We don't have synonyms for sunbow yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are unbow.
(n) :
A bow or arc of prismatic colors like a rainbow, caused by refraction through a spray of water from a cataract, waterfall, fountain, etc., rather than through droplets of rain.
(n) :
A bow or arc of prismatic colors like a rainbow, caused by refraction through a spray of water from a cataract, waterfall, fountain, etc., rather than through droplets of rain.