Resolving turbulence in the boundary layer of Titan to interpret Cassini-Huygens measurements and to prepare Dragonfly explorations

Characterizing the dynamics of Titan's Planetary Boundary Layer by turbulence-resolving modeling is a means to broaden the knowledge on this key part of the atmosphere in contact with the surface and to bridge the gap from the environmental conditions unveiled during the Huygens descent twenty years ago to the atmospheric diversity to be experienced and explored by Dragonfly about ten years from now.

We leverage large-eddy simulations for Titan in which turbulent dynamics in the Planetary Boundary Layer is resolved (using the WRF hydrodynamical solver) with the full daytime cycle of Titan environmental conditions represented by online radiative transfer and soil modeling inherited from Titan global-climate modeling (Titan PCM).

While our large-eddy simulations reproduce the correct vertical extent of the mixed PBL estimated by Huygens instruments during the descent of the probe in the morning, the mixed PBL is predicted to extend up to 2.2 km above the surface in the afternoon, close to the largest possible values reported in the literature based on dune spacing and interpration of the Huygens descent profile (including recent revisit of the dataset). This value is compliant with the profile obtained by global-climate modeling in similar conditions; at the same time, global-climate modeling is found to yield significantly distinct estimates for the vertical extent of the mixed PBL depending on the environmental conditions of the considered site as well as model assumptions. 

Large-eddy simulations offer a good plateform to explore the plausible atmospheric dynamics to be experienced by Dragonfly. Turbulent variability of wind and temperature, vertical variations of turbulent kinetic energy and vertical eddy heat flux, possible occurrence of convective vortices -- all within reach of Dragonfly's measurements which, we are able to argue using large-eddy simulations as illustrative predictions, would be particularly interesting to perform during flights.

The possible interest of large-eddy simulations extends well above the altitudes at which Dragonfly will be able to fly in the Titan atmosphere. Our turbulence-resolving simulations of daytime Titan PBL also showcase gravity wave activity above the top of the mixed PBL, with wave packets propagating above the mixing layer as a result of perturbations caused by dry, turbulent, convective plumes.