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A.P. Showman (Univ. Arizona)
The question of what causes the persistent east-west jets on Jupiter, Saturn, Uranus, and Neptune remains a major unsolved problem in planetary science. Two-dimensional models of turbulent motion show that, under the appropriate conditions, small-scale turbulence reorganizes itself into large-scale jets. And since the 1970s it has been repeatedly suggested that moist convection (i.e., thunderstorms) can supply the small-scale turbulence to the cloud layer. However, published dynamical models generally induce the turbulence with random perturbations of vorticity or mass that are broadly distributed (with some characteristic wavelength) over the entire globe. In contrast, Jovian thunderstorms are localized and produce isolated clouds that expand in a few days to diameters of 1000--5000 km. Moreover, published studies have used forcing that does not interact with the flow, whereas Jovian thunderstorms typically occur within cyclonic regions, indicating that the large-scale circulation modulates the storm locations. Here, I present full nonlinear shallow-water simulations to test the hypothesis that realistic thunderstorms can drive Jupiter's cloud-level atmospheric jets. Using the premise that thunderstorms transport moist air into the cloud level, I parameterize the storms as localized mass sources that are episodically added to the model's cloud-level flow. Like Jupiter, the simulations generate numerous midlatitude vortices, primarily anticyclones; the wind speeds often reach 50-100 m/sec. Unlike Jupiter, the flow generally develops a robust westward jet at the equator. Midlatitude jets form in some cases but not in others, depending on the storm parameters. Interestingly, several simulations develop jets with latitudinal potential-vorticity gradients that strongly violate the barotropic stability criterion; eddies and/or vertical stratification (i.e., the spatially variable free-surface height) may help stabilize these jets against instabilities. These simulations generally support the suggestion that thunderstorms can help drive Jupiter's mid-latitude jets. But the repeated failure of shallow-water models to produce eastward equatorial jets suggests that the eastward equatorial jets on Jupiter and Saturn result from 3-dimensional effects. This work is supported by NSF grant AST-0206269.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.