Variation Characteristic Of GPS-retrieved Precipitable Water Vapor And Its Relationship With Summer Precipitation Over The Tibet Plateau

Japan International Cooperation Agency (JICA project) which established 24 ground-based GPS stations over Tibetan Plateau and around area was carried out from 2005 to 2009. In this article, by using GPS-retrieved atmospheric precipitable water vapor (PWV) data of JICA from 2011 to 2013 and the hourly AWS (Automatic Weather Station) precipitation, we focus on analyzing the diurnal variation characteristics and the relationship with precipitation at Naqu, Dingqing, Linzhi and Longzi stations which locate in the mid-east Tibet and Gaize station in the west on the basic of revealing the seasonal variance over Tibet Plateau and its east mountains. The results are obtained as follows:(1) By comparing the datus of GPS and Radiosonde station locating in the same place, the average RMSE between GPS and Radiosonde PWV is 0.29 cm and it trends to decrease with the altitude increasing. The error is larger in 12:00 UTC than in 0:00 UTC. The correlation coefficients pass the significance test of 0.001 and the relationship at 0:00 UTC is better.(2) PWV may decrease from southeast to northwest. PWV in summer is the largest, then follows autumn. It is the least in winter. The research of PWV monthly variation series by using harmonic analysis indicates that the peak usually appears in early July in Sichuan, north Yunnan and mid-east Tibet while in the middle of July in the west Tibet.(3) PWV has significant diurnal variation and circular character on the Tibet Plateau. The result of harmonic analysis shows that the diurnal (24h) cycle is the main signal of PWV diurnal variation in each season and it also presents characteristic of semidiurnal (12h) to a certain extent in Tibet. The maximum ordinarily appears in the afternoon or at night local time while the low area is often from before dawn to the forenoon of the next day, and the time in the west of Tibet is later than in the east. But the diurnal cycle is main in the eastern mountains and the peak appears earlier. By analizing the variations of 5 stations in Tibet, diurnal cycle gets stronger during the day with rain while the semidiurnal is weaker.(4) The diurnal variation of precipitation has significant characteristic of diurnal cycle. And precipitations often occur at 16:00～18:00 UTC at Naqu, Dingqing and Linzhi station while occur at 11:00～12:00 UTC at Longzi station. The peak of rainfall frequency is usually later than hourly accumulative precipitation. The cumulative precipitation is effected by semidiurnal cycle and the diurnal cycle is weak at Gaize station locating in the west of Tibet.(5) PWV accumulates gradually and decreases rapidly before and after precipitation. The time when PWV begins to rise is generally earlier than hourly accumulative precipitation and rainfall frequency. The precipitation reaches the peak while PWV is still high. The 3 h average PWV before rainfall would rise with precipitation increasing. With the increase of PWV, precipitation probability after 1 h will increase gradually, and rapidly rising after PWV reaches a certain value.