冰雪晶碰并勾连增长的实验与观测分析

冰雪晶碰并勾连成雪花或雪团的过程是降水的重要机制之一。冰雪晶形态不同导致各类晶动力特性的差异, 因此, 无论从理论、数值试验还是室内模拟研究这一过程都有很大困难。该文就室内、外场试验和自然云的观测, 讨论分析其发生机理和条件。结果表明:冰晶的增长过程有一个与云滴碰并增长相似的加速过程, 冰晶碰并过冷滴形成霰 (雹胚), 凇附长成冰雹; 冰雪晶相互碰并勾连、攀附增长为雪花或雪团, 都是降水质点加速增长的重要过程。此过程仅在水面饱和、过饱和条件下发生, 而水面欠饱和、无液滴 (无云) 时, 冰晶很薄、晶型简单, 无碰并勾连、攀附现象。冰晶在液滴存在的云雾中伴随气流对流、乱流运动中接触碰并勾连成雪花或雪团, 其碰并勾连效率既受晶体形状影响, 亦受晶体表面附着力的影响, 其机制有勾连亦有粘连, 晶型多样, 以相同晶型为主, 温度范围广 (-3~-17 ℃); 其中-13~-17 ℃碰并勾连效率最高, 该层的枝、星状晶是勾连、攀附的主要区域, 亦为冰晶繁生的主要区域、生长率最快, 是人工增雨播撒人工冰核催化效率较高的温度段。

Observation and Analysis of the Aggregation Growth Among Ice snow Crystals

The process of the ice-snow crystal running together plays an important part in origination of precipitation. It is a focus in cloud physics research. It discusses the cloud and fog that happens in lab and field observations. Result shows that as the droplets coalesce and grow, there is a speed up process also in ice crystals growth, such as snowflake, snow-circular, graupel and hail that coalesce super-cooled droplets and aggregate among ice-snow crystals. Experiments show, in the lab the shapes are fixed basically in temperature of -3.5- -20 ℃. The aggregate among ice-snow crystals only happens in saturated or super-saturated water level vapor pressure in the lab. While in insaturation (no liquid droplets) it does not happen. In 1 m3 chamber their shapes are dendritic and stellar, and their aggregated mechanism is caught by branches appearing in -13- -17.℃ While in 96 m3 chamber the shapes are needle, columnar, dendritic and stellar, the mechanism is adhesion and adhering appearing in -5- -18 ℃. In field observations of natural cloud and fog, the mechanism of the ice-snow crystal running together is adhesion and adhering appearing in -3- -17 ℃, and their shapes are dendritic, stellar, sector, plate, tabular, needle, columnar and columnar bean.The 1 m3 chamber is 1.76 m high and 0.88 m wide, the experiment is made in-3.5- -20 17. The 96 m3 chamber is 14.8 m high, 3.0 m wide, the experiment is made in-5- -18 ℃. The temperature is measure by three Pt resistance thermometers with the difference of 0.1 ℃ to compare with standards one. The sample that burnt for Silver Iodide pyrotechnic is made by deposit method. A great deal ice crystal aggregates into each other appearing in -13- -17 ℃ in 1 m3 chamber when the sample is exposured for 1 minute. But for other temperatures of -3.5- -20 ℃ it does not. While in 96 m3 chamber the aggregation process happens among ice crystals. Their sizes are lager than 1 m3, because the ice crystals could be maintained for a few hours, and there exists torrent and the ice crystal could move with air in 96 m3 chamber. In field observation, the measurement is made by PMS, 2D-C, 2D-P on aircraft in cloud, and by electronic microscope and the samples making for deposit on ground. The shapes of snowflake aggregation vary in cloud in nature because they could exist long in cloud and exhibit convective and torrent features. The ice crystal could interact with them. Another field observation is made by artificial dispersing fog by liquid nitrogen, and samples are measured by electronic microscope for deposit on ground. The ice-snow crystals running into snowflakes by the mechanism of adhering in -3— -8 ℃, and their shapes are hollow columnar, columnar and needle.