标题： 天王星和海王星上的风暴与对流：甲烷丰度的影响通过三维云分辨模型揭示 显示英文标题

标题： Storms and convection on Uranus and Neptune: impact of methane abundance revealed by a 3D cloud-resolving model

摘要： 天王星和海王星的大气主要由分子氢和氦组成。在上部对流层，甲烷是第三种主要分子，并且会凝结，从而在CH4中产生垂直梯度。这种可凝结物种比H2和He重，因此由于凝结导致的平均分子量变化成为抵消干对流和湿对流的一个因素。由于观测还显示甲烷丰度存在纬向变化，可以预期从一个纬度到另一个纬度会有不同的垂直梯度。本文研究了这一甲烷垂直梯度对大气环流的影响，特别是对冰巨星对流层中湿润对流风暴的形成和抑制作用。我们开发了一个三维云分辨模型来模拟对流过程。利用我们的模拟，我们得出结论，深对流层中典型的干对流速度相对较低（大约为1米/秒），但足以维持甲烷的向上运输，而在甲烷凝结层的湿对流受到强烈抑制。先前的研究得出了一个关于甲烷蒸汽量的分析标准，超过这个量湿对流应该被抑制。我们首先通过数值方法验证了这个分析标准。然后我们表明临界甲烷丰度控制着湿对流风暴的抑制和形成，并得出结论这些风暴的强度和间歇性应取决于甲烷丰度和饱和度。在冰巨星中，干对流较弱，而湿对流受到强烈抑制。然而，当足够多的甲烷通过干对流和湍流扩散被输送到上方时，偶尔会出现湿润的对流风暴。由于海王星内部热流较强，这类风暴在海王星上比在天王星上更频繁。我们的结果可以解释冰巨星中观测到的云的偶发性。 显示英文摘要

摘要： Uranus and Neptune have atmospheres dominated by molecular hydrogen and helium. In the upper troposphere, methane is the third main molecule and condenses, yielding a vertical gradient in CH4. This condensable species being heavier than H2 and He, the resulting change in mean molecular weight due to condensation comes as a factor countering dry and moist convection. As observations also show latitudinal variations in methane abundance, one can expect different vertical gradients from one latitude to another. In this paper, we investigate the impact of this methane vertical gradient on the atmospheric regimes, especially on the formation and inhibition of moist convective storms in the troposphere of ice giants. We develop a 3D cloud-resolving model to simulate convective processes. Using our simulations, we conclude that typical velocities of dry convection in the deep atmosphere are rather low (of the order of 1 m/s) but sufficient to sustain upward methane transport, and that moist convection at methane condensation level is strongly inhibited. Previous studies derived an analytical criterion on the methane vapor amount above which moist convection should be inhibited. We first validate this analytical criterion numerically. We then show that the critical methane abundance governs the inhibition and formation of moist convective storms, and we conclude that the intensity and intermittency of these storms should depend on the methane abundance and saturation. In ice giants, dry convection is weak, and moist convection is strongly inhibited. However, when enough methane is transported upwards, through dry convection and turbulent diffusion, sporadic moist convective storms can form. These storms should be more frequent on Neptune than on Uranus, because of Neptune's internal heat flow. Our results can explain the observed sporadicity of clouds in ice giants.