Effect of Unidirectional Vertical Wind Shear on Tropical Cyclone Intensity Change—Lower‐Layer Shear Versus Upper‐Layer Shear

Abstract

In this study, a quadruply nested, nonhydrostatic tropical cyclone (TC) model is used to investigate how the structure and intensity of a mature TC respond differently to imposed lower‐layer and upper‐layer unidirectional environmental vertical wind shears (VWSs). Results show that TC intensity in both cases decrease shortly after the VWS is imposed but with quite different subsequent evolutions. The TC weakens much more rapidly for a relatively long period in the upper‐layer shear than in the lower‐layer shear, which is found to be related to the stronger storm‐relative asymmetric flow in the middle‐upper troposphere and the larger vertical vortex tilt in the former than in the latter. The stronger storm‐relative flow in the former imposes a greater ventilation of the warm core in the middle‐upper troposphere, leading to a more significant weakening of the storm. The storm in the lower‐layer shear only weakens initially after the VWS is imposed but then experiences a quasi periodic intensity oscillation with a period of about 24 hr. This quasi periodic behavior is found to be closely related to the boundary layer thermodynamic “discharge/recharge” mechanism associated with the activity of shear‐induced outer spiral rainbands. There is no significant intensity oscillation for the storm embedded in the upper‐layer shear, even though outer spiral rainbands develop quasi periodically also. The boundary layer inflow is very weak in that case and the low equivalent potential temperature air induced by downdrafts in outer spiral rainbands therefore cannot penetrate into the inner core but remains in the outer region.

Intensity Change of Binary Tropical Cyclones (TCs) in Idealized Numerical Simulations: Two Initially Identical Mature TCs

“…This strongly suggests that the upperlayer VWS is a good indicator of the intensity change of the two interacting strong TCs. Note that the more detrimental effect of the upper-layer VWS on the intensity of a single mature TC has been also found in previous studies (e.g., Xu and Wang 2013;Fu et al 2019).…”

“…Figure 3 shows the mean potential temperature anomaly in the eye region of the WTC in the four binary experiments. The potential temperature anomaly was calculated as the perturbation relative to the environment, which was the mean potential temperature from the TC surface center to 600-km radius, the same as that in Fu et al (2019). Following Zhang and Chen (2012), we also calculated the central sea level pressure rises induced by the upper-level (between the 9-and 16km heights, ∆ ), lower-level (between the 2-and 6-km heights, ∆ ) and total-level warm core (∆ ) using the hydrostatic equation with the results shown in Fig.…”

Influence of Saharan Dust on the Large‐Scale Meteorological Environment for Development of Tropical Cyclone Over North Atlantic Ocean Basin

“…Figure 6 shows the spatial distributions of the averaged VWSs between 200 and 850 hPa in the boreal summers from 2000 to 2018 over the study region. Environmental VWS is defined as the absolute difference of the wind vectors between 200 hPa (upper troposphere) and 850 hPa (lower troposphere) which is more representative of the troposphere‐deep shear experienced in tropical storms (Aiyyer & Thorncroft, 2006; Chen et al, 2006; Fu et al, 2019; Gray, 1968; Heymsfield et al, 2006; Kossin, 2017; Zeng et al, 2008, 2010). The VWS ranges from −13.4 to 8.6 m s −1 .…”

“…These opposite findings, which were obtained from model simulation studies, motivated us to investigate the potential connections between dust and the meteorological parameters which affect the formation and development of TCs from an observational viewpoint. Previous studies have revealed that the strong low‐level relative vorticities, mean SSTs higher than 27°C, and VWS from 200 to 850 hPa less than the 10–15 m s −1 are necessary for TC genesis and intensification (Bracken & Bosart, 2000; Fu et al, 2019; Vimont & Kossin, 2007; Webster et al, 2005). VWS is a detrimental factor for the formation of TCs, especially at low latitudes and for small TCs (Aiyyer & Thorncroft, 2006; Bracken & Bosart, 2000; DeMaria, 1996; Fu et al, 2019; Jones, 1995; Kossin, 2017; Murakami et al, 2018; Zeng et al, 2008).…”

“…Previous studies have revealed that the strong low‐level relative vorticities, mean SSTs higher than 27°C, and VWS from 200 to 850 hPa less than the 10–15 m s −1 are necessary for TC genesis and intensification (Bracken & Bosart, 2000; Fu et al, 2019; Vimont & Kossin, 2007; Webster et al, 2005). VWS is a detrimental factor for the formation of TCs, especially at low latitudes and for small TCs (Aiyyer & Thorncroft, 2006; Bracken & Bosart, 2000; DeMaria, 1996; Fu et al, 2019; Jones, 1995; Kossin, 2017; Murakami et al, 2018; Zeng et al, 2008). Bracken and Bosart (2000) have demonstrated that TCs generally occur in the equatorward entrance regions of jet streaks in conjunction with a cyclonic vorticity maximum in regions where the VWS is minimized.…”