Based on the precipitation observation data of meteorological stations in the source region of the Yellow River (SRYR) in the past ten years, July is selected as the largest month of precipitation, the maximum positive and negative abnormal year's corresponding to July are 2012 and 2015 respectively. The Lagrange Flexible Particle Dispersion Model (FLEXPART) is driven by NECP reanalysis data to simulate the backward trajectories of target particles in these two months. The characteristics and differences of water vapor transport under abnormal conditions are emphatically analyzed, and the contribution rate of each moisture source to the regional rainfall is calculated quantitatively. The results show that, in July 2012, the moisture transportation to the SRYR is mainly conducted by South Branch which contains two routes entering the SRYR from the southern side of the Qinghai-Tibetan Plateau (QTP):one is the trans-equatorial transport path, that is to say, the Somali jet carries moisture from the Arabian Sea and finally enters the SRYR by way of the Indian Peninsula and Bay of Bengal; the other is that particles carry moisture from the South China Sea and finally enters the SRYR by way of the Sichuan Basin. On the contrary, the North Branch which means moisture enters the SRYR from the Western or northern side of the QTP plays a dominant role in July 2015, including two typical paths as well:one is that the Easterly jet carries the moisture from the South China Sea and finally enters the SRYR by the Bay of Bengal and Indian Peninsula; the other is that the Westerly Jet carries moisture from the northern Africa or eastern European Plain and finally enters the SRYR, via Central Asia. The characteristics of specific humidity variation during the movement of particles show that the southern foot of the Himalayas, the Sichuan Basin, the Bay of Bengal and the northern Tibetan Plateau are potential moisture sources for precipitation in the SRYR. Moreover, the estimation of moisture sources contributions to the precipitation in SRYR shows that:the arid and semi-arid grassland areas on the northern side of the TP are main sources of precipitation for the SRYR in July of dry year. The contribution rate is 52.9%, which is much higher than the other four potential moisture sources. While the contributions of the three main sources in the wet year are far less significant than those in the dry year. No matter what type of precipitation, the southwestern QTP and the northern side of the QTP provide the main external water vapor for the main precipitation in the SRYR.
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