Step 1: Comprehension of the Task
The inquiry requires identifying the ancient Indian scholar who employed the concept of "Lunar mansions" (Nakshatras) specifically to forecast seasonal rainfall.
Step 2: In-depth Analysis
Although numerous ancient texts address agriculture and weather, the most thorough and structured treatise on astrology and astronomy, encompassing meteorology, is the Brihat Samhita authored by Varāhmihira (6th century CE).
Varāhmihira allocated several sections to predicting rainfall (Garbhalakshana, Pravarshana). His methodologies were grounded in extensive observations, including the celestial positions of the Sun and Moon within various Nakshatras (lunar mansions), cloud formations, wind direction, and biological indicators.
While Parāśara's Krishi Parashara also addresses rainfall, Varāhmihira's work is recognized as the most authoritative and detailed in its use of astronomical phenomena like lunar mansions for this predictive purpose.
Step 3: Conclusion
Varāhmihira stands as the foremost ancient authority who extensively utilized lunar mansions for the prediction of rainfall.