title: Numerical simulation of the impact of reforestation on winter meteorology and environment in a semi-arid urban valley, Northwestern China.
abstract: Since 1999 Chinese government has made great effort to reforest the south and north mountains surrounding urban Lanzhou - a city located in a river valley, Northwestern China. Until 2009 obvious land use change occurred, with 69.2% of the reforested area been changed from grasslands, croplands, barren or sparsely vegetated land to closed shrublands and 20.6% from closed shrublands, grasslands, and croplands to forests. Reforestation changes land-surface properties, with possible impact on the evolution of atmospheric variables. To understand to what extent the local meteorology and environment could be affected by reforestation in winter, and through what processes, two sets of simulations were conducted using the Weather Research and Forecasting model (WRF) and the FLEXible PARTicle (FLEXPART) dispersion model for a control case with high-resolution remotely sensed land cover data for 2009 and a scenario assuming no reforestation since 1999. Results suggested that the changes in albedo, surface exchange coefficient and surface soil heat conductivity related to reforestation led to the changes in surface net radiation and surface energy partitioning, which in turn affected the meteorological fields and enhanced the mountain-valley wind circulation. Replacement of shrublands and grassland with forest in the south mountain through reforestation play a dominant role in the enhancement of mountain-valley wind circulation. Reforestation increased the amount of air exchanged between the valley and the outside during the day, with the largest hourly increase of 10% on calm weather days and a monthly mean hourly increase of 2% for the study period (Dec. 2009). Reforestation affected the spatial distribution of pollutants and slightly improved the urban air quality, especially in the eastern valley. Results from this study provide useful information for future urban air quality management and reforestation plan, and some experience for cities with similar situations in the world.
authors.affiliation: Clod and Arid Regions Environmental and Engineering Research Institute, Key Laboratory of Land Surface Process and Climate Change in Cold & Arid Regions, Chinese Academy of Sciences, Lanzhou 730000, China. Electronic address: yyu@lzb.ac.cn.
authors.forename: Ye
authors.initials: Y
authors.lastname: Yu
authors.affiliation: Clod and Arid Regions Environmental and Engineering Research Institute, Key Laboratory of Land Surface Process and Climate Change in Cold & Arid Regions, Chinese Academy of Sciences, Lanzhou 730000, China; College of Environmental Science & Engineering, Nankai University, Tianjin 300071, China. Electronic address: 18766167147@163.com.
authors.forename: Jianjun
authors.initials: J
authors.lastname: He
authors.affiliation: Clod and Arid Regions Environmental and Engineering Research Institute, Key Laboratory of Land Surface Process and Climate Change in Cold & Arid Regions, Chinese Academy of Sciences, Lanzhou 730000, China.
authors.forename: Suping
authors.initials: S
authors.lastname: Zhao
authors.affiliation: Clod and Arid Regions Environmental and Engineering Research Institute, Key Laboratory of Land Surface Process and Climate Change in Cold & Arid Regions, Chinese Academy of Sciences, Lanzhou 730000, China; Weather Modification Office, Qinghai Meteorological Bureau, Xining 810001, China.
authors.forename: Na
authors.initials: N
authors.lastname: Liu
authors.affiliation: Clod and Arid Regions Environmental and Engineering Research Institute, Key Laboratory of Land Surface Process and Climate Change in Cold & Arid Regions, Chinese Academy of Sciences, Lanzhou 730000, China.
authors.forename: Jinbei
authors.initials: J
authors.lastname: Chen
authors.affiliation: College of Environmental Science & Engineering, Nankai University, Tianjin 300071, China.
authors.forename: Hongjun
authors.initials: H
authors.lastname: Mao
authors.affiliation: College of Environmental Science & Engineering, Nankai University, Tianjin 300071, China.
authors.forename: Lin
authors.initials: L
authors.lastname: Wu
keywords: Air pollution; Complex terrain; Land use change; Lanzhou; Mountain-valley circulation; WRF