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不同高度垂直风切变环境下热带气旋外核区对流尺度上升运动特征对比分析

Characteristics of convective-scale updrafts in the outer core of numerically simulated tropical cyclones: lower-layer shear versus upper-layer shear

  • 摘要: 利用理想数值模式资料,对比分析了不同高度垂直风切变(简称风切变)影响下热带气旋外核区对流尺度上升运动的特征。结果表明,上升运动在高层和低层风切变环境中呈现不同的特征:(1)低层风切变试验顶高达到12 km以上的上升运动的比例更高,上升运动的垂直质量输送和最大垂直速度更大,与较大的对流有效位能、边界层相当位温和边界层水汽混合比有关。此外,低层风切变、热带气旋主环流诱发的局地风切变和地面冷池之间的相互作用对上升运动的发生发展也产生积极影响。(2)上升运动的径向倾斜与上升运动的顶高和非对称径向流场有关。低层风切变试验穿透性对流上升运动占比大,顺着风切变象限(简称顺风切)的上升运动受到高层非对称径向出流的影响随高度径向向外倾斜,逆着风切变象限(简称逆风切)的上升运动受到高层非对称径向入流的影响随高度径向向眼区倾斜;高层垂直风切变试验大部分上升运动受到顺风切右侧和逆风切象限对流层高层非对称径向入流及顺风切左侧对流层中低层非对称径向入流的影响随高度径向向眼区倾斜。(3)低层风切变试验对流层低层以相对对称的径向入流为主,大部分上升运动向眼区移动;高层风切变试验顺风切低层为径向入流,上升运动径向向眼区移动,逆风切低层为径向出流,上升运动径向向外移动。(4)低层风切变试验的上升运动的热力浮力较大,但动力浮力以负值为主,导致总浮力从负值到正值分布;高层风切变试验的上升运动的热力浮力较小,动力浮力和总浮力以正值为主。

     

    Abstract: The characteristics of convective-scale updrafts in the outer core of tropical cyclones (TC) simulated under lower- and upper-layer vertical wind shear are compared in this study. The results show that there are quite a few notable differences between the two experiments. More deep updrafts develop up to the tropopause in the lower-layer shear experiment, with their top heights above 12 km. Vertical mass transport, maximum vertical velocity associated with outer-core updrafts are statistically significantly larger in the lower-layer sheared TC than in the upper-layer sheared TC, which is related to larger convective effective potential energy, equivalent temperature and water vapor mixing ratio in the boundary layer. Furthermore, the interaction among the lower-layer vertical wind shear, local wind shear induced by the circulation of TC and surface cold pools is also favorable to the initiation and development of updrafts. The radial tilts of the updrafts is associated with updraft top height and asymmetric radial flows. Updrafts with top heights exceeding 12 km dominates in lower-layer shear experiment, the preferential radial tilt is radially outward induced by upper-level asymmetric radial outflow in downshear quadrants but radially inward induced by upper-level asymmetric radial inflow in upshear quadrants. However, in upper-layer shear experiment, most of updrafts prefer to tilt radially inward in height, the largest proportion of the updrafts originate from in upshear quadrants and downshear right quadrant, another small proportion of updrafts with top heights less than 8 km originates from downshear left quadrant, which induced by asymmetric radial inflow at upper-level of troposphere in upshear quadrants and downshear right quadrant, and asymmetric radial inflow in low-mid troposphere in downshear left quadrant, respectively. In the lower-layer shear experiment, updrafts in all quadrants tended to move radially inward induced by radial inflow in lower levels. However, in the upper-layer shear experiment, updrafts in upshear quadrants tended to move radially outward induced by radial outflow in lower levels, updrafts in downshear quadrants tended to move radially inward induced by radial inflow in lower levels. Most of updrafts in lower-layer shear experiment exhibit larger positive thermal buoyancy and negative dynamic buoyancy, resulting in a negative to positive distribution of total buoyancy. In contrast, in the upper-layer shear experiment, most of the updrafts exhibit smaller positive thermal buoyancy and positive dynamic buoyancy, resulting in positive total buoyancy.

     

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