Air-sea interactions on Titan: Lake evaporation, atmospheric circulation, and cloud formation

Abstract

Titan's abundant lakes and seas exchange methane vapor and energy with the atmosphere via a process generally known as air-sea interaction. This turbulent exchange process is investigated with an atmospheric mesoscale model coupled to a slab model representation of an underlying lake. The impact of lake size, effective lake mixed layer depth, background wind speed, air-lake temperature differential, and atmospheric humidity on air-sea interaction processes is studied through dozens of two-dimensional simulations. The general, quasi-steady solution is a non-linear superposition of a very weak background plume circulation driven by the buoyancy of evaporated methane with a stronger opposing thermally direct (sea breeze) circulation driven by the thermal contrast between the cold marine layer over the lake and the warmer inland air. The specific solution depends on the value of selected atmosphere and lake property parameters, but the general solution of the superposition of these two circulations is persistent. Consistent with previous analytical work of others, the sensible heat flux and the latent heat flux trend toward opposite and equal values such that their ratio, the Bowen ratio, approaches −1.0 in most, but not all, of the quasi-steady state solutions. Importantly, in nearly all scenarios, the absolute magnitude of the fluxes trends toward very small values such that the equilibrium solution is also nearly a trivial solution where air-sea energy exchange is ~3 W m−2 or less. In all cases, a cool, moist, and statically stable shallow marine layer with nearly calm winds and small turbulent flux exchanges with a colder underlying lake is produced by the model. The temperature of the lake, the marine properties of the air, and the strength of the sea breeze depends on the initial conditions and to a lesser degree, the boundary conditions.