Abstract
The freezing-probability of a supercooled water drop, which grows by the capture of cloud droplets, due to the collision of ice crystals formed on ice nuclei was numerically studied in clouds with various conditions. In all cases, the probability increases about 100 times during staying of the drop at cloud top and most of collisions with ice crystals take place when the drop stays near cloud top. The accumulated number of ice crystals colliding with the drop increases with the vertical velocity of air-current and the height of cloud top. The accumulated number of colliding ice crystals is larger in clouds of small water content of cloud droplets than in clouds of large water content after a sufficiently long time, although it tends to increase with the water content in early stage of the growth of the drop. Most favourable water content for the collision is dependent upon the velocity of updraft and the time length. In clouds which are lower than 3000m in the height of cloud top and which are a few m/s in the velocity of updraft, at most one drop of 10 to 100 supercooled water drops can collide with one ice crystal in 1.0 to 1.5 hour.
