Analysis of Convection-Symmetric Instability and Triggering Mechanism for Extreme Rainstorms on the Northern Slope of the Middle Kunlun Mountains

This paper analyzes the extreme rainstorm that occurred on May 6 and 7, 2020, on the north slope of the central Kunlun Mountains. This method uses ERA-5 0.25°×0.25° reanalysis data to calculate the wet potential vorticity and the frontier-generating function. The aim of this study is to better understand atmospheric instability during the rainstorm process and clarify the role of the frontal system in triggering convective activity. The results showed the following: (1) During the heavy rainstorm period, the rainstorm area was controlled by the divergence superposition area caused by two stream jets at 200 hPa, the Central Asian vortex and the slowly-moving shear line north of the plateau at 500 hPa. At 700 hPa, topographic convergence lifting was observed, along with a clash between the cold air parcel in front of the surface high and the warm air parcel from the Tarim basin. These conditions were conducive to developing the cloud cluster in the shallow mountain area on the northern slope of the middle Kunlun Mountains. (2) The rainstorm process was divided into two phases. During the first phase (EP1), an observable convective instability layer was observed in the lower troposphere over the Yutian to Qiemo area. The second phase (EP2-1) witnessed a brief but heavy rainfall at the Cele station. The convective instability in the lower troposphere gradually transitioned to symmetric instability. The variation in Mpv2 at the lower troposphere was caused by the wet atmospheric baroclinic and vertical shear of the lower horizontal wind. Following the initial heavy rainfall, the lower layer atmosphere over the Cele to Luopu area became warmer and more humid than non-precipitation owing to the release of condensation’s latent heat. This caused the stratification stability of the lower layer atmosphere to shift to convective instability in the second stage (EP2-2). (3) Uplifting of terrain convergence and frontogenesis at the lower–upper troposphere in the northern plateau were primarily responsible for forming mesoscale cloud clusters in the first stage. The cold front in the lower troposphere triggered the release of convective unstable energy, leading to rapid cloud development near Qiemo. Simultaneously, the airflow that had piled up in front of the hill ascended to the vicinity of 500 hPa, where the cold front underwent frontogenesis, briefly confronting the warm front on the northern side of the plateau to form a cold occluded front. The vertical movement near this front enhanced the rapid development of convective clouds. These clouds gradually merged with the convective clouds near Qiemo, resulting in first-stage precipitation (EP1) from Tian to Qiemo. During the second short-time heavy rainfall stage over the Cele area (EP2-1), a cold front of 700 hPa in the lower troposphere frontogenesis and the warm, wet inflow air met with the cold air mass of evaporation under the cloud and inflow behind the cloud. This interaction caused the warm air mass to climb further along the bottom cold pool, rapidly developing the cloud to its mature stage and resulting in a brief but heavy rainfall over the Cele area. Ultimately, the Central Asian low vortex gradually entered the northern slope of the middle Kunlun Mountains and the cold front frontogenesis in the lower and middle troposphere, further strengthening the development of the upward movement. This was identified as an important factor for continuous precipitation from the Cele to Luopu area in the second stage (EP2-2).