Evolution of collision sites with increasing number of collisions and the contour plot of the geopotential. The red points denote collision sites on the saltation routes, and the white points indicate the initial locations where particles directly enter orbit and no longer collide with the surface during the simulation. The black solid lines indicate the contour lines of the geopotential. The color spectrum (blue to yellow) corresponds to the geopotential ranging from −0.1398 to −0.091 J kg⁻¹. Panels a, b, c, and d show the location distribution of particles at the initial moment, the first collision, the second collision, and the last collision, respectively.