A Study On The Effects Of Vertical Wind Shear In Different Layers On The Tropical Cyclone Intensity Change

The environmental vertical wind shear(VWS)is considered a key factor to the intensity change of tropical cyclones(TCs).It is of great importance to have a profound understanding on the effect of VWS on TC structure and intensity changes for the improvement of TC intensity forecasting.Observations show that VWS is detrimental to TC intensity,and statistical studies also verify the negative correlation between TC intensity change and VWS.Meanwhile,the dependence of TC intensity change on the shear profile is being aware of.Both statistical analysis and numerical modeling studies have demonstrated that the TC intensity is more sensitive to the low-level shear.However,it is not clear about how the TC intensity change depends on the shear profile.In this study,a quadruply-nested,nonhydrostatic TC model is used to simulate a mature TC embedded in the unidirectional environmental VWSs at different heights,and compare the effects of upper-layer and lower-layer shears on the simulated TC intensity change.The simulation results show that TC intensity in both cases decreased shortly after the VWS was imposed,but with quite different subsequent evolutions.The TC weakened 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 mid-upper troposphere,the larger vertical vortex tilt,and a greater ventilation of the warm core in the former.The storm-relative flow across the warm core is much stronger due to the slow movement of the TC embedded in the upper-layer shear,which leads to more significant“upper-level ventilation”.The storm in the lower-layer shear only weakened initially after the VWS was imposed,but then experienced a quasi-periodic intensity oscillation with a period of about 24 hours.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.The downdrafts-induced environmental low-entropy air flushed the boundary layer and then spiraled into the inner core with the inflow,leading to the dilution of eyewall convection,resulting in a thermodynamics efficiency of the energy cycle,which is consistent with the “low-level ventilation”.There is no significant intensity oscillation for the storm embedded in the upper-layer shear,even though outer spiral rainbands developed 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.Based on the dependence of the ventilation effect on the shear profile,we used the Lagrangian diagnostics to further analyze the impact of VWS on the ventilation pathways.Trajectories of both warm core and boundary layer low-entropy air are mapped from the three-dimensional physical space to the two-dimensional entropy-temperature space.The mass/heat flux vectors in this space depict the thermodynamic characteristics of the targeted air parcles.The trajectory analysis results show that the upper-layer shear can drive a significant“upper-level ventilation” to rapidly advect the TC warm core to the outer core region on the downshear side.Moreover,the TC warm core structure is destroyed in the upper troposphere,leading to the blocking of the ascending parcels in the eyewall.As a result,the environmental low-entropy air parcels are hard to move upward along the eyewall even they can intrude the inner core region,suggesting that the “low-level ventilation” does not work effectively in the upper-layer shear case.However,in the lower-layer shear case,there is clear inward heat flux from the outer core region to the eyewall downshear-right and strong upward motion along the eyewall to the downshear left.Such trajectory of the boundary layer low-entropy air is in agreement with the “low-level ventilation” pathway in the entropy-temperature space.However,the “upper-level ventilation” effect is much weaker than that embedded in the upper-layer shear case.The Lagrangian diagnostics provides further evidence for aforementioned numerical simulation results.According to the air trajectories of the upper-level warm core and the environmental low-entropy air in the inflow boundary layer,the dependence of the ventilation effect on the shear profile is emphasized.Furthermore,the results strongly suggest that the ventilation effect of VWS depends closely on the shear profile,which determines the TC intensity change through the corresponding ventilation effect.