Quadrant-Dependent Evolution of Low-Level Tangential Wind of a Tropical Cyclone in the Shear Flow

Abstract

This study investigates the quadrant-by-quadrant evolution of the low-level tangential wind near the eyewall of an idealized simulated mature tropical cyclone embedded in a unidirectional shear flow. It is found that the quadrant-averaged tangential wind in the right-of-shear quadrants weakens continuously, while that in the left-of-shear quadrants experiences a two-stage evolution: a quasi-steady stage followed by a weakening stage after the imposing of vertical wind shear. This leads to a larger weakening rate in the right-of-shear and a stronger jet in the left-of-shear quadrants. The budget analysis shows that the quadrant-dependent evolution of tangential wind is controlled through the balance between the generalized Coriolis force (GCF; i.e., the radial advection of absolute angular momentum) and the advection terms. The steady decreasing of the GCF is primarily responsible for the continuous weakening of jet strength in the right-of-shear quadrants. For the left-of-shear quadrants, the quasi-steady stage is due to the opposite contributions by the enhanced GCF and negative tendency of advections cancelling out each other. The later weakening stage is the result of both the decreased GCF and the negative tangential advection. The combination of storm-relative flows at vortex scale and the convection strength both within and outside the eyewall determines the evolution of boundary layer inflow asymmetries, which in turn results in the change of GCF, leading to the quadrant-dependent evolution of low-level jet strength and thus the overall storm intensity change.

Hydrometeor Size Sorting in the Asymmetric Eyewall of Hurricane Matthew (2016)

“…Cha (2018) conducted a similar analysis on Hurricane Matthew and found the wavenumber‐1 maximum roughly in the same region of the eyewall, though slight differences in the location may be due to different methods used. Previous composite and modeling studies show that the strongest tangential winds in a sheared eyewall typically create a wavenumber‐1 asymmetry left of the shear vector with a maximum in the DL quadrant (Reasor et al, 2013; Uhlhorn et al, 2014; Zhang et al, 2013), a result of inward advection of angular momentum downshear (Gu et al, 2016). The wavenumber‐1 wind maximum in the downshear to DR region in Matthew could have contributions from other environmental or dynamic forcings.…”

The Evolution of Vortex Tilt and Vertical Motion of Tropical Cyclones in Directional Shear Flows

“…Large-scale vertical wind shear (VWS), also referred to as deep-layer shear, is commonly defined as the difference between horizontal wind vectors in the 200-and 850-hPa layers, averaged over an area in an annular region or within a given radius from the tropical cyclone (TC) center. Although deep-layer shear is responsible for TC genesis (Gray 1968;Tuleya and Kurihara 1981), structure (Black et al 2002;Corbosiero and Molinari 2003;Reasor et al 2013;Zhang et al 2013;DeHart et al 2014;Gu et al 2016), and intensity change (Simpson and Riehl 1958;DeMaria 1996;Frank and Ritchie 2001;Wu and Braun 2004;Riemer et al 2010;Tang and Emanuel 2010;Gu et al 2015), it alone is not enough to represent the overall vertical structure of a large-scale environmental flow. For example, observational analysis (Wang et al 2015) and idealized simulations (Finocchio et al 2016) both found that low-level shear is more destructive to TC intensification than is deep-layer shear.…”

Buoyancy of convective-scale updrafts in the outer cores of sheared tropical cyclones

“…Important roles of environmental vertical shear in TC structure and intensity changes have long been known. Large vertical shear is generally negative to TC intensification (DeMaria 1996;Zeng, Wang, and Chen 2010;Riemer, Montgomery, and Nicholls 2010;Gu, Tan, and Qiu 2016;Wang et al 2015), and produces inner-core asymmetric convection with stronger convection downshear-left (Jones 1995;Rogers et al 2003). Recently, ambient vertical shear has also been found to cause outer-rainband formation in the downshear quadrant (Li, Wang, and Duan 2017).…”