新疆中天山一次城市暴雪过程诊断分析

采用NCEP逐日4次1°×1°再分析资料和Micaps常规观测等资料,对2011年3月发生在新疆中天山城市暴雪过程进行天气学诊断分析。诊断计算包括：中尺度分析、水汽通量、水汽通量散度、水平散度、垂直速度、高低空风场、螺旋度、假相当位温等。结果表明：暴雪是南北两支锋区在中亚地区交汇后东移发展造成的,降雪前乌鲁木齐城区出现东南大风,地面强烈减压升温为暴雪天气触发不稳定能量提供了热力条件,500 hPa有＞30 m·s^-1的西南急流,700 hPa存在低空切变,散度和垂直速度表现为明显的高层辐散、低层辐合的对称结构。降雪强盛期整层呈现上负下正的垂直螺旋度对,θ_se低能舌伸至200 hPa,700 hPa至400 hPa维持θ_se高能舌区,湿层厚度高达300 hPa。这种物理量场的配置有利低层湿空气聚合及向上的抬升运动,为暴雪的产生提供了必要条件。此次冷空气以偏西路径影响城区,在冷空气明显的条件下,受城市热岛效应影响,强降雪容易发生在温度较高的城区,同时降水中心倾向于出现在锋区的位置。

Diagnostic analysis of an urban snowstorm process in the middle of Tianshan mountains of Xinjiang

Based on the daily NCEP reanalysis data（ 1°×1° ） in four times and the MICAPS conventionally obser- vational data,an urban snowstorm process on March of 2011 in the middle of Tianshan mountains of Xinjiang was diagnosed. The diagnostic study includes a mesoscale analysis, calculations of moisture flux and its divergence, hor- izontal divergence, vertical speed, upper and lower wind fields, helicity and pseudo-equivalent potential tempera- ture. The results indicate that the snowstorm is caused by the eastward movement of the north and south frontal zones after joining up in the central Asia region. The strong southeast wind occurs before snowfall at the urban area of Urumqi. The sharp decompression and warming on the ground provide thermodynamic conditions for the insta- bility energy. There is the southwesterly jet whose wind speed is greater than 30 m. s - 1 at 500 hPa, and there exists low-level shear lines at 700 hPa. The vertical distribution of divergence and vertical velocity presents obvious sym- metrical structure of lower-level convergence and upper-level divergence. In the period of heavy snowstorm, it shows a couple of vertical helicity with upper negative and lower positive. The low energy tongue ofθSe extends upward to 200 hPa, while the high energy tongue of θSe maintains from 700 hPa to 400 hPa. The deep moist layer reaches to 300 hPa. This kind of configuration of the physical quantity field accelerates the convergence of wet air in low level and their lifting movement upwards, which provides the necessary conditions for generation of snow- storm. The cold air influences the urban area of Urumqi along a western path. Under the cold air condition, the strong snowstorm usually occurs at the central urban areas with higher temperature due to the effect of urban heat island. Meanwhile, the rainfall center is prone to form in the location of frontal zone.