Does strong vertical wind shear certainly lead to the weakening of a tropical cyclone?

Abstract

This is a preliminary pilot study giving an important insight into the feasible improvement of existing prediction of tropical cyclone (TC) intensity change by revisiting the relevant impact of vertical wind shear (VWS), where VWS is commonly defined as the environmental horizontal wind difference between the top and bottom of the shear layer. Macroscopically, strong VWS has been concluded to be detrimental to TC intensification. However, this study, from the microscopic point of view, shows that given the same thermal forcing, strong VWS does not certainly lead to TC weakening. Considering the magnitude, direction or orientation of VWS alone is no longer comprehensively physical enough for predicting the change in TC intensity. Instead, the whole profile of VWS (VWSP), that is the environmental horizontal winds at all levels within the shear layer, determines. The ventilation of dry and low-entropy air that are directly or indirectly driven by the environmental flows in the upper-, mid-and lower-troposphere could modify TC intensity in various extent. The VWSP is proposed to be a new potential proxy or update of the current TC intensity change prediction. Tropical cyclone recurvature: an intrinsic property? Geophys. Res. Lett. 43 8769-74 Cram T A, Persing J, Montgomery M T and Braun S A 2007 A lagrangian trajectory view on transport and mixing processes between the eye, eyewall, and environment using a high-resolution simulation of Hurricane Bonnie (1998) J. Atmos. Sci. 64 1835-56 DeMaria M 1996 The effect of vertical shear on tropical cyclone intensity change J. Atmos. Sci. 53 2076-88 DeMaria M 2009 A simplified dynamical system for tropical cyclone intensity prediction Mon. Weather Rev. 137 68-82 DeMaria M and Kaplan J 1999 An updated statistical hurricane intensity prediction scheme (SHIPS) for the Atlantic and eastern North Pacific basins Weather and Forecasting 14 326-37 DeMaria M, Mainelli M, Shay L K, Knaff J A and Kaplan J 2005 Further improvements to the statistical hurricane intensity prediction scheme (SHIPS) Weather and Forecasting 20 531-43 Donelan M A et al 2004 On the limiting aerodynamic roughness of the ocean in very strong winds Geophys. Res. Lett. 31 L18306 Emanuel K A 1989 The finite-amplitude nature of tropical cyclogenesis J. Atmos. Sci. 46 3431-56 Emanuel K A, DesAutels C, Holloway C and Korty R 2004 Environmental control of tropical cyclone intensity J. Atmos. Sci. 61 843-58 Finocchio P M and Majumdar S J 2017 A statistical perspective on wind profiles and vertical wind shear in tropical cyclone environments of the Northern Hemisphere Mon. Weather Rev. 145 361-78 Finocchio P M, Majumdar S J, Nolan D S and Iskandarani M 2016 Idealized tropical cyclone responses to the height and depth of environment vertical wind shear Mon. Weather Rev. 129 2155-75 Frank W M and Ritchie E A 2001 Effects of vertical wind shear on the intensity and structure of numerically simulated hurricanes Mon. Weather Rev. 129 2249-69 Ge X, Li T and Peng M 2013 Effects of vertical shears and midlevel dry air on tropical...

Effects of vertical shear on intensification of tropical cyclones of different initial sizes

“…Energy input from the surface may have significant effects on convection near the TC center (Xu and Wang, 2010a;Chan et al, 2019). The surface latent flux was not significantly different near the TC center when comparing large and small TCs (Figure 9), indicating that the surface latent heat flux did not directly influence the changes in intensity and that those changes were more related to the dynamic and thermodynamic processes associated with VWS.…”

Impact of Dry Midlevel Air on the Tropical Cyclone Outer Circulation

“…11c overlaps with the boundary layer, which accelerates the inflow speed of the main secondary circulation. A stronger boundary layer inflow favors the angular momentum accumulation in the rainbands, which makes the TC outer circulation grow (Chan and Chan 2013).…”

“…Furthermore, the lower inertial stability in the outer weaker wind regions (compared to the core) provides less resistance to the radially inward motion in response to the heating gradient. In other words, the radial heating gradient change can be relatively small at large radii (compared to near the core), but there will be a response of radial momentum inflow in this low-inertial-stability region (Chan et al 2019).…”

Seasonal to Interannual Variability of Vertical Wind Shear and Its Relationship with Tropical Cyclogenesis in the Mozambique Channel

“…Gray [1] listed six basic conditions favoring TC genesis, namely: (i) sea surface temperature (SST) exceeding 26 • C, (ii) enhanced mid-troposphere (700 hPa) relative humidity, (iii) conditional instability, (iv) enhanced lower troposphere relative vorticity, (v) weak vertical environmental wind shear (VWS) and (vi) a displacement by at least 5 • latitude away from the equator. Chan et al [2] suggested that VWS is one of the main governing factors of both tropical cyclogenesis and tropical cyclone intensity. Moreover, some studies indicate that the uncertainties in forecasting the environmental wind shear values prevent the improvements in TC intensity forecast [3,4].…”

“…Although some literature has shown cases of cyclones intensifying under moderate to strong VWS (e.g., [8][9][10]), in general large values of VWS are considered to be detrimental to the formation as well as the intensification of TCs, due to the "ventilation" effect [1,[11][12][13]. Chan et al [2] have indicated that the reduction of ventilation above the boundary layer due to vertical alignment is crucial to accumulate the energy within the inner core region of a TC. Nolan and McGauley [6] studied the relationship between VWS and TC genesis events within the latitudes 20 • S and 20 • N from 1969 to 2008, and found that VWS values in the range of 1.25-5 m/s are the most favorable for TC genesis, although previous studies suggested VWS values between 5-10 m/s (e.g., [14]).…”