Moisture Sources and Contributions of Source Regions to the Northeast Cold Vortex Rainstorm in June 2021

In this paper, we first analyze the moisture transport and convergence characteristics of an early summer cold vortex rainstorm in Northeast China using the traditional Eulerian method. Subsequently, we use a Lagrangian particle dispersion model, FLEXPART, to quantify the contribution from various moisture sources. This helps us reveal the origins of moisture and their roles in this rainstorm process. The results show that during this early summer northeast cold vortex (NECV) rainstorm, the subtropical high pressure was positioned to the south. The northeast region was mainly influenced by the NECV and the weak high-pressure ridge in the Japan Sea downstream of it. In the lower troposphere, the cyclonic flow between the subtropical high and the weak high-pressure ridge in the Japan Sea contributed to a low-level jet northward along the coastal areas of China. This action was conducive to the northward transport of moisture from the southern part of Northeast China, which intersected with cold–dry airflow from the rear of the NECV in Liaoning and Jilin provinces, triggering heavy precipitation. Lagrangian trajectory tracking showed that >60% of the target air masses originated from the Eurasian continental region to the west and northwest of the precipitation area. These air masses started at a high elevation but significantly decreased in height as they passed through the East China Sea and the Yellow Sea, merging into the southward warm–wet airflow, which then flowed into the target precipitation area. The remaining air masses stemmed from Lake Baikal and its eastern region, as well as the South China Sea and the south–central continental region of China. These air masses maintained a relatively low height both at their origin and during transit. The quantitative contribution of moisture from the source regions to the target regions showed that the East China Sea–Yellow Sea to the Northwest Pacific made the largest contribution (37.04%), followed by East–Central China (30.05%). These regions demonstrated significant moisture uptake and low loss along the way, resulting in a substantial moisture contribution. The local precipitation area ranked third in moisture uptake. Lake Baikal and its eastern region, along with central–western Eurasia, also contributed, though most of the moisture from central–western Eurasia was lost during long-distance transport. The Indian peninsula to the South China Sea made the smallest moisture contribution. In conclusion, the Lagrangian method provided a clearer picture of the main sources of moisture and their contribution to the rainstorm process compared with the traditional Eulerian method.