中昆仑山北坡极端暴雨对流—对称不稳定性及其触发机制分析

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

本文利用ERA-50.25°×0.25°再分析资料,通过计算2020年5月6~7日中昆仑山北坡极端暴雨天气过程中湿位涡和锋生函数,给出暴雨过程大气不稳定性演变特征,明确锋面系统在对流触发中的作用,并得出以下结论:(1)本次中昆仑山北坡暴雨期间,200 hPa高空两支急流造成辐散叠加区,500 hPa中亚低涡、高原北部切变线缓慢移动过程中,配合低层700 hPa地形辐合抬升、地面高压前冷气团与塔里木盆地暖气团交汇,为中昆仑山北坡浅山区云团发展提供有利的动力、热力条件。(2)暴雨分为两个阶段,第一阶段(EP1)于田至且末一线降水期间,对流层低层暴雨区为对流不稳定大气层结;第二阶段(EP2-1)策勒地区短时强降水期间,策勒对流层低层逐渐由对流不稳定大气转为条件对称不稳定大气层结,对流层低层Mpv2变化由大气的湿斜压性和低层水平风的垂直切变所造成;受前期降水和凝结潜热释放影响,第二阶段(EP2-2)策勒至洛浦一线低层增暖增湿,对流层低层转为对流不稳定大气层结。(3)地形辐合抬升是第一阶段降水中尺度云团生成的主要原因,低层冷锋锋生触发对流不稳定能量释放,且末附近云团迅速发展,同时塔里木盆地气流沿地形爬坡至高原北部,在500 hPa附近冷锋锋生,与高原北侧暖锋短时间对峙形成冷式锢囚锋,锋面附近垂直运动增强使得对流云团快速发展,与且末附近对流云团合并加强,造成第一阶段(EP1)于田至且末一线降水天气;第二阶段(EP2-1)策勒短时强降水期间,对流层低层700 hPa冷锋锋生,云团移动方向暖湿入流气流与云下蒸发和云后入流冷气团相遇,暖气团沿底层冷池进一步爬升,使得云团迅速发展至成熟阶段,造成策勒地区出现短时强降水天气;随着中亚低涡逐渐进入研究区,对流层低层—中层冷锋锋生,进一步加强了上升运动的发展,是第二阶段(EP2-2)策勒至洛浦一线持续性降水天气的重要原因。

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-50.25°×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).