expression, noun-modifying (irregular)
Prenominal phrase describing a period or place with frequent or heavy rain. Often used with words like 日, 季節, 地域.
Rainy days have been continuing.
This region has entered its rainy season.
Phrase composed of 雨 (rain) + の (attributive particle) + 多い (many, frequent). The exact origin as a set phrase is uncertain, but it follows a common pattern for descriptive prenominal expressions.