Evolution of Meteorological Conditions during a Heavy Air Pollution Event under the Influence of Shallow Foehn in Urumqi, China

The air pollution in Urumqi which is located on the northern slope of the Tianshan Mountains in northwestern China,is very serious in winter.Of particular importance is the influence of terrain-induced shallow foehn,known locally as elevated southeasterly gale(ESEG).It usually modulates atmospheric boundary layer structure and wind field patterns and produces favorable meteorological conditions conducive to hazardous air pollution.During 2013-17,Urumqi had an average of 50 d yr-1 of heavy pollution(daily average PM2.5 concentration>150μg m-3),of which 41 days were in winter.The majority(71.4%)of heavy pollution processes were associated with the shallow foehn.Based on microwave radiometer,wind profiler,and surface observations,the surface meteorological fields and boundary layer evolution during the worst pollution episode in Urumqi during 16-23 February 2013 are investigated.The results illustrate the significant role of shallow foehn in the building,strengthening,and collapsing of temperature inversions.There were four wind field patterns corresponding to four different phases during the whole pollution event.The most serious pollution phase featured shallow foehn activity in the south of Urumqi city and the appearance of an intense inversion layer below 600 m.Intense convergence caused by foehn and mountain-valley winds was sustained during most of the phase,resulting in pollutants sinking downward to the lower boundary layer and accumulating around urban area.The key indicators of such events identified in this study are highly correlated to particulate matter concentrations and could be used to predict heavy pollution episodes in the feature.

近10 a中亚低涡背景下新疆短时强降水环境场分析

为更好地把握中亚低涡造成新疆短时强降水天气过程的机理特征,为新疆短时强降水预报预警提供参考和依据,利用FNL资料、探空资料等,运用天气学原理、动力诊断分析方法等,研究了中亚低涡系统不同部位发生的三类短时强降水的环境场特征及环流配置。结论表明:(1)短时强降水发生在500 h Pa中亚低涡前部西南气流下为最多。(2)A指数在新疆地区短时强降水降水预报中意义不大。(3)中亚低涡背景下新疆短时强降水天气的配置一般属于高空冷平流强迫类。(4)三类短时强降水形势配置的共性:在低涡背景下,对流层中高层都有显著气流存在,低空有切变线存在,近地面往往有辐合线,700 hPa左右往往有湿舌。异性:第I类短时强降水湿度条件较好,第Ⅱ类短时强降水高低空温差较大,第Ⅲ类短时强降水垂直风切变较强。

中天山北坡焚风时空分布特征及预报效果分析

焚风是一种特殊的山地气候现象,是由地形造成的高温、低湿的强下坡风,在天山北坡乌鲁木齐及其周边地区频繁发生。焚风影响飞机起降、陆路运输,对建筑设施以及当地人民生命财产安全有着潜在威胁,并且在重污染天气的形成过程中起重要作用。目前对中天山北坡的焚风研究,多局限于乌鲁木齐焚风天气个例的分析,对焚风的时空分布特征的认识不够细致深入,对焚风的形成机理依然缺乏全面系统的认识。本研究采用2018—2020年中天山北坡9个气象站逐时风速风向资料、欧洲细网格数值预报产品,分析乌鲁木齐及其周边焚风的时空分布特征,并检验了欧洲细网格数值预报产品对焚风最大风速的预报效果。结果表明:(1)近3年乌鲁木齐南郊乌拉泊出现大于等于17.00m·s^(-1)的焚风1213时次,其次是西郊八钢(480时次)、峡谷内部柴窝堡(329时次)和乌鲁木齐市区(101时次)。焚风着陆点春季偏向南郊乌拉泊以北地区,冬季则偏向其以南地区;(2)上述4站平均瞬时风速由大到小依次为乌拉泊21.24m·s^(-1)、八钢20.21m·s^(-1)、乌鲁木齐19.61 m·s^(-1)、柴窝堡18.36 m·s^(-1)。秋季八钢平均风速大于乌拉泊。4个站点出现东东南(ESE)风向频率最高;(3)近3年焚风天气过程共发生27次,其中春季发生14次(51.85%)、冬季7次(25.93%)、秋季6次(22.22%);(4)与南郊乌拉泊相比,乌鲁木齐市区焚风过程持续时间相对较短;(5)欧洲细网格10 m风场预报产品对八钢、乌鲁木齐、乌拉泊焚风最大风速的预报相关性较高,相关系数均达0.80以上,但预报偏差较大;欧洲细网格850 hPa风场预报产品对八钢、乌鲁木齐、乌拉泊焚风最大风速预报偏差较小。本研究可为中天山北坡焚风天气监测预警提供科技支撑。