February 17, 2009 | NCAR scientist Fei Chen is collaborating with colleagues at China’s Institute of Urban Meteorology to explore how growth in Beijing is changing the city’s summer rainfall patterns, focusing specifically on the relationship between urban expansion, aerosols, and summer rainfall, using climatologic data from the period 1980–2005. They propose that the dynamic and thermal effects of urbanization may be equally as important as aerosol-cloud interactions in modifying summer precipitation in major metropolitan areas.

With more than 10 million residents, the Beijing metropolitan area is one of the 10 largest in the world, having grown rapidly over the past 20 years. Aridity and water shortages are increasingly serious problems. Heavy summer rainfall in the Miyun reservoir area, a major source of Beijing’s water supply, has decreased in recent years.

Fei and his colleagues found a statistical correlation between Beijing’s rapid expansion since 1981 and summer rainfall reduction between 1981 and 2005, with the largest decrease in rainfall centered around the Miyun reservoir. Less rainfall from moderate, heavy, and extreme precipitation accounts for a large contribution of the total decreasing trend.

However, in contrast to previous studies on how pollution reduces urban rainfall, the researchers found no convincing evidence that summer rainfall reduction is due to an increase in aerosols. Using a high-resolution mesoscale model (PSU/NCAR MM5) to look at weather, land-surface, and urban interactions, they simulated two heavy summer rainfall events in Beijing. The results show that urban expansion, by replacing vegetated areas, can produce evaporation, higher surface temperatures, larger sensible heat fluxes, and a deeper boundary layer. This leads to less water vapor, more mixing of water vapor in the boundary layer, and less energy available for showers and thunderstorms.

The model simulations also found that increasing green vegetation coverage in the Beijing area would lead to more rainfall. “This study has given us clues on how to better plan urban development and green planting to mitigate the shortage of water resources in megacities,” Fei says.

The research was published in Journal of Geophysical Research in January.

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