An in-situ Case Study on Micro Physical Properties of Aerosol and Shallow Cumulus Clouds in North China
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摘要: 2014年8月15日，山西省人工降雨防雹办公室在山西忻州开展了气溶胶和浅积云的飞机观测，本文利用机载云物理资料，详细分析了华北地区气溶胶、云凝结核（CCN）和浅积云微物理特性及其相互影响。主要结论有：（1）此次过程的边界层高度约为3600 m，不同层结情况下，0.1～3 μm尺度范围内的气溶胶粒子浓度Na、有效直径Da和CCN数浓度的垂直廓线明显不同，近地面Na可达2500 cm−3。（2）CCN的主要来源为积聚模态、爱根模态或者核模态的气溶胶颗粒，0.2%过饱和度下，气溶胶活化率（AR）在各高度层的结果变化不大；0.4%过饱和度下，AR随着高度增加而降低。（3）后向轨迹模式分析表明，2 km以下的气溶胶主要来自于当地城市排放，由细颗粒污染物组成；2 km以上的气溶胶主要来源于中国西北和蒙古地区的沙漠，由亚微米沙尘组成，溶解度相对较低，可作为潜在的冰核。（4）本文细致分析了两块相邻浅积云（Cu-1和Cu-2）的云物理特性。Cu-1云底高度约4500 m，云厚约600 m，云体松散，夹卷较多；云中液态含水量（LWC）基本保持在0.5 g m−3，云粒子浓度Nc平均值为278.3 cm−3，云滴有效直径Dc整体在15 μm以内；毛毛雨滴粒子浓度最大值为0.002 cm−3，云中几乎无降水粒子；粒子谱宽随着高度增加而增大，主要集中在30 μm以内。Cu-2云底高度约3900 m，云厚约1200 m，云体密实；云中过冷水丰沛，LWC有多个超过1 g m−3的区域，云顶附近出现冰晶，云中粒子从凝结增长状态直接进入到混合相态；积云内部粒子水平分布不均，同一高度Nc相差较大，最大可达1240 cm−3。Dc随着高度增加而增大；粒子谱宽随着高度增加而拓展，最大可达1100 μm，谱型由单峰向多峰转变；降水粒子和冰晶图像大多为霰粒子、针状和板状。
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关键词:
- 气溶胶 /
- CCN(cloud concentration nuclei) /
- 浅积云 /
- 飞机观测
Abstract: An aircraft observation on aerosol and shallow cumulus clouds in Xinzhou was carried out by the Weather Modification Office of Shanxi Province on 15 August 2014. In this paper, the microphysical properties of aerosols, CCN (cloud concentration nuclei), shallow cumulus clouds, and their interactions in North China are analyzed in detail, based on the airborne cloud physics data. Main results are as following: (1) In this case, the height of the boundary layer is about 3600 m, and the aerosol particle concentration (Na) near surface can reach 2500 cm−3. The vertical profiles of Na, aerosol effective diameter (Da) and CCN number concentration are obviously different under different stratification conditions. (2) The main source of CCN is aerosol particles in accumulation mode, Aegean mode or nuclear mode. The aerosol AR (activation rate) through the vertical layer does not change much under the 0.2% supersaturation condition, while decreases with height under the 0.4% supersaturation condition. (3) HYSPLIT4 (Hybrid Single-Particle Lagrangian Integrated Trajectory) model analysis shows that aerosols below 2 km mainly come from local urban emissions, which are composed of fine particulate pollutants. Above 2 km, aerosols mainly come from deserts in northwestern China and Mongolia, which are composed of submicron sand and dust. They can be potential IN (ice nuclei) due to their low solubility. (4) The physical characteristics of two adjacent shallow cumulus clouds (Cu-1 and Cu-2) are also analyzed. Cu-1 is loose with a lot of entrainment. The cloud base height and cloud thickness are about 4500 m and 600 m, respectively. The LWC (liquid water content) in Cu-1 is basically maintained at 0.5 g m−3, while the average Nc (cloud particle concentration) is 278.3 cm−3 and Dc (cloud effective diameter) is overall within 15 μm. The maximum Nd (drizzle droplet number concentration) is 0.002 cm−3, with almost no precipitation particles in Cu-1. The particle spectrum width increases with height, and ED (effective diameter) is mainly concentrated within 30 μm. Cu-2 is much denser than Cu-1, with cloud base height at 3900 m and cloud thickness of 1200 m. There is plenty of supercooled water in Cu-2, and LWC is over 1 g m−3 at multiple areas. Ice crystals appear near the cloud top, and particle growth states change from condensation to mixed phase directly. The horizontal distribution of particles in Cu-2 is uneven, and Nc at the same height differs greatly, with the maximum value up to 1240 cm−3. Dc increases with height. The particle spectral width expands with height, up to 1100 μm, and the spectral pattern changes from a single peak to multi-peaks. The images of precipitation particles and ice crystals are mostly graupel, needle-shaped, and plate-shaped.-
Key words:
- Aerosol /
- CCN (cloud concentration nuclei) /
- Shallow cumulus cloud /
- Aircraft observation
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图 1 2014年8月15日飞机探测的（a）飞行轨迹、（b）飞行高度（蓝线，单位：m）及对应温度（黑线，单位：°C）的时间序列、（c）积云照片、（d）穿云过程中机窗上出现的积冰照片
Figure 1. Aircraft observation on 15 August 2014: (a) Flight trajectory; (b) time series of flight altitude (blue line, units: m) and temperature (black line, units: °C); (c) photo of a cumulus; (d) in-cloud photo of ice through aircraft window
图 2 2014年8月15日16时（北京时）4500 m高度以下（a）Na，（b）Da，（c）温度（实线，单位：°C）、相对湿度（虚线），（d）气溶胶粒子谱的垂直分布。图d中的N和D分别表示粒子的数浓度和直径
Figure 2. Vertical profiles of (a) Na (aerosol particle number concentration), (b) Da (aerosol particle effective diameter), (c) temperature (solid line, units: °C) and RH (Relative humidity, dashed line), and (d) spectrum of aerosol particles measured below the height of 4500 m at 1600 BJT (Beijing time) 15 August 2014. In Fig. d, N, D represent number concentration and diameter of particle, respectively
图 3 2014年8月15日（a）CCNN1、（b）CCNN2、（c）不同过饱和度活化率的垂直廓线
Figure 3. Vertical profiles of (a) CCNN1, (b) CCNN2, and (c) activation ratio (AR) at SS=0.2% and SS=0.4% on 15 August 2014. CCNN1 and CCNN2 represent the cloud condensation nuclei number concentration at supersaturations (SS) of 0.2% and 0.4%
图 4 观测区域不同高度气团48 h后向轨迹分布，终止时间为2014年8月15日16时（北京时，下同）。绿线、蓝线、红线、黑线和黄线分别表示1200 m、2000 m、2800 m、3600 m、4400 m的后向轨迹
Figure 4. 48-h backward trajectories of air mass at five height levels in the measurement area, ending at 1600 BJT (Beijing time) 15 August 2014. The green, blue, red, black, and yellow lines represent the backward trajectories at 1200 m, 2000 m, 2800 m, 3600 m and 4400 m height, respectively
图 5 2014年8月15日飞机探测的两块积云（Cu-1、Cu-2）中（a）LWC、（b）Nc、（c）Dc、（d）Nd、（e）Nr随时间的变化
Figure 5. Time series of (a) LWC (cloud water content), (b) Nc (cloud particle number concentration), (c) Dc (cloud droplet effective diameter), (d) Nd (drizzle droplet number concentration), and (e) Nr (rain droplet number concentration) for the two cumulus clouds (Cu-1, Cu-2) observed by the aircraft on 15 August 2014
图 6 2014年8月15日两块积云（Cu-1、Cu-2）中（a）LWC、（b）Nc、（c）Dc、（d）Nd、（e）Nr的垂直分布特征
Figure 6. Vertical profiles of (a) LWC, (b) Nc, (c) Dc, (d) Nd, (e) Nr for the two cumulus clouds (Cu-1, Cu-2) on 15 August 2014
图 7 2014年8月15日两块积云（Cu-1、Cu-2）中不同高度粒子谱分布
Figure 7. Distributions of droplet size at different altitudes in the two cumulus clouds (a) Cu-1 and (b) Cu-2 on 15 August 2014
图 8 2014年8月15日两块积云中不同高度、温度的CIP和PIP图像：（a）Cu-1的PIP粒子图像；（b–d）Cu-2的CIP粒子图像；（e–g）Cu-2的PIP粒子图像。CIP和PIP的图像宽度分别代表1550 μm和6200 μm的测量范围
Figure 8. CIP (Cloud Imaging Probe) and PIP (Precipitation Imaging Probe) array probe images at various sampling altitudes and temperatures in the two cumulus clouds: (a) Cu-1 PIP image; (b–d) Cu-2 CIP images; (e–g) Cu-2 PIP images. The width of the CIP array is 1550 μm and that of the PIP array is 6200 μm
表 1 DMT云物理探测系统的各探头参数列表
Table 1. List of all the DMT (Droplet Measurement Technology) instruments on the aircraft during the research flights
探头
名称分档
数量测量范围
/μm误差范围 主要探测粒子类型 PCASP 30 0.1～3 ±(10%～
15%)气溶胶 CCN Counter 20 0.75～10 ±10% 不同过饱和度下云凝结核 Hot-wire LWC ±15% 液态水含量 CDP 30 2～50 ±20% 霾、云滴、冰晶 CIP 62 25～1550 ±20% 冰雪晶、大云滴 PIP 62 100～6200 ±20% 云和降水粒子 AIMMS 温、风、湿、压、经纬度 
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