Kathīb means a sand dune. Ẓahr literally means back, but it may also denote the top or outward elevated surface of something. Ẓahr al-kathīb is a construct phrase combining height with a desert landscape.
The preposition ʿalā makes the whole phrase locative. Instead of stating merely that another meeting took place, the poet places it on the crest of a dune, making elevation and the surrounding landscape perceptible.
This is a spatial extension of meaning, not a reference to an actual human back. Similar transferred expressions occur naturally when Arabic describes the parts of landscapes and objects.