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Impact of Megacity Shanghai on the Urban Heat-Island Effects over the Downstream City Kunshan |
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| Authors: | Han-Qing Kang Bin Zhu Tong Zhu Jia-Li Sun Jian-Jun Ou |
| Institution: | 1. Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, Nanjing University of Information Science & Technology, Nanjing, China 2. CIRA/Colorado State University, Fort Collins, CO, USA 3. NOAA/NESDIS/STAR/JCSDA, College Park, MD, USA 4. Jiangsu Climate Centre, Nanjing, China 5. Shanghai Marine Meteorological Centre, Shanghai, China |
| Abstract: | The impact of upstream urbanization on the enhanced urban heat-island (UHI) effects between Shanghai and Kunshan is investigated by analyzing seven years of surface observations and results from mesoscale model simulations. The observational analysis indicates that, under easterly and westerly winds, the temperature difference between Shanghai and Kunshan increases with wind speed when the wind speed \(<\) 5 m s \(^{-1}\) . The Weather Research and Forecasting (WRF) numerical model, coupled with a one-layer urban canopy model (UCM), is used to examine the UHI structure and upstream effects by replacing the urban surface of Shanghai and/or Kunshan with cropland. The WRF/UCM modelling system is capable of reproducing the surface temperature and wind field reasonably well. The simulated urban canopy wind speed is a better representation of the near-surface wind speed than is the 10-m wind speed at the centre of Shanghai. Without the urban landscape of Shanghai, the surface air temperature over downstream Kunshan would decrease by 0.2–0.4 \(^{\circ }\) C in the afternoon and 0.4–0.6 \(^{\circ }\) C in the evening. In the simulation with the urban landscape of Shanghai, a shallow cold layer is found above the UHI, with a minimum temperature of about \(-0.2\) to \(-\) 0.5 \(^{\circ }\) C during the afternoon hours. Strong horizontal divergence is found in this cold layer. The easterly breeze over Shanghai is strengthened at the surface by strong UHI effects, but weakened at upper levels. With the appearance of the urban landscape specific humidity decreases by 0.5–1 g kg \(^{-1}\) within the urban area because of the waterproof property of an urban surface. On the other hand, the upper-level specific humidity is increased because of water vapour transferred by the strong upward vertical motions. |
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