Imported and Storm-Generated Near-Ground Vertical Vorticity in a Simulated Supercell*
Imported and Storm-Generated Near-Ground Vertical Vorticity in a Simulated Supercell*
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DOI：
10.1175/jas-d-13-0123.1
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发表时间：
2014-07
影响因子：
3.1
通讯作者：
Johannes M. L. Dahl;M. Parker;Louis J. Wicker
中科院分区：
文献类型：
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作者：
Johannes M. L. Dahl;M. Parker;Louis J. Wicker
利用高分辨率的超级单体模式，将近地面垂直涡度矢量分解为正压和非正压两部分，研究了近地面垂直涡度的来源。这样，环境和风暴产生的涡度的作用可以被隔离。采用一种新的拉格朗日技术，其中物质流体体积元素跟踪分析的环境涡线段的重排。这一贡献被解释为正压涡度，风暴生成涡度被解释为已知总涡度和正压涡度之间的残差。在模拟中，近地面垂直涡度的发展是一种外流现象。有明显的气旋性切变涡的“河流”起源于下降气流的底部，流入发展中的近地涡旋。这些垂直涡度河流的起源主要是水平斜压生产，这是最大的最低几百米AGL。随后，这种水平涡度向上倾斜，而包裹仍在下降。正压涡度仍然主要是流向沿着分析的轨迹，并没有获得一个大的垂直分量的包裹到达地面。因此，输入到风暴中的环境涡量只占近地垂直涡量的一小部分。
The authors use a high-resolution supercell simulation to investigate the source of near-ground vertical vorticity by decomposing the vorticity vector into barotropic and nonbarotropic parts. This way, the roles of ambient and storm-generated vorticity can be isolated. A new Lagrangian technique is employed in which material fluid volume elements are tracked to analyze the rearrangement of ambient vortex-line segments. Thiscontributionisinterpretedasbarotropicvorticity.Thestorm-generatedvorticityistreatedastheresidual between the known total vorticity and the barotropic vorticity. In the simulation the development of near-ground vertical vorticity is an outflow phenomenon. There are distinct ‘‘rivers’’ of cyclonic shear vorticity originating from the base of downdrafts that feed into the developing near-ground vortex. The origin of these rivers of vertical vorticity is primarily horizontal baroclinic production, which is maximized in the lowest few hundred meters AGL. Subsequently, this horizontal vorticity is tilted upward while the parcels are still descending. The barotropic vorticity remains mostly streamwise along the analyzed trajectories and does not acquire a large vertical component as the parcels reach the ground. Thus, the ambient vorticity that is imported into the storm contributes only a small fraction of the total near-ground vertical vorticity.