The Interplay between the Location of the City and the Background Winds in Modifying the Rainfall Patterns over an Eastern Indian Tropical City

Urban areas significantly influence rainfall patterns, often intensifying rainfall downwind of cities. Depending on various factors, cities can suppress, deflect, split, or intensify incoming storms, with some studies even highlighting storm initiation due to urban heat and dynamics. The location and magnitude of these impacts are largely dependent on the city’s geographic setting, topography, and prevailing wind-flow regime. In India, urban areas have been associated with enhanced monsoon rainfall extremes, with recent research indicating that urban warming is more pronounced over developing tier-II cities than over established metropolitan centers. Limited studies in India have explored the influence of background flow regimes on the preferential locations of rainfall intensification. Against this backdrop, Bhubaneswar, a developing tier-II tropical city in eastern India, serves as an insightful case study.

Bhubaneswar lies approximately 40–50 km inland from the Bay of Bengal, with an average elevation of 45 m above sea level. The city’s terrain rises westward toward the Eastern Ghats and slopes downward eastward toward the ocean, creating a temperature gradient warmer on the western side and cooler on the eastern side due to proximity to the sea. The region receives nearly 80% of its annual rainfall during the monsoon season (JJAS), with most heavy rainfall events driven by low-pressure systems over the Bay of Bengal. 

This study utilizes the Weather Research and Forecasting (WRF) model to simulate seasonal monsoon rainfall over Bhubaneswar under varying city-size scenarios. A total of 88 rainfall events were simulated, of which 47 cases exhibited increased rainfall due to urban expansion, while 41 cases showed a reduction. Initial findings indicate that drawing a definitive conclusion on whether urban expansion consistently amplifies or diminishes rainfall is challenging.

During the monsoon season, the region experiences winds from all directions except the north, with southwesterlies being the most dominant. A detailed classification of events based on the prevailing wind regimes provided critical insights. The analysis revealed significant rainfall enhancement over the city and its downwind areas under easterly wind conditions, often associated with cyclonic circulations over the Bay of Bengal. Conversely, westerly winds were found to reduce downwind rainfall. Notably, rainfall enhancement predominantly occurred on the right side of the prevailing wind direction in the case of westerlies.

The study underscores the prominent role of urban location, topography, and prevailing winds in shaping the magnitude and spatial distribution of urbanization-driven rainfall changes during the monsoon. Identifying the preferential location of rainfall enhancement during different wind conditions is crucial for flood mapping and mitigation.