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D.M.G.C. Luz (Observatorio Astronomico de Lisboa, Portugal), F. Hourdin (Laboratoire de Meteorologie Dynamique, Paris)
With a specific angular momentum which is much larger than that for an atmosphere co-rotating with the surface, Titan's stratosphere is in a state of superrotation. While the mean meridional circulation carries momentum poleward from the equatorial regions, the maintenance of superrotation requires that a transient, non-axisymmetric component of the atmospheric circulation be invoked in order to transport momentum back to the equator. Waves generated by barotropic instability of the high-latitude jets are a perfect candidate mechanism for this.
We present a numerical study of Titan's stratospheric circulation, with special emphasis on the impact of non-axisymmetric barotropic waves on angular momentum and trace species. We have used a 2D latitude-altitude, axially-symmetric GCM coupling dynamics, haze microphysics and a simple model of photochemistry. Barotropic waves have been included under the form of a parameterization, which is based on their horizontal mixing properties as obtained from a shallow-water model of the stratosphere.
The 2D climate model is able to reproduce well both the main characteristics of the circulation and the latitudinal contrasts in the distributions of trace species measured by Voyager 1. The parameterization of barotropic waves is adaptive, responding in real time to changes in the degree of barotropic stability of the zonal circulation.
The time constant for horizontal eddy mixing varies strongly with the altitude. The greatest mixing occurs at the solstice between 1 and 0.01~mb, where the time constant is less than one Titan day. An important seasonal variation is also present, with the time constant decreasing by two orders of magnitude between the solstice and the equinox.