Observational study on the initial formation process of the Mei-yu frontal disturbance in the eastern foot of the Tibetan plateau in middle-late June 1992

In this research, a case study is conducted on the formation process of the Mei-yu frontal disturbance in the eastern foot of the Tibetan Plateau. The target period is middle-late June of 1992, during which the Mei-yu front is re-intensified after the decaying phase observed in middle June. The re-intensification process of the Mei-yu front occurs in accordance with the approach of the migrating upper level trough to the north of the Plateau. When the upper level trough is situated to the northwest or north of the Plateau, the lower level high pressure area becomes apparent. Subsequently, the low pressure area on the northeast of the Plateau deepens. At the same time, a shallow cold air mass observed below the 700 hPa level formed in the southeast of the low pressure area. The appearance of the cold air mass might be related to the development of low and high pressure systems to the north-northeast of the Plateau. Concurrently, a lower level strong westerly wind appears along the northern periphery of the Plateau, and turns into northwesterly or northerly wind along the eastern periphery of the Plateau. A shear line formed in the northeastern or eastern foot of the Plateau between this northwesterly wind and the southerly wind prevailing over the North-Middle China Plain. This shear line in the lower layer changed into the Mei-yu frontal disturbance after similar to00UTC 21 June. Note that the wind system along the northern-eastern periphery of the Plateau mentioned above is considered to be an ageostrophic wind system, accompanied by the transient small scale low and high pressure systems that migrate clockwise along the northern-eastern periphery of the Plateau. The synchronic appearance of the cold air mass and the shear line is considered to be an effective trigger for the formation of the initial Mei-yu frontal disturbance. Further, the coupling of the upper level migrating trough and the lower level shear line also can be important for the evolution of the Mei-yu frontal disturbance.