Convective clouds on Titan may play an important role in climate dynamics, atmospheric chemistry, and the overall volatile cycle. To study the formation and evolution of these clouds, we have developed the Titan Regional Atmospheric Modeling System (TRAMS). TRAMS is a three‐dimensional, time‐dependent, coupled fully compressible dynamic and microphysical model capable of simulating methane and ethane clouds in Titan's atmosphere. In initial model tests over a two‐dimensional domain, a warm bubble or random temperature perturbations trigger a parcel of air to rise. For an initial methane profile with a 60% surface humidity, convection occurs for positive temperature perturbations of 1 K or greater. Cloud tops are between 25 and 35 km, consistent with observations of the south polar clouds. For a drier methane environment in the lower atmosphere, characteristic of the Huygens landing site, convection does not occur, but a layer of stratiform clouds is able to form at altitudes around 10 km.

Titan's equatorial regions are covered by eastward propagating linear dunes [1][2][3] . This direction is opposite to mean surface winds simulated by Global Climate Models (GCMs), which are oriented westward at these latitudes, similar to trade winds on Earth 1,4 . Different hypotheses have been proposed to address this apparent contradiction, involving Saturn's gravitational tides 1 , large scale topography 4 or wind statistics 5 , but none of them can explain a global eastward dune propagation in the equatorial band. Here we analyse the impact of equinoctial tropical methane storms developing in the superrotating atmosphere (i.e. the eastward winds at high altitude) on Titan's dune orientation. Using mesoscale simulations of convective methane clouds 6,7 with a GCM wind profile featuring superrotation 8 , we show that Titan's storms should produce fast eastward gust fronts above the surface. Such gusts dominate the aeolian transport, allowing dunes to extend eastward. This analysis therefore suggests a coupling between superrotation, tropical methane storms and dune formation on Titan. Furthermore, together with GCM predictions and analogies to some terrestrial dune fields, this work provides a general framework explaining several major features of Titan's dunes: linear shape, eastward propagation and poleward divergence, and implies an equatorial origin of Titan's dune sand.A major surprise in the exploration of Titan by Cassini was the discovery of large dune fields in the equatorial regions, which cover close to 15% of Titan's surface 1,3? . These giant dunes are linear and parallel to the equator, and are probably composed of hydrocarbon material. The analysis of dune morphology around obstacles and dune terminations indicates an eastward dune propagation with some regional variations 1,2 (see also Fig. 3b and Supplementary Fig. 5). However, Global Climate Models (GCMs) predict that annual mean surface winds are easterlies (westward) at low latitudes 4 , as trade winds on Earth 9 . Therefore, Titan's dune orientation is opposite to predicted mean winds, raising a major enigma.Given the non-linear dependence of sediment transport on wind speed, the only way to propagate dunes eastward would be the occurrence of episodic fast westerly (eastward) gusts 5 . Above 5 km, Titan's troposphere is in superrotation with fast eastward winds at any latitude. Pumping momentum from this superrotation down to the surface might provide a solution. However, Titan's tropospheric circulation is essentially confined into a 2 km boundary layer 10 . Incidentally, this boundary layer circulation may explain the dune spacing of around 2 km 10,11 . Dry convection is therefore limited to the first 2 km and unable to reach the fast eastward winds 10 . The Koln GCM has been shown to generate episodic fast eastward gusts at the equator during Titan's equinoxes 5 . However, this result has not been reproduced by any published GCM, including the Titan IPSL GCM (used here), which faithfully reproduces the superrotation and the thermal str...

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