The Impact Of Different Characteristics Of Environmental Vertical Wind Shear On The Activities Of The Outer Rain Belt Of Tropical Cyclone

Outer rainbands are an important convective structure in the outer core of tropical cyclones.Active outer rainbands often produce severe weather,such as heavy precipitation and strong wind.By analyzing the results of high-resolution numerical simulations,this thesis reveals the characteristics of "direct" and "indirect" responses of outer rainband activities to environmental vertical wind shear(VWS)and discusses corresponding physical mechanisms.The main conclusions of this thesis are summarized as follows:(1)The convective activity in tropical cyclone outer rainbands shows different "direct responses" to the direction of low-level mean flow,which mainly affects the outer rainband activity by regulating near-surface enthalpy fluxes and divergence in the boundary layer.The VWS aligned with the mean surface winds environment is conducive to forcing more active outer rainband activities,but the precipitation characteristics of the outer rainbands are different in different stages.Stronger convergence near the surface on the left side of the shear is beneficial to the development of outer rainband convection at the beginning of the simulation under VWS counter-aligned with the mean surface winds.In contrast,higher entropy near the surface in downshear quadrants is beneficial to the occurrence of stronger precipitation in the outer core region under VWS aligned with the mean surface winds.However,with the enhancement of the ventilation effect produced by the asymmetric airflow in the middle to upper troposphere,outer rainband convection under VWS aligned with the mean surface winds gradually weakens due to the dry air intrusion.Comparatively,the precipitation coverage of outer rainbands shows a gradual radial expansion trend under VWS counter-aligned with the mean surface winds,mainly related to an increase in surface fluxes in the downshear semicircle,especially in the downshear-right quadrant.(2)The activity of tropical cyclone outer rainbands also shows different "direct responses" to the VWS height.The proportion of deep convection in outer rainbands is larger in a lower-layer shear environment.On the one hand,the occurrence and development of deep convection are related to the boundary layer equivalent potential temperature in the downshear semicircle;on the other hand,the interaction between localized shear and near-ground cold pools promotes the development of deep convection.Stratiform clouds in tropical cyclone outer rainbands are broader in the upper-layer shear environment due to stronger upper-tropospheric asymmetric outflow.The outflow strengthens the outward transport of water vapor and ice particles from the inner core,beneficial to the development of stratiform clouds.(3)Tropical cyclone outer rainband activity shows different "indirect responses" to the VWS magnitude.The moisture stability distribution in the outer core can regulate the convective activity of outer rainbands.In moderate-to-strong environmental shear,convective-scale downdrafts on the left side of the shear reduce the equivalent potential temperature near the ground through downdraft ventilation.In the downshear-right quadrant,the local environment is conditionally unstable due to the boundary-layer recovery.In addition,upward transports of water vapor caused by outer rainband convection contribute the most to the equivalent potential temperature maximum in the midlevel outer core in the downshear-left quadrant,and the outward transport of high-entropy air from the inner core by the shear-forced asymmetric outflow played a secondary role in this maximum.As a result,the local environment in the downshear-left quadrant in the outer core is potentially stable.The above-mentioned asymmetric distribution of moist instability will affect the convective activity of outer rainbands,and such an asymmetric instability distribution becomes more significant in larger environmental shear.