A simple coupled map lattice (CML) model for the formation dynamics of ripples by wind-blown sand, based on the model by Nishimori and Ouchi (1992).
The model incorporates the elementary sand transport processes of grain saltation (i.e. 'downwind' transport) and surface creep (i.e. diffusion) in a nonlinear reaction-diffusion-type process represented on a lattice grid.
In Python with Pillow and Matplotlib.