Titan's stratospheric composition driven by condensation and dynamics
Titan's stratospheric composition driven by condensation and dynamics
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DOI：
10.1029/2004je002282
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发表时间：
2004-12-23
影响因子：
4.8
通讯作者：
Rannou, P
中科院分区：
文献类型：
--
作者：
Hourdin, F;Lebonnois, S;Rannou, P
使用轴对称大气环流模型研究了土卫六平流层中化合物和有机雾霾的大气输送。先前的研究表明，以全球哈德来环流为主的经向环流既造成了强烈的平流层纬向流的产生，又造成了高纬度地区化学物质和雾霾的积累。修改后的成分反过来又增强了经向环流和赤道与极地的热对比。本文详细分析了导致观测到的大气成分垂直和纬度变化的传输过程。结果表明，冬季极地涡旋中平流层上层空气的快速下沉与正压行星波（模型中参数化）的纬度混合之间的竞争控制着化合物的垂直梯度。低纬度地区的极地富集程度（取决于特定物种，为 1.4 至 20 倍）主要由经向平流增加从对流层上升的清洁空气中化合物浓度的方式控制，其中大部分化合物通过冷凝而被去除（对流层顶的温度接近 70 K）。观测和模拟对比之间的一致性为模拟动力学提供了间接但有力的验证，从而证实了对大气超旋转提出的解释。研究还表明，通过测量大气成分，卡西尼-惠更斯任务将为土卫六的大气环流提供强有力的约束。
Atmospheric transport of chemical compounds and organic haze in the stratosphere of Titan is investigated with an axisymmetric general circulation model. It has been shown previously that the meridional circulation, dominated by global Hadley cells, is responsible both for the creation of an intense stratospheric zonal flow and for the accumulation of chemical compounds and haze in high latitudes. The modified composition in turn intensifies the meridional circulation and equator-to-pole thermal contrasts. This paper analyzes in detail the transport processes responsible for the observed vertical and latitudinal variations of atmospheric composition. It is shown that the competition between rapid sinking of air from the upper stratosphere in the winter polar vortex and latitudinal mixing by barotropic planetary waves (parameterized in the model) controls the vertical gradient of chemical compounds. The magnitude of polar enrichment (of a factor 1.4 to 20 depending on the particular species) with respect to low latitudes is mostly controlled by the way the meridional advection increases the concentrations of chemical compounds in the clean air which is rising from the troposphere, where most of the chemical compounds are removed by condensation ( the temperature at the tropopause being close to 70 K). The agreement between the observed and simulated contrasts provides an indirect but strong validation of the simulated dynamics, thus confirming the explanation put forward for atmospheric superrotation. It is shown also that by measuring the atmospheric composition, the Cassini-Huygens mission will provide a strong constraint about Titan's atmospheric circulation.