Upper-tropospheric flow transitions during rapid tropical cyclone intensification

摘要

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 equatorward extremity. During these episodes, the flow is characterized by weak inertial stability. It is proposed that this allows extremely rapid and large-scale changes to occur in the upper-level environment of the storm. The new environment provides seemingly favourable conditions of reduced wind shear, development of a downstream trough very near the storm, and access for the storm outflow to the tropical easterlies and mid-latitude westerlies. A global shallow-water model, initialized with objective analyses at the 200 hPa 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. Using the technique it seems possible, as a first approximation, to isolate the environment from the vortex development. It is shown that flow transitions are mostly independent of the presence of the TC. Short term, 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 is the distinguishing feature of the intensification, and this occurs during an upper-tropospheric flow transition. It is shown: (i) that the reduction in ventilation is associated with flow transitions, (ii) that these transitions directly influence the development of the upper vortex of the TC via downstream development of a weak environmental trough, and (iii) that the transitions indirectly influence the vortex development by allowing the momentum and mass sources to operate more efficiently to assist in both the development of the vortex and the outflow channels at small radii. Several examples of flow transitions during intensification are presented to support the proposed hypothesis.