Tropical Cyclones Resist Destruction from Vertical Shear
(To appear in the January Issue of Bulletin of the American Meteorological Society)
It is well known that a tropical cyclone (TC) vortex in a vertically sheared environment has rising motion on its downshear side, sinking motion on its upshear side, and tends to tilt downshear. Thus, vertical shear is considered inimical to TC development. However, many TCs have been observed to intensify in strongly sheared environments. These observations prompt two fundamental questions: (a) Why can TCs intensify against the destructive action of vertical shear? (b) What are the roles of other physical processes in the interaction of TCs with shear?
We addressed the above questions by isolating the forced vertical circulations (FVCs) induced by vertical shear, diabatic heating and boundary layer (BL) processes. This was achieved by applying a recently developed potential vorticity inversion and quasi-balanced omega (i.e., vertical motion) (PV-w) equations system to a nested-grid cloud-resolving simulation of Hurricane Andrew (1992) with the finest grid size of 6 km.
With the model output of diabatic heating rates, the PV-w-diagnosed FVC exhibits a vertical circulation typical of TCs, with bottom cyclonic inflows and midlevel slantwise and upper-level anticyclonic outflows (Fig. 1a). This FVC acts to oppose the shear-forced vertical tilt by coupling the lower to upper-level vortex flows in the eyewall. The BL processes produce low-level radial inflows and weak ascent in the eye that are similar to the Ekman pumping leading to the spin-down of a cyclonic vortex (Fig. 1b). In contrast, the shear-FVC shows a counter-shear vertical circulation with rising motion on the downshear side of the eyewall, sinking motion on the upshear side of the eyewall, and easterly flows across the radius of maximum wind (RMW) aloft as well as westerly flows across the RMW below (Fig. 1c). This wavenumber-1 vertical motion asymmetry accounts for the development of more clouds and precipitation that are often observed on the downshear-left side of the eyewall.
Moreover, we can easily show with idealized experiments that the shear-FVC is proportional in intensity to vertical shear and vortex strength. Of importance is that a westerly shear of 10 m s-1 in the 850-200 hPa layer could force a vertical motion couplet of more than ± 0.3 m s-1 in the eyewall and horizontal counter-shear flows of about ± 2.5 m s-1 in the eye (Fig. 2). This suggests that the shear-FVC could reduce as much as 40% of the destructive influence of environmental shear in the inner-core region.
The above results are supported by previous observational and modeling studies showing that the inner-core vortices of TCs can remain upright while the outer portions may be markedly tilted downshear. Such a vortex-restoring effect helps explain why some environmental air is forced to flow around a TC, as if it were an “obstacle,” rather than flowing through it. More studies are needed to examine the relative importance of shear- and diabatic-heating-FVCs in resisting the destruction of vertical shear during different developing stages of TCs. - Da-Lin Zhang (University of Maryland), and Chanh Q. Kieu. “The Vertical Shear Induced Secondary Circulation in Tropical Cyclones.” 11th Conference on Mesoscale Processes, Albuquerque, New Mexico, 24-29 October 2005.
Figure 1. Schematic of the forced vertical circulations (red lines) by (a) latent heat release; (b) the frictional process in the BL; and (c) vertical wind shear ( in blue) in vertical cross sections. Orange shadings denote convective clouds in the eyewall; a darker shading in (c) is used to indicate stronger convective development on the downshear side. Green lines in (c) represent isentropic surfaces. The radius of maximum wind (RMW) is plotted in black.
Figure 2. West-east vertical cross sections of the shear-forced vertical motion, at intervals of 0.05 m s-1 (solid/positive, dotted/negative), and in-plane flow vectors after adding a westerly vertical shear of 1.5 x 10-3 s-1 to an axisymmetric balanced hurricane vortex. Dashed lines are isentropic surfaces at intervals of 10 K.