Condensation clouds in substellar atmospheres have been widely inferred from spectra and photometric variability. Up until now, their horizontally averaged vertical distribution and mean particle size have been largely characterized using models, one of which is the eddy diffusion-sedimentation model from Ackerman & Marley (2001) that relies on a sedimentation efficiency parameter, $f_{\rm sed}$, to determine the vertical extent of clouds in the atmosphere. However, the physical processes controlling the vertical structure of clouds in substellar atmospheres are not well understood. In this work, we derive trends in $f_{\rm sed}$ across a large range of eddy diffusivities ($K_{zz}$), gravities, material properties, and cloud formation pathways by fitting cloud distributions calculated by a more detailed cloud microphysics model. We find that $f_{\rm sed}$ is dependent on $K_{zz}$, but not gravity, when $K_{zz}$ is held constant. $f_{\rm sed}$ is most sensitive to the nucleation rate of cloud particles, as determined by material properties like surface energy and molecular weight. High surface energy materials form fewer, larger cloud particles, leading to large $f_{\rm sed}$ ($>$1), and vice versa for materials with low surface energy. For cloud formation via heterogeneous nucleation, $f_{\rm sed}$ is sensitive to the condensation nuclei flux and radius, connecting cloud formation in substellar atmospheres to the objects' formation environments and other atmospheric aerosols. These insights could lead to improved cloud models that help us better understand substellar atmospheres. For example, we demonstrate that $f_{\rm sed}$ could increase with increasing cloud base depth in an atmosphere, shedding light on the nature of the brown dwarf L/T transition.
A new astrobiological model of the atmosphere of Titan
我们目前的结果的调查形成的含氮分子在土卫六的大气。我们扩展了以前的模型（Li et al. 2015，2016），以覆盖对流层顶以下的区域，因此新模型将土卫六表面的大气层处理到1500公里的高度。我们考虑冷凝和升华的影响，使用连续的，数值稳定的方法。这与气溶胶及其冷凝物的沉降和霾粒子的形成的参数化处理相结合。这些过程影响较重的物种，如含氮分子的丰度，但对较轻分子的丰度影响较小。去除分子以形成气溶胶也在确定混合比中起作用，特别是HNC、HC3N和HCN的混合比。我们发现很好的协议与最近检测到的混合比C2H5CN，与冷凝起着重要的作用，在确定该分子的丰度低于500公里。特别令人感兴趣的是丙烯腈（C2H3CN）的化学性质，Stevenson等人（2015年）提出丙烯腈是一种可以在缺氧环境中形成生物膜的分子。随着烟雾形成的列入，我们发现我们的模型预测丙烯腈与现有的观测结果很好的协议。
Hot and cloudy: High temperature clouds in super-Earths and sub-Neptunes
Leiden Observatory（莱顿天文台）；SRON Netherlands Institute for Space Research（荷兰空间研究所）；Tsinghua University（清华大学）；Jet Propulsion Laboratory, California Institute of Technology（加州理工学院喷气推进实验室）
