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庐山两次冷雾过程宏微观物理特征

Macroscopic and Microscopic Physical Characteristics of Two Supercooled Fog Processes in the Lushan Mountain

  • 摘要: 为研究冷雾演变机制和宏微观物理特征,2016年1月和12月在江西省庐山气象局布设雾滴谱仪和自动气象站进行雾综合观测,结合观测获取的资料和NCEP 1°×1°再分析资料,分析了2016年1月16~17日(个例1)、2016年12月25~27日(个例2)两次冷雾的宏微观结构。结果表明,两次冷雾的发展演变与冷锋的移动密切相关,从形成阶段到发展阶段800 hPa以下由西南风转为偏北风,冷锋前缘到达,近地面气温骤降;两次冷雾的成熟阶段近地面雨停且风力减弱,个例2出现锋面逆温层;两次冷雾的消散阶段900~500 hPa转为一致的偏北气流,800 hPa以下风速增大。个例1和2的过程平均雾滴谱均呈双峰分布,主峰均位于4.9 μm,次峰分别位于8.9、11.0 μm。个例1和2均出现了主峰位于10~14 μm的瞬时雾滴谱,出现频率分别为12.4%和46.3%。个例1和2中均有暖雾向冷雾的转换,冷雾与暖雾相比各粒径段雾滴数密度均有所上升,尤其是粒径14 μm以下的雾滴数量增长较为明显。个例1全过程雾滴数浓度与平均直径的相关性较弱,可能受雾滴碰并、雾滴竞争水汽等因素影响。个例2全过程雾滴数浓度、平均直径与含水量均为正相关关系,说明此次雾过程以凝结核活化和凝结增长为主。个例2西南低空急流强劲、近地面降温明显、有逆温层存在,雾滴谱较宽,雾滴谱10~14 μm粒径之间峰值更为突出,雾滴数浓度、平均直径、含水量皆大于个例1。

     

    Abstract: Comprehensive fog observation campaigns were conducted in the Lushan Meteorological Bureau of Jiangxi Province with a fog drop spectrometer and an automatic weather station in January and December 2016. This study investigated the macro- and microphysical characteristics of supercooled fog and elucidated its evolution mechanism. Combining observational data with the NCEP 1°×1° reanalysis data, the macro- and microphysical characteristics of two supercooled fog events (case 1 occurred on January 16 and 17, 2016; case 2 happened on December 25–27, 2016) were analyzed. Results revealed that the evolutions of the two supercooled fogs were strongly correlated with the movement of the cold front. From the formation to the development stages, the dominant wind direction below 800 hPa changed from southwest to northerly, the front edge of the cold front arrived at the study area, and temperature near the surface decreased sharply in both cases. At the mature stages of the two supercooled fog events, the rain stopped at the near-surface, and the wind force weakened; meanwhile, the front inversion layer appeared in case 2. During the dissipation stages, wind direction in 900–500 hPa changed to north in both cases and wind speed below 800 hPa increased. For microphysical characteristics, average droplet spectra exhibited bimodal distributions in both cases, with main peaks observed at 4.9 μm. However, secondary peaks were noted at 8.9 and 11.0 μm in cases 1 and 2, respectively. Both cases presented instantaneous droplet spectra with the main peaks occurring at 10–14 μm, and the frequencies were 12.4% and 46.3% for cases 1 and 2, respectively. Furthermore, both cases exhibited a transition from warm fog to supercooled fog. Compared with the warm fog, the number density of supercooled fog droplets increased for each droplet size, especially for droplets with particle size below 14 μm. The correlation between fog droplet number concentration- and average diameter was weak in the whole process of case 1, which may be affected by factors such as fog droplet collision–coalescence and droplet competition for water vapor. Meanwhile, case 2 showed a positive correlation among fog droplet number concentration, average diameter, and water content in the whole process, which indicated that the fog process was dominated by condensation nucleus activation and condensation growth. A strong low-level jet in the southwest, an obvious temperature drop in the near-surface, and a temperature inversion layer were all observed in case 2, resulting in a wider droplet spectrum, with peaks between 10 and 14 μm diameter more prominent in the droplet spectrum, and higher number concentration, average diameter, and water content than case 1.

     

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