Modeling the microphysics of CO2 ice clouds within wave-induced cold pockets in the martian mesosphere
Modeling the microphysics of CO2 ice clouds within wave-induced cold pockets in the martian mesosphere
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模拟火星中间层波浪引起的冷区中二氧化碳冰云的微观物理
DOI：
10.1016/j.icarus.2014.04.022
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发表时间：
2014
期刊：
影响因子：
3.2
通讯作者：
F. Lefévre
中科院分区：
文献类型：
--
作者：
C. Listowski;A. Määttänen;F. Montmessin;A. Spiga;F. Lefévre
用一维微物理模型模拟了火星中间层CO2冰云，该模型包括一个适合于火星中间层高过饱和度的晶体生长速率。白天在赤道周围60-80公里高度观测到的这些云存在观测限制（晶体半径和不透明度）。夜间中层云被解释为CO2冰云，也在南半球低纬度地区（海拔1090 -100公里）出现。从模拟和观测证据来看，人们认为中间层云是在热潮汐产生的温度最低值内形成的，重力波的传播允许产生过饱和层（冷袋）。因此，在模型中使用的重力波扰动的温度分布，以启动成核和维持CO2冰晶的生长。我们表明，这是可能的，以重现观测到的有效半径白天和夜间的云。晶体的大小主要取决于云形成的高度和过饱和度的幅度。云的时间和空间行为由冷袋的范围和寿命控制。在冷袋消失后，云迅速蒸发，这意味着重力波活动和CO2云形成之间有很强的相关性。只要使用典型的尘埃条件，模拟的不透明度仍然远远低于观察到的。在白天较低的云层中，沙尘暴条件下通常达到的中间层尘埃负荷增加，通过向大气提供凝结核，允许更大的云不透明度，接近观测值。然而，CO2冰云没有检测到在沙尘暴季节，而且，由于快速沉降的尘埃粒子，外源性供应（陨石通量）似乎有必要解释白天和夜间的中间层CO2冰云沿着他们的整个观察期间的不透明。
Mesospheric CO2ice clouds on Mars are simulated with a 1D microphysical model, which includes a crystal growth rate adapted to high supersaturations encountered in the martian mesosphere. Observational constraints (crystal radius and opacity) exist for these clouds observed during the day around the equator at ∼60–80 km altitude. Nighttime mesospheric clouds interpreted as CO2ice clouds have also been characterized at low southern latitudes, at ∼90–100 km altitude. From modeling and observational evidence, it is believed that mesospheric clouds are formed within temperature minima created by thermal tides, where gravity wave propagation allows for the creation of supersaturated layers (cold pockets). Thus, temperature profiles perturbed by gravity waves are used in the model to initiate nucleation and maintain growth of CO2ice crystals. We show that it is possible to reproduce the observed effective radii for daytime and nighttime clouds. Crystal sizes are mainly governed by the altitude where the cloud forms, and by the amplitude of supersaturation. The temporal and spatial behavior of the cloud is controlled by the extent and lifetime of the cold pocket. The cloud evaporates fast after the cold pocket has vanished, implying a strong correlation between gravity wave activity and CO2cloud formation. Simulated opacities remain far below the observed ones as long as typical dust conditions are used. In the case of the lower daytime clouds, the enhanced mesospheric dust loading typically reached during dust storm conditions, allows for greater cloud opacities, close to observed values, by supplying the atmosphere with condensation nuclei. However, CO2ice clouds are not detected during the dust storm season, and, because of fast sedimentation of dust particles, an exogenous supply (meteoritic flux) appears necessary to explain opacities of both daytime and nighttime mesospheric CO2ice clouds along their whole period of observation.