Convective Entrainment and Dry Downdrafts on Jupiter

R. D. Baker, G. Schubert (UCLA)

One of the more puzzling results from the Galileo entry probe into Jupiter's atmosphere is the low water abundance it measured. The probe entered a relatively cloud-free region (5 tex2html_wrap_inline14 m hotspot) of the Jovian atmosphere and the temperature profile within the cloud-free region is nearly dry adiabatic. One possible explanation for these observations is that the Galileo probe entered a dry convective downdraft (Showman and Ingersoll, 1996). However, the removal of water vapor from an air parcel on Jupiter makes the parcel less dense than its environment, and thus low humidity regions of the atmosphere will resist sinking. We propose an alternative hypothesis for the formation of the dry, cloud-free entry site. Instead of a thermally-driven dry convective downdraft (i.e., the downdraft is a direct result of radiative cooling), we suggest a ``mechanically-driven'' situation where convection entrains dry, cloud-free stable air originally located above the cloud tops into the deep Jovian atmosphere. Merging convective downdrafts may trap stable air between them and drag the stable air downward, thereby producing a conduit of less dense air that extends into the deep atmosphere. Two-dimensional numerical simulations that address convective entrainment of overlying stable air for Jupiter-like conditions are presented.

Showman, A. P. and A. P. Ingersoll (1996): Bull. Am. Astron. Soc., 28, 1141.