Mesoscale Modelling of Titan’s Shangri-La reigon:the impact of surface properties on surface winds

1. Introduction
Titan’s weather is particularly active at all spatial scales: global (e.g., super-rotating winds), intermediate / mesoscale (e.g., convective thunderstorms, air-sea circulations), local (e.g., turbulence in the planetary boundary layer, the atmospheric layer in direct contact with the surface). The aeolian environment of Titan is active, with a fifth of Titan’s surface being covered by dunes fields and the organic dust particles at the surface being potentially transported by wind circulations at all scales. In 2034 NASA’s Dragonfly quadcopter will land and explore Selk impact crater structure in the dark Shangri-La region [1], characterized by a surface rich in organic dust particles organized as dune fields whose peculiar morphology probably involves two distinct wind regimes that remain to be understood [2].

2. Model
To study the near-surface dynamics, a new model was developed based on the Weather-Research Forecast (WRF) non-hydrostatic dynamical core [3] and coupled with the LMD Titan PCM physics package [4, 5]. The domain of interest is centered on Selk crater and Shangri-La region. The horizontal resolution is set to 5 km. Synthetic high-resolution topography and surface characteristic maps such as albedo, thermal inertia and surface roughness, set constant in the PCM respectively to 0.2, 335 J K⁻¹ m⁻³ and 5 cm, were created based on Cassini SAR images. The meteorological fields initial states are taken from LMD Titan PCM outputs. The mesoscale boundary conditions are forced with the LMD Titan GCM.

3. Results
Fig 1 shows Snapshots map of horizontal winds 10 m above local surface (m s⁻¹) at noon. The topography, plains and mountains, affects the direction of the surface winds. The vast majority of the wind is between ± 0.5 m s⁻¹, consistent with observations. The high mountains are able to engender mountain waves with horizontal wind reaching 1.0 m s⁻¹. The diurnal cycle has an impact on the amplitude and direction of wind, with preferably anabatic winds during the day and katabatic winds at night. Different synthetic topography maps were tested and will be presented. The impact of albedo, thermal inertia and surface roughness spatial variability due to terrain type of these parameters will be discussed. The near-surface dynamics seasonal variability is sensitive to the seasonal variability of the Hadley cell between roughly 100 and 400 km. At both Equinoxes, the Hadley cell will be composed of a cell in each hemisphere, whereas is northern winter, for example, there will only have a large cell with a jet in the Northern Hemisphere. This seasonal variability will affect the tropospheric circulation and therefore the near-surface winds at the synoptic scale. To capture this variability at the mesoscale level, five solar longitudes were selected, thought to be representative of each four region and will be discussed.

Figure 1: Snapshots map of horizontal winds 10 m above local surface (m s⁻¹ ) at noon in the center of the domain. The red circle represents the landing site of Dragonfly [1].

References
[1] Lorenz, R. D. et al. Selection and Characteristics of the Dragonfly Landing Site near Selk Crater, Titan. Planet. Sci. J. 2, 24 (2021).
[2] Malaska, M. J. et al. Geomorphological map of the Afekan Crater region, Titan: Terrain relationships in the equatorial and mid-latitude regions. Icarus 270, 130–161 (2016).
[3] Skamarock, W. C. & Klemp, J. B. A time-split nonhydrostatic atmospheric model for weather research and forecasting applications. Journal of Computational Physics 227, 3465–3485 (2008).
[4] de Batz de Trenquelléon, B., L. Rosset, J. V. d’Ollone, S. Lebonnois, P. Rannou, J. Burgalat, and S. Vinatier The New Titan Planetary Climate Model. I. Seasonal Variations of the Thermal Structure and Circulation in the Stratosphere. The Planetary Science Journal, 6, 78. (2025)
[5] de Batz de Trenquelléon, B., P. Rannou, J. Burgalat, S. Lebonnois, and J. V. d’Ollone The New Titan Planetary Climate Model. II. Titan’s Haze and Cloud Cycles. The Planetary Science Journal, 6, 79. (2025)