金星南极涡旋的位势涡度
Potential Vorticity of the South Polar Vortex of Venus
AI总结：
本研究绘制金星南极区域Ertel位势涡度（EPV）分布图，分析上云层南极涡旋的三种形态并初步估算下云层EPV，发现EPV形态由相对涡度垂直分量决定，上云层存在弱环状结构而下云层更均匀，为理解金星极涡动力学提供支撑。
摘要：
金星大气在两极均存在高度多变的暖涡旋，其形成机制与特性背后的本质仍未明确。位势涡度是当平流过程主导摩擦与非绝热加热作用时的守恒量，也是模拟平衡流的常用物理量。作为理解涡旋动力学的一步，我们绘制了金星南极区域的Ertel位势涡度（EPV）分布图。我们基于此前对金星快车（Venus Express）搭载的VIRTIS仪器获取数据的云运动与热结构分析，研究了上云层（压强P约240毫巴；高度z约58千米）处南极涡旋的三种形态。此外，我们结合此前的风场测量结果，以及先驱者金星号（Pioneer Venus）和VIRA模型的静力稳定度数据，初步估算了下云层（压强P约2200毫巴；高度z约43千米）的EPV。上、下云层的EPV值分别约为10^-6和10^-8 K·m²·kg⁻¹·s⁻¹，是估算误差的3倍。EPV分布图呈现的形态主要由相对涡度的垂直分量结构决定，这与3.8或5微米图像中观测到的、与云顶大气热结构相关的涡旋形态形成对比。部分上云层EPV分布图呈现出弱环状结构，而下云层的EPV场则更为均匀。
英文摘要：
Venus' atmosphere shows highly variable warm vortices over both of the planet's poles. The nature of the mechanism behind their formation and properties is still unknown. Potential vorticity is a conserved quantity when advective processes dominate over friction and diabatic heating, and is a quantity frequently used to model balanced flows. As a step toward understanding the vortices' dynamics, we present maps of Ertel's potential vorticity (EPV) at Venus' south polar region. We analyze three configurations of the South Polar Vortex at the upper cloud level ($P\sim 240 mbar$; $z\sim 58 km$), based on our previous analyses of cloud motions and thermal structure from data acquired by the VIRTIS instrument onboard Venus Express. Additionally, we tentatively estimate EPV at the lower cloud level ($P\sim 2200 mbar$; $z\sim 43 km$), based on our previous wind measurements and on static stability data from Pioneer Venus and the VIRA model. Values of EPV are on the order of $10^{-6}$ and $10^{-8} K m^2 kg^{-1} s^{-1}$ at the upper and lower cloud levels, respectively, being 3 times larger than the estimated errors. The morphology observed in EPV maps is mainly determined by the structures of the vertical component of the relative vorticity. This is in contrast to the vortex's morphology observed in 3.8 or 5 $μm$ images which are related to the thermal structure of the atmosphere at the cloud top. Some of the EPV maps point to a weak ringed structure in the upper cloud while a more homogenous EPV field is found in the lower cloud.
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