Supporting information for Transition between dune collision patterns — a numerical simulation study

Barchan dunes often occur in swarms on Earth and extraterrestrial bodies. Collisions among barchan dunes have been extensively investigated since it has been one of the main causes for the dune swarm to achieve size and migration speed selection. As a localized simplification of dune collision, the double-dune collision can be divided into coalescence and ejection. Effect of the relative sizes of two dunes on the collision patterns have been widely studied, whereas the influence of inter-dune space has been less mentioned. Here, the Real-Space Cellular Automaton Laboratory (ReSCAL) model is applied to comprehensively investigate the effects of inter-dune space and relative dune size on the dune collision patterns. A parameter space is established to include the abovementioned effects, where a transition surface distinguishing coalescence and ejection is determined. As confirmed by field observations and subaquatic experiments, a comprehensive depiction of transition between coalescence and ejection is provided that may be used to bring new constraints on the morphodynamics of barchan dune fields. The supporting information contains four texts: physical scaling in ReSCAL (T1), distinguishing upstream and downstream dunes (T2), data processing of the double–dune collision cases in the field (T3) and of subaquatic experiments (T4), as well as Video 1 that illustrates the lateral offset between two dunes caused by the periodic calving along the horns of large barchans.