The intersection of atmospheric physics and atmospheric chemistry explores the formation process of freezing rain

Schematic diagram of the formation process of freezing rain. (The background is a photo of the frozen rain in Hengshan Mountain, provided by the research group)

Recently, “Atmospheric Environment” published the results of the cooperation between Li Xingyu, senior engineer of the Institute of Atmospheric Physics of the Chinese Academy of Sciences, and Pan Yuepeng, a researcher. The researchers explored the effects of chemical composition on the formation process of freezing rain.

Freezing rain is a common catastrophic weather in winter and spring, which is mainly distributed in the southwest and central China mountainous areas in China. Frozen rain exists in the form of super-cold water in the atmosphere, and freezes immediately after colliding with objects below 0 °C, which is a huge harm to air transport, transmission lines, communication equipment and agriculture and forestry industries.

It is the essential feature of freezing raindrops to remain supercooled (without freezing) in an atmosphere with a temperature below 0 °C. Previous studies have suggested that freezing rain does not freeze because of the lack of ice cores.

In fact, freezing rain is the melting of snow falling into the warm layer, and the ice core is always present in the super-cold water. Experiments have found that increasing the concentration of chemicals in the droplets is conducive to keeping the droplets supercooled, and this “chemical effect” has also been reproduced by numerical models. However, whether this “chemical effect” is real in nature has been lacking direct evidence from field observations.

Since 2015, Li Xingyu and Pan Yuepeng have cooperated to carry out comprehensive observation experiments on the physics and chemistry of extreme precipitation in Hengshan (Nanyue Mountain) in Hunan Province, collecting samples of precipitation in different phases such as freezing rain, and carrying out long-term follow-up measurements of their physical characteristics and chemical composition.

Through the analysis of the winter samples in the past 7 years, they found that the freezing rain was acidic as a whole (pH<5), in addition to nitric acid and sulfuric acid, the contribution of organic acids was close to 20%; compared with the alpine rain at home and abroad, the ion concentration in the freezing rain was significantly higher; with the increase of the concentration of ions (especially ammonium nitrate), the ambient temperature of the freezing rain showed a decreasing trend.

“Higher ion content may reduce the freezing temperature of raindrops, prolong the time when raindrops exist in the form of super-cold water and the distance they fall vertically, and the scope of harm is closer to the surface”, said Professor Ronald Stewart, an expert in freezing rain, saying that as the global climate warms, the freezing rain belt will move north, and the above “chemical effects” are likely to be further amplified in the north where atmospheric pollution is relatively serious.

Dissertation partners include the National Climate Centre and the University of Manitoba, Canada. The research has received strong support and technical guidance from Guo Xueliang, a researcher at the Institute of Atmospheric Sciences, and experts from the China Electric Power Research Institute. (Source: China Science Daily Cui Xueqin)

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