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The Microphysical Structure of a Heavy Fog Event in North China
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摘要: 基于华北雾-霾综合观测试验资料，分析了2011年12月4日河北涿州一次浓雾过程爆发性增强的微物理特征及形成机理。结果表明：此次浓雾过程除具有均压场、地面辐射降温、逆温层、静稳天气等特征外，还具有雾微物理过程出现爆发性增强的特征，10 min内，小雾滴浓度显著增加，含水量增大了3个量级，雾滴谱由15 μm拓宽到35 μm，能见度由500 m骤降至70 m。夜间地面长波辐射冷却效应导致近地层雾的形成，而近地层雾的形成反过来快速地增强了地面长波辐射冷却效应，促使大量小雾滴的形成和碰并过程的产生，这是一种正反馈效应；大量雾滴形成释放的潜热，促使雾体抬升和向下长波辐射增强，又使地面长波辐射冷却效应减弱，产生负反馈效应。相对于南京辐射雾过程，此次涿州浓雾的小雾滴粒子数浓度高，液态水含量明显偏小，这与华北高浓度气溶胶和弱水汽输送有关。Abstract: Based on comprehensive observations of haze in North China during 2011, the genesis and microphysical structure characteristics of a dense fog process are analyzed. Results show that the fog process is under the control of high-pressure uniform pressure field when the wind speed on the ground is small. The inversion layer near the ground, sufficient water vapor and radiation cooling are important causes for the fog process. After the occurrence of fog, the ground temperature decreased obviously, and the inversion layer also increased, accompanied by the rise of the high relative humidity area, and the thickness of the fog body increased continuously. Compared with the radiation fog process in Nanjing during winter, the concentration of small particle number is larger and the content of liquid water is lower. This is because of the high aerosol concentration in North China and the weak water vapor transport. The average aerosol number concentration in this fog process is three times of that in Nanjing. The concentration of cloud condensation nuclei is positively related to the aerosol number concentration. With the decrease of temperature, aerosol particles continue to aggregate and grow, and become droplets. The increasing number of droplets competes for water vapor, which makes it difficult big particles for to form in the fog. The long-wave radiation cooling effect at night results in the formation of near-ground fog, which in turn enhances the cooling effect of long wave of the fog rapidly, and promotes the formation and collision of a large number of small droplets, providing positive feedback. The latent heat released by the formation of droplets promotes the lifting of the fog body and the enhancement of downward long-wave radiation, and makes the cooling on the ground weakened, which is a negative feedback. This fog process shows characteristics of burst enhancement. Within 10 minutes, the number and density of droplets increased significantly, the water content increased by three orders of magnitude, the droplet spectrum widened from 15 μm to 35 μm, and the visibility plummeted from 500 m to 70 m. The explosive growth of fog is due to the increase of long-wave radiation on the top of fog, which makes the temperature drop continuously and the supersaturation increase. The water vapor condensation and droplet condensation lead to the rapid growth of droplets and a double peak spectrum distribution. The emergence of large drop number concentration further accelerates the collision process and further widens the spectrum width.
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Key words:
- heavy fog;
- microphysical structure;
- burst enhancement
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图 2 2011年12月3—4日涿州观测点浓雾形成前后系留气球探测的边界层气象要素垂直分布
Fig. 2 Boundary meteorological profiles observed by tethered balloon before and after the fog formation at the observation site of Zhuozhou on 3-4 Dec 2011
图 3 2011年12月4日涿州观测点雾平均直径、最大直径、雾滴数浓度、液态水含量、地面净辐射(正值表示吸收，负值表示向外辐射)、气温、相对湿度、风速、风向、能见度的时间变化
Fig. 3 Temporal variation of the equivalent diameter, maximum diameter, number concentration, liquid water content, ground net radiation (the positive denotes absorption, the negative denotes emission), temperature, relative humidity, wind speed, wind direction, visibility at the observation site of Zhuozhou on 4 Dec 2011
图 4 2011年12月4日涿州观测点雾爆发性增长期间的滴谱变化
Fig. 4 Size distributions of fog during its explosive growth stage at the observation site of Zhuozhou on 4 Dec 2011
图 5 2011年12月4日涿州观测点地面气温及地面净辐射随时间变化
Fig. 5 Temporal variations of surface temperature and surface net radiation at the observation site of Zhuozhou on 4 Dec 2011
图 7 2011年12月4日涿州观测点气溶胶粒子数浓度、平均直径和云凝结核数密度随时间变化
Fig. 7 Temporal variations of aerosol number concentration, mean diameter and CCN number concentration at the observation site of Zhuozhou on 4 Dec 2011
表 1 涿州观测点观测仪器一览表
Table 1 Instruments used in the observation site of Zhuozhou
观测仪器 型号 探测要素范围 观测频率 自动气象站 ZQZ-C型 气温：-50~50℃，风向：0~360°，风速：0~60 m/s，雨强：0~4 mm/min，相对湿度：0~100%，气压：400~1100 hPa 1 min 能见度仪 PWD20 10~2000 m 1 min 扫描电迁移率粒径谱仪 SMPS 3936 0.01~1 μm 3 min 雾滴谱仪 FM-100 2~50 μm 1 s 系留探空 XMS-2 温度：-50~50℃，湿度：0~100%，风速：0~60 m/s，风向：0~360° 2 s 辐射表 CNR1 1 min 
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表 2 2011年12月4日华北雾过程与2006—2007年冬季南京辐射雾过程微物理参数比较
Table 2 Comparison of microphysical parameters of between the fog process in North China on 4 Dec 2011 and the radiation fog process in Nanjing during winter of 2006-2007
地点 统计量 数浓度/cm-3 液态水含量/(g·m-3) 等效直径/μm 最大直径/μm 能见度/m 最大值 939.8 0.27 5.6 50.0 1000 涿州 最小值 0.3 2.4×10-6 2.6 4.0 70 平均值 99.2 0.014 3.4 6.0 最大值 883.0 0.46 8.40 50.00 1000 南京[35] 最小值 1.0 1.57×10-5 3.00 4.00 15 平均值 89.0 0.028 3.72 12.20 387
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