Origins of barchan dune asymmetry: insights from numerical simulations

TL;DR

This study uses numerical simulations to explore how various environmental factors like wind regimes, topography, and dune interactions lead to asymmetry in barchan dunes on Earth and Mars.

Contribution

It systematically investigates the specific conditions under which different types of barchan asymmetry develop, providing a comprehensive understanding of their formation mechanisms.

Findings

Bimodal wind regimes cause limb extension when divergence angle exceeds 90°.

Extended limbs evolve into seif dunes if secondary to primary transport ratio exceeds 25%.

Topography induces limb elongation and lateral migration of dunes.

Abstract

Barchan dunes --- crescent-shaped dunes that form in areas of unidirectional winds and low sand availability --- commonly display an asymmetric shape, with one limb extended downwind. Several factors have been identified as potential causes for barchan dune asymmetry on Earth and Mars: asymmetric bimodal wind regime, topography, influx asymmetry and dune collision. However, the dynamics and potential range of barchan morphologies emerging under each specific scenario that leads to dune asymmetry are far from being understood. In the present work, we use dune modeling in order to investigate the formation and evolution of asymmetric barchans. We find that a bimodal wind regime causes limb extension when the divergence angle between primary and secondary winds is larger than $90^{\circ}$, whereas the extended limb evolves into a seif dune if the ratio between secondary and primary transport rates is larger than 25%. Calculations of dune formation on an inclined surface under constant wind direction also lead to barchan asymmetry, however no seif dune is obtained from surface tilting alone. Asymmetric barchans migrating along a tilted surface move laterally, with transverse migration velocity proportional to the slope of the terrain. Limb elongation induced by topography can occur when a barchan crosses a topographic rise. Furthermore, transient asymmetric barchan shapes with extended limb also emerge during collisions between dunes or due to an asymmetric influx. Our findings can be useful for making quantitative inference on local wind regimes or spatial heterogeneities in transport conditions of planetary dune fields hosting asymmetric barchans.