AMS Cloud Physics Conference, 7-11 July 2014
Numerical experiment on cloud formation in two-component hygroscopic aerosol system
Akihiro Hashimoto, Katsuya Yamashita, and Masataka Murakami
Natural aerosol is composed of many kinds of chemicals. When a cloud forms in the atmosphere, a variety of physicochemical properties of aerosol particles should affect the generation of droplets. To precisely understand a cloud formation, an approach considering the multi-chemical-component aerosols is needed. This motivates the authors to develop a new version of multi-dimensional bin microphysics model in which internal and external mixing of two kinds of aerosols with different hygroscopitity are seamlessly represented. Droplet nucleation and subsequent diffusional growth are formulated by k-Köhler theory. This model is applied to a numerical experiment assuming adiabatically ascending air parcel in order to investigate the sensitivity of cloud droplet formation and growth to the number concentration ratio of two types of CCN, ammonium sulfate (AS) and sodium chloride (SC) particles. For the initial number concentrations, two values, 3000 and 9000 cm-3 for AS, and eight values, 1, 3, 10, 30, 100, 300, 1000 and 3000 cm-3 for SC are adopted. A lognormal size spectrum is assumed at the initial for the both types of CCN. Median diameter and variance are given as 0.1 mm and 0.6, respectively, for AS, and 1 mm and 1.0, respectively, for SC. Ascending rate of air parcel is set to 0.3, 1, 3 or 10 m s-1. Combination of initial number concentration of AS and SC, and ascending rate of air parcel yields 64 cases of simulation to be performed. The Initial air pressure, temperature and humidity are given as 850 hPa, 16 oC and 80%, respectively.
In the simulations, cloud droplets start to form at about 500 m above the initial point and their number concentration increases till super-saturation ratio attains its maximum. After that, the super-saturation ratio gradually decreases toward water saturation
The sensitivity experiments of the 64 cases revealed that there are three regimes of cloud droplet formation in the two-component hygroscopic aerosol system. In Regime I where SC number concentrations are less than 10 cm-3 and are much less than the AS particles, the AS particles control the behavior of system. Although the SC particles have much higher hygroscopicity than the AS particles, the number concentration of SC is too small to affect the cloud formation process. In Regime II where the number concentrations of SC range from 10 to 300 cm-3 and are still far less than that of AS, the SC particles become effective in cloud droplet formation processes due to their higher hygroscopicity. In Regime III where the number concentrations of SC particles are larger than 1,000 cm-3, but are still less than that of AS, the SC particles determine the behavior of system. The influence of ascending rate on cloud droplet formation becomes weak in Regime III, since the excess water vapor, which is produced by adiabatic cooling in the ascending air parcel, cannot get large, due to the consumption of water vapor by strongly hygroscopic SC particles. The differences in cloud formation among the three regimes should affect subsequent cloud and precipitation processes. The approach considering multi-chemical-component aerosols should be extended to the 2D and 3D cloud system simulation involving cloud dynamics, thermodynamics, aerosol transport, and gravitational sedimentation of precipitation particles.