ALBERT

Description: A major contribution to intensity changes of tropical cyclones (TCs) is believed to be associated with interaction with dry environmental air. However, the conditions under which pronounced TC-environment interaction takes place are not well understood. As a step towards improving our understanding of this problem we analyze the flow topology of a TC in vertical wind shear in an idealized, three-dimensional, convection-permitting numerical experiment. A set of distinct streamlines, the so-called separatrices, can be identified under the assumptions of steady and layer-wise horizontal flow. The separatrices are shown to divide the flow around the TC into distinct regions. The separatrix structure in our numerical experiment is more complex than the well-known flow topology of a non-divergent point vortex in uniform background flow. In particular, one separatrix spirals inwards and ends in a limit cycle, a meso-scale dividing streamline encompassing the eyewall above the inflow and below the outflow layer. Air with the highest values of moist entropy resides within this limit cycle supporting the notion that the eyewall is well protected from intrusion of dry environmental air despite the adverse impact of the vertical wind shear. This "moist envelope" is distorted considerably by the vertical wind shear, and the shape of the moist envelope is closely related to the shape of the limit cycle. A simple kinematic model based on a weakly divergent point vortex in background flow is presented. The model is shown to capture the essence of many salient features of the flow topology in the idealized experiment. A regime diagram representing realistic values of TC intensity and vertical wind shear can be constructed for this simple model. The results indicate distinct scenarios of environmental interaction depending on the ratio of storm intensity and shear magnitude. Further implications of the new results derived from the flow topology analysis for TCs in the real atmosphere are discussed.