Interaction of Trace Species and Three‐Dimensional Circulation in Titan's Middle Atmosphere

Abstract

The atmosphere of Titan, Saturn's largest moon, is home to a suite of hydrocarbons and nitriles that are primarily produced high in the atmosphere. These trace molecules are distributed away from their high altitude sources by Titan's atmospheric circulation, resulting in a seasonality in their stratospheric abundance. In this paper, we simulate the distribution of trace molecules in Titan's middle atmosphere using the Titan Atmospheric Model, a three‐dimensional General Circulation Model. Using an observationally constrained, fixed abundance at the model top (500 km), the simulated molecules are advected throughout the atmosphere and subjected to a pressure‐dependent lifetime informed by published photochemical models. We find that enrichment in the high winter latitudes is initiated by downward transport by the meridional overturning circulation, with a pressure‐dependent impact from transient eddies in both the horizontal and vertical directions that act to mix molecules down‐gradient. We confirm that molecular tongues equatorward of the polar vortex are enabled by three‐dimensional horizontal mixing processes, with the most notable tongues in simulated , , and HCN; due to its shorter chemical lifetime, N does not exhibit a substantial tongue. Finally, we quantify the contributions of adiabatic descent, longwave radiative cooling, and enrichment from the vertical convergence of air to identify the drivers of ice cloud production. We find that, contrary to the prevailing paradigm, the dominant driver of ice cloud production is (im)balance between radiative cooling and adiabatic warming, with the enrichment from downward transport of molecules being several orders of magnitude less important.