临夏州2月一次极端暴雪过程湿位涡特征分析

Characterisation of the wet level vortex during an extreme snowstorm in Linxia State in February

摘要:

2018年2月18日，临夏州出现2月历史最强一次降雪天气过程，强降雪伴有强降温，对设施农业、交通出行、电力、畜牧业和人们生活等带来不利影响。本文利用高空、地面常规观测资料、ERA5（0.25oｘ0.25o)全球再分析格点资料，对这场极端暴雪天气过程进行诊断分析。结果表明：印度低槽与巴尔喀什湖冷槽是这场极端降雪天气的主要影响系统，印度低槽北抬东移，巴尔喀什湖低槽东移南压，在青海高原至临夏州，形成东北―西南向的切变线，该切变线的形成对临夏州强降雪有增幅作用；水汽主要来源于孟加拉湾地区，随着印度低槽北抬东移，水汽沿槽前西南气流输送到青藏高原东南侧，后经四川盆地，在临夏州形成明显的水汽辐合区，水汽辐合区与临夏州主要降雪区有很好的对应关系，水汽通道主要集中在 650hPa～350hPa，水汽辐合区位于650hPa～400hPa。降水前临夏州上空等θe线陡立密集, 湿斜压性增强，垂直涡度显著发展，伴随着强烈的上升运动，导致大气对流不稳定能量释放, 有利于临夏州短期内强降水发生；湿位涡等值线密集带是冷暖气流的过渡带，临夏州暴雪中心位于MPV1等值线正值密集带和MPV2负值得到较大增长的区域，MPV2负值增大,大气湿斜压性增强导致下滑倾斜涡度发展，是形成此次暴雪的另一个重要原因。

Abstract:

On 18 February 2018, one of the strongest snowfall weather processes in the history of February occurred in Linxia Prefecture, and the strong snowfall was accompanied by a strong temperature drop, which adversely affected facility agriculture, traffic and travel, electricity, animal husbandry and people's lives. In this paper, this extreme snowstorm weather process was diagnosed and analysed using routine observations at high altitude and on the ground, and ERA5 (0.25ox0.25o) global reanalysis grid point data. The results show that the Indian trough and the cold trough of Lake Balkhash are the main influencing systems of this extreme snowfall weather, the Indian trough lifts north and moves east, the trough of Lake Balkhash moves east and presses south, and forms a northeast-southwest oriented shear line from the Qinghai Plateau to Linxia Prefecture, and the formation of this shear line has an increasing effect on the strong snowfall in Linxia Prefecture; the water vapour mainly comes from the Bay of Bengal region, and with the Indian trough lifting north and moving east, the water vapour increases along the Bay of Bengal. As the Indian trough lifts northward and moves eastward, the water vapour is transported to the southeast side of the Tibetan Plateau along the southwestern airflow in front of the trough, and then passes through the Sichuan Basin, forming an obvious water vapour convergence zone in Linxia Prefecture, and there is a very good correspondence between the convergence zone and the main snowfall area of Linxia Prefecture, with the water vapour channel mainly concentrating at 650hPa～350hPa, and the water vapour convergence zone is located at 650hPa～400hPa. The isotropic θe line is steeply and densely packed over Linxia Prefecture before precipitation. Before the precipitation, the isotropic θe lines were steep and dense over Linxiazhou, the wet obliquity pressure was enhanced, the vertical vorticity developed significantly, accompanied by a strong upward movement, leading to the release of unstable energy from atmospheric convection, which was conducive to the occurrence of strong precipitation in Linxiazhou in the short term; the dense zone of wet vortex isotropic contours is the transition zone of cold and warm airflow, and the centre of the blizzard in Linxiazhou was located in the area where the positive value of MPV1 isotropic contour was dense and negative value of MPV2 got a large growth, and the negative value of MPV2 was enlarged. The increase of MPV2 negative value and the development of downward tilted vorticity is another important reason for the formation of this blizzard.