Flattening of Aeolian Ripples

Aeolian ripple patterns shape nearly every wind-exposed sand surface. Despite the different formation origins of megaripples, impact ripples and the newly discovered aerodynamic ripples on Earth [1], their disappearance under strong winds is conventionally blamed on the same mechanism, by the fluid-entrainment hypothesis. By revealing its shortcomings and inconsistencies, the need for an update of our understanding of ripple flattening is pointed out. Based on recently discovered grain-scale characteristics of aeolian sand transport [2], we propose a robust new hypothesis for impact ripple disappearance, which we call surface-melting hypothesis. It states that impact ripples cannot form when mid-air collisions play a substantial role in the transport process. Since the latter is correlated with a scaling crossover in the total mass-transport rate as a function of surface shear stress [2], the surface-melting hypothesis predicts an upper bound on the wind- strength regime that allows impact ripples to form. We will show that it stands up well to a comparison with original and literature data, does not suffer from conflicts and inconsistencies with the disappearance of other ripple types and thus allows for a coherent and profound understanding of the stability regimes of aeolian ripples in general. We present and discuss a tentative phase diagram of ripple existence in the parameter space of Shields-number and grain diameter which, in addition to summarizing our theoretical and experimental findings, predicts the disappearance of the recently introduced aerodynamic ripples in the aerodynamically rough regime, characterized by Re_𝑝 ≳ 20.

[1] Yizhaq, H., Tholen, K., Saban, L. et al. Coevolving aerodynamic and impact ripples on Earth. Nat. Geosci. 17, 66–72 (2024).
[2] T. Pähtz and O. Durán. Unification of Aeolian and Fluvial Sediment Transport Rate from Granular Physics. Phys. Rev. Lett. 124, 168001 (2020).