EFFECTS OF VERTICAL WIND SHEAR ON INTENSITY AND STRUCTURE OF TROPICAL CYCLONE

CHEN Qi-zhi; FANG Juan

doi:10.3969/j.issn.1006-8775.2012.02.007

Abstract:

In this study, the effect of vertical wind shear (VWS) on the intensification of tropical cyclone (TC) is investigated via the numerical simulations. Results indicate that weak shear tends to facilitate the development of TC while strong shear appears to inhibit the intensification of TC. As the VWS is imposed on the TC, the vortex of the cyclone tends to tilt vertically and significantly in the upper troposphere. Consequently, the upward motion is considerably enhanced in the downshear side of the storm center and correspondingly, the low- to mid-level potential temperature decreases under the effect of adiabatic cooling, which leads to the increase of the low- to mid-level static instability and relative humidity and then facilitates the burst of convection. In the case of weak shear, the vertical tilting of the vortex is weak and the increase of ascent, static instability and relative humidity occur in the area close to the TC center. Therefore, active convection happens in the TC center region and facilitates the enhancement of vorticity in the inner core region and then the intensification of TC. In contrast, due to strong VWS, the increase of the ascent, static instability and relative humidity induced by the vertical tilting mainly appear in the outer region of TC in the case with stronger shear, and the convection in the inner-core area of TC is rather weak and convective activity mainly happens in the outer-region of the TC. Therefore, the development of a warm core is inhibited and then the intensification of TC is delayed. Different from previous numerical results obtained by imposing VWS suddenly to a strong TC, the simulation performed in this work shows that, even when the VWS is as strong as 12 m s-1, the tropical storm can still experience rapid intensification and finally develop into a strong tropical cyclone after a relatively long period of adjustment. It is found that the convection plays an important role in the adjusting period. On one hand, the convection leads to the horizontal convergence of the low-level vorticity flux and therefore leads to the enhancement of the low-level vorticity in the inner-core area of the cyclone. On the other hand, the active ascent accompanying the convection tends to transport the low-level vorticity to the middle levels. The enhanced vorticity in the lower to middle troposphere strengths the interaction between the low- and mid-level cyclonical circulation and the upper-level circulation deviated from the storm center under the effect of VWS. As a result, the vertical tilting of the vortex is considerably decreased, and then the cyclone starts to develop rapidly.

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CHEN Qi-zhi, FANG Juan. EFFECTS OF VERTICAL WIND SHEAR ON INTENSITY AND STRUCTURE OF TROPICAL CYCLONE [J]. Journal of Tropical Meteorology, 2012, 18(2): 172-186, https://doi.org/10.3969/j.issn.1006-8775.2012.02.007

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EFFECTS OF VERTICAL WIND SHEAR ON INTENSITY AND STRUCTURE OF TROPICAL CYCLONE

Key Laboratory of Mesoscale Severe Weather, Department of Atmospheric Sciences, Nanjing University, Nanjing 210093 China

Received Date: 2011-09-30

Rev Recd Date: 2012-02-16

Abstract: In this study, the effect of vertical wind shear (VWS) on the intensification of tropical cyclone (TC) is investigated via the numerical simulations. Results indicate that weak shear tends to facilitate the development of TC while strong shear appears to inhibit the intensification of TC. As the VWS is imposed on the TC, the vortex of the cyclone tends to tilt vertically and significantly in the upper troposphere. Consequently, the upward motion is considerably enhanced in the downshear side of the storm center and correspondingly, the low- to mid-level potential temperature decreases under the effect of adiabatic cooling, which leads to the increase of the low- to mid-level static instability and relative humidity and then facilitates the burst of convection. In the case of weak shear, the vertical tilting of the vortex is weak and the increase of ascent, static instability and relative humidity occur in the area close to the TC center. Therefore, active convection happens in the TC center region and facilitates the enhancement of vorticity in the inner core region and then the intensification of TC. In contrast, due to strong VWS, the increase of the ascent, static instability and relative humidity induced by the vertical tilting mainly appear in the outer region of TC in the case with stronger shear, and the convection in the inner-core area of TC is rather weak and convective activity mainly happens in the outer-region of the TC. Therefore, the development of a warm core is inhibited and then the intensification of TC is delayed. Different from previous numerical results obtained by imposing VWS suddenly to a strong TC, the simulation performed in this work shows that, even when the VWS is as strong as 12 m s-1, the tropical storm can still experience rapid intensification and finally develop into a strong tropical cyclone after a relatively long period of adjustment. It is found that the convection plays an important role in the adjusting period. On one hand, the convection leads to the horizontal convergence of the low-level vorticity flux and therefore leads to the enhancement of the low-level vorticity in the inner-core area of the cyclone. On the other hand, the active ascent accompanying the convection tends to transport the low-level vorticity to the middle levels. The enhanced vorticity in the lower to middle troposphere strengths the interaction between the low- and mid-level cyclonical circulation and the upper-level circulation deviated from the storm center under the effect of VWS. As a result, the vertical tilting of the vortex is considerably decreased, and then the cyclone starts to develop rapidly.

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EFFECTS OF VERTICAL WIND SHEAR ON INTENSITY AND STRUCTURE OF TROPICAL CYCLONE

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