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The Impact of Urbanization on Indian Monsoon Rainfall
Reference
Kishtawal, C.M., Niyogi, D., Tewari, M., Pielke Sr., R.A. and Shepherd, J.M. 2010. Urbanization signature in the observed heavy rainfall climatology over India. International Journal of Climatology 30: 1908-1916.

Background
The authors note that the early studies of Changnon (1968), Landsberg (1970) and Huff and Changnon (1972) yielded "evidence of warm seasonal rainfall increases of 9-17% downwind of major cities," and that the Metropolitan Meteorological Experiment of the 1970s suggested there were 5-25% increases in precipitation downwind of urban centers during the summer months (Changnon et al., 1991). In addition, they state that "recent research continues to show credible evidence that surface temperature increases (Kalnay and Cai, 2003), cloud enhancement (Inoue et al., 2004), and precipitation anomalies (Hand and Shepherd, 2009; Shem and Shepherd, 2009; Shepherd et al., 2009) may be linked to urban environments."

What was done
In a study of the Indian subcontinent (8.2°N to 35.35°N, 68.5°E to 97°E), Kishtawal et al. (2010) assessed the impacts of urbanization on the region's rainfall characteristics during the time of the Indian summer monsoon by analyzing in situ and satellite-based precipitation and population datasets.

What was learned
The five researchers say their study showed "a significantly increasing trend in the frequency of heavy rainfall climatology over urban regions of India during the monsoon season," adding that "urban regions experience less occurrences of light rainfall and significantly higher occurrences of intense precipitation compared to non-urban regions."

What it means
In their book entitled Dire Predictions: Understanding Global Warming, Mann and Kump (2008) write that most climate model simulations of global warming indicate that "increases are to be expected in the frequency of very intense rainfall events." Throughout most of the world, however, such has not been found to have been the case to this point in time; and in places where there may have been a tendency for such to occur, the results of Kishtawal et al. (2010), and the papers they cite in the introduction to their study, suggest that urbanization may have been the cause of the observed increases in intense rainfall events, while the study of Hossain et al. (2009) suggests that the proliferation of large dams may also be causing the same to occur. Thus, it is becoming ever more difficult to be able to distinguish what could be the primary cause of any net increase in the global frequency of intense rainfall events that might yet be detected over land.

References
Changnon, S.A. 1968. The LaPorte weather anomaly -- fact or fiction? Bulletin of the American Meteorological Society 49: 4-11.

Changnon, S.A., Shealy, R.T. and Scott, R.W. 1991. Precipitation changes in fall, winter, and spring caused by St. Louis. Journal of Applied Meteorology 30: 126-134.

Hand, L. and Shepherd, J.M. 2009. An investigation of warm season spatial rainfall variability in Oklahoma City: Possible linkages to urbanization and prevailing wind. Journal of Applied Meteorology and Climatology 48: 251-269.

Hossain, F., Jeyachandran, I. and Pielke Sr., R. 2009. Have large dams altered extreme precipitation patterns? EOS, Transactions, American Geophysical Union 90: 453-454.

Huff, F.A. and Changnon, S.A. 1972. Climatological assessment of urban effects on precipitation at St. Louis. Journal of Applied Meteorology 11: 823-842.

Inoue, T. and Kimura, F. 2004. Urban effects on low level clouds around the Tokyo metropolitan area on clear summer days. Geophysical Research Letters 31: 10.1029/2003GL018908.

Kalnay, E. and Cai, M. 2003. Impact of urbanization and land use on climate change. Nature 423: 528-531.

Landsberg, H.E. 1970. Man-made climate changes. Science 170: 1265-1274.

Mann, M.E. and Kump, L.R. 2008. Dire Predictions: Understanding Global Warming. DK Publishing, Inc., New York, New York, USA.

Shem, W. and Shepherd, M. 2009. On the impact of urbanization on summertime thunderstorms in Atlanta: two numerical model case studies. Atmospheric Research 92: 172-189.

Shepherd, J.M., Carter, W.M., Manyin, M., Messen, D. and Burian, S. 2009. The impact of urbanization on current and future coastal convection: A case study for Houston. Environment and Planning B: In press.

Reviewed 23 March 2011