What makes serene a long vowel word?

Answer 1

The ending silent E gives the second E a long E sound, to rhyme with marine.

(suh-reen)

The same effect is heard in the words gene (jeen) and scene (seen).

Answer 2

The word "serene" contains the long e vowel sound, which is typically represented by a single vowel followed by a silent e. In this case, the e at the end of the word serves to make the preceding vowel long rather than short.