Abstract: The northeast of the Qinghai-Tibet Plateau is the intersection transitional zone of the Qinghai-Tibet Plateau and the Loess Plateau, also the transitional zone of agriculture and animal husbandry, the terrain is complex, and it is often affected by various weather systems, so the response of climate change is very sensitive. It is characterized by the fragile ecological environment, soil erosion and water loss, pasture degeneration, land desertification and various environmental problems. Therefore, it is very meaningful for further understanding the climatic characteristics and climate change in the Qinghai-Tibet Plateau by analyzing the climate change and its trends in the northeast of the plateau. However, the recent discussion about cloud cover and its related factors in the northwest or in the whole plateau was insufficient. So it is necessary to study the temporal change of cloud cover and more detailed weather characteristics related to cloud cover over the northeast of the Qinghai-Tibet Plateau. It is of great significance for implementing artificial precipitation and developing air resources.
Based on the daily cloud cover data observed by 22 meteorological stations and the NCAR/NCEP reanalysis data in the northeast of the Qinghai-Tibet Plateau (32°-37° N, 99°-104° E) during the period from 2001 to 2011, the development characteristics of cloud cover and the relationship between cloud cover and vapor flux in the northeast of the Qinghai-Tibet Plateau in recent 10 years were studied in terms of the statistical analysis and the HYSPLIT_4 Model. The results showed that the total cloud cover and the low cloud cover over the study area was decreased from the south to the north. There was an increase trend of total cloud cover and low cloud cover over the study area during the period from 2001 to 2011, especially in spring and summer. When rain or snow occurred, appear probability of cirrus was the highest, then that of altostratus, and that of cumulus was the lowest. Appear probability of convective cloud was higher than that of nonconvective cloud over the northeast of the Qinghai-Tibet Plateau. Water vapor is one of the main factors affecting the distribution of precipitation and the change of convective cloud. Water vapor flux came mainly from 700 hPa layer, and water vapor flux could reflect the low cloud cover well.

Key words: HYSPLIT Mode, cloud cover, cloud form, water vapor, Qinghai-Tibet Plateau