Meteorological Characteristics of a Continuous Ice-covered Event on Ultra-High Voltage Transmission Lines in Yunnan Region in 2021

: Yunnan plays a pivotal role in transmitting electricity from west to east within China's Southern Power Grid. During January 7-13, 2021, a large-scale continuous ice-covering event of ultra-high voltage (UHV) transmission lines occurred in Qujing area in eastern Yunnan Province. Based on ERA5 reanalysis data and meteorological observation data of UHV transmission line icing in China Southern Power Grid, the synoptic causes of the icing are analyzed comprehensively from the aspects of weather situation, vertical stratification of temperature and humidity, local meteorological elements, and atmospheric circulation indices. The results show that the East Asian trough is strong, and the blocking high leads the northern air flow in front of the ridge to the south, and the cold air enters the ridge and joins the warm and wet air brought by the subtropical high pressure of 50-90 ° E. When the warm and cold air masses confront each other to form a quasi-stationary front over the northern mountains of Qujing, the alternating dominance of the warm and cold air is the main reason for the repeated ice accumulation of the power lines. Indices such as the Siberian High, East Asian Trough, and 50-90 ° E Subtropical High provided clear precursor signals within 0-2 days in advance of the icing events.


Introduction
The main meteorological disasters (road icing, ice accumulation on power lines, crop frost damage, etc.) in winter in the south are caused by the persistent freezing rain and snow during the cold wave [1] .
Electricity is the driving force behind the rapid and healthy development of the national economy.
Overhead line icing refers to the weather phenomenon where freezing rain, freezing fog, or wet snow freezes on the power lines.The main environmental factors contributing to this phenomenon include quasi-stationary fronts, atmospheric vertical structures, and inversion layers.It is also influenced by terrain, altitude, and the power lines.Overall, the distribution of icing disasters on power lines shows a pattern of more rime in the north and more silver thaw in the south [2] .Overhead line icing has always been one of the most serious meteorological disasters for UHV power transmission lines in southern China, posing a direct threat to the operation and maintenance of these power lines.
China experiences frequent incidents of wire icing, particularly during the winter months.In January and early February 2008, southern China confronted an unprecedented period marked by extremely low temperatures, freezing rain, and extensive snowfall.Continuous freezing rain and freezing fog led to long-lasting and extensive wire icing in the southern power grid, with maximum ice thickness exceeding 100 mm [3] .It caused significant disasters in 20 provinces (districts), such as Hunan, Guizhou, Guangxi, and Hubei, affecting over 100 million people and resulting in direct economic losses of over 150 billion RMB [4] .During the freezing rain and snow event in southern China, a total of 506 transmission towers in the 500 kV transmission lines collapsed, and transmission tower structures at various levels of the power grid suffered severe damage [5] .Extreme freezing rain and snow weather events can bring about significant harm and it is of great importance to analyse the causes of the ice accumulation.
Currently, many researchers have conducted extensive studies on the freezing rain and snow weather in southern China, focusing on various aspects such as atmospheric circulation, local meteorological elements, thermodynamic structure, and moisture transport [1] .For example, previous research has analyzed the causes of the low-temperature freezing rain and snow weather events at the end of December 2020 and the beginning of January 2021 by examining atmospheric circulation anomalies.Both cold wave weather events occurred under the influence of the "two troughs and one ridge" circulation pattern.During these cold wave processes, there was a transition from a west-east trough to a north-south trough, which rapidly transported cold air southward.Furthermore, the continuous southward displacement of polar vortices and the presence of a blocking high-pressure system near the Ural Mountains played a crucial role in facilitating the deep southward movement of the cold air [6] .Additionally, Zhu Jun et al [7] emphasized the climatic background and summarized the meteorological and climatic characteristics, meteorological element conditions, and the relationship between geographical environmental factors and wire icing in Guizhou from 2008 to 2011.They found that Guizhou's wire icing is closely related to anomalies in mid-high latitude atmospheric circulation, the northward shift of the western Pacific subtropical high, and the influence of the lower-tropospheric inversion layer.Furthermore, regarding moisture transport, the freezing rain and snow weather event in Guizhou in 2008 was influenced by an Ω-shaped blocking pattern in the mid-high latitude westerlies.The subtropical high-pressure system was positioned to the west-north, allowing cold air to move southward.A quasi-stationary front was maintained in Guizhou for an extended period, facilitating the northward transport of moisture by southward-flowing air currents [8] .In Yunnan, during the wire icing in January and February 2008, the quasi-stationary front near Kunming was the most significant influencing weather system [9] .Additionally, some researchers used micro-meteorological factors as input data and icing mass as output, constructing analytical models and non-analytical models from polynomial regression, time series analysis (including stationary and non-stationary analysis), and machine learning perspectives [10][11][12][13][14][15] .However, these new artificial intelligence-based input factors are closely linked to the previous exploration of the physical mechanisms affecting icing.
From January 7th to January 13th, 2021, a cold wave invaded, as cold air descended southward, it met with warm and moist airflows from the southwest.During this cold wave event, the main areas affected by wire icing were concentrated in the eastern part of Yunnan and the central-northern and western regions of Guizhou.Multiple UHV transmission lines in Qujing Yunnan and other regions, experienced icing, which had adverse effects on power transmission and communication.This prolonged period of temperature drop had a wide-reaching impact, particularly on the transmission lines.Yunnan is a crucial channel for transmitting electricity from west to east in China's southern power grid.Qujing, a city with a high-altitude mountainous terrain in Yunnan, experienced rapid ice accumulation and significant icing thickness on the Kunliulong transmission line.This section of the Southern Power Grid is considered an area with special anti-icing requirements due to its unique characteristics.Often, there was a temperature difference of 6-8 ℃ between the actual on-site temperature and the weather forecast temperature.Even after manual de-icing efforts, the lines experienced repeated icing, posing considerable challenges for the power department.Therefore, this study takes this icing event as an example and systematically investigates the meteorological factors contributing to the icing of UHV transmission lines.A comparison is also made with the icing event that occurred in Guangxi in 2015.

Data and Methods
This study focuses on typical transmission line towers, denoted as A-H (specific tower names are replaced with A-H to protect power security information), located in the mountainous areas of Fuyuan, Zhanyi, and Huize in eastern Qujing, Yunnan Province.These towers have experienced severe and rapid ice accumulation over the years.Their geographical coordinates range from approximately  The temperature and ice thickness data used in this study were obtained from the UHV transmission line icing observation system.Data from the fifth generation of the European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis of the global climate (ERA5) for the period from January 5th to January 14th, 2021 were used to analyze the precipitation conditions in the tower area, the weather situation during the icing process, and the temperature-humidity vertical structure.The data covers the range of 40-160°E and 10-70°N with a resolution of 0.25°×0.25°.
To identify large-scale circulation precursor signals for UHV transmission line icing, this study also calculated several atmospheric circulation indices, including the Siberian High-Intensity Index, East Asian Trough Intensity Index, and Subtropical High Index at 50~110°E.The Siberian High-Intensity Index is calculated as the standardized sea-level pressure over the region of 80-120°E and 40-65°N [16]   .The East Asian Trough Intensity Index is calculated from the standardized 500 hPa height field over the region of 110-145°E and 25-45°N [17] .Due to the high-pressure system that affected Qujing, Yunnan, being located west of 110°E, the subtropical high-pressure index was calculated for the range of 10-70°N and 50-110°E.The area index is defined as the number of grid points within this region where the 500 hPa geopotential height is greater than 588 gpm.The intensity index is the cumulative difference between grids with potential heights >588 gpm and 587 gpm.The western ridge point is determined as the longitude of the westernmost location where the 588 gpm contour of geopotential height is found.

3.1The icing process overview
Fig. S1 provides an overview of the dual icing events that were observed from January 7th to January 13th, 2021.Table S1 presents data concerning the maximum ice thickness, time of occurrence, temperature, and humidity for the first stage (January 7th to January 10th) and the second stage (January 10th to January 13th) of icing for eight tower pylons designated as A to H. Icing was initiated at approximately 08:00 on January 7th (hereafter all time zones are provided as UTC+8).During this period, a maximum ice thickness of 26.44 mm was attained by Pylon A on January 9th, with an average rate of ice growth of 0.57 mm/h and a minimum temperature of -5.7 ℃.Pylon B reached a maximum ice thickness of 11.71 mm, exhibiting an average growth rate of 0.71 mm/h.Pylon E reached a maximum ice thickness of 31.52 mm, exhibiting an average growth rate of 0.52 mm/h, and with 100 % relative humidity.Details for the other pylons are outlined in Table 1.To ensure the safety of power transmission lines, a "direct current de-icing" operation was implemented.However, due to the persistent influx of frigid air, the transmission lines experienced a subsequent icing event on January 10th.Pylon A's ice thickness reached 27.61 mm, and the temperature dropped to -8.00 ℃.As the intensity of the cold air abated and relative humidity diminished, resulting in reduced moisture content in the atmosphere, a gradual ice melt was observed by January 12th.Due to micro-topographical influences, there were variations in the initial icing onset times and times to reach maximum icing thickness among the eight pylons.Additionally, de-icing operations were carried out upon achieving a specific ice thickness threshold.However, the second ice accumulation exhibited relatively milder conditions, with some of the ice naturally melting.The temperature of the second stage was lower than that of the first stage.

Analysis of the atmospheric circulation pattern during the icing process
On January 5th, 2021, at 08:00, the large-scale geopotential height field at 500 hPa showed a "ridge-trough" pattern in the Eurasian middle and high latitudes (Fig. 2).To the east of the Urals, there was a high-pressure ridge, and there were two cold vortexes, one over Lake Baikal and one over the Okhotsk Sea.The Western Pacific subtropical high was shifting northwestward.By January 6th, 2021, at 08:00, the cold vortex over Lake Baikal had strengthened and moved eastward, pressing southward into the northeastern region of China.This led to the formation of a blocking high-pressure system, and due to the northwest airflow behind the trough, cold air was continuously transported southward.The southward movement of the trough in the southern branch weakened, and the Western Pacific subtropical high shifted eastward.On January 7th, the northeastern cold vortex further intensified, leading to increased wind speeds.The horizontal trough transformed into a vertical one and the southwestern airflow ahead of the southern trough transported maritime moisture onto the continent.
By January 8th, the northeastern cold vortex weakened and continued to move eastward, eventually merging with the cold vortex over the Sea of Okhotsk.The southern trough weakened and transformed into a high-pressure ridge around 90 °E while still carrying maritime moisture.On January 9th, the blocking high-pressure system began to gradually break down, allowing cold air to continue moving southward.On January 10th, the blocking high-pressure system completely broke down, and a low-pressure system from Mongolia moved eastward and southward.The trough continued to deepen, with Qujing located ahead of it.A new surge of cold air moved eastward and southward.By January 11th, the trough deepened further as it moved eastward.Qujing remained situated behind the trough, experiencing northwest airflow.The Iranian high-pressure system shifted eastward and northward.On January 12th, the trough continued to move eastward over the sea, weakening.Qujing remained under northwest airflow behind the trough.On January 13th, a high-pressure system gradually strengthened in the 70-90 °E region, and a blocking high-pressure system began to form, making it increasingly difficult for the cold air to move southward.Fig. 3 shows the 750 hPa potential height field and temperature field, where the contour line is the potential height field, the color filled in is the temperature field, and the vector field is the water vapor flux.From January 5th to 6th, influenced by the blocking high-pressure system over the Sea of Okhotsk and Siberia, and from January 7th to 8th, as the trough associated with the Sea of Okhotsk cold vortex gradually shifted from a horizontal to a vertical orientation, the first round of cold air moved southward from the north.The Qujing area in Yunnan was located near the frontal zone, with dry and cold air to the north of the front and warm, moist air to the south.These two air masses, with opposing characteristics, converged in the airspace above Qujing.When the cold air mass prevailed over the warm air mass, a rapid temperature drop occurred.This led to the freezing of supercooled liquid droplets or supercooled fog droplets, resulting in the icing of power lines.Conversely, when the warm air mass was dominant over the cold air mass, temperatures rose, causing the ice to melt.From January 10th to 11th, the Yenisei River cold vortex shifted westward.The trough ahead of it guided another round of cold air southward along the southeastern side of Xinjiang, Qinghai, and the eastern Tibetan Plateau, affecting Qujing in Yunnan.By January 12th, these two cold air masses converged, leading to prolonged cold temperatures.From January 13th to 14th, the subtropical high-pressure system strengthened, and weak warm advection moved northward.The cold vortex over the Sea of Okhotsk shifted northward and weakened, allowing warm air to dominate the Qujing area in Yunnan.The early stages of the cold air process from January 7th to January 13th, 2015, were characterized by the transformation of a horizontal trough into a vertical trough, which deepened southward.This shift in weather patterns led to a significant influx of cold air, resulting in the outbreak of frigid weather.As the blocking high-pressure system gradually broke down and the Mongolian low-pressure system moved eastward and southward, the trough continued to deepen.Additionally, a new surge of cold air developed from the southeast of Xinjiang and the Tibetan Plateau.The convergence of these two cold air masses intensified the cooling effect.As the Yenisei River cold vortex moved westward, it brought a weak cold air mass that influenced the Qujing area in Yunnan.
Additionally, the southern flow behind the high-pressure system transported moisture from the sea northward.In the first stage, there was a strong cold air mass, and a significant amount of moisture was transported from the south to the north.In the second stage, although the cold air mass was still strong, the center of the Indian high-pressure system shifted towards the southwest, resulting in less moisture transport.Therefore, in the first stage, the icing on the power lines occurred rapidly, leading to a greater ice thickness, while the opposite was observed in the second stage.

Comparison of ice accumulation temperature and humidity conditions
The temperature and humidity conditions of the ice-covering process are analyzed in the Supplementary Information, and a comparison of the temperature and humidity conditions of the two phases is presented below.Fig. 4 and 5 depict the latitudinal temperature and humidity vertical profiles at the peak times of maximum ice accumulation during the first stage (January 9, 08:00) and the second stage (January 11, 03:00).In these figures, red arrows represent warm, moist air, while blue arrows indicate dry, cold air.Qujing exhibits a 'cold-warm-cold' vertical structure in both stages, which is considered a typical vertical pattern for freezing rain weather conditions.The presence of a pronounced inversion layer in the atmospheric structure and abundant moisture are conducive to the growth and maintenance of ice accumulation on power lines.In the second stage, the inversion layer is deeper compared to the first stage, and the southward-moving moist and cold air is stronger than in the first stage.Additionally, the temperature in the second stage is lower.However, the moisture content is not as abundant as in the first stage.Therefore, the ice accumulation in the second stage soon melts with the warm air sinking and warming.

Correlation analysis of ice thickness and large-scale atmospheric circulation indices
The strong cold air brought by the Siberian high-pressure system invades southern China.When the cold air in the front of the high pressure meets the warm air from the south, if the cold air is stronger than the warm air, it forms a cold front.The cold surge brought by the cold front has a significant impact on the winter cooling process in China.When the high-pressure ridge at 70-90°E is strong and eastward in winter, the southwest airflow in the rear of the high-pressure ridge guides warm and moist airflow to the southwestern region.In areas with higher terrain, the accumulation of cold air forces the warm and moist air masses to rise, forming a quasi-stationary front, making it easier for power lines to accumulate ice.Therefore, the strength of the cold air is judged by the Siberian high-pressure index and the East Asian trough intensity index, and the strength of the warm and moist air is judged by the high-pressure index, to analyze the variation of cold and warm air during the icing process.Fig. 6 shows the changes in various indices during the icing period.In this context, a high Siberian high-pressure index corresponds to a strong Siberian cold high-pressure system, and a low East Asian trough index corresponds to a strong East Asian trough.In the pre-icing period, the Siberian high-pressure index decreased, and the intensity of the East Asian trough continued to strengthen.In the second stage, the Siberian high-pressure was weaker compared to the first stage, but the intensity of the East Asian trough was stronger.Overall, during the second stage, the cold air was stronger than in the first stage.Among the various indices related to the high-pressure system at 70-90°E, except for the Low Western Ridge Point Index, which corresponds to the high-pressure system shifting westward, all other high indices indicate a strong high-pressure system.It can be observed that during the icing period, the high-pressure system was shifted eastward and higher in intensity.It is because the source of moisture transport to Qujing during this icing event was mainly influenced by the high-pressure ridge at low latitudes in the 50-90°E region.It is quite different from the dominant moisture factors in the 2015 icing event in Guangxi.
To further investigate the relationships between the Siberian High, East Asian Trough, 50-90 °E Subtropical High, and ice cover thickness, an analysis of their lead-lag correlations is presented in Fig.

7.
The results indicate that when the East Asian Trough's intensity and the area of the subtropical high are leading by one day, their correlations with the maximum ice cover thickness are the lowest (correlation coefficients of -0.86 and -0.83, respectively).On the other hand, when the 850 hPa subtropical high-intensity index leads by two days, it exhibits the lowest correlation (correlation coefficient of -0.82).Conversely, the subtropical high intensity, Siberian High, and subtropical high ridge points show the highest correlations with ice cover thickness when leading by one day (correlation coefficients of 0.46, 0.92, and 0.86, respectively).Overall, the analysis suggests that various indices can provide relatively clear precursor signals for the daily maximum ice cover thickness when leading by 0 to 2 days in advance.
In the case of the power line icing process in eastern Qujing, Yunnan Province, we found that the  (1) In terms of large-scale circulation patterns: The early part of this cold air process is a horizontal trough turning vertical.The Northeast cold vortex rapidly moved eastward into the sea, and its strength led to the continuous rebuilding and eastward movement of the East Asian trough.As the blocking high-pressure system collapsed and reformed, cold air descended southward from the east side of Xinjiang and the Tibetan Plateau.The 50-90°E subtropical high-pressure system, which was eastward and strong, guided the southwest airflow to transport warm and moist air from the ocean to the southwestern region of China.Over Yunnan, the forces of cold and warm air masses were roughly equal, leading to their confrontation.The dominance of either the cold or warm air mass caused the front to oscillate back and forth.Under the influence of mountain ranges and the flow field, this led to the formation of a quasi-stationary front.When the cold air mass dominated and there was abundant moisture, it favored ice accumulation on power lines, while when the warm air mass dominated, it contributed to ice melting.
(2) In terms of the vertical temperature and humidity structure: During the initial stage of icing formation, there is a strong warm and moist airflow moving northward in the upper part of the boundary layer.Meanwhile, near the surface, cold air is moving southward and creating an inversion layer.In the first stage, this inversion layer persists for a longer duration compared to the second stage.
Additionally, in the first stage, the cold air is stronger, and there is more abundant moisture.Therefore, the icing thickness is slightly greater in the first stage than in the second stage.
(3) In terms of the local meteorological elements: From the perspective of temperature and water vapor flux, the second stage has lower temperatures, while the first stage has a higher water vapor flux directed towards the Qujing area.In the early stages of both icing phases, precipitation occurs.In high-altitude, cold mountain areas where the temperature is below 0 ℃, the raindrops freeze into a transparent or semi-transparent layer of ice on the power lines.Water vapor in the air can also directly condense, contributing to icing growth and affecting the rate of icing growth.
(4) In terms of the atmospheric circulation indices: The intensity of cold air is characterized by the Siberian High Pressure Index and the East Asian Trough Index, and the intensity of warm and humid air is characterized by the Subtropical High Index.During the icing period, the Siberian High and East Asian Trough exhibited heightened intensity, while the Subtropical High also leaned toward being strong with an eastward bias.Indices such as the Siberian High, East Asian Trough, and 50-90 °E Subtropical High provided clear precursor signals within 0-2 days in advance of the icing events.
(5) Comparison with the cause of ice cover in Guangxi District in 2015: Although the driving factors for cold air were both influenced by the East Asian trough, there was one more cold vortex in the pre-formation period of the East Asian trough in January 2021.In January 2021, the moisture source for Yunnan's Qujing region was mainly the oceanic moisture carried by the 50-90 °E subtropical high, while in January 2015, the moisture source for the mountainous areas of Guilin, Guangxi, was mainly the oceanic moisture carried by the Western Pacific subtropical high.
During this event, the mountainous areas near the front zone in Qujing were subjected to the combined effects of cold air and warm, moist airflow, leading to the formation of ice accumulation on power lines.Due to the quasi-stationary nature of the front, it was not entirely stationary, and its movement influenced the icing conditions on the power lines.When cold air prevailed over warm air and moisture was abundant, it was more favorable for ice accumulation on the power lines.Conversely, when warm air dominated over cold air, it led to a higher likelihood of ice melting.It can be observed that during the icing period, the process was primarily driven by cold air, while the ice melting in the second stage was dominated by warm air.
This paper provides initial guidance on ice accumulation prediction using the example of transmission line ice accumulation in the mountainous area of Qujing in 2021.In the future, a more comprehensive analytical method will be established, combined with multi-modal forecasting, to improve the accuracy of ice cover prediction and provide scientific and technological support.
103.33 °E to 104.34 °E and 25.74 °N to 26.01 °N.From January 7th to 13th, 2021, due to the influence of a southern trough and cold air, Qujing experienced a significant temperature drop.It resulted in a continuous icing event on the UHV transmission lines in the mountainous areas of Fuyuan, Zhanyi, and Huize.The region with severe ice accumulation is mainly concentrated along the Kunliulong Line, with the probability distribution of the maximum ice thickness shown in Fig. 1.

Figure 1 .
Figure 1.The probability distribution of the maximum ice thickness of transmission lines in Yunnan during the continuous icing process from January 7th to January 13th, 2021.

Figure 2 .
Figure 2. The 500 hPa large-scale circulation pattern (daily 8:00 AM from January 5th to January 14th, 2023).The green dots represent the areas where power transmission lines experienced icing, and the brown solid lines represent the locations of low-pressure troughs."G" represents high-pressure centers, and "D" represents low-pressure centers.

Figure 3 .
Figure 3. 750 hPa large-scale circulation situation field.Contours are potential height fields, filled colors are

Figure 4 .Figure 5 .
Figure 4.The latitudinal temperature (a) and humidity (b) vertical profiles on January 9th at 08:00 during latitudinal distribution of vertical temperature and humidity layers more intuitively reflects the strength and characteristics of cold and warm air masses.During the first stage, a well-hydrated cold air mass resulted in thicker power line icing.Furthermore, we have identified the lead-lag correlations between various atmospheric circulation indices and icing thickness.Indices such as the Siberian High, East Asian Trough, and 50-90 °E Subtropical High provided clear precursor signals within 0-2 days in advance of the icing events.Through this case analysis, we can contemplate developing a more precise method for ice accumulation prediction by considering factors like vertical temperature and humidity profiles, the intensity of cold and warm air masses, and the attributes of dry and moist air masses.

Figure 6 .Figure 7 . 5 Conclusion
Figure 6.The changes in various indices during the icing process (horizontal axis represents 08:00 Beijing time)