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T. Bosak, A. P. Ingersoll (California Institute of Technology)
Vertically propagating waves carry indirect information about the dynamic and static stability of Jupiter's deep atmosphere. We are using mesoscale waves observed as linear wave patterns in jovian clouds with wavelength clustered around 300km (Flasar and Gierasch, J.Atmos.Sci.,43,2683-2707,1986) as a probe of dynamical and thermal conditions in the otherwise unobservable regions. Time sequences of the Galileo images show that the waves move relative to the background zonal flow.
Ingersoll and Koerner (Bull. Am. Astron. Soc., 21,943,1989) suggested shear instability as the mechanism that selects the preferred wavelength of the features. The deep vertical shear layer extending from about 0.4 bars to about 5 bars and static stability as measured by the Galileo probe allowed us to test this hypothesis quantitatively. We are numerically analyzing linear stability of gravity waves with non-zero horizontal phase speeds in atmospheric conditions constrained by the probe measurements.
We have found that, if the static stability in the shear layer is very low (but positive), a strong and deep vertical shear of the zonal wind as measured by the Galileo probe (Atkinson, D.H., et al., JGR, 103, 22911-22928,1998) can generate propagating gravity wave instabilities. Very small values of static stability that support the development of the instabilities are within the range of values measured by the Galileo probe as interpreted by Seiff et al. (JGR, 103, 22857-22889,1998). The largest growth rates of the unstable modes are of the order of hours, and their horizontal wavelengths are 350±100 km. There is a good match between the modeled and the observed wavelengths and timescale of the waves.
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