From 雨(あま) (ama-) + 曝(さら)し (sarashi, “連(れん)用(よう)形(けい) (ren'yōkei, “stem or continuative form”) of the verb 曝(さら)す (sarasu).”). The sarashi changes to zarashi as an instance of rendaku (連濁).
あまざらし • (amazarashi) ←あまざらし (amazarasi)?