Working toward Better Forecast of Freezing Rain in Southern China
【 Big Mid Small 】
Under particular conditions, extreme low-temperature, precipitation and freezing-weather events can occur over an extensive area to form a combined meteorological disaster. For example, in January 2008, southern China experienced record-breaking long-lasting severe low temperatures, precipitation and freezing weather, which seriously affected transportation, energy supply, power transmission, agriculture, and people’s everyday lives. It was reported that the disaster eventually affected more than 100 million people and caused a direct economic loss of more than 54 billion Chinese Yuan.
Since the beginning of the current century, this type of disaster, referred to as extensive cold–precipitation–freezing events (ECPFEs), has occurred more frequently in southern China. In January 2011 and January 2018, for example, pronounced ECPFEs swept across southern China and caused unexpected, huge economic losses.
"Although ECPFEs have been investigated for individual cases, an objective definition for them is thus far lacking," explains Prof. PENG Jingbei, the first author of a study on this topic recently published in Advances in Atmospheric Sciences. "It is necessary to comprehensively consider these three elements of temperature, precipitation and freezing weather for an objective definition of ECPFEs." PENG is from the Institute of Atmospheric Physics at the Chinese Academy of Sciences.
Freezing rain—a kind of precipitation where supercooled water droplets freeze immediately after collision with surface objects with temperatures below 0°C—is the most hazardous form of freezing weather. It is known that an inversion layer is the usual vertical temperature profile typical of freezing rain.
"The zonally elongated region to the south of the Yangtze River is a place conducive to warm and wet air gliding up the near-surface cold air with temperatures below 0°C," says Prof. PENG. "That is why freezing rains often occurs to the south of the Yangtze River."
This study reveals that long-term confrontation between stable and extensive cold air from the north and warm and moist air from the south facilitates the concurrence of extensive and persistent low-temperature, precipitation and freezing weather.
"A large-scale tilted ridge and trough pairing over mid- and high-latitude Eurasia [to the north] and intensified subtropical westerlies along the southern foot of the Tibetan Plateau [to the south] is crucial for the appearance of an ECPFE," says the corresponding author of the study, Prof. BUEH Cholaw. "These circulation anomalies and their precursory signal are of essential importance for medium- and extended-range forecasting of ECPFEs. In the future, we plan to continue this research with a numerical weather prediction model."
Citation: Peng, J.-B., C. Bueh, and Z.-W. Xie, 2021: Extensive cold-precipitation-freezing events in southern China and their circulation characteristics. Adv. Atmos. Sci., 38(1), 81–97, https://doi.org/10.1007/s00376-020-0117-4.