Presentation #401.06 in the session Titan: Up High, Down Low.
Titan is the only place beyond Earth that hosts a hydrological cycle. Due to the very low temperatures at Titan’s surface, the flowing element is not water but methane. The Cassini-Huygens mission brought unprecedented insights on this methane cycle, with radar cartography of many methane lakes and seas, images of clouds and riverbeds, and proofs of rain. However, this cycle is not completely understood yet. Open questions remain on the presence of methane reservoirs and on why global climate models do not fairly reproduce clouds and rain.
To better understand all the steps of the methane cycle, we investigate each physical process with a mesoscale climate model. The first step of this investigation focuses on the evaporation above methane lakes, giving insights on the quantity of evaporated methane and on its transport by local winds. This investigation started with the 2D mtWRF model described in Rafkin & Soto (2020). Since then we added many improvements, with the addition of heating/cooling by solar and infrared radiations, and the implementation of a third spatial dimension.
First, the addition of a radiative transfer scheme reveals the significant role of solar and infrared radiations on the lake evaporation and on the induced wind circulation. Though notably small in magnitude compared to the Earth, radiation fluxes on Titan are of the same order of magnitude as the other energy fluxes involved in the evaporation of lakes and the local circulation (i.e. the latent and sensible heat fluxes). We show that, through the modification of the energy balance, the implementation of radiation enhances the evaporation of methane with a higher latent heat flux, increases strongly the lake temperature, and strengthens the local sea breeze that transports methane vapor over the shores. We also observe a significant day-night evolution in the evaporation efficiency, the lake temperature and the sea breeze structure.
Our second improvement is the addition of a third dimension. In particular, this improvement creates differences in how the simulated circulation preserves mass-continuity. 2D simulations are equivalent to a lake with an infinite extension along one dimension. In 3D, an irregular shoreline allows the air circulation to intensify at some places. This increases the evaporation efficiency above the windiest sections of the lake and modifies the transport of methane in the atmosphere. In addition, Titan lakes are surrounded by high rims likely to affect the local wind circulation and the transport of vapor over land. Near future work is to add this topographical feature in the 3D simulations.