General circulation and transport in Saturn鈥檚 upper troposphere and stratosphere

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• 作者：A. James Friedsonp>ap>p> ; p>p>Andrew.Friedson@jpl.nasa.govp> ; Julianne I. Mosesp>bp>
• 关键词：Saturn ; Atmospheres ; Dynamics ; Atmospheres ; Structure
• 刊名：Icarus
• 出版年：2012
• 期刊代码：90_00191035
• 类别：ph
• 出版时间：April, 2012
• 卷：218
• 期：2
• 页码：861-875
• 文件大小：1178 K

摘要

We present a model for the general circulation and dynamical transport in Saturn鈥檚 upper troposphere and stratosphere and derive the effective advective circulation and eddy transport coefficients required for use in two-dimensional (latitude-altitude) photochemistry-transport models. A three-dimensional Outer-Planet General Circulation Model (OPGCM) is used to generate the transport data. We find that the OPGCM adequately captures the global-scale, pole-to pole temperature contrast, but overestimates mid- and high-latitude temperatures in the summer hemisphere by 鈭? K. In addition, the model reproduces the local temperature minimum seen at the equator in Cassini Composite Infrared Spectrometer (CIRS) 0.1-mbar data but not the local maximum in 1-mbar temperatures, suggesting that it is capturing the phase of Saturn鈥檚 Semiannual Oscillation associated with a temperature minimum at the equator but not the opposite phase. The meridional circulation at low latitudes is found to be dominated by a seasonally reversing Hadley circulation, characterized by upwelling near the equator, cross-equatorial flow from summer to winter hemisphere, and strong subsidence centered near 25掳 latitude in the winter hemisphere. The cross-equatorial flow induces an asymmetry in which the equatorial jet is found to be stronger in the winter than in the summer stratosphere. The location of the subsidence near 25掳N for L_s 鈭?#xA0;310掳 coincides with local maxima in acetylene, diacetylene, and methylacetylene mixing ratios measured by Cassini/CIRS (Guerlet, S., Fouchet, T., B茅zard, B., Moses, J.I., Fletcher, L.N., Simon-Miller, A.A., Flasar, F.M. [2010]. Icarus 209, 682-695). This result supports the suggestion by Guerlet et al. (2010) that the hydrocarbon abundances are enhanced at this latitude by pronounced downward transport of hydrocarbon-rich air from above. The lateral eddy diffusion coefficient is found to typically be 鈭?0p>5p>-10p>6p> mp>2p> sp>鈭?p> at mid-latitudes, implying meridional eddy transport time scales of order 100-1000 years.