New Parcel Model with Detailed Cloud Microphysics

A new parcel model with detailed cloud microphysics has been developed on the bases of Chen's microphysical model, in order to study the effect of aerosol particles on the initiation of water and ice clouds. This model has the hydrometeor categories for droplets and ice particles, each of which is divided into multi-dimensional bin to express a variety of properties of hydrometeors. The model assumes that each of droplets is represented with the mixture of pure water, soluble and insoluble matters. If the mass of pure water is extremely small, the particle is assumed to be dry aerosol particle. For ice particle, pure water, soluble and insoluble matters, aspect ratio, and volume are assumed as its components. The detailed expression of hydrometeor property allows us to investigate the sensitivity of nucleation of droplet and ice particle to the chemical components of soluble matters and the size of insoluble portion of a particle. And, the successive change in the density of ice particle in its growth process also becomes possible to be simulated.

Using this model, new formulations for heterogeneous ice nucleation are being tested. For deposition mode, the nucleation rate is formulated so as to depend on the size of insoluble particle and its surface area partially covered by solution, as well as the super-saturation for ice. For immersion freezing mode, the effect of insoluble particle size is introduced in addition to the temperature dependency so that the probability of active site occurrence on the surface of ice nucleus can be formulated. In the numerical simulation for a test, larger ice nuclei (insoluble particles) initiate ice particles faster through the deposition mode, and enhance the nucleation through the immersion freezing mode. Finally, consumption of water vapor becomes larger in the case of larger ice nuclei. As a result, it is revealed that the size of ice nucleus affects the timing of ice initiation, number concentrations of ice particles, and the amount of water vapor.

We plan to further improve the numerical model through a comparison with the experimental results from the cloud simulation chamber which has been developed by Meteorological Research Institute, Tsukuba, Japan, to simulate cloud processes in adiabatically ascending air parcel.