a-fortification.
A process by which the variable vowel in a Strong noun is strengthened by changing it to an equivalent long vowel. [1]
Used mostly for the Subjective inflection and to indicate proper noun formation of common nouns.
Variable noun changes
a → â,
i → ê,
u → ô.
[1]. If you wish for an in-depth explanation for such a phenomenon, look up Paul Strack’s Eldamo, a scholarly collection of every word ever made by Tolkien, also includes grammar.