Modeling the effects of Urbanization on Local Climates in the Pearl River Delta

by Rebecca Herrera

It has become clear that cities and other urban localities experience warmer temperatures than their rural counterparts. The urban heat islands of megatropolises can have more extreme effects on a specific area than global temperature rises have on the same area. This phenomena was studied by Wang et al. in the Pearl River Delta region of Southern China in the coastal zone of the Guangdong Province, an area that has experienced dramatic economic development over the last thirty years. The authors attempt to gain a more thorough and comprehensive understanding of the effects of urbanization on regional and local climactic indicators over an extended period of time. The results showed an overall average temperature increase, an decrease in daytime temperature ranges, a decrease in near surface water vapor quantities, a decrease in the annual number of precipitation days, an increase in annual precipitation, and a decrease in average wind speeds across urbanized zones.

The Pearl River Delta of Southern China has experienced remarkable economic growth and urbanization over the past thirty years; urban areas account for over 60% of land use in the region, making it an apt and appropriate place to study the effects of urbanization on local climates. The need to study the consequences of urbanization on local and regional climate is important as more of the world’s population migrates to urban areas and as more urban areas develop. The authors studied urbanization and climate indicators through a model called the Advanced Research core of the Weather Research and Forecasting system that conducted a pair of simulations using two different representations of urbanization on the Pearl River Delta region. The efficacy of the model was ensured against results of previous models along with prior data from the year in question.

The results showed an increase in average temperatures in all seasons while there was a decrease in the temperature range. Changes in the surface albedo through the shading effect likely increased solar radiation absorption while tall buildings likely trapped such radiative heat in synthetic ‘street canyons.’ The temperature range decreased because the average minimum temperature rose more than the average maximum temperature. ‘Surface roughness’ also increased, according to the authors. Because of the highly mixed distribution of buildings in urban areas as well as the high concentration of buildings in urban areas, friction and drag on winds increases and average wind speeds fell in urban areas. Surface water vapor concentrations in urban areas also fell as urban areas have a high area of impervious surfaces, which decrease the amount of water available for evaporation. Finally, while the model showed a decrease in the number of days with precipitation, it did show an increase in overall precipitation, indicating an increase in localized ‘extreme’ weather events.

It is evident that urbanization does in fact have a significant effect on local and regional climate in the Pearl River Delta. It is also likely that these findings will hold true for other urban areas across the world. It is necessary to continue studying the effects of urbanization on the planet as more individuals migrate to cities. Additional parameters to consider for future studies include the effect urbanization and urban head has on airborne pollutants.

Wang, X., Liao, J., Zhang, J., Shen, C., et al. 2014. A Numerical Study of Regional Climate Change Induced by Urban Expansion in the Pearl River Delta, China. Journal of Applied Meteorology and Climatology, 53, 346-362.

http://journals.ametsoc.org/doi/abs/10.1175/JAMC-D-13-054.1

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