Atmos. Chem. Phys., 11, 9395-9414, 2011
www.atmos-chem-phys.net/11/9395/2011/ doi:10.5194/acp-11-9395-2011 © Author(s) 2011. This work is distributed under the Creative Commons Attribution 3.0 License. |

Research Article

12 Sep 2011

Received: 28 August 2010 – Published in Atmos. Chem. Phys. Discuss.: 16 November 2010

Revised: 23 May 2011 – Accepted: 19 August 2011 – Published: 12 September 2011

Abstract. A major impediment to the intensity forecast of tropical cyclones (TCs) is
believed
to be associated with the interaction of TCs 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 here the flow topology of a TC immersed in an environment
of vertical wind shear in an idealized, three-dimensional, convection-permitting
numerical experiment. A set of distinct streamlines, the so-called manifolds,
can be identified under the assumptions of steady and layer-wise horizontal flow.
The manifolds are shown to divide the flow around the TC into distinct regions.
Revised: 23 May 2011 – Accepted: 19 August 2011 – Published: 12 September 2011

The manifold structure in our numerical experiment is more complex than the well-known manifold structure of a non-divergent point vortex in uniform background flow. In particular, one manifold spirals inwards and ends in a limit cycle, a meso-scale dividing streamline encompassing the eyewall above the layer of strong inflow associated with surface friction and below the outflow layer in the upper troposphere. From the perspective of a steady and layer-wise horizontal flow model, the eyewall is well protected from the intrusion of environmental air. In order for the environmental air to intrude into the inner-core convection, time-dependent and/or vertical motions, which are prevalent in the TC inner-core, are necessary. Air with the highest values of moist-entropy resides within the limit cycle. This "moist envelope" is distorted considerably by the imposed vertical wind shear, and the shape of the moist envelope is closely related to the shape of the limit cycle. In a first approximation, the distribution of high- and low-

Motivated by the results from the idealized numerical experiment, an analogue model based on a weakly divergent point vortex in background flow is formulated. The simple kinematic model captures the essence of many salient features of the manifold structure in the numerical experiment. A regime diagram representing realistic values of TC intensity and vertical wind shear can be constructed for the point-vortex model. The results indicate distinct scenarios of environmental interaction depending on the ratio of storm intensity and vertical-shear magnitude. Further implications of the new results derived from the manifold analysis for TCs in the real atmosphere are discussed.