Geographic Causes And Numerical Study Of The Daily Variation Of Low Level Jet In The Meiyu Frontal Jet Rainstorm

Meiyu frontal rainfall including jet-related rainstorms experiences pronounced daily variation (DV). Scientists tried to seek explanations from the DV of low-level jet as a member of the Meiyu precipitating system. Based on case study and climate analysis, this work arrives at the central cause of DV of the Meiyu rainfall, which lies in the regional difference in DV of the lower-level temperature field of east Asia, and the cause relates essentially to 1) distinct topography in this region and 2) a particular cloud pattern during the wet season. Because of the existence of Meiyu precipitating cloud band the temperature DV is faint inside the Meiyu trough and on account of large-scale prominent terrain available, significant temperature DV occurs on both sides of the trough, leading to the reinforcement of the subtropical high and middle latitude transformed high, and the deepening and narrowing of the trough in the night with the result that the relative intensity and precipitating function of the Meiyu frontal rainfall system are changed at day and night, resulting in precipitation DV. And the DV of lower-level systems, e.g., jet stream, is among the intermediate changes happening after the DV is displayed in the Meiyu pressure field. Regional difference in lower-level temperature DV is a climatic phenomenon that often emerges in this wet season of east Asia.The effect of Nanling ridge on the DV of LLJ is conducted in terms of MM5V3.5. Results indicate that LLJ becomes strongest at 5a.rn.and weakest at 14p.m. by the influence of the Nanling ridge. Regional difference in lower-level temperature DV and pressure DV are greatly weakened and DV of LLJ is distinctly faint without the Nanling ridge. This result show that Nanling ridge acts as a critical role in the establishment of DV of the lower-level temperature and pressure,which give rise to the DV of LLJ in South China.