An in-situ Case Study on Micro Physical Properties of Aerosol and Shallow Cumulus Clouds in North China

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 (N_a) near surface can reach 2500 cm⁻³. The vertical profiles of N_a, aerosol effective diameter (D_a) 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 N_c (cloud particle concentration) is 278.3 cm⁻³ and D_c (cloud effective diameter) is overall within 15 μm. The maximum N_d (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 N_c at the same height differs greatly, with the maximum value up to 1240 cm⁻³. D_c 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.