Improvement of Quantitative Analysis for Immersion Freezing Nucleation Experiments with MRI Cloud Simulation Chamber

Heterogeneous ice nucleation can occur via several different mechanisms. Although some expressions of these mechanisms have been implemented into the numerical model and actively investigated the reproducibilities for weather forecasting and climate prediction, our fundamental knowledge of the primary ice formation is still not sufficient. The properties of the precipitating cloud systems are dependent upon the size distribution of cloud droplets for warm clouds and partitioning between supercooled droplets and ice crystals for cold clouds. Therefore, the activation processes of atmospheric aerosol functioning as cloud condensation nuclei (CCN) and ice nuclei (IN), which determine an initial droplet size distribution and number concentration of ice crystals, are important.

Immersion freezing mode is a key process for forming ice in mixed-phase clouds. The particles immersed in supercooled water contributing to heterogeneous ice nucleation at a certain freezing temperature should have been activated as an effective CCN before that. It is required to investigate by connecting the cloud droplet formation and the subsequent process of the ice crystal formation.

A cloud simulation chamber facility run by the Meteorological Research Institute (MRI) has been used to investigate the details of the fundamental processes of cloud formation. Physicochemical properties, such as size distribution, cloud condensation nuclei and ice nuclei (IN) abilities of aerosol particles were measured using aerosol and cloud particle detectors and instruments. We focus on the temperature-deterministic ice nucleation active surface site (INAS) density as a method of description of the heterogeneous ice nucleation ability of Atmospheric aerosol. In this study, we attempted to improve the procedure of analysis and derivation of INAS.

In the adiabatic expansion experiment, the time evolution of both dry aerosol particle, cloud droplet and ice crystal size distribution are continuously measured throughout the before cloud formation, during the continued cooling, and after ice crystals formation. By closure analysis of those data, it is possible to distinguish the particles previously activated as CCNs and immersed in supercooled water, the particles activated as IN, and the interstitial particles still suspended in the chamber at any moment. Then, modified INAS is derived by appropriately evaluating total surface area of aerosol particles immersed in supercooled water at that moment. In calculating, we also consider about the fallout of each type of particles to the bottom of the chamber. We are currently progressing the quantitative evaluation of this method.