Estimating the criterion for determining water vapour sources of summer precipitation on the northern Tibetan Plateau

According to the precipitation and delta O-18 data obtained during the GEWEX Asian Monsoon Experiment-Tibet Intensive Observation Period, based on the knowledge that VO is lower in precipitation formed from ocean air mass vapour than that from local evaporation vapour, the water vapour sources can be identified from the 6180 values in precipitation. We attempt to give the identification criterion of VO values in precipitation. The threshold values chosen to distinguish between ocean and local sources are delta O-18 < -20 parts per thousand and delta O-18 > -13 parts per thousand respectively. According to this criterion, the proportion of local evaporation-formed precipitation and ocean air-mass-formed precipitation in total precipitation was estimated. The average value of precipitation at three sites (NODA, Amdo and AQB) is 249.76 mm. Among this, precipitation formed directly by the ocean air mass vapour is 80.08 mm at most. Precipitation formed by water vapour evaporated from local places is 117.05 mm at least. That is to say that precipitation formed directly by the ocean air mass vapour accounts for 32.06% of the total precipitation at most. Precipitation formed by water vapour evaporated from local places accounts for 46.86% of the total precipitation at least. At least 21.8% of the total precipitation came from water vapour that was evaporated on the way and transported by the monsoon circulation. Copyright (c) 2005 John Wiley & Sons, Ltd.