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M. H. Wong, S. K. Atreya (Univ. Michigan), P. N. Romani (NASA GSFC)
We introduce a one-dimensional model of a downdraft with entrainment in order to explain the unexpected behavior of condensible volatiles (CVs) along the Galileo Probe descent path through Jupiter's atmosphere. The three-layer equilibrium cloud condensation model (ECCM) predicted constant mixing ratios of the CVs, from deep in Jupiter's well-mixed interior up to the lifting condensation levels (LCLs) [e.g.\ Weidenschilling and Lewis, \it Icarus\/ \bf20\rm, p465, 1972; Atreya and Romani in \it Planetary Meteorology\rm, pp17-68, 1985]. Above the LCLs, the ECCM dictates saturation-limited mixing ratio profiles. However, in the Galileo Probe entry site, the CV mixing ratios were strongly depleted and did not reach their well-mixed values until much deeper than the LCLs. Ammonia and hydrogen sulfide seem to have reached their deep abundances at 8 bar and 15 bar, respectively, while the abundance of water increased along the probe descent and possibly did not level off even at the probe's deepest measurement, at 22 bar [Niemann et al., \it Science\/ \bf272\rm, p846, 1996]. Several hypotheses have been advanced to explain the depletions of the CVs, as well as their recoveries at varying depths. Niemann et al.\ [1996] suggested the mechanism of a downdraft (consistent with the probe's descent into a 5-\mum hotspot), and in Atreya et al. [in \it The Three Galileos\rm, pp249-260, 1998], we extended the idea using lateral mixing (entrainment) to explain the staggered CV recovery. Showman and Ingersoll [\it Icarus\/ \bf132\rm, p205, 1998] examined the energetics of lateral mixing and column-stretching. In this work we present the first quantitative analysis of the effects of entrainment upon the CV abundances within a Jovian downdraft. The simulation is based on Niewiadomski's [\it Acta Geophysica Polonica\/ \bf27\rm, p277, 1979] model of entrainment in a terrestrial thunderstorm updraft-downdraft system, but is adapted to the larger scale, multiple-CV system on Jupiter. We use the probe site CV mixing ratio profiles to find the entrainment rate, which can then yield the depth of the base of the downdraft. The downdraft depth is expected to lead to an estimate of the water mixing ratio in Jupiter's well-mixed deep atmosphere.
The author(s) of this abstract have provided an email address for comments about the abstract: mike.wong@umich.edu