Superrotation of Titan’s Stratosphere Driven by the Radiative Heating of the Haze Layer
Superrotation of Titan’s Stratosphere Driven by the Radiative Heating of the Haze Layer
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DOI：
10.3847/1538-4357/ac54ab
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发表时间：
2022-02
期刊：
影响因子：
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通讯作者：
Motoki Sumi;S. Takehiro;W. Ohfuchi;H. Nomura;Y. Fujii
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文献类型：
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作者：
Motoki Sumi;S. Takehiro;W. Ohfuchi;H. Nomura;Y. Fujii
土卫六的平流层被观察到处于超旋转状态，大气的旋转速度比表面快很多倍。土卫六大气层的另一个特征是存在厚厚的薄雾层。本文利用一个大气环流模式进行了数值试验，探讨了霾层对平流层超旋的影响。我们采用了一个半灰色的土卫六的大气辐射模型，其考虑了混浊颗粒对阳光的吸收。没有考虑甲烷的相变或季节变化。我们的模型与辐射参数调整土卫六产生了一个全球性的东风在赤道附近的更大的速度在更高的海拔，除了在70公里左右，105个地球日。虽然大气不处于平衡状态，但纬向风廓线与观测廓线基本一致。通过改变辐射模式的参数，我们发现最大纬向风速的强度和位置分别与霾层的光学厚度和高度密切相关。我们的实验分析表明，准静止平流层超旋转是由从表面解耦的纬向环流和向赤道输送角动量的涡旋之间的平衡维持的。这与所谓的吉拉斯机制不同，但类似，在吉拉斯机制中，动量是从表面提供的。这种结构可以解释在80公里高度处的无风区。
Titan’s stratosphere has been observed in a superrotation state, where the atmosphere rotates many times faster than the surface does. Another characteristic of Titan’s atmosphere is the presence of a thick haze layer. In this paper, we performed numerical experiments using a general circulation model to explore the effects of the haze layer on the stratospheric superrotation. We employed a semigray radiation model of Titan’s atmosphere following McKay et al., which takes account of sunlight absorption by the haze particles. The phase change of methane or seasonal changes were not taken into account. Our model with radiation parameters tuned for Titan yielded a global eastward wind around the equator with larger velocities at higher altitudes, except at around 70 km, after 105 Earth days. Although the atmosphere is not in an equilibrium state, the zonal wind profiles are approximately consistent with the observed one. By changing the parameters of the radiation model, we found that the intensity and the location of the maximum zonal wind velocity highly depended on the optical thickness and the altitude of the haze layer, respectively. Analysis of our experiments suggests that the quasi-stationary stratospheric superrotation is maintained by the balance between the meridional circulation decoupled from the surface and the eddies that transport angular momentum equatorward. This is different from, but similar to, the so-called Gierasch mechanism, in which momentum is supplied from the surface. This structure may explain the no-wind region at about 80 km in altitude.