Abstract This study analyzes GPS dropsonde data in multiple tropical cyclones from 1997 to 2017 to investigate the boundary layer structure with a focus on helicity distribution. A helicity-based method for boundary layer height is developed and evaluated by comparing it to other boundary layer height scales including the inflow layer depth, height of the maximum tangential wind speed and thermodynamic mixed layer depth. Our dropsonde composites confirmed the radial variations of these boundary layer heights seen in previous studies. The results show that the boundary layer height defined by the maximum vertical gradient of helicity is closest to the height of the maximum tangential wind speed or jet height and is located between the inflow layer depth and thermodynamic mixed layer height in all intensity groups. All three kinematic height scales generally decrease with storm intensity at a given radius. These kinematic height scales converge in the major hurricane group, while the inflow layer depth is much larger than the other two height scales in the tropical storm group. The maximum normalized helicity is located at 100–200 m altitude which is close to the height of the maximum inflow. Both front-back and downshear-upshear asymmetries are observed in the 0–1 km layer integrated helicity in the inner core region of a storm, and the helicity on the front and downshear sides is larger in all intensity groups. The results also show that the helicity magnitude is generally larger in the boundary layer of stronger storms. Application of helicity to quantify turbulent characteristics in the boundary layer is discussed.

中文翻译：

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下落式探空仪复合材料热带气旋边界层的螺旋度分布

摘要 本研究分析了 1997 年至 2017 年多个热带气旋的 GPS 下投探空仪数据，以研究边界层结构，重点研究螺旋度分布。通过将边界层高度与其他边界层高度尺度（包括流入层深度、最大切向风速高度和热力学混合层深度）进行比较，开发并评估了一种基于螺旋度的边界层高度方法。我们的下落式探空仪复合材料证实了之前研究中看到的这些边界层高度的径向变化。结果表明，由最大垂直螺旋梯度定义的边界层高度最接近最大切向风速或射流高度，并且在所有强度组中都位于流入层深度和热力混合层高度之间。所有三个运动高度尺度通常随着给定半径的风暴强度而降低。这些运动高度尺度在主要飓风组中会聚，而流入层深度远大于热带风暴组中的其他两个高度尺度。最大归一化螺旋度位于接近最大流入高度的 100-200 m 高度。在风暴核心区的0-1 km层综合螺旋度中观察到前后和下切变上切变不对称，并且在所有强度组中前和下切变侧的螺旋度都较大。结果还表明，在强风暴的边界层中，螺旋度大小一般较大。讨论了螺旋度在边界层中量化湍流特性的应用。