Effects Of Vertical Wind Shear On The Development Of Tropical Cyclone

Tropical cyclones (TCs) are severe weather systems causing natural disasters to life and property in many coastal areas. In the past decades, the track prediction errors of TCs have been greatly reduced while the accuracy of the intensity prediction is still relatively poor. The vertical wind shear (VWS) is one of the major factors affecting the development of TCs. Therefore, the understanding of the effects of VWS on TCs is significant for the improvement of TC intensity prediction.By using the daily NCEP reanalysis data for the period of 2000-2010 and TC best track from JMA, the statistical relationship between VWS and TC intensity during the genesis and development period is examined. It is found that TCs usually intensify in the sheared environment, and the VWS mostly concentrate in the middle and upper troposphere, while shear of middle and lower troposphere is weak and has less impact. The intensity of TCs is weak and intensification of TCs is inhibited in the environment with strong shear. There is no necessary relationship between VWS and the maximum intensity of TC, and the intensity of TC is not depended on the direction of VWS. It is found that there is some relationship between the TC intensity change and the angle between different levels of VWS, this interesting event needs further investigation.The dynamic processes and mechanism of the effect of VWS on TCs is investigated via the numerical simulations. Firstly, a series experiments with VWSs of different intensity is carried out. The results indicate that weak shear tends to facilitate the development of TC while the strong shear appears to inhibit the intensification of TC. As the vertical wind shear is imposed on the tropical cyclone, the vortex of the cyclone tends to tilt in the vertical direction with the significant tilting occurring 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 beneficial to the burst of convection. In the case of the weak shear, the vertical tilting of the vortex is weak and the increased ascent, static instability and relative humidity occurs in the area close to the TC center. Therefore, the convection is active in the TC center region and facilitates to the enhancement of vorticity in the inner core region and then the intensification of TC. In contrast, due to the strong vertical wind shear, the increase of the ascent, static instability and relative humidity induced by the vertical tilting is mainly appeared in the outer region of TC in the case with stronger shear, 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 warm core is inhibited and then the intensification of TC is delayed. The simulation performed in this work shows that, even the vertical wind shear is as strong as 12 ms-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 the 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 vertical wind shear. As a result, the vertical tilting of the vortex is considerably decreased, and then the cyclone starts to develop rapidly.Another series experiments is carried out to investigating the effect of VWSs on different levels. The results showed that during early developing period, the TC develops more quickly in the low-level shear environment compared to high-level shear environment. The tilting of vortex in the middle and lower troposphere is significant when the VWS concentrates in the low-levels. Consequently, the upward motion is considerably enhanced in the downshear side and beneficial to the burst of convection and the intensification of TC during the period of TC genesis. After the initial development period, the intensification of TC is inhibited and the size of TC is large when the VWS concentrates in the low-level. There aer two mainly reasons:1) the amplitude of vortex tilting is small when the VWS concentrates in the high-levels. Consequently, the contribution of diabatic heating is concentrated and close to the TC center, which benefit for the development of warm core and intensification of TC. However, the convections spread widely and tilt outward with height when the VWS is concentrated in the low-levels. Consequently, the contribution of diabatic heating to the development of warm core is relative weak, the intensify rate of TC reduces. 2) Corresponding to the development of vertical motion, the strong precipitation spreads widely when VWS concentrates in the low-levels. The equivalent potential temperature decreases accompany with the precipitation. Accompany entrance of air with low equivalent potential temperature to the core area of TC, the intensification of TC is suppressed. The diagnostic result of angular momentum showed that high angular momentum was translated outward by the strong outflow in the middle and lower troposphere, which reduce the increase of mean tangential wind, inhibit the intensification of TC. In the strong shear environment, the amplitude of tilting is greater when the VWS concentrates in the low-level than the VWS concentrates in the high-level. Thus, the rapid development of TC is delayed after a longer adjusting period.