WEST LAFAYETTE, Ind. - Two Purdue University studies show that urbanization changes storm patterns and rainfall amounts, highlighting the need for urban planning and infrastructure design that considers how the landscape will affect the weather.

In two separate papers, teams led by Dev Niyogi, Indiana state climatologist and professor in the departments of Agronomy and Earth, Atmospheric, and Planetary Sciences, studied storm patterns over the coastal megacity of Mumbai, India, and the mountainous city of San Miguel de Tucumán, Argentina, to determine how urban development affected storms in those regions. The Mumbai study was done in collaboration with the Indian Institute of Technology Bombay, while the study in Argentina was done with the University of Alabama in Huntsville.

The researchers expected to see that Mumbai’s added heat and buildings significantly disrupted storms. But they expected to see little impact in San Miguel de Tucumán since the terrain around the city is rugged, which likely makes the storms turbulent before they reach the city.

In Mumbai, Niyogi said, the urban landscape disrupted rainfall, creating pockets and ribbons of rain that would intensify downpours in some parts of the city. Mumbai and other Indian cities have experienced significant flooding in recent years, possibly exacerbated by the way the cities affect storms. The researchers also found the storms organize themselves over the city in clusters. This organization showed that meteorologists should focus on small, 100-kilometer-squared areas with rain gauges or satellite images to best model future storms.

The study, published in Nature - Scientific Reports, used satellite data to track storm patterns and model the ways in which Mumbai was altering those patterns. Niyogi said the results highlight the need to understand how sprawling urban landscapes will affect severe weather, helping inform flood monitoring efforts and future critical infrastructure decisions.

“Understanding how these storms are changing by interacting with a city helps improve forecasting,” Niyogi said. “But it also gives an idea about how infrastructure design will need to be considered since cities will change their own rainfall patterns. We may need to think about things like storm water drainage and the location of drains, for example. Certain parts of a city might receive more rainfall, and that could lead to flooding if proper planning isn’t considered.”

San Miguel de Tucumán’s urban development also influenced regional rainfall patterns, according to results published in the journal Geophysical Research Letters. Satellite data and models showed that urbanization resulted in 20-30 percent less precipitation downwind of the city and an eastward shift in precipitation upwind. Again, Niyogi said the effect cities will have on rainfall changes needs to be taken into account before large-scale developments continue in the mountain regions where water is already a scarce resource.

“Even in complex terrain, we see really significant changes coming from the effects of the city,” Niyogi said. “In the long term, as these mountain communities evolve and they try to balance development and water needs, their landscape changes will have a profound impact on water availability.”

Despite differences in each storm studied, precipitation in Mumbai and San Miguel de Tucumán both fell into fairly predictable patterns - ribbons or pockets of heavy rain in India and a skirting of the city in Argentina.

“These are very complex environments, yet we see that these storms beautifully organize themselves into nice structures that we can understand,” Niyogi said. “Almost everything around us seems chaotic and unpredictable, yet we see these patterns emerge in the natural systems. That means that we don’t need to study every storm in every situation. Solutions can emerge from snapshots of our understanding and perhaps have universal validity. This similarity helps develop models and guidance that can have broad utility as we design prediction systems for the next generation of cities and their infrastructure.”

The National Science Foundation Division of Atmospheric and Geospace Sciences, Indian Ministry of Earth Science National Monsoon Mission, the U.S. Department of Agriculture National Institute of Food and Agriculture, and the Indo-U.S. Science & Technology Forum supported the research.

Writer: Brian Wallheimer, 765-532-0233, bwallhei@purdue.edu

Source: Dev Niyogi, 765-494-6574, climate@purdue.edu

ABSTRACT

Increased Spatial Variability and Intensification of Extreme Monsoon Rainfall due to Urbanization Supantha Paul1, Subimal Ghosh1, 2, Micky Mathew2, Anjana Devanand1, Subhankar Karmakar1,3 and Dev Niyogi1,4 Interdisciplinary Program in Climate Studies, Indian Institute of Technology Bombay, Mumbai Department of Civil Engineering, Indian Institute of Technology Bombay, Mumbai Centre for Environmental Science and Engineering, Indian Institute of Technology Bombay, Mumbai Department of Earth, Atmospheric, and Planetary Sciences and Department of Agronomy - Crops, Soils, Water Sciences, Purdue University, West Lafayette, IN While satellite data provides a strong robust signature of urban feedback on extreme precipitation; urbanization signal is often not so prominent with station level data. To investigate this, we select the case study of Mumbai, India and perform a high resolution (1 km) numerical study with Weather Research and Forecasting (WRF) model for eight extreme rainfall days during 2014-2015. The WRF model is coupled with two different urban schemes, the Single Layer Urban Canopy Model (WRF-SUCM), Multi-Layer Urban Canopy Model (WRF-MUCM). The differences between the WRF-MUCM and WRF-SUCM indicate the importance of the structure and characteristics of urban canopy on modifications in precipitation. The WRF MUCM simulations resemble the observed distributed rainfall. WRF-MUCM also produces intensified rainfall as compared to the WRF-SUCM and WRF-NoUCM (without UCM). The intensification in rainfall is however prominent at few pockets of urban regions, that is seen in increased spatial variability. We find that the correlation of precipitation across stations within the city falls below statistical significance at a distance greater than 10 km. Urban signature on extreme precipitation will be reflected on station rainfall only when the stations are located inside the urban pockets having intensified precipitation, which needs to be considered in future analysis.

ABSTRACT Urban Modification of Convection and Rainfall in Complex Terrain B.M. Freitag, U.S. Nair, D. Niyogi Despite a globally growing proportion of cities located in regions of complex terrain, interactions between urbanization and complex terrain and their meteorological impacts are not well understood. We utilize numerical model simulations and satellite data products to investigate such impacts over San Miguel de Tucumán, Argentina. Numerical modeling experiments show urbanization results in 20-30% less precipitation downwind of the city and an eastward shift in precipitation upwind. Our experiments show changes in surface energy, boundary layer dynamics, and thermodynamics induced by urbanization interact synergistically with the persistent forcing of atmospheric flow by complex terrain. With urbanization increasing in mountainous regions, land-atmosphere feedbacks can exaggerate meteorological forcings leading to weather impacts that require important considerations for sustainable development of urban regions within complex terrain.

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