This study investigates the anomalous signals near the tropopause before the overshooting convective system (OCS) onset over the Tibetan Plateau (TP). It is found that the tropopause height is stable at the maximum height seven and five days before the OCS onset. It then decreases significantly one day before and on the day of the OCS onset. The upward motion in the troposphere is the strongest five days before the OCS onset. From one day before and after the OCS onset, there are strong ascending motions at 500–300 hPa but weak descending motions near the tropopause. It is proposed that the descending of the tropopause height on the day of the OCS onset is caused by frequent tropopause fold events over the eastern TP associated with frequent cold trough intrusion from the north and the southeastward movement of upper-level westerly jet stream. The decrease of the tropopause height is accompanied by the intrusion of stratospheric air with higher potential vorticity (PV). Positive potential vorticity anomalies on 350 K isentropic surface can be noted in the region where the tropopause height decreases one day before and on the day of the OCS onset. With the deepening of the tropopause fold on the day of the OCS onset, there is not only downward motion near the tropopause in the area behind of the fold but also upward motion in the troposphere beneath the folding region. In addition, the upward displacement of isentropic surfaces leads to an upper-level cold pool, which causes a reduction in static stability beneath the PV anomaly on the day of the OCS onset. The upper-level PV anomalies and their associated strong instability in the middle troposphere can trigger convective activities by the release of potential instability on the day of the OCS onset. The overshooting convection is more likely to occur due to lower tropopause height, although upward motion in the troposphere is not the strongest.

中文翻译：

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青藏高原对流系统超调爆发前对流层附近异常信号的分析

这项研究调查了青藏高原（TP）上的超调对流系统（OCS）发生之前对流层顶附近的异常信号。发现在OCS发作前七天和五天，对流层顶高度稳定在最大高度。然后，它在OCS发作的前一天和当天显着降低。对流层中的向上运动是在OCS发生前五天最强的。从OCS发生前后的第一天开始，对流层顶附近在500–300 hPa处有强烈的上升运动，但有较弱的下降运动。有人提出，OCS发病当天对流层顶高度的下降是由于东部TP上频繁的对流层顶褶皱事件引起的，并伴有北部高空西风急流向北和向东南方向的频繁冷槽侵入。对流层顶高度的降低伴随着平流层空气的侵入而具有较高的潜在涡度（PV）。在OCS发作的前一天和一天中，对流层顶高度下降的区域中，可以注意到350 K等熵表面上的正势涡度异常。随着OCS发作当天对流层顶褶皱的加深，不仅在对折层后面的对流层顶附近存在向下运动，而且在对折区下方的对流层中也存在向上运动。此外，等熵面的向上位移会导致上层冷池，这会导致OCS发作当天PV异常以下的静态稳定性降低。在对流层中层，高层PV异常及其相关的强烈不稳定性可通过在OCS发生之日释放潜在的不稳定性来触发对流活动。由于对流层顶高度较低，对流层对流更有可能发生，尽管对流层中的向上运动不是最强。