superposition of environmentally-induced divergence with the mass source. However, formation of the upper level vortex - used here as an indicator of the vertical development of the real storm - is the unique and distinguishing feature of the observed intensification. This development occurs during an upper tropospheric flow transition which reduces the environmental upper level winds and maintains the outflow channels. We propose that this allows the inner core convection to operate efficiently and effectively to produce the intensification.

Several examples of flow transitions during intensification are presented to support the proposed hypothesis - including Tropical Cyclone Tracy, which devastated Darwin, Australia on Christmas Eve, 1974. Discussion focuses on the associated physical processes.