The Difference Of Urbanization Effect Over Different Climate Zones And The Impact Of Urban Expansion On Regional Climate

By analyzing the observational data and the results of numerical simulations, the topics involved the difference of urbanization effect over different climate zones and the impacts of urban expansion on regional climate are studied in this work, the primary contents and conclusions include the following three parts:(1) By using the land use/land cover (LULC) category data, the meteorological stations in the Beijing-Tianjin-Hebei (BTH), the Yangtze River Delta (YRD) and the Pearl River Delta (PRD) are quantitatively divided into urban and rural stations in accordance with percentages of the urban type grid points. Surface temperature, wind speed and sunshine hours among the three regions show significant different features. By analyzing the urban heat island (UHI), urbanization warming rate (UWR) and the contribution of the urbanization to surface warming (CUSW) over the three regions for the recent30years (1980-2009), it is found that the UHI is strong in PRD, followed by BTH, and it is weak in YRD. However, the UWR is large in YRD, followed by PRD, and it is small in BTH. Due to the different background temperature trends, the CUSW is large in PRD, followed by YRD, and it is small in BTH. Further analyses show that the UWR in spring is large in YRD, followed by BTH, and it is small in PRD, while the UWR in summer is large in YRD, followed by PRD, and it is small in BTH. In autumn and winter, the UWR is large in PRD, followd by YRD, and it is small in BTH. In the other hand, the CUSW in spring is large in YRD compared with the other two regions while it is large over PRD in summer, autumn and winter. Further, it is found that the impact of urbanization on surface warming mainly through affecting the nocturnal temperatures, and the UWR and CUSW during the daytime exhibit different characteristics, as they may be affected by other factors. In addition, the results of the numerical simulations prove the existence of difference of the urbanization effect among different regions.(2) By analyzing the results of two groups of WRF simulation, it is found that the urban expansion over the YRD region leads to an increase trend of surface energy incomes and temperatures over the eastern China, and the magnitude of the increasing trend over the eastern part of the YRD is largest, while the magnitudes over the western and southwestern parts of the YRD are small. The urban expansion over the YRD also leads to the increase of hot weather frequency and the increasing magnitudes of the northern, eastern and central parts of the YRD are large. Further, the impacts of urban expansion on air temperatures have obviously diurnal variations characteristics. The westerly anomalies caused by the urban expansion lead to the weakening of the local prevailing easterly winds over the regions to the west of120.5°E, while easterly anomalies caused by the urban expansion lead to the strengthening of the local prevailing easterly winds over the regions to the east of120.5°E. As a result, a convergence belt formed over the region to the east of Taihu Lake. As the urban expansion changes the original natural vegetation covers and further causes the decreases of the near-surface atmospheric humidity and rainfall amount. The magnitude of summer precipitation decreasing trend is large in the central part of the YRD region, followed by the eastern, southern and southeastern parts, while the magnitude is small in the southwestern part of the YRD region.(3) The Yangtze River Delta has experienced significant urban expansion in recent years, whereas the Meiyu belt of China has a decadal northward shift trend during the same period, so it is of interest to assess how the urban expansion affects the Meiyu precipitation and reveal the underlying possible physical mechanisms. The urban extents over the YRD region in the years2001and2010are established based on the land use/land cover category data and nighttime lights image. Two parallel groups of10-summer (2001-2010) numerical simulations have been carried out with the urban extents over the YRD region in2001and2010. Results show that the urban expansion in the YRD region tends to result in Meiyu precipitation increase (decrease) over Huaihe River Basin (Yangtze River Basin) with the intensity of0.2-1.2mm d-961. Further analysis indicates that the spatial-temporal pattern of the changed Meiyu precipitation induced by the urban expansion resembles the3rd EOF mode of the observed Meiyu precipitation. Analyses of the possible underlying physical mechanisms reveal that the urban expansion in the YRD region leads to the changes in the surface energy budget and warming (cooling) of tropospheric (stratospheric) air temperature over eastern China. Anomalous upward (downward) motion and moisture convergence (divergence) over Huaihe River Basin (Yangtze River Basin) occur, corresponding to the increases (decreases) of the Meiyu precipitation over Huaihe River Basin (Yangtze River Basin)..