Diagnostics of Titan's stratospheric dynamics using Cassini/CIRS data and the 2-dimensional IPSL circulation model
Diagnostics of Titan's stratospheric dynamics using Cassini/CIRS data and the 2-dimensional IPSL circulation model
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DOI：
10.1016/j.icarus.2008.05.010
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发表时间：
2008-10-01
期刊：
影响因子：
3.2
通讯作者：
Hourdin, F.
中科院分区：
文献类型：
--
作者：
Crespin, A.;Lebonnois, S.;Hourdin, F.
本研究讨论了泰坦平流层的动力学，基于卡西尼航天器上的CIRS仪器的观测结果与皮埃尔-西蒙·拉普拉斯研究所开发的二维环流模型的结果的比较，该模型可在http://www.lmd.jussieu.fr/ titanDbase获得[Rannou， P., Lebonnois， S., Hourdin， F., Luz， D., 2005]。航天广告，36,2194-21981。比较的目的是评估模型的能力和解释观测结果，包括：(1)利用卡西尼/CIRS的温度反演和惠更斯/DWE获得的惠更斯着陆点纬向风垂直剖面的动力和热结构；(2)根据Cassini/CIRS数据推断的平流层气体垂直和纬度剖面。模拟的热结构与从观测（卡西尼/CIRS和基于地球的观测）推断的热结构相似。然而，平流层上层（0.05 mbar以上）在2D-CM中系统地太热，因此平流层顶区域没有很好地代表。这种偏置可能与雾霾的结构和该地区的误报辐射效应有关，如氰化氢（HCN）的冷却效应。2D-CM产生强烈的大气超旋转，在海拔200至300公里（0.1-1毫巴）的冬季高纬度地区，纬向风达到200毫秒（-1）。模拟的纬向风与掩星观测、卡西尼/CIRS和惠更斯/DWE反演的风场吻合良好。热结构的变化与经向环流和极涡扩展的变化相耦合，从而影响化学分布，特别是在冬季极地地区。当使用较高海拔的霾源时，模拟得到的经向环流较弱，极地涡旋引起的垂直和水平混合在纬度上的扩展较少。模拟的化学分布与赤道地区的观测结果总体上很吻合。观测到的C2H2和HCN垂直梯度的差异可能是HCN相对循环强度和化学损失的一个指标。在南纬15度的低平流层中，乙烯的负垂直梯度不能用简单的一维模型来模拟，因为在这种情况下，平流层中部必须有一个强大的光化学汇。这里可以用平流层低处从冬季极向赤道的动力平流和乙烯不凝结的事实来解释。在冬季极（北纬80度）附近，一些化合物（C4H2, C3H4）在观测到的丰度垂直剖面中表现出（内部）最小值，而2D-CM剖面则沿大气柱混合良好。这个最小值可以诊断出经向环流的强度，以及冬季极涡的空间扩展，那里存在强烈的下降运动。在夏季半球，观测到的平流层丰度在纬度上是均匀的，而模式在夏季极上保持了春季单体的残余富集，这是由于在南纬40-50度南纬1 -50毫巴之间的次级经向翻转造成的。这种结构的强度以及时空扩展是一个难题，这可能与水平混合过程的可能错误描述有关。由于模型的受限二维性质。这一限制也应作为其他差异的可能来源牢记在心。(C) 2008爱思唯尔公司版权所有。
The dynamics of Titan's stratosphere is discussed in this study, based on a comparison between observations by the CIRS instrument on board the Cassini spacecraft, and results of the 2-dimensional circulation model developed at the Institute Pierre-Simon Laplace, available at http://www.lmd.jussieu.fr/ titanDbase [Rannou, P., Lebonnois, S., Hourdin, F., Luz, D., 2005. Adv. Space Res. 36, 2194-21981. The comparison aims at both evaluating the model's capabilities and interpreting the observations concerning: (1) dynamical and thermal structure using temperature retrievals from Cassini/CIRS and the vertical profile of zonal wind at the Huygens landing site obtained by Huygens/DWE; and (2) vertical and latitudinal profiles of stratospheric gases deduced from Cassini/CIRS data. The modeled thermal structure is similar to that inferred from observations (Cassini/CIRS and Earth-based observations). However, the upper stratosphere (above 0.05 mbar) is systematically too hot in the 2D-CM, and therefore the stratopause region is not well represented. This bias may be related to the haze structure and to misrepresented radiative effects in this region, such as the cooling effect of hydrogen cyanide (HCN). The 2D-CM produces a strong atmospheric superrotation, with zonal winds reaching 200 ms(-1) at high winter latitudes between 200 and 300 km altitude (0.1-1 mbar). The modeled zonal winds are in good agreement with retrieved wind fields from occultation observations, Cassini/CIRS and Huygens/DWE. Changes to the thermal structure are coupled to changes in the meridional circulation and polar vortex extension, and therefore affect chemical distributions, especially in winter polar regions. When a higher altitude haze production source is used, the resulting modeled meridional circulation is weaker and the vertical and horizontal mixing due to the polar vortex is less extended in latitude. There is an overall good agreement between modeled chemical distributions and observations in equatorial regions. The difference in observed vertical gradients of C2H2 and HCN may be an indicator of the relative strength of circulation and chemical loss of HCN. The negative vertical gradient of ethylene in the low stratosphere at 15 degrees S, cannot be modeled with simple 1-dimensional models, where a strong photochemical sink in the middle stratosphere would be necessary. It is explained here by dynamical advection from the winter pole towards the equator in the low stratosphere and by the fact that ethylene does not condense. Near the winter pole (80 degrees N), some compounds (C4H2, C3H4) exhibit an (interior) minimum in the observed abundance vertical profiles, whereas 2D-CM profiles are well mixed all along the atmospheric column. This minimum can be a diagnostic of the strength of the meridional circulation, and of the spatial extension of the winter polar vortex where strong descending motions are present. In the summer hemisphere, observed stratospheric abundances are uniform in latitude, whereas the model maintains a residual enrichment over the summer pole from the spring cell due to a secondary meridional overturning between 1 and 50 mbar, at latitudes south of 40-50 degrees S. The strength, as well as spatial and temporal extensions of this structure are a difficulty, that may be linked to possible misrepresentation of horizontally mixing processes, due to the restricted 2-dimensional nature of the model.This restriction should also be kept in mind as a possible source of other discrpancies. (C) 2008 Elsevier Inc. All rights reserved.