Based on the observational datasets of rime and glaze from 743 stations in China and the atmospheric circulation data from the NCEP–NCAR reanalysis during 1954–2009, large-scale atmospheric and oceanic conditions for extensive and persistent rime and glaze events were examined with a composite analysis. Results show that rime events mostly occur in northern China while glaze events are mainly observed in southern China. The icing events are accompanied by low temperature and high humidity but not necessarily by above-normal precipitation. The Asian low, blocking highs, strong moisture transport, and an inversion layer related to major abnormal circulation systems contribute to the occurrence and persistence of icing events in China. The Ural blocking high plays a major role in the glaze events, and the Okhotsk blocking high is closely related to the rime events. For glaze events, extratropical circulation anomalies and the southward outbreak of cold air play a dominant role. In contrast, the strong northward transport of warm and moist airflows plays a leading role and the blocking high and the southward outbreak of extratropical cold air take a supporting role for rime events. There is nearly an equal chance for occurrences of rime events under La Niña and El Niño backgrounds. However, glaze events more likely occur under the background of La Niña. Additionally, the sea surface temperatures from the tropical Indian Ocean to the tropical northwestern Pacific Ocean also contribute to the occurrence and maintenance of icing events in China.
As important disastrous weather phenomena, icing events, especially extensive and persistent icing events, have great impacts on power industry and communication networks and have caused heavy socioeconomic losses. For example, unprecedented snow and icing hazards hit most of China, especially southern China, from 10 January to 2 February 2008. Over 100 million people were affected, with the direct economic loss exceeding 110 billion RMB (L. Wang et al. 2008; Z. Wang et al. 2008). Since the early twentieth century, icing-prone countries, including the United States, Canada, Japan, and others, have conducted intensive studies on the general environmental conditions for icing events (e.g., Brooks 1920) and their synoptic and climatic characteristics (Young 1978; Cooper and Marwitz 1980; Huffman and Norman 1988; Bell and Bosart 1988; Cortinas 2000; Jones et al. 2004). Studies have also been carried out for the physical mechanisms and synoptic processes for the formation of icing events (Okada 1914; Stewart and King 1987, 1990; Forbes et al. 1987; Czys et al. 1996; Zerr 1997) and the physical processes of ice growth and icing modeling (Makkonen 1981, 1984, 2000; Poots 1996; Farzaneh and Savadjiev 2005).
Rime and glaze are the two major types of icing events in China, which occur mostly in severe winter and early spring conditions. As explained in China Meteorological Administration (2003), rime is formed by rapid freezing of supercooled fog drops or sublimation of supercooled water vapor drops when they strike an object at a temperature below freezing. Glaze is formed by freezing of supercooled water drops or drizzles. Rime and glaze have obviously different physical characteristics. Rime is white or milky and opaque granular deposit of ice, while glaze is generally clear and smooth. Rime has densities of 0.2–0.3 g cm−3, while glaze is much denser, harder, and more transparent, with densities of 0.8–0.9 g cm−3. Generally, low air temperature and high humidity are favorable for occurrence of rime and glaze (Tan et al. 1985; Wang et al. 2011), and wind is also a very important factor. Breeze or calm air helps the formation of glaze and soft rime, which are feathery. They are called surface hoar when deposited onto an existing snowpack and hoar frost otherwise. Strong wind is favorable for hard rime or rime mushrooms on mountains (Whiteman and Garibotti 2013).
In China, rime and glaze not only occur on ridges and mountaintops. Glaze is generally observed in southern China as results of freezing rain and drizzle. Rime is mostly observed in northern China or the mountainous areas above 1000 m (Fig. 2a) (Tan et al. 1985; Zhang 1991; Zhao et al. 2010; Wang 2011). In general, freezing fog or conditions with low temperature and high humidity are necessary for the occurrence of rime. However, for stations on ridges and mountaintops, cloud immersion with a temperature below freezing is the main cause of rime or glaze. According to the analysis of Wang (2011), the annual number of rime days is more than 1 in 40.2% of all observational stations. These stations are almost located to the north of the Yangtze River, especially in northern northwest China, northeast China, and north China. No or little rime is observed in the rest of stations. Stations with annual number of rime days above 5 account for 14.8% of all observational stations (Fig. 1a). On the other hand, in only 17.6% of the all observational stations is the annual number of glaze days above 1, and glaze is mainly observed between the Yellow River and the Yangtze River and in the region to the south of the Yangtze River valley (Fig. 1b). Thus, neither rime nor glaze icing is evenly distributed; they tend to occur in their preferred regions. In this paper, rime and glaze are collectively referred to as icing events.
In China, glaze often causes great losses by breaking electric or communication wires, damaging trees and crops, leading to freezing roads, and so on. Though rime is relatively light, the long-lasting or severe rime weather also results in similar impacts; for example, rime events always destroy the power facilities in the Three Gorges mountain area (Tan et al. 1985). However, previous studies have mostly focused on the microphysical process of icing, icing accretion conditions, and modeling and calculation of the radial thickness of ice; few studies have been conducted to explore the atmospheric and oceanic conditions of the occurrence and persistency of extensive icing events. Since the disastrous icing event that took place in early 2008, efforts have been devoted to characterizing the involved atmospheric circulation, oceanic background, and external factors. It is shown that, first, the abnormal atmospheric circulation over Eurasia, especially the abnormal development and maintenance of the Ural blocking high and the transverse trough from Lake Baikal to Lake Balkhash, was the direct cause of the widespread icing hazard in 2008 (Tao and Wei 2008; Ding et al. 2008; Zhao and Sun 2008; Li et al. 2008). Moreover, the unusually northward position of the northwestern Pacific subtropical high and the stable but active southern branch trough of the westerlies over the southern Tibetan Plateau provided moisture for the icing event (Yang et al. 2008; Gu et al. 2008; Gao et al. 2008). A combination of multiple anomalies of atmospheric circulation eventually led to the disastrous weather events in early 2008 (Li et al. 2008; Wen et al. 2009). Second, a La Niña event developed to its mature phase during the winter of 2007/08. Although the observed patterns of atmospheric circulation over the extratropics and the abnormal precipitation and temperature in China were different from the typical features associated with La Niña events (Li et al. 2008; Ding et al. 2008), the East Asian winter monsoon was much stronger than that normally associated with the La Niña event. The condition favored the development of low temperatures in China and provided important environmental conditions for the large-scale icing event (Ding et al. 2008; Zhang et al. 2008). Additionally, the northwestern Pacific subtropical high in winter was generally located farther north than normal during the peak season of La Niña, providing more moisture favorable for icing process (Zhang et al. 2012). Last, the Arctic Oscillation (AO) and the Middle East jet stream also played an important role in the temperature and precipitation anomalies over central-southern China. Indeed, the intensification and southward shift of the Middle East jet stream and the abnormally active AO might be other possible contributors to the freezing and icing weather event in China in early 2008 (D. Wang et al. 2008; Wen et al. 2009).
Previous studies have given a sound explanation for the causes of the China icing hazard in early 2008. However, one case alone is insufficient to make robust conclusions. It is necessary to analyze more events to describe icing climatology, atmospheric circulation related to the occurrence and persistence of icing events, and possible external impacting factors. Through identifying similar circulation patterns and signals of external factors from predicted fields, prediction of icing events may become possible.
2. Data and methodology
Daily data of glaze and rime from 743 Chinese meteorological stations during 1954–2009, compiled by the National Meteorological Information Center (NMIC), were used in this paper. The definitions and criteria of identification and observations of rime and glaze follow China Meteorological Administration (2003). The definitions of rime and glaze by the NMIC are almost the same with those in the American Meteorological Society glossary. The data indicate whether glaze or rime occurs on a certain day. Quality control of the data was provided by the NMIC. Stations with records less than 30 yr were eliminated and eventually the data from 669 stations were available for this analysis. The distribution of stations analyzed was given in Fig. 1. The daily temperature and precipitation datasets in the same stations and time period were also analyzed.
The National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis daily data from 1954 to 2009 were used in this study. The variables include sea level pressure, winds, geopotential height, and absolute and relative humidity (Kalnay et al. 1996). The monthly-mean sea surface temperature (SST) from the National Oceanic and Atmospheric Administration Extended Reconstructed Sea Surface Temperature (ERSST; Smith and Reynolds 2003) with a horizontal resolution of 2° in both latitude and longitude was also analyzed.
b. Criteria for icing event identification
If the number of stations where rime (or glaze) exists on two consecutive days reaches 20 (15 for glaze), an extensive rime (or glaze) event is identified. Otherwise, the event ends. If the interval between two extensive events (i.e., the time from the ending date of the first event to the starting date of the second event) is 3 days or less, the two events are merged with the starting date of the first event as the starting date of the merged event and the ending date of the second event as the ending date of the merged event. Finally, an extensive and persistent event is defined as an extensive event that lasts for at least 8 days. As shown in Tables 1 and 2, 28 rime events and 19 glaze events were identified, collectively referred to as icing events. An integrated index was derived from the mathematical mean of the standardized duration and number of stations affected for each event. Details about the criteria and events can be found in Wang (2014). It should be pointed out that the spatial continuity of stations with rime or glaze is not necessary to identify an extensive and persistent event given the uneven distribution of stations where rime or glaze occurs. Examination of the daily distribution of rime or glaze indicates that, during an event, two or more rime (or glaze) concentration zones were first seen in scattered regions. They developed, moved with a corresponding circulation pattern, and merged forming a rime or glaze event.
c. Case composite method and significance test
Case composite method was mainly used in this analysis. First, the departures of composite variables from climatology were calculated for the period of each rime (or glaze) event. Next, the mean departures of each period were derived arithmetically. Finally, the Student’s t test was used to assess statistical significance. The null hypothesis is that the mean value of the composite results is equal to zero. Based on this hypothesis, the t-statistic equation is established hereunder:
where is the composite departure of a variable, is the standard deviation of the departure, and N represents the sample size.
3. Distributions of icing events and related environmental conditions
a. Spatial distribution
Figure 2 shows the percentage of numbers of rime or glaze events to total numbers of both rime and glaze events at each station. It is clear that rime events mainly occur in northern China, because most stations to the north of the Yangtze River are affected by more than 20% of the total rime events. A ratio above 50% is seen in most of the stations in northern Xinjiang, central and southern northeast China, the middle reach of the Yellow River, north China, and the Huaihe River basin. In contrast, glaze events mostly occur in areas from the south of the Yangtze River to northern south China and present more evident regional characteristics. Most stations from the south of the Yangtze River to northern south China are affected by over 50% of the total glaze events. There are also glaze events in the areas from the middle-lower reaches of the Yellow River to the Huaihe River basin, but the frequency is obviously lower. The features shown in Fig. 2 are similar with those in Fig. 1, which is understandable because extensive and persistent icing events always affect the stations where rime or glaze occurs frequently.
b. Air temperature, precipitation, and relative humidity
As shown in Figs. 3a and 3d, during rime and glaze events the mean air temperature in most of China is significantly below normal. This indicates that low temperature is favorable for the occurrence of rime and glaze. It is also obvious that the amplitude of temperature anomalies is larger for glaze events than for rime events. This phenomenon is related to the atmospheric circulation that controls rime and glaze events, and details are discussed in section 4.
During rime events, precipitation is below normal in northeast China and parts of southwest China but above normal in most of the other regions (Fig. 3b). Similar patterns are also observed for glaze events (Fig. 3e). As rime usually occurs with fog but not with precipitation, the pattern of percentage of precipitation anomalies is not meaningful for rime events. On the contrary, glaze generally occurs with freezing rain or drizzle. To the south of the Yangtze River, where glaze is observed frequently, the composite percentages of precipitation anomalies do not exceed the confidence level, indicating that glaze events are not necessarily accompanied by above-normal precipitation. Thus, the above-normal precipitation observed in southern China during early 2008 was not typical for icing events.
As shown in Figs. 3c and 3f, the occurrence of icing events in China is accompanied by high humidity in the lower atmosphere. Regions with high humidity are mainly located in northern and eastern China when a rime event occurs. A high humidity zone, which is favorable for glaze events, is just located to the south of the Yangtze River.
Overall, the environmental condition with low temperature and high humidity favors the occurrence of rime and glaze events. As pointed out by Wang et al. (2011), a daily mean temperature range between −3° and 0°C and relative humidity ≥80% are most favorable for the occurrence of icing in China. Additionally, freezing rain or drizzle is always accompanied by glaze, but precipitation is not necessarily greater than normal.
4. Abnormal atmospheric conditions associated with icing events
During rime events, the middle and upper troposphere over Eurasia shows an anomalous pattern featuring “low in west and high in east” (Fig. 4a). A deep trough persists over the Asian continent to the south of 60°N, while a strong positive center of 500-hPa geopotential height anomaly appears over the Sea of Okhotsk, which reaches 40 geopotential meters (gpm). The center corresponds to the Okhotsk blocking high, showing that cold air is accumulated over the region. This pattern creates a favorable condition for cold air to move southward and leads to severe icing weather in northern China when moisture is adequate. For glaze events, the 500-hPa anomalous geopotential height shows development of a strong Ural blocking high with an unusual pattern featuring “low in south and high in north,” similar to the icing event in early 2008. As shown in Fig. 4d, almost all of extratropical Eurasia is dominated by the strong Ural blocking high, oriented nearly zonally, with positive center values over 120 gpm. On the other hand, the Asian continent is dominated by a deep trough, centered over western northwest China with central values below −90 gpm. Steered by the East Asian trough, the cold air accumulated over the Urals breaks out toward East Asia, leading to a sharp temperature drop in China and providing low-temperature conditions and cold air for a glaze event. When the cold air meets with the strong warm and moist flow from the lower latitude, icing events may occur. In addition, only with such a strong blocking high and the deep trough can cold air intrude southward to the lower latitudes and affect southern China. In brief, blocking highs are important for icing events in China, with the Ural blocking high affecting glaze events and the Okhotsk blocking high influencing rime events.
As seen from Fig. 4b, when a rime event occurs, a strong abnormal low-level cyclonic circulation appears over East Asia. It corresponds to the large-scale trough over the Asian continent, indicating that the system is deep and well developed. Under its influence, the subtropical westerlies around 20°N are unusually stronger than normal, favorable for the eastward transport of water vapor from the Arabian Sea, the Bay of Bengal, and the South China Sea. An abnormal anticyclone exists from the Indochina Peninsula to the Maritime Continent, and another abnormal anticyclone appears over the subtropical western Pacific Ocean, showing that the northwestern Pacific subtropical high is stronger and is located farther northward than normal. Additionally, anomalous easterlies prevail over the mid–high latitudes of East Asia, spurring strong moisture transport from the East China Sea, the Sea of Japan, and the northwestern Pacific to China. During rime events, the southerlies are so strong that they reach higher latitudes and meet with the cold air over northern China.
However, the situation is different for glaze events. As shown in Fig. 4e, strong anomalous northerlies control most of eastern East Asia, presenting a stronger-than-normal East Asian winter monsoon and intense southward outbreak of cold air. Thus, the warm and moist flow does not extend far north, and warm flow meets with cold air to form glaze events over southern China.
Inversion layer is a unique feature for icing. When there is an inversion layer, a “cold–warm–cold” vertical temperature distribution can be observed in the atmosphere. Snow and ice crystals fall from the cold upper layer into the warm middle layer and melt into water drops. When these water drops reach the cold layer near the surface, they are cooled rapidly. However, they remain in the liquid status instead of being frozen before contacting objects at the surface because of their tiny dimensions and rapid falling. Finally, the water drops reach the surface and form rime or glaze. During a rime event (Fig. 4c), a large-scale inversion layer exists over eastern China. It centers on the Huaihe River basin, north China, and northeast China, and the difference in temperature between the 925- and 500-hPa levels can reach −1.5°C. For a glaze event (Fig. 4f), the inversion layer is even more extensive and stronger. As both eastern and northern China are dominated by an inversion layer, the difference in temperature can be as large as −7°C over the middle-lower reaches of the Yangtze River.
The above analysis indicates that at least four major abnormal circulation systems control the occurrence and persistence of icing events in China. They include blocking patterns, the Asian low, inversion layer, and strong abnormal southerlies that transport warm moisture northward. However, differences are found between rime and glaze events. For rime events, the Okhotsk blocking high is dominant. Southerlies with northward transport of moisture are particularly strong, reach higher latitudes, and meet with the cold air over northern China. On the contrary, for glaze events, a strong and stable Ural blocking high, instead of the Okhotsk blocking high, allows the outbreaking cold air to reach lower latitudes, while the southerlies and the northward moisture transport from the mid- to lower latitudes are comparatively weaker. Cold and warm flows merge over southern China. In other words, the northward transport of warm and wet flow may play a leading role and the outbreak of cold air plays a supporting role for rime events. In contrast, the glaze events in China are dominated by the outbreak of cold air toward the south, merging with the northward transport of warm and moist air. Differences in abnormal patterns, intensity of atmospheric circulation systems, and interaction between the lower and higher latitudes may give rise to the distinctions in location, climatology, and physical characteristics between rime and glaze events.
By comparison, it can be noted that the intensity of the blocking highs and the Asian low at 500 hPa, the anomalous northerlies at 850 hPa, and the inversion layer are all much stronger for glaze events than for rime events. These differences in atmospheric circulation cause larger negative temperature anomalies for glaze events than for rime events, as shown in section 3.
5. Oceanic background for icing events
Figure 5a shows the distribution of composite SST anomalies (SSTA) during the rime events in China. Significant negative anomalies are observed over the equatorial eastern and western Pacific, and positive but insignificant anomalies are over the central equatorial Pacific. This pattern is similar with the developing or decaying phase of La Niña, with a “− + −” pattern from the tropical Indian Ocean to the South China Sea, the central equatorial Pacific, and the eastern Pacific. As a response, ascending flow occurs over the warm oceanic areas, and descending flow occurs over the cold oceanic areas. Because of the effect of the Coriolis force, a cyclone (an anticyclone) forms in the low-level atmosphere over the warm (cold) oceanic areas. Based on the SSTA pattern shown in Fig. 5a, ascending flow should be over the central equatorial Pacific and descending flow over the eastern and western Pacific. The Bay of Bengal, the South China Sea, and the northwestern Pacific should be dominated by an anomalous anticyclone, favorable for the northward transport of warm moisture from the oceans to support icing events in China. Obviously, all these features are consistent with those observed in Fig. 4b, suggesting the important impact of oceanic background on icing events in China. The distribution of SSTA during glaze events shows similar features (Fig. 5b). However, the areas exceeding the 95% confidence level are much smaller than those for rime events. Furthermore, the SSTA pattern during glaze events is more like the pattern of the mature phase of La Niña, as negative SSTAs extend to the central Pacific. Overall, La Niña is favorable for the occurrence and maintenance of extensive and persistent icing events in China. Previous studies (Mu and Li 1999; Chen 2002; Zhang et al. 1996) have pointed out that, when La Niña develops, the East Asian winter monsoon is stronger than normal and the meridionality of the atmospheric circulation in mid–high latitudes increases. Then, cold air breaks out southward frequently. This atmospheric response connects La Niña with icing events in China.
To further understand the impact of oceanic background on icing events in China, the SSTA pattern for each glaze or rime event is also analyzed. As shown in Tables 1 and 2, 15 rime events occurred under the background of El Niño, while 13 rime events were observed under the background of La Niña. For glaze, 7 events occurred during El Niño and 12 events during La Niña. This feature proves that glaze and rime events can occur under either La Niña or El Niño. However, La Niña is more favorable for occurrences of glaze events, as in the early 2008 event. Additionally, as shown in the above composite patterns, the SSTA extending from the tropical Indian Ocean to the tropical northwestern Pacific likely plays an important role. Statistical analysis shows that, among the 28 rime events, 9 occurred with positive SSTA over the region and 19 with negative SSTA. Among 19 glaze events, 7 were observed when the SSTA was positive, while 12 events were under the condition of negative SSTA. Thus, the negative SSTA from the tropical Indian Ocean to the tropical northwestern Pacific favors the occurrence and maintenance of both rime and glaze events in China. This pattern of SSTA may affect the icing events in China by exciting anomalous anticyclones from the Bay of Bengal to the subtropical northwestern Pacific and favoring eastward and northward shifts of warm and moist westerlies and southerlies, which provide abundant water vapor for the icing events in China.
An analysis of 28 rime events and 19 glaze events in China indicates that rime events occur mostly in northern China, especially in northern Xinjiang, central and southern northeast China, the middle reach of the Yellow River, north China, and the Huaihe River basin. However, glaze events are mainly observed from the south of the Yangtze River to northern south China. These icing events are accompanied by low temperature and high humidity, but not necessarily by above-normal precipitation.
Blocking highs, the Asian low, strong warm and moist water vapor transport, and an inversion layer are the four essential conditions for the formation and maintenance of icing events in China. The blocking highs contribute to the accumulation and southward outbreak of cold air. The abnormal cyclonic circulation corresponding to the Asian low is favorable for transporting abundant moisture to China. Strong warm and moist water vapor transport ensures a moist condition for icing events, and inversion layer allows formation of ice, instead of precipitation. Among other conditions, the Ural blocking high mostly affects glaze events, while the Okhotsk blocking high is closely related to rime events. Overall, the glaze events in China are dominated by extratropical abnormal circulation and outbreak of cold air, accompanied by warm and moist flow from the lower latitudes. On the contrary, the strong northward transport of warm and moist flow from the lower latitudes plays a leading role while the extratropical abnormal circulation pattern only plays a secondary role in rime events. Differences in patterns and intensity of abnormal atmospheric circulation may lead to the distinctions in types, locations, and physical characteristics of icing events in China.
There is a nearly equal chance for occurrences of rime events under either La Niña or El Niño background. However, glaze events more likely occur under the background of La Niña. Additionally, the SSTAs from the tropical Indian Ocean to the tropical northwestern Pacific also contribute to the occurrence and maintenance of icing events in China.
The authors thank the editor and three anonymous reviewers whose comments were helpful for improving the overall quality of the paper. This research was funded by the National Basic Scientific Research Plan of China (Grants 2014CB953900 and 2012CB955901), the National Natural Science Foundation of China (Grants 40905036 and 41375081), and the LASW State Key Laboratory Special Fund (2013LASW-A05).