A Numerical Investigation of a Supercell Tornado: Genesis and Vorticity Budget
A Numerical Investigation of a Supercell Tornado: Genesis and Vorticity Budget
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DOI：
10.2151/jmsj.2010-203
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发表时间：
2010-04
影响因子：
3.1
通讯作者：
A. Noda;H. Niino
中科院分区：
文献类型：
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作者：
A. Noda;H. Niino
通过对1977年5月20日发生在美国俄克拉荷马州的德尔城风暴的高分辨率（水平均匀网格尺寸为70 m）数值模拟，研究了超级单体龙卷风形成机制及其涡度收支。模拟50分钟后，在距地面约1.8 km处形成了一个由风暴上升气流和与环境和风暴引起的水平气流相关的垂直风切变之间的非线性相互作用产生的中低气压。中低压的作用是加强底层的上升气流，并通过与环境风相关的水平涡度倾斜和斜压过程产生的水平涡度产生低层中气旋。与低空中气旋相关的低压在1.5 km AGL处产生超过43 m s -1的上升气流。此外，小尺度涡旋（飑前涡旋）沿阵风锋发展。当低层上升气流增强时，其中一个位于上升气流下方的风暴前涡旋迅速成长为一个大型龙卷风。随着龙卷风的发展，与龙卷风强烈旋转有关的向下压力梯度力加强了其北侧龙卷风规模的下降气流。发展的下沉气流压缩了龙卷风的垂直涡，最终导致其消散。我们还沿着典型的气包轨迹进行了涡度预算分析。成熟旋涡中的空包主要来自于西北方向AGL 10 ~ 500 m层。成熟龙卷风内部的垂直涡度最初是在下降过程中通过水平涡度的倾斜产生的，水平涡度的倾斜通过涡管的拉伸而增强。
The mechanism of supercell tornadogenesis and its vorticity budget are investigated by means of a high-resolution (horizontally uniform grid size of 70 m) numerical simulation of the Del City storm, which occurred in Oklahoma, USA, on May 20, 1977. After 50 min of the simulation, a meso-low, which is generated by nonlinear interaction between the storm updraft and vertical wind shear associated with both environmental and storm-induced horizontal flow, develops at around 1.8 km above ground level (AGL). The meso-low acts to strengthen the underlying updraft, and generates a low-level mesocyclone via the tilting of horizontal vorticity associated with the environmental wind and that generated by baroclinic processes. In turn, a pressure depression associated with this low-level mesocyclone generates an updraft exceeding 43 m s -1 at 1.5 km AGL. In addition, small-scale vortices (pretornadic vortices) develop along a gust front. When the low-level updraft strengthened, one of these pretornadic vortices located immediately beneath the updraft shows a rapid growth into a major tornado. As the tornado develops, a downward pressure gradient force associated with an intense rotation of the tornado strengthens a tornado-scale downdraft on its north side. The developed downdraft compresses the vertical vortex of the tornado, eventually resulting in its dissipation. We also performed a vorticity budget analysis along a typical air-parcel trajectory. Air parcels in the mature tornado vortex originate mainly from the northwest in a layer between 10 and 500 m AGL. Vertical vorticity within the mature tornado is initially produced during descent via tilting of the horizontal vorticity, which is enhanced by stretching of the vortex tube.