A wind systems analysis of rainfall variation over semi-arid region in China

The Loess Plateau in China is located in a typical summer rain region with a rainfall season. It is one of the wheat belts, and is located in semi-arid region with a large variation in precipitation. Thus, the authors analyze the year-to-year variation of precipitation and wind systems transferring the vapor into the Loess Plateau. Used data are monthly average precipitation, wind, geopotential height and vapor transport calculated by wind vector and specific humidity. The monthly average precipitation is analyzed for 14 years from 1979 to 1992 at 22 observation sites using the data provided by the National Center for Atmospheric Research. The wind vector, geopotential height and vapor transport are analyzed based on the re-analysis data of the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR). The results are summarized as follows. There are some differences in rainfall system between July and August, which cause rainfall over the Loess Plateau. In high precipitation year, southeasterly wind originated from southwesterly monsoon can flow into the inland due to the poleward shift of subtropical jet stream in July. It transports a large amount of water vapor into disturbance with polar-front jet trough staying in the northeastward of the Loess Plateau. Under this convective instability, it is expected that convective clouds produce precipitation around the Loess Plateau. In August, water vapor is transported into the Loess Plateau by the convergence of low-level jet from the North Pacific subtropical high and southwesterly monsoon. On the other hand, subtropical jet stream shifts southward, flowing over the Loess Plateau and its ridge-line appears over Japan at upper troposphere in low precipitation year. Westward extending North Pacific subtropical high along the ridge-line tends to weaken the southeasterly arm of southwesterly monsoon into the Loess Plateau, while the Taiwan convergence zone is intensifies and southwesterly monsoon towards the Korean Peninsula is forced. Therefore, water vapor is transported into the Loess Plateau mainly from the East China Sea. This is expected to be the mechanistic process of low precipitation in the Loess Plateau