In winter and early spring, freezing rain is an increasingly frequent weather disaster in many regions of the world, sometimes occurring even in the tropics. Since it has a significant negative impact on electrical transmission infrastructure, transportation systems, and the general public, understanding the causes triggering this phenomenon is essential. A research team led by the Institute of Atmospheric Physics (IAP) at the Chinese Academy of Sciences has now investigated the relation between air pollution and the emergence of such weather events.
In order for freezing rain to occur, raindrops need to remain supercooled (below 0°C) in the atmosphere. While previous studies have attributed the presence of supercooled drops in the atmosphere to the lack of ice nuclei, recent research has found that this is in fact not the case, since ice nuclei remain in supercooled drops when snow melts and forms these drops at even higher and warmer atmospheric levels. According to the IAP researchers, elevated soluble chemicals help the drops stay supercooled through a chemical effect known as “freezing point depression.”
The scientists reached this conclusion by conducting a long-term measurement of the physical and chemical composition of freezing rain at Mount Heng in southcentral China. Since the elevation of the sampling site is 1265.9 meters, above the top of the atmospheric boundary layer, low temperatures in the winter favor the occurrence of supercooled fog and freezing rain. However, chemical analyses over a seven-year period have found that the freezing rain at Mount Heng was acidic, with higher concentrations of soluble ions and organic matter than those in rain. The study revealed that, as the concentrations of soluble ions (particularly ammonium nitrate) increase, the ambient temperatures of freezing rain showed decreasing trends.
“This study presented the first chemical view on freezing rain, a physical phenomenon in traditional belief,” said study co-author Ronald Stewart, an expert in Atmospheric Physics at the University of Manitoba in Canada. “The higher ionic concentration may depress the temperature needed to freeze raindrops and this process prolongs the existence of freezing rain.”
“Against the background of global warming, the influencing area of the freezing rain will shift northward, and the effect of freezing point depression induced by air pollution in northern China may be amplified,” concluded co-author Xiaying Zhu, a climate scientist at the Beijing Climate Center.
The study is published in the journal Atmospheric Environment.