1. Key Laboratory for Semi-Arid Climate Change of the Ministry of Education, Lanzhou University, Lanzhou 730000, P. R. China
2. Laboratory for Climate and Ocean-Atmosphere Studies, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing 100871, P. R. China
3. Yale School of Forestry and Environmental Studies, Yale University, New Haven, Connecticut 06511, United States
4. College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, P. R. China
5. School of Geography and Planning, and Guangdong Key Laboratory for Urbanization and Geo-Simulation, Sun Yat-sen University, Guangzhou 510275, P. R. China
6. Gansu Provincial Maternity and Child Care Hospital, Lanzhou 730050, P. R. China