15 February 2016

Urbanization Impact on Summer Precipitation: Urban Heat Island versus Aerosol Effects

Summary

Urbanization changes the land cover and increases pollution emissions. Both changes impact the atmosphere, regional circulation, the water cycle, and climate. Studies have tackled how land cover changes and emission changes separately affect clouds and precipitation, but fewer studies have quantified their combined effects. Researchers at the Department of Energy’s Pacific Northwest National Laboratory and collaborators from Nanjing University in China selected the Greater Beijing Metropolitan Area (GBMA)—one of the 10 largest megacities in the world—to investigate the combined impacts of land use and human-caused pollution emissions from urbanization on a heavy rainfall event in the area. They conducted convection-resolving ensemble simulations using the WRF-Chem model coupled with a single-layer Urban Canopy Model and found that the simulation with the urbanization effect included generally captured the spatial pattern and temporal variation of the rainfall event. They found an improvement of precipitation in the experiment including the aerosol effect on both clouds and radiation. The expanded urban land cover and increased aerosols have an opposite effect on precipitation processes, with the latter playing a more dominant role. This leads to suppressed convection and rainfall over the upstream (northwest) area, and enhanced convection and more precipitation in the downstream (southeast) region of the GBMA. In addition, in this rainfall event they found that the influence of the aerosol indirect effect (clouds and precipitation) overwhelmed the direct effect (sunlight scattering and absorbing) on precipitation. Increased aerosols lead to more and smaller cloud droplets, which favors evaporative cooling and reduces updrafts and suppresses convection over the upstream (northwest) region in the early stages of the event. As the rainfall system propagates southeastward, more latent heat is released because of a larger number of smaller cloud drops freezing when they are lofted above the freezing level. This process is responsible for the increased updraft strength and convective invigoration over the downstream (southeast) area.

Contact
Yun Qian
Acknowledgments

The contribution of Shi Zhong and Xiu-Qun Yang in this study is supported by the National Basic Research program of China (2010CB428504), Jiangsu Collaborative Innovation Center for Climate Change, and the Scholarship Award for Excellent Doctoral Student granted by China Scholarship Council. The contributions of Yun Qian, Chun Zhao, and Ruby Leung in this study are supported by the U.S. Department of Energy’s Office of Science as part of the Atmospheric System Research (ASR) program and the Regional and Global Climate Modeling Program. The Pacific Northwest National Laboratory is operated for DOE by Battelle Memorial Institute under contract DE-AC05- 76RL01830. This study used computing resources from the PNNL Institutional Computing. All model results are stored at a PNNL cluster and available upon request. Please contact Yun Qian (yun.qian@pnnl.gov).