Roll of the Tibetan Plateau concerning the formation of the Baiu front over China and the occurrence of the Baiu season

虽然白云锋面被认为是准静止的副热带锋面，但与白云锋面相对应的雨带或云带并不总是出现在东亚地区。因此，白云锋面的形成或更新过程应存在于天气场日变化中。通过对青藏高原周边大气环流系统的深入个例分析，发现青藏高原北部与高层槽向青藏高原的东南推进相对应，在更新后的白牛锋出现之前，在青藏高原北部形成了一个东西向的高压系统，从而促进了青藏高原上空白牛锋的形成和演变。当高空槽到达青藏高原以北时，青藏高原东北部的这个高压系统和低压系统就会加强。同时，浅寒是…青藏高原东北角的低压系统与中国平原北部的高压系统之间出现了更多的联系。此时，青藏高原东北外围地区出现较强的西风-北风，青藏高原以东出现一条南北向风切变线。这条低层切变线或正涡度带是由于青藏高原东北缘出现非地转西北偏北风而形成的。在此期间，还观察到了高原圈闭的高压和低压系统。随着冷气团的东移，切变线的方向变得接近东西走向。冷空气团南侧的局地强温度梯度和南风带来的水汽补给可能是切变线向白云锋区演化的重要原因。较少

Though the Baiu (Mei-yu) front is considered to be a quasi-stationary subtropical front, rainfall zone or cloud band which corresponds to the Baiu front does not always appear over east Asia. Therefore, formation or renewal process of the Baiu front should exist in the day-to-day variation in the synoptic field. The purpose of this study is to detect the stmospheric circulation system around the Tibetan Plateau which contributes to the formation and/or evolution process of the Baiu front over China from intimate case analysis.Before the appearance of the renewed Baiu front, an east-west oriented high pressure system in the lower level is formed on the north of the Tibetan Plateau in accordance with the southeastward advancing of upper level trough to the Tibetan Plateau. This high pressure system and low pressure system to the northeast of the Tibetan Plateau are intensified when the upper level trough reaches just to the north of the Tibetan Plateau. Simultaneously, a shallow cold are … More a emerges between the low pressure system at the northeast corner of the Tibetan Plateau and a high pressure system around the North China Plain. In this time, a strong westerly-northerly wind is observed along the northern-eastern periphery of the Tibetan Plateau, and a north-south oriented wind shear line between the northerly and southerly wind appears to the east of the Tibetan Plateau. This lower level shear line or positive vorticity zone is established due to the emergence of ageostrophic northwesterly-northerly wind along the northeastern periphery of the Tibetan Plateau. In this time, plateau-trapped high pressure and low pressure systems are also observed. With the eastward migration of the cold air mass, the direction of the shear line becomes nearly an east-west orientation. It seems that the localized strong temperature gradient to the south of the cold air-mass and the water vapor supply due to the southerly wind could contribute to the evolution of the shear line to the Baiu front. Less