Shear-Induced Vertical Circulations in Tropical Cyclones

It is well known that a tropical-cyclone (TC) vortex in a vertical-sheared environment has rising (sinking) motion on its down- (up-) shear side and it tends to tilt downshear. Thus, vertical shear is considered inimical to the TC development. However, many TCs have been observed to intensify in strong-sheared environments. Two fundamental questions that still remain poorly understood are: (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 address the above questions by isolating the forced vertical circulations (FVCs) associated with the vertical shear, diabatic heating and boundary-layer (BL) processes. This is achieved by applying a recently developed potential vorticity inversion and quasi-balanced omega (i.e., vertical motion) (PV-ω) 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, the PV-ω-diagnosed FVC exhibits a typical vertical circulation of TCs with bottom cyclonic inflows, midlevel slantwise and upper-level anticyclonic outflows. 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 radial inflows in BL and weak ascent in the eye that are similar to the Ekman pumping leading to the spin-down of a cyclonic vortex. In contrast, the shear-FVC shows a counter-shear vertical circulation with rising (sinking) motion on the downshear (upshear) side of the eyewall, and easterly (westerly) flows across the radius of maximum wind (RMW) aloft (below) (Fig. 1). 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. 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^-3 s^-1 could force a vertical motion couplet of more than ± 0.3 m s-1 in the eyewall and the horizontal counter-shear flows of about ± 2.5 m s-1 in the eye. This suggests that the shear-FVC could reduce as much as 30 – 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 downshear-tilted. Such a vortex-restoring effect helps explain why some environmental air is forced to flow around a TC, making it more like an “obstacle.” 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.