Mesoscale climate modeling above a methane lake on Titan

Titan is the only known place beyond the Earth to have lakes and seas. On Earth, we know that liquid surfaces are constantly subject to evaporation and that they strongly modify the local climate and drive the planet water cycle. Then, what climate could we expect on Titan in the environment of methane lakes? And how their evaporation does control the methane cycle? We investigate this using mesoscale climate modeling. Currently there are no advanced mesoscale or large eddy simulation models of Titan. The first models came out only very recently, by Rafkin & Soto (2020), Lavely et al (2021) and Spiga et al (2020). Each focuses on a specific point (methane evaporation, topography, turbulence), but none of them handles all of the key processes yet.

We use the mtWRF 2D model first described in Rafkin & Soto (2020), which simulates the effect of a methane lake on the local climate. The model previously lacked the implementation solar insolation and radiative transfer. However, previous results in Rafkin & Soto (2020) suggested that these effects could be important on the lake-induced wind circulation. We thus added a simple gray radiative transfer scheme to the model. It takes into account solar radiation scattering through the atmosphere [Adamson 1975] and reflection at the surface, as well as IR radiation emission and absorption in the atmosphere and at the surface [Schneider et al 2012].

Simulations with and without radiative transfer both show the formation of a sea breeze (with surface winds from the lake to the land), which extends well outside the limits of the lake. The simulation with radiative transfer shows the formation of the strongest winds during daytime and at the lake shores (because of an increase of the temperature difference between the quickly warming land and the slowly warming lake).

Methane evaporation is also the most efficient on the lake shores, where and when winds are the strongest. Methane vapor is then spread over land by the winds.

The diurnally-varying insolation induces an oscillation on the land and lake surface temperatures. The lake is slower to react because of the increase of evaporation during the day, which has a cooling effect opposite to the solar warming. As they undergo stronger evaporation, the side parts of the lake stabilize at a lower temperature than the center. The resulting mean lake surface temperature is increased by a few Kelvins compared to the case without radiative transfer.

Our short term next step is to investigate the effects of seasons on lake evaporation and the local atmospheric circulation. Strong winds are caused by evaporative effects on lakes at Titan’s poles, but this could also happen on wetlands at all latitudes, and in particular at Dragonfly’s landing site [Niemann et al 2010]. To help prepare for mission flight operations, we therefore aim to model evaporation above wetlands in the near future.

This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No 101022760.