Characteristics Of The Outer Rainband Stratiform Sector In Sheared Tropical Cyclones

Idealized numerical simulations are conducted in this study to comparatively investigate the characteristics of the stratiform sector in the outer rainbands of tropical cyclones in easterly wind shears with different heights and magnitudes.Consistent with the results in previous studies,the outer rainband stratiform sector of the tropical cyclones simulated in the experiments is generally located downshear left.Upper-layer vertical wind shear tends to produce stronger asymmetric outflow at upper levels in the downshear-left quadrant than lower-layer shear.This stronger asymmetric outflow transports more water vapor radially outward from the inner core to the outer core at upper levels in the downshear-left quadrant in the upper-layer shear experiment.More depositional growth of both graupel and cloud ice thus occurs downshear left in upper layers in the outer core,yielding more diabatic heating and stronger upward motions,particularly in the stratiform-dominated part of the stratiform sector in the upper-layer shear experiment.Resultingly,a better-organized stratiform sector in the outer rainbands is found in the upper-layer shear experiment than in the lower-layer shear experiment.The diabatic heating associated with the stratiform sector produces strong midlevel outflow on the radially inward side of the heating core,and weak midlevel inflow on the radially outward side of the heating core,with lower-level inflow beneath the midlevel outflow and upper-level inflow above.The upper-layer shear tends to produce a deeper asymmetric inflow layer in the outer rainband stratiform sector,with more significant lower-level inflow and tangential jets in the upper-layer shear experiment.Outer rainband stratiform sector is more organized and concentrated on the downshear-left as wind shear increases.Stronger vertical wind shear can force stronger asymmetric outflow at upper levels,which causes not only more water vapor transported from the inner core to the outer core,but also more graupel and cloud ice directly transported to the outer rainband region.As a result,radar reflectivity of 20-30 d BZ is more concentrated in upper layers in stronger shear experiment.As the height of the shear increases,the height of the tangential jet ascends.Because stronger vertical wind shear can force a stronger and deeper low-level inflow,it contributes to the occurrence of maximum asymmetric tangential wind at a higher height.It is further found that the asymmetric structures in outer rainbands have an important influence on the change in vortex-scale tangential wind speed.Tangential momentum budgets indicate that the eddy term plays an important role in increasing the tangential wind within the boundary layer,and the positive eddy contribution mainly results from asymmetric stratiform clouds and convective clouds in the outer rainbands.In lower-layer shear experiments,the positive eddy term contributing to tangential wind increases with the wind shear enlargement,because convective clouds are enhanced with the wind shear enlargement.In upper-layer shear experiments,the stronger positive eddy term contributing to tangential wind is also observed in stronger shear experiment.This is because the stratiform clouds become more organized on the downshear left,leading to stronger wavenumber-1 descending inflow in lower layers.The comparison of upper-and lower-layer shear experiments show that the eddy term contributing to the strengthening of vortex-scale tangential wind by discrete convection in lower-layer shear experiments is more significant than that by the wavenumber-1 stratiform clouds in upper-layer shear experiments,leading to a more rapid enlargement in tropical cyclone scale in lower-layer experiments.The structural features of the outer rainband stratiform sector differ significantly in observations,and the results of this study provide a possible explanation for such differences.