The regular ablative of nox is nocte. At least in the temporal sense noctu is a synonym of nocte. Are nocte and noctu fully interchangeable as temporal expressions? In particular, can I attach adjectives or pronouns to noctu the way I can with nocte? For example, I can say quadam nocte or nocte atra et procellosa, but I don't know whether I can use noctu in such a way.
Adverbs are not normally modified by adjectives. And 'noctu,' occurs mostly in Livy, usually on its own, but sometimes modified by the adverb: secretly 'clam.' And occasionally with numquam, fere, saepe, diu. (never, almost, often, for some time.)
But the ablative 'nocte,' in an adverbial phrase can have an adjective. The phrase 'sub nocte' in the descent to the underworld 'Lonely, at nightfal, they went unseen through the shadow,' has a transferred epithet, and adverbial phrase.
Ibant obscuri sola sub nocte per umbram.
Adjectives qualifying 'nocte' : proxima nocte, 'next night,' and Adverbs qualifying the complete averbial phrase are found: fere nocte, clam nocte 'almost by night,' 'secretly by night.'