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球调和函数和相应的大气环流拓扑模型

Spherical Harmonics and Topological Models of Atmospheric Circulation

  • 摘要: 球调和函数的Nodal集(零集)将球面剖分成许多小块。剖分出的顶点数V,边数E,面数F和球面拓扑的欧拉示性数\; \chi 之间的关系为\; \chi =V-E+F=2 。若球面上流场为有旋场,则Nodal集的物理意义为球面的垂直涡度为零,Nodal集将球面分成正负涡度相间的小块,分别代表气旋、反气旋。对仅有纬向气流的有旋场,Nodal集既是垂直涡度为零,也是纬度加权纬向气流的最大值集。若球面上流场为无旋梯度场,则纬向的Nodal集是等位势线或等压线,南北的经向环流就和Nodal集相垂直,Nodal集是水平散度为零,剖分球面成正负水平散度相间的小块。据此球面上的三圈环流(纬向环流、经向环流、Hadley环流)与行星风带等大气环流,从拓扑上定性地就知道它们的模型。然后再从流场临界点性质加以验证。

     

    Abstract: The Nodal sets (zero sets) of spherical harmonics divide the spherical surface into numerous small regions through their intersections, leading to the formation of vertices (V), edges (E), and faces (F). The relationship among these elements and the Euler characteristic (\; \chi ) of spherical topology is expressed as \; \chi =V-E+F=2 . If the flow field on the spherical surface is vortical, the Nodal sets physically correspond to locations where the vertical vorticity on the sphere is zero. These Nodal sets divide the sphere into alternating regions of positive and negative vorticity, which represent cyclonic and anticyclonic systems, respectively. For vortical fields consisting solely of the zonal flow, the Nodal sets coincide with the locations of zero vertical vorticity and the latitude-weighted maxima of the zonal flow. If the flow field on the spherical surface is a nonvortical gradient field, the zonal Nodal set represents lines of constant potential or isobars. The meridional circulation, such as the north–south meridional flow, is perpendicular to the Nodal set. In this case, the Nodal set corresponds to regions of zero horizontal divergence and divides the spherical surface into alternating areas of positive and negative horizontal divergence. Based on these considerations, qualitative models of atmospheric circulation, such as meridional and zonal flows, Hadley circulation, and the three-cell circulation system with its associated planetary wind belts, can be inferred from a topological perspective on the sphere. These conceptual models can be validated through analysis of the properties of critical points in the flow field.

     

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