Terrestrial Planetary Atmospheres and Climate Extremes: From Earth to Titan

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At first glance Titan, a moon of Saturn with a surface temperature of -290F, looks nothing like Earth. But closer inspection reveals some oddly familiar scenes: photos from the Cassini mission appear to show seas, lakes, dry lake beds, and drainage channels. As on earth these surface features come about through the action of precipitation, but Titan's raindrops are made of liquid methane instead of liquid water. Also as on Earth, raindrops on Titan form in clouds, albeit clouds consisting of condensed methane cloud droplets, which in turn come from the convergence of methane vapor, carried by the winds from regions in which liquid methane evaporates from the surface. In short, Titan has a methane-based "hydrological cycle", complete with methane ice and snow, liquid methane on the surface, methane clouds and rain, and methane vapor, at temperatures low enough for methane to change phase just as water does on Earth. Recognizing the commonalities of Earth's water cycle and Titan's methane cycle, this project treats Titan as an Earth analog, a completely independent expression of hydrological cycle physics and dynamics which can be used to develop a deeper understanding of the water cycle and climate of our world. While many features of the water cycle have methane analogs on Titan, the behavior of the analogs is different in ways that suggest the same underlying dynamics but with different values of a few key parameters. For example, Earth has an intertropical convergence zone (ITCZ), a narrow low-latitude band of clouds roughly parallel to the equator, and Titan has a similar feature. In both cases the ITCZ has a seasonal cycle in which it shifts north and south over the course of the year. But on Earth the shifts are modest and the ITCZ remains in the tropics, while on Titan the ITCZ migrates all the way from the north pole to the south pole. This dramatic difference can be explained by the difference in planetary rotation rate between Earth and Titan, as Earth rotates 16 times faster than Titan. Idealized simulations of Earth at much slower rotation rates by the PI and others show a comparable widening of the north-south migration of the ITCZ. Thus, as far as ITCZ migration is concerned, rotation rate can be regarded as a dynamical parameter which can be varied to transition from an Earth-like to a Titan-like state. Two other parameters are considered in the work: the amount of vapor in the atmosphere and the amount of liquid at the surface. On earth the water vapor in a typical atmospheric column would amount to between one and ten centimeters if it were condensed to liquid form, but on Titan the methane vapor in a column of the atmosphere is higher by a factor of perhaps a hundred (although methane is still only about two percent of Titan's atmosphere, which is about 98% nitrogen). The water vapor content of Earth's atmosphere increases with temperature so that, somewhat counterintuitively, when it comes to column vapor content Titan resembles a much warmer earth. Regarding surface liquid, Titan resembles a very dry earth, as Earth's surface is dominated by deep oceans while Titan has only lakes and small seas. The project examines the extent to which variations in the three parameters can cause an Earth-like atmospheric circulation and water cycle to take on Titan-like behaviors. One such behavior is Titan's relatively meager cloud cover, which occurs despite the high methane vapor amount and precipitation which can come in heavy downpours.The work is conducted largely through computer simulations. The suite of models used includes ICON, a global cloud resolving model from the Max Planck Institute for Meteorology (MPI), the Isca model from the University of Exeter, and atmospheric column models which can simulate convection in a simplified setting. A ground hydrology model including subsurface flow is constructed for Isca as part of the project. A key element of the modeling strategy is a modification to the equation for saturation specific humidity (the Clausius-Clayperon equation) which allows column water vapor to be increased without increasing mean temperature.The work has broader impacts through a number of educational activities connected to the project. The PI incorporates results of the research in his undergraduate teaching through a "weather in a tank" suite of hands-on fluid dynamics devices. In addition, the project supports development of a radial inflow device that demonstrates the importance of angular momentum conservation on fluid flow in the atmosphere and ocean, designed by students as a class project. Classes also include a field trip on Santa Monica bay in a small research vessel, and funds from this project are used to cover costs of the trip. The project provides support and training to two graduate students, and funds are provided in the budget to allow the students to visit the foreign collaborators at Exeter and MPI. Finally, the project supports development of a land hydrology component model for use with Isca, an open-source model.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.