Abundance of condensable species at planetary cold traps: The role of moist convection

Abstract

We examine the interplay of moist convection and particle growth in the tropopause region of the atmospheres of Earth, Uranus and Neptune, in an attempt to understand the diverse vertical distributions of these planets' major condensable species. On Earth, water is undersaturated in the lower stratosphere relative to the mixing ratio expected from average temperature profiles near the tropopause cold trap. On Uranus, methane is largely depleted in the upper stratosphere by photochemistry, but plausibly close to its saturated value at the tropopause. On Neptune methane is oversaturated by 2–3 orders of magnitude in the lower stratosphere. Paradoxically, moist convection has been invoked to undersaturate the Earth's stratosphere and oversaturate Neptune's. A simple one-dimensional model of moist convection is used to address this problem, along with analytic expressions for growth timescales of particles from the vapor. For the Earth, we also derive downward fluxes for Junge layer aerosols to assess their role in dehydrating the stratosphere. We reach the following conclusions. (1) The downward flux of aerosols is not sufficient to dehydrate the Earth's stratosphere by chemical desiccation, but aerosols may play a role as nucleating particles for ice crystals in the cold anvil model of Danielsen (1982, Geophys. Res. Lett. 9, 605). (2) The source of Neptune's oversaturated stratosphere is convective penetration of the tropopause by methane moist convective columns. However, certain restrictive conditions are required to initiate and maintain moist convection. Conditions in the troposphere of Uranus, on the other hand, are not conducive to the initiation of moist convection, and gaseous methane closely follows the vapor pressure law. (3) The action of moist convection in undersaturating Earth's stratosphere while oversaturating Neptune's can be understood in terms of very different particle size distributions for the condensable on the two bodies. Methane particles remain small and are carried up through Neptune's columns; on Earth water droplets grow rapidly and fall out. Additionally, the conditions appropriate for radiative cooling of terrestrial anvils, which accounts for the undersaturation, do not apply on Neptune.