The Himachal Pradesh region experienced an unprecedented 436% more rainfall than normal from July 7th to 10th, 2023, resulting in devastating landslides, flash floods, and significant socio-economic losses. Using rainfall observations and reanalysis data, this study investigates the dynamics of the extreme precipitation event that caused the disastrous 2023 flood in Himachal Pradesh, India. Prior to the flood, moderate rainfall events saturated the mountainsides, making them more vulnerable to landslides and flash floods. Local features such as rapid convergence, vertical velocity, relative humidity, relative vorticity, and moisture flux accumulation were observed to decrease following the extreme precipitation event, indicating a short-duration cloud burst event. Further, the study reveals a large-scale circulation pattern linking the extratropic and monsoonal low, which is favorable for extreme precipitation in the Himachal Pradesh region. This pattern includes a transient merging of eastward-propagating trough in the mid-upper-level, which extends southward and a monsoon low moving westward across India. The key elements of this transient merging feature consist of a Rossby wave train structure and a blocking high west of the features. The Rossby wave breaking effect occurred downstream of the blocking high two days prior to extreme precipitation event occurrence. This allowed high potential vorticity stratospheric air to intrude the equator and troposphere, causing surface pressure change and enhanced vertical motion, thereby advecting high amount of moisture causing extreme precipitation leading to catastrophic floods. Overall, the findings of this study hold direct implications for early warning systems and disaster preparedness in the face of extreme precipitation-induced flood situations triggered by such transient merging features.

中文翻译：

---

西部扰动与季风低压之间短暂融合的多尺度动态：与喜马偕尔邦 2023 年 7 月洪水的联系

2023年7月7日至10日，喜马偕尔邦地区的降雨量史无前例地比正常情况多出436%，导致毁灭性的山体滑坡、山洪爆发和重大社会经济损失。本研究利用降雨观测和再分析数据，调查了导致 2023 年印度喜马偕尔邦灾难性洪水的极端降水事件的动态。在洪水发生之前，中等强度的降雨使山坡饱和，使山体更容易遭受山体滑坡和山洪爆发。极端降水事件后，快速辐合、垂直速度、相对湿度、相对涡度和水汽通量积累等局地特征均出现减少，表明发生了短时云爆发事件。此外，该研究揭示了连接温带低压和季风低压的大规模环流模式，这有利于喜马偕尔邦地区的极端降水。这种模式包括中高层向东传播的低压槽短暂合并并向南延伸，以及季风低压向西移动穿过印度。这种瞬态合并特征的关键要素包括罗斯贝波列结构和特征西侧的阻塞高地。罗斯比波破碎效应发生在极端降水事件发生前两天的阻塞高点下游。这使得高位涡平流层空气侵入赤道和对流层，引起表面压力变化和增强的垂直运动，从而平流输送大量水分，导致极端降水，导致灾难性洪水。总体而言，这项研究的结果对面对由这种短暂合并特征引发的极端降水引发的洪水情况的早期预警系统和备灾具有直接影响。