Seasonal and Geographical Variations in CCN Ability of Atmospheric Aerosols and Cloud Droplet Concentrations in Japan Based on In Situ Aircraft Measurements

Aerosol particles acting as cloud condensation nuclei (CCN) and ice nuclei (IN) determine the microphysical structures of cloud and precipitation, and affect a short-range precipitation forecast and climate change projection. Also an efficiency of hygroscopic seeding is dependent upon the characteristics of background CCN as well as physico-chemical properties of seeding particles and cloud types. Therefore we have investigated the physico-chemical properties and CCN ability of background aerosols and cloud microphysical structures using an instrumented aircraft (B200T) over several places in Japan since 2008.

Number concentrations of CN ranged from 400 – 30,000 cm^-3 and water supersaturation spectra of CCN concentrations, Nccn=C * SSw^k, showed a large seasonal, geographical and altitudinal variations; C ranged from 200 to 4,000 cm^-3 and k ranged from 0.55 to 2.2. The dry diameter of aerosol particles activated as CCN at 0.1 (1.0) % corresponded to 0.3 (0.07) mm and the number concentrations of CCN activated at SSw=1% and aerosol particles larger than 0.1 mm measured by PCASP showed a good correlation. Estimated hygroscopicity of the atmospheric aerosols acting as CCN was on the order of 0.1 and small aerosol particles activated at high SSw had a large variability of hygroscopicity. The hygroscopicity of aerosol particles activated at high SSw is as low as about 0.01 in the air mass strongly influenced by East Asian air pollutions and forest fires while aerosol particles activated at low SSw in the air mass strongly affected by dust storms had the hygroscopicity of about 0.01 .

Typical maximum cloud droplet number concentrations near cloud bases were 200~1,500 cm^-3. The ratio of cloud droplet number concentration and CCN number concentration activated at SSw=1.0% increased with decreasing the CCN number concentration and increasing updraft velocity. The estimated maximum SSw near cloud bases ranged from 0.2 ~ 1.0% and also increased with decreasing CCN number concentration and increasing updraft velocity.