Current understanding of tropical cyclone structure and intensity changes - a review

Current understanding of tropical cyclone (TC) structure and intensity changes has been reviewed in this article. Recent studies in this area tend to focus on two issues: (1) what factors determine the maximum potential intensity (MPI) that a TC can achieve given the thermodynamic state of the atmosphere and the ocean? and (2) what factors prevent the TCs from reaching their MPIs? Although the MPI theories appear mature, recent studies of the so-called superintensity pose a potential challenge. It is notable that the maximum intensities reached by real TCs in all ocean basins are generally lower than those inferred from the theoretical MPI, indicating that internal dynamics and external forcing from environmental flow prohibit the TC intensification most and limit the TC intensity. It remains to be seen whether such factors can be included in improved MPI approaches. Among many limiting factors, the unfavorable environmental conditions, especially the vertical shear-induced asymmetry in the inner core region and the cooling of sea surface due to the oceanic upwelling under the eyewall region, have been postulated as the primary impediment to a TC reaching its MPI. However, recent studies show that the mesoscale processes, which create asymmetries in the TC core region, play key roles in TC structure and intensity changes. These include the inner and outer spiral rainbands, convectively coupled vortex Rossby waves, eyewall cycles, and embedded mesovortices in TC circulation. It is also through these inner core processes that the external environmental flow affects the TC structure and intensity changes. It is proposed that future research be focused on improving the understanding of how the eyewall processes respond to all external forcing and affect the TC structure and intensity changes. Rapid TC intensity changes (both strengthening and weakening) are believed to involve complex interactions between different scales and to be worthy of future research. The boundary-layer processes are crucial to TC formation, maintenance, and decaying. Significant progress has been made to deduce the drag coefficient on high wind conditions from the measurements of boundary layer winds in the vicinity of hurricane eyewalls by Global Positioning System (GPS) dropsondes. This breakthrough can lead to reduction of the uncertainties in the calculation of surface fluxes, thus improving TC intensity forecast by numerical weather prediction models.