  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.
