Simple kinematic models for the environmental interaction of tropical cyclones in vertical wind shear 1Department of Meteorology, Naval Postgraduate School, Monterey, CA, USA
12 Sep 2011
2NOAA's Hurricane Research Division, Miami, FL, USA
*current address: Institut für Physik der Atmosphäre, Johannes Gutenberg-Universität, Mainz, Germany
Received: 28 August 2010 – Published in Atmos. Chem. Phys. Discuss.: 16 November 2010 Abstract. A major impediment to the intensity forecast of tropical cyclones (TCs) is
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-θe air around
the TC at low to mid-levels is governed by the stirring of convectively modified
air by the steady, horizontal flow.
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.
Citation: Riemer, M. and Montgomery, M. T.: Simple kinematic models for the environmental interaction of tropical cyclones in vertical wind shear, Atmos. Chem. Phys., 11, 9395-9414, doi:10.5194/acp-11-9395-2011, 2011.