Maintenance and development of the Ural high and its contribution to severe cold wave activities in winter 2020/21

Jingei Peng , , Shuqing Sun , Bomin Chen

a International Center for Climate and Environment Sciences, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China

b Shanghai Climate Center, Shanghai, China

Keywords:Winter 2020/21 Severe cold wave Ural ridge Energy dispersion Quasi-stationary wave

ABSTRACT Two successive severe cold waves invaded eastern China from the end of 2020 to early 2021, leading to an extensive, severe, and persistent drop in temperature. The paper investigates the features and formation mechanisms of the two cold waves. The main results are as follows: (1) An anticlockwise turning of the transverse trough was observed in both cold waves. However, a broad ridge was maintained over the Ural area from mid-December 2020 till mid-January 2021. No breakdown or discontinuous westward shift of the blocking high was observed,which is different from typical cold waves in eastern Asia. (2) The maintenance and strengthening of northerly winds in front of the Ural high led to an increase in baroclinicity in-situ. In the downstream region, the gradient of the geopotential height contour in the south of the transverse trough rapidly increased and the advection of cold temperature consistently enhanced and advanced southwards. This in turn caused the intensification and southward expansion of the Siberian high. (3) Energy propagation of the quasi-stationary wave was a reason for the development and persistence of the Ural blocking. Prior to the occurrence of the two cold waves, the energy of the low-frequency stationary wave originating from near 0°E (or even to the west) propagated eastwards, which helped the Ural ridge intensify and maintain. Meanwhile, it also contributed to the development of the trough downstream of the ridge and resulted in the anticlockwise turning of the transverse trough, providing a favorable condition for the southward outbreak of cold air.

1. Introduction

Cold wave activities are important weather processes that influence the climate of China in winter. Extensive studies on the sources and routes of cold air, key circulation systems and relevant external forcing of cold waves in eastern Asia have been carried out ( Tao, 1959 ;Qiu and Wang, 1983 ; Qiu and Zhao, 1983 ; Ding, 1990 ; Xie et al., 1992 ;Park et al., 2011 ; Wang and Chen, 2014 ). In particular, a common feature of most cold waves invading eastern Asia is the breakdown or discontinuous retreat of the Ural high, leading to an anticlockwise turning of the transverse trough downstream, which has long been recognized( Zhu et al., 1992 ).

Two successive severe cold waves invaded eastern China from the end of 2020 to early 2021, leading to an extensive, severe, and persistent drop in temperature and considerable effects on the economy and daily lives of people. Their features and formation mechanisms have received widespread attention ( Zheng et al., 2021 ; Dai et al., 2021 ; Wang et al.,2021 ).

Regarding the features of large-scale circulation, the existence and activity of a Ural blocking high plays a very important role in the outbreak of cold waves in eastern Asia. Its development and decay not only influence the orientation of the trough downstream, but also closely relate to the evolution of the Siberian high and cold waves. However, no breakdown or discontinuous retreat of the blocking high was observed in the present case, as demonstrated later in the paper. On the contrary,during the period from mid-December 2020 to early January 2021, an intense high-pressure system was consistently maintained around the Ural Mountains. How, then, did this persistent Ural blocking or ridge affect the process downstream, causing the deepening of the trough to its east? How did the ridge maintain and strengthen? And what are the possible underlying physical mechanisms? This study attempts to answer these questions.

2. Data and methods

The data used were: (1) daily observations from the NCEP—NCAR reanalysis ( Kalnay et al., 1996 ), consisting of geopotential height at 500 hPa and winds and temperature at 1000 hPa for the period December—January 1951—2021; (2) surface minimum temperature at 699 observation stations in China, provided by the National Meteorological Information Center (available to downloaded from http://data.cma.cn/data/cdcdetail/dataCode/SURF_CLI_CHN_MUL_DAY_V3.0.html ). The climatological annual cycle was calculated for the period 1981—2010. In order to investigate the relationship between high-frequency perturbations and low-frequency weather systems, Lanczos filtering (cut-offperiod of eight days) was adopted( Duchon, 1979 ).

The eastward propagation of wave energy is a source of energy for the development of a downstream trough (or ridge). Estimating the wavelength of the stationary waveLswill help determine whether the energy is favorable for the development of a trough or ridge at a specific longitude ( Yeh, 1949 ; Enomoto et al., 2003 ). For a barotropic and nondivergent atmosphere,Lscan simply be estimated byLs=whereuis the zonal wind andβis the meridional variation of Coriolis parameter. We used this formula to estimate theLsfrom mid-December 2020 to mid-January 2021.

3. Severe cold waves and associated circulation features

Two successive cold waves are investigated in this paper —namely,that which occurred during 28—31 December 2020, and the other being that which took place during 6—8 January 2021. Fig. 1 shows the temperature drops during the total process of each of the two cold waves.The isoline denotes the difference between the minimum temperature in the cold wave process and that of the day just before the outbreak of the cold wave. The red spots represent the stations with minimum temperature equal to or lower than the historical record. It can be seen that the area where the temperature drop exceeds 12°C extends far to the south of the Yangtze River basin for the first cold wave ( Fig. 1 (a)).And for the second cold wave, the stations with minimum temperature lower than the historical record are spread over a vast area ( Fig. 1 (b)).

Fig. 1. Temperature drop during the entire process of the two cold waves: (a) 28—31 December 2020; (b) 6—8 January 2021 (units: °C). Red dots indicate stations with a minimum temperature equal to or lower than the historical record.

The setup and maintenance of the blocking situation in the midtroposphere is the main feature of circulation that induces the occurrence of a severe cold wave. Fig. 2 shows the average 500 hPa geopotential height and its anomalies during the two cold wave processes. There was a ridge over the Ural Mountains extending to the polar region during 28—31 December 2020. The center of polar vortex was biased towards northeastern Asia and the central North Pacific, and a so-called “inverse Ω” flow pattern was formed ( Fig. 2 (a)). Northerly winds prevailed at the front of the ridge over the Ural Mountains, leading to a southward shift and accumulation of polar cold air, which provided the necessary condition for the southward breakout of this polar cold air. As for the second cold wave ( Fig. 2 (b)), there were ridges over the area to the east of the Ural Mountains and over the central Atlantic, respectively. The polar vortex was biased towards the Asian region, and a transverse trough was tilted from northeastern Asia to Lake Baikal. The common features of the two can be briefly summarized as follows: in the middle and high latitudes, a broad ridge area was situated at approximately 30°—105°E with a blocking high centered over the Ural Mountains area. Strong northerlies prevailed in front of the ridge. A northeast—southwest-tilting transverse trough lay to the southeast of Lake Baikal. The northerly winds consistently brought polar cold air to the mid latitudes of Asia.

Fig. 2. Mean 500 hPa geopotential height (contours) and its anomalies (shaded)averaged for (a) 28—31 December 2020 and (b) 6—8 January 2021. Units: m.

The outbreak of cold waves usually relates to the decay of a blocking high upstream, which may lead to an anticlockwise turning of the transverse trough and southward shift of the Siberian high in the lower troposphere. However, the evolution of the present cases is different. As can be seen in Fig. 2 (three-day mean 500 hPa height field), a broad ridge presented over the midlatitudes (60°—90°E) in both cold wave cases. The daily evolution of geopotential height at 500 hPa from 27 to 30 December 2020 and from 5 to 7 January 2021 is shown in Fig. 3 .Even during the whole period from mid-December 2020 to early January 2021, a high was consistently maintained over the Ural Mountains area(though the high experienced strengthening, weakening, and strengthening again in the meantime). Strong cold advection associated with intense northerly flows in front of the blocking high led to a steady southward shift of the cold-air mass. Therefore, it is meaningful to study the effects and maintenance mechanism of the Ural blocking high during this period.

Fig. 3. Daily geopotential height (contours) and its anomalies (shaded) on (a) 27, (b) 29, and (c) 30 December 2020, and on (d) 5, (e) 6, and (f) 7 January 2021.Units: m.

4. Role of the Ural blocking high in the outbreak of cold waves

4.1. Enhancement of baroclinicity of the trough downstream

As mentioned above, a broad ridge was maintained from mid-December 2020 till the end of the second cold wave. Its persistence and strengthening brought a pronounced effect on the development of the downstream trough. The strong northerly flows in front of the ridge and associated southward advance of cold air was able to enhance the meridional gradient of geopotential height at the bottom of the downstream trough. The meridional gradient of temperature also increased from the beginning to the extreme of the first cold wave ( Fig. 3 (a—c)).We calculated the meridional difference in geopotential height at 500 hPa between 30°N and 40°N along 120°E. At the beginning of the first cold wave (27 December 2020), the meridional difference in geopotential height was 218 m, while it reached 365 m at the extreme of the cold wave (30 December 2020). Similar features were observed during the second cold wave ( Fig. 3 (d—f)). The geopotential height at 500 hPa between 30°N and 40°N along 120°E increased from 394 m (5 January 2021) to 521.5 m (6 January 2021). Fig. 4 shows the temporal evolution of the 500 hPa height field and temperature advection at 1000 hPa.There were very strong cold advections in the two cold wave events,which were located right to the east of the ridge. Strong cold advection appeared with wide meridional extension, closely following a rapid strengthening of the high, which also indicates an increase in baroclinicity in the lower troposphere.

It can be concluded from the above analysis that the maintenance and development of the Ural blocking high or ridge led to an enhancement of baroclinicity at the front of the high and bottom of the downstream trough, which, in turn, favored the development and southward advance of cold air.

4.2. Effect of energy dispersion

The analysis above has displaced the quasi-stationary characteristic of the Ural blocking high. The formation and influence of the stationary wave is discussed in this section. First, an eight-day low-pass filtering for 500 hPa geopotential height was performed and a Hovm?ller diagram of its anomalies to the climatology drawn ( Fig. 5 (a)). It can be seen that positive anomalies were maintained in the area of 50°—60°E,whereas negative anomalies persisted within 90°—150°E throughout the 35-day period from 10 December 2020 to 15 January 2021. The phase of the wave train basically remained unchanged, which is reminiscent of the pattern of a stationary wave, albeit with the strength varying during this period. If we use the movement of the maxima of the anomalies of geopotential heightΦ′(positive or negative centers in Fig. 5 (a)) to indicate the eastward propagation of the maxima ofΦ′2, and hence the propagation of energy, the mean velocity of the eastward propagation in the first and second cold wave can be estimated at 16° and 12° of longitude per day, respectively. The mean velocity of energy propagation was about 15° of longitude per day from around the middle of December 2020 to the middle of January 2021.

Fig. 4. Time—latitude cross section of (a) 60°—90°E mean 500 hPa geopotential height (units: m) and (b) 105°—120°E mean temperature advection at 1000 hPa (units: °C d ? 1 ) from 22 December 2020 to 10 January 2021. Grey shading denotes the 28—31 December 2020 and 6—8 January 2021 cold waves.

Taking the 500 hPa geostrophic westerlies in January 2021 averaged within the area of 0°—120°E and 45°—55°N as?u,Lsis estimated to be 96°.This means that if an intense development of low pressure occurs at 0°E,the development of a trough at 96°E is expected. This, of course, is but a simple physical explanation. In fact, how to decide the value of?uis a matter of multiple choices.

The above analysis has shown that the eastward propagation of the stationary wave not only caused the maintenance and reinforcement of the ridge (or blocking high), but also favored the development of the transverse trough downstream and the outbreak of the cold waves.

Fig. 5 (b) shows the activity of eight-day high-pass waves. Apparently, successive high-frequency waves moved eastwards rapidly.Positive and negative centers alternated in a specific area. Negative centers appeared to the east of 110°E, i.e., the location of the developing trough at the outbreak of the first cold wave. This indicates that the activity of high-frequency waves, together with the eastward propagation of low-frequency stationary wave energy, jointly contributed to the development of the cold waves.

Fig. 5. Time—longitude cross section of 40°—60°N mean eight-day (a) low-pass and (b) high-pass filtering of 500 hPa geopotential anomalies from 10 December 2020 to 10 January 2021. Units: m. Grey shading denotes the 28—31 December 2020 and 6—8 January 2021 cold waves.

5. Conclusion

At the end of 2020 and on into early 2021, the eastern part of China experienced the intrusion of two successive cold waves. A drastic drop in temperature exceeding its historical extreme occurred at many stations from the north to the south. The area with intense temperature variation(interdiurnal variation larger than 6°C) extended to the south of 20°N.The zero line of minimum temperature advanced to South China.

Regarding the features of circulation, a broad ridge (or blocking high) was maintained over the Ural Mountains area. No breakdown or discontinuous westward shift of the blocking high was observed, which is different from the typical situation for cold waves in eastern Asia.It is therefore worthwhile answering the question as to what process promoted the turning of the transverse trough.

Firstly, the maintenance and intensification of the Ural blocking and the northerly winds at its front were a strong of the increase in baroclinicity. Cold advection was reinforced and marched to the south consistently. These processes led to the strengthening and southward extension of the Siberian high. And secondly, the dispersion of stationary wave energy was a key factor in the maintenance and development of the Ural blocking high. Before the beginning of the two cold waves, the eastward propagation of low-frequency stationary waves originating at 0°E (or even to its west) successively reinforced the Ural high. It also promoted the development of a trough or the anticlockwise turning of the transverse trough downstream. Thus, it favored the southward invasion of cold air.

The above conclusions are based on an investigation into two cold wave cases. To verify the broader applicability of the conclusions, more cases need to be studied. Furthermore, numerical model experiments would be helpful to further examine the process and effect of stationary wave energy propagation. This will be the next step in the work of our group.

By comparing the temperature drop during each entire process( Fig. 1 ) and the geopotential height at 500 hPa ( Fig. 2 ), remarkable differences between these two cases were found. The characteristics and mechanisms underlying these differences should be studied in detail in the future.

Funding

This research was funded by a National Key Research and Development Program Project [grant number 2018YFC1505601 ] and the National Natural Science Foundation of China [grant number 41975072 ].

Acknowledgments

We are grateful to Professor Liren Ji for his encouragement and helpful discussions during the course of this work.