Effects of ambient vertical wind shear on the inner-core asymmetries and vertical tilt of a simulated tropical cyclone

Abstract

The distributions of precipitation, radar reflectivity, and vertical motion associated with tropical cyclones frequently exhibit a significant degree of wavenumber-one asymmetry within the intense inner core. Recent observational and numerical studies attributed the convective asymmetry to the ambient vertical wind shear in which the storms were embedded. However, the precise mechanisms by which the environmental shear controls the convective asymmetry are not yet fully established. Although a previous numerical study by the author suggested that a thermal-wind adjustment to the shear-induced vortex tilt may be a primary mechanism responsible for the asymmetry, the reason why the rainfall maxima were observed predominantly in the downshear-left quadrant (looking downshear) in the Northern Hemisphere rather than right downshear remains to be answered. In the present study, an analytical formula for the amplitude of shear-induced vertical motion asymmetry developed in the previous study based on the proposed mechanism was applied to the numerical simulation data obtained for Typhoon Chaba in 2004. As a result, high correlation coefficients were calculated between simulated and formula-predicted vertical motion asymmetries, especially at low levels where the diabatic enhancement of upward motion was not so significant, confirming that the proposed mechanism may be a primary triggering mechanism for the vertical motion asymmetry. In the study, to discover the processes that govern the directional preference for the rainfall maxima, a Lagrangian trajectory approach was applied to the simulation data. The results suggested that asymmetric water vapor distribution caused by shear-induced vertical motion was a primary factor to locate the rainfall maxima on the downshear-left side rather than right downshear of the storm center. In addition, it was found that the near-collocation in azimuth of upward motion maxima and vortex tilts, which was commonly observed in this and other simulations, was inextricably linked to the formation mechanism for the asymmetric water vapor distribution. © 2008, Meteorological Society of Japan.