。
She's a singer whose popularity is skyrocketing.
Literal
She [topic-は] popularity [subject-が] rapid-rise [possessive-の] singer is.
急上昇 (きゅうじょうしょう) combines 急 ('sudden, rapid') with 上昇 ('rise, ascent') to mean 'skyrocketing.' The clause 人気が急上昇 ('popularity is rapidly rising') is compressed into a noun modifier via の — a compact construction common in journalistic Japanese where entire clauses are squeezed into pre-noun position to create dense, informative descriptions.