Temporal relations between precipitable water vapour and precipitation during wet seasons based on nearly two decades of data from the Lhasa River valley, Tibetan Plateau

摘要

Hourly precipitation and ground-based Global Positioning System sensing precipitable water vapour (PW) measurements are analysed to better understand the relationships between PW and precipitation at small time scales. The measurements were made during the wet seasons-from May to September each year-between 2001 and 2017 at the Lhasa River Valley, a region representative over the Tibetan Plateau. In addition, we validate the power law expected from the critical phenomena in atmospheric precipitation using hourly PW and precipitation observations made in the region. The results show that peaks in PW, low cloud cover and precipitation values, and high frequencies of lightning usually occur under conditions of convective activities during the wet season which extends from May to September at the Lhasa River Valley. PW is found to have considerable autocorrelation periods that are twice longer as those in tropical regions. In addition, composite analysis results show that precipitation increases with PW. In the time range extending 36 to 7 hr prior to precipitation events, PW increases slowly. This phenomenon is likely related to synoptic-scale atmospheric activities. From 6 to 0 hr prior to precipitation events, PW increases rapidly-a change that is likely to be associated with mesoscale convective activities. The peak of PW is about 1 hr before the peak of precipitation, indicating that PW increases stimulate precipitation occurrences. Furthermore, our results show that the power law represents well the relationship between PW and precipitation with a PW critical value of 21.5 mm for Lhasa River valley. Also, this critical value is a tipping point for the number distribution of PW measurements. When the PW is less than the critical value, the number of PW measurements increases gradually along with the growth of PW. Otherwise, it drops rapidly.