1. Institute of Arid meteorology,CMA; Key Open Laboratory of Arid Climate Change and Disaster Reduction of CMA; Key Laboratory of Arid Climate Change and Disaster Reduction of Gansu Province,Lanzhou 730020,China; 2. College of Atmospheric Science,Lanzhou University,Lanzhou 730000;Key Laboratory of Arid Climate Change and Reducing Disaster of Gansu Province,Lanzhou 730020,China;