Simple kinematic models of tropical cyclones in vertical shear

Interaction with dry environmental air is one major contribution to intensity changes of tropical cyclones (TCs). However, it is not well understood under which conditions pronounced interaction takes place, and thus when storm weakening should be expected. As a step towards improving our understanding of this problem we consider here the interaction of a TC with its environment using a simple quasi-steady and quasi-horizontal fluid dynamics model. Under said assumptions, a set of distinct streamlines, so-called separatrices, which divide the flow around the TC in distinct regions is readily identified.

We examine next the separatrix structure of a TC in an idealized numerical experiment in some detail. It is found that the flow structure is more complex than the well-known flow topology of a point vortex in a uniform background flow. Nonetheless, the distribution of the high theta_e ‘vortex' air is closely related to the separatrices, giving some credence to the simplified quasi-steady and quasi-horizontal flow model. In particular, a separatrix encompasses the eyewall at all height levels and indicates that the eyewall is well protected from the environment despite the adverse impact of vertical wind shear. Moreover, examination of the separatrix structure provides improved insight into the formation mechanism of vortex-scale downdrafts. A recent companion study has suggested that the flushing of the inflow layer with low theta_e air by these downdrafts primarily governs the intensity evolution of the TC. It is of particular interest that a distinct source region for the environmental air involved in the formation of these vortex-scale downdrafts can be identified. If such source regions could be identified in the real atmosphere, targeted observations should prove helpful to improve intensity forecast for TCs in vertical wind shear.

Finally, a simple extended kinematic model is presented and is shown to capture the essence of many of the salient features of the flow topology in the numerical experiment. Preliminary results of an extensive trajectory analysis using the full, time-dependent flow field of the numerical experiment are presented and discussed in the light of the simple kinematic models.