碘化银核化过程的数值模拟研究

THE NUMERICAL SIMULATION OF AGI NUCLEATION IN CLOUD

根据DeMott等给出的不同成核机制下AgI成核效率的实验结果 ,通过数值模拟的方法 ,研究了AgI粒子在云室、层状云和对流云中的核化过程。通过对云室的模拟 ,发现云滴浓度和云雾持续时间是造成不同云室检测的AgI成核率相差悬殊的主要原因。数值试验结果表明AgI的成核方式在层状云和对流云中有很大不同 :层状云中AgI主要以接触冻结、浸没冻结等慢核化过程为主 ,而对流云中则以凝结冻结过程为主。

Generally, the mechanism of ice-forming process by AgI can be identified as deposition, contact freezing, condensation freezing and immersion freezing. The total nucleation activity is the sum of contributions from different nucleation modes. In 1995, P.J.DeMott etc published their dynamic chamber experiment results, in which they identified the effect of the four ice-forming mechanisms and gave out the quantitative results of them. In our research, the experiments results of P.J.DeMott etc were applied in box model, one-dimension stratiform cloud model and three-dimension convective cloud model. Results show that the dominant ice-forming mechanism in different clouds is quite different and explain the reason why the nucleation rate tested by different cloud chamber is inconsistent. The detail demonstration will be given in the following part. In box model, the nucleation process of AgI in cloud chamber was studied. Its results included the following three parts: (1) AgI nucleation activity is depended on cloud droplets concentration and cloud holding time. The cloud droplets concentration and the cloud holding time is quite different in different cloud chamber, so the nucleation rate tested by different cloud chamber is extreme variance. These results have been approved by the Bigg cloud chamber, the 2 m 3 CSU cloud chamber and the 96 m 3 cloud chamber. (2) Just as other experiments showed, temperature and humidity have notable effect on AgI nucleation activity. Normally, if the temperature is lower and the humidity is higher, the AgI nucleation process is often more effective. (3) Though the nucleation rates tested by different cloud chamber are different, the normalized results are quite similar. In stratiform cloud simulation research, the one-dimension cloud model developed by Hu zhijin etc (1986) was used. In this model, water vapor, cloud water, ice crystal, snow, graupel, rain and their 18 interactive actions were considered. Besides these, variables involved AgI nucleation process have been added, and then the simulation capability of seeding AgI in stratiform cloud was obtained. By the simulation research of seeding AgI in stratiform cloud, the main nucleation modes were found. Owing to low updraft and long resident time, contact freezing and immersion freezing play a very important role in AgI nucleation process under stratiform cloud conditions. In convective cloud research, a three-dimension cloud model developed by CAMS was used. By the similar method, the model was modified and used to the convective cloud modified research. Results show that condensation freezing is the dominant nucleation modes in convective cloud. In conclusion, humidity, temperature, cloud droplets concentration and cloud holding time are the main influence factors in AgI nucleation process. These factors are quite different in different cloud environment, so the dominant nucleation mechanism is different in different cloud conditions. These also explain the reason why AgI nucleation rate tested by different cloud chamber is inconsistent.