On the formation and wind enhancement mechanisms of a Mongolian cyclone that caused a transmission line galloping trip in Gansu province

Shuanglong Jin ,Shuanglei Feng ,Xiaolin Liu ,Bo Wang ,Zongpeng Song ,Wei Cui ,Cong Wang

a State Key Laboratory of Operation and Control of Renewable Energy & Storage Systems, China Electric Power Research Institute Co., Ltd., Beijing, China

b Electric Power Meteorology State Grid Corporation Joint Laboratory, Beijing, China

c Northwest Branch of State Grid Corporation of China, Xi’an, Shaanxi Province, China

Keywords:Mongolian cyclone Kinetic energy State grid Transmission line galloping trip

ABSTRACT Influenced by strong winds associated with a southeastward-moving Mongolian cyclone, a severe transmission line galloping occurred in Baiyin City, Gansu Province, on 14 April 2020.This caused a tripping incident of the transmission line in this region.Based on the hourly, 0.5° × 0.5°, ECMWF ERA5 reanalysis data, this study investigated the formation mechanisms of the Mongolian cyclone and its associated strong winds.Results from the vorticity budget indicate that the convergence-related vertical stretching and the upward transport of cyclonic vorticity governed formation of the Mongolian cyclone in this event; whereas, tilting and export of cyclonic vorticity from the central region of the cyclone mainly decelerated the cyclone’s formation.The kinetic energy(KE) budget shows that the wind associated with the Mongolian cyclone was mainly enhanced by the positive work of the pressure gradient force.Unlike some typical strong wind events in Northwest China, during this event, no significant downward momentum transportation from the upper troposphere was found.The vertical transport of KE exerted a slightly favorable effect on the KE increase around the location where the transmission line galloping trip appeared.In contrast, the horizontal transport mainly caused an export of KE from this region,which applied an overall negative effect on the wind enhancement associated with the Mongolian cyclone.

1.Introduction

As the world’s largest power grid, the State Grid (SG) covers about 88% of the Chinese national territory ( www.sgcc.com.cn ).Over this vast area, the SG has established numerous power facilities and a large number of transmission lines with different voltages, that supply power to a population of over 1.1 billion in China.Statistical studies show that natural disasters exert a huge threat to the SG, as they cause more than 40% of the total power grid faults each year ( Yang et al., 2009 ;Sun et al., 2011 ).Among these, meteorological disasters account for the largest proportion, and, under a warming climate, a notable increasing trend in the numbers of meteorological disasters is found, particularly in the recent 30 years ( IPCC, 2014 ).Consequently, this causes an increasing trend of meteorological disaster–related SG faults ( Song et al.,2019 ).Strong wind is one of the most severe meteorological disasters that causes severe power grid faults, not only in China ( Xie and Li, 2006 ;Yang et al., 2010 ; Luo et al., 2016 ; Song et al., 2019 ), but also all over the world ( Cevik et al., 2019 ; Zhang et al., 2019 ).

For China, strong winds tend to occur in all regions ( Qiu et al., 2013 ;Miao et al., 2020 ).Northwest China (including Xinjiang, Gansu, and Qinghai) is one of the areas where strong winds appear frequently.This poses a huge threat to the power facilities in this region ( Yang et al.,2010 ; Song et al., 2019 ).Of the three regions in Northwest China, Gansu has the least frequency of strong winds ( Qiu et al., 2013 ), particularly for its southern section (however, strong winds have also led to SG faults in this region).This is partly because the main weather systems that cause strong winds show obvious differences among the three regions in Northwest China.Compared to those of Xinjiang and Qinghai( Wang et al., 2011 ; Li et al., 2012 ; Lu et al., 2014 ; Ding et al., 2019 ;Fu et al., 2020 ), strong winds in Gansu are much less discussed.Thus,the mechanisms governing the formation of strong winds in Gansu still remain unclear.As a result, one primary purpose of this paper is to enhance understanding of the underlying mechanisms for the strong winds in Gansu, on the basis of a typical event that induced an SG failure in Baiyin City, Gansu Province (small black rectangles in Fig.1 ).A deeper understanding of the key mechanisms that govern the formation of strong winds in Gansu would improve the guarantee of service for the safe operation of the SG in Gansu and other similar regions.

The remaining part of the paper is structured as follows: In Section 2 ,the data and methods are described; in Section 3,an overview of the event and a synoptic analysis are presented; in Section 4,results of vorticity and kinetic energy (KE) budgets are discussed; and finally, the conclusion and discussion are provided in Section 5 .

2.Data and methods

2.1.Data

This study uses station-observed three-hourly surface wind form the China Meteorological Administration for analyzing the strong wind event.Hourly 0.25° × 0.25°ERA5 reanalysis provided by the European centre for Medium-Range Weather Forecasts ( Hersbach and Dee, 2016 )are used for synoptic analyses as well as vorticity and KE budgets.The reanalysis data cover a total of 37 vertical levels (from 1000 hPa to 1 hPa).

2.2.Vorticity and KE budgets

Prior to analysis, for Eqs.(1) and (2) we calculated the ratio of the sum of the right-hand side terms to the left-hand side term (i.e., the time derivative) within targeted regions.It was found that, for the vorticity budget, the ratio is between 1.16 and 1.25, and for the KE budget the ratio is between 1.12 to 1.28.Therefore, overall, the balance of Eqs.(1) and (2) is good, and thus the budget results can be used for further investigation.

3.Overview of the strong wind event

Between 0800 UTC and 0900 UTC 14 April 2020, due to strong winds, a severe transmission line galloping occurred in Baiyin City,Gansu Province (small black rectangles in Fig.1 ).This resulted in a tripping incident of the transmission line over this region.During the event,in the upper troposphere, Baiyin City and its surrounding regions were governed by a ridge ( Fig.1 (a)) that was behind a trough over central China.The upper-level jet broke into several sections, with Baiyin City located within its discontinuous zone, which featured relatively weak cold temperature advection and divergence.This was not a favorable upper-tropospheric condition for strong ascending motions.In the middle troposphere, Baiyin City was controlled by strong northerly wind and its associated warm advection, which was ahead of a shortwave trough around Lake Baikal ( Fig.1 (b)).According to the geostrophic omega equation ( Holton, 2004 ), warm advection acted to promote ascending motions in this region.In the lower troposphere, as Fig.2 shows,there was a southward-moving Mongolian cyclone ( Huang et al., 2016 ).This cyclone formed around Lake Baikal ( Fig.2 (a)) at 1200 UTC 13 April 2020, and, as it moved southward, its associated vorticity enhanced notably, which reflects its rapid development.Due to the development and southeastward approaching of the Mongolian cyclone, isolines of geopotential height became more concentrated in Baiyin City (not shown).According to the geostrophic relationship, more concentrated isolines of geopotential height meant geostrophic wind (real wind can be decomposed into the geostrophic wind and geostrophic deviation) was enhanced ( Holton, 2004 ).Moreover, intensification and southeastward movement of the cyclone caused a notable lowering of pressure in the regions ahead of the cyclone’s track.This made the high ridge and its associated northwestward-pointing pressure gradient force (which were located southeast of Baiyin City) disappear (not shown), which contributed to acceleration of northwesterly strong winds in Baiyin City.As Fig.1 (c, d), from 0300 UTC to 0600 UTC 14 April 2020, surface wind enhanced rapidly from north to south in the regions around Ningxia and Gansu (e.g., stations 52575, 52594, 53517, 53512, 53609, and 53707),and meanwhile the component of northerly wind increased notably.According to an SG observation station (~50 km away from the location where the transmission line trip appeared), wind speed increased gradually from 1200 UTC 13 April (when the cyclone formed) to 0200 UTC 14 April (when the cyclone accelerated its southeastward movement), and then from 0200 UTC 14 April to 0800 UTC 14 April (around the time when the transmission line trip appeared), the wind speed increased more rapidly.During this period, the maximum wind speed appeared at 0700 UTC 14 April (~9.5 m s), and, about one hour later, the transmission line trip occurred (the wind speed was ~8.9 m s1 ).

4.Vorticity and kinetic energy budgets

4.1.Vorticity budget of the Mongolian cyclone’s formation

As discussed in Section 3,the strong wind that caused the transmission line galloping in Baiyin City occurred southwest of the Mongolian cyclone.Its evolution was consistent with the cyclone’s evolution and displacement.Therefore, understanding the formation of the Mongolian cyclone is necessary.We first calculated the vorticity budget 12 h before the cyclone’s formation (during which time its central region mean vorticity showed monotonic growth) using Eq.(1),and then averaged it within the central region of the cyclone (the red dashed box shown in Fig.2 (a)).As the horizontally integrated vorticity over a region equals the velocity circulation along the boundary line of the region( Holton, 2004 ), the central region’s averaged vorticity is an effective indicator of the Mongolian cyclone ( Fu et al., 2017 ).

As Fig.3 (a) shows, the central-region averaged TOT was positive from 0000 UTC to 1200 UTC 13 April 2020, which means that cyclonic vorticity within the central region of the Mongolian cyclone increased during this period ( Fig.3 (b)).Convergence-related ( Fig.3 (b)) vertical stretching (i.e., STR in Fig.3 (a)) and upward transport ( Fig.3 (b)) of cyclonic vorticity (i.e., VAV in Fig.3 (a)) were the most and second-most dominant factors for the cyclone’s formation.In contrast, tilting and outward horizontal transport of cyclonic vorticity from the central region of the cyclone mainly acted as detrimental factors that decelerated the cyclone’s formation.

4.2.Kinetic energy budget of the wind enhancement

As Fig.2 shows, the western section of the Mongolian cyclone and the regions around Baiyin City all experienced notable wind enhancement.This ultimately produced the strong winds that caused the transmission line galloping.The fastest enhancement in wind speed mainly occurred after the cyclone’s formation.In this section, we focus on the longitudes between 103°E and 107°E (which contained the regions with fastest wind enhancement) ( Fig.2 ) during the period from 1200 UTC 13 April (i.e., formation of the Mongolian cyclone) to 0900 UTC 14 April 2020 (around the time when the transmission line galloping trip occurred).

As the green solid lines in Fig.4 show, in the lower troposphere below 650 hPa, two strong positive-TOTcenters appeared –one located around 37°N, corresponding to Baiyin City, and the other around 48°N, corresponding to the western section of the Mongolian cyclone.This means that KE within these two regions experienced rapid growth from 1200 UTC 13 April to 0900 UTC 14 April 2020, which can be confirmed by the notable wind enhancement shown in Fig.2.Of the two regions, the work on rotational wind done by the pressure gradient force( Fig.4 (a)) was the dominant factor for wind enhancement, and the pressure gradient force’s work on divergent wind acted as the second most favorable factor ( Fig.4 (b)).Comparing the cyclone’s western section with the region around Baiyin City, the former had a larger pressure gradient force, because the geopotential height gradient in the western section of the cyclone was larger than that southwest of the cyclone(not shown).This is an important reason why the former had a faster increase in KE ( Fig.4 (a)).As there were no strong descending motions(not shown), for this event, downward momentum transportation was not important.This can be confirmed by the vertical transport of KE( Fig.4 (d)), which only showed slightly favorable effects on the increase in KE.The horizontal transport of rotational and divergent wind KE was relatively weak ( Fig.4 (c,e)).Overall, it applied a negative effect, which decelerated the wind enhancement in this region.

5.Conclusion and discussion

On 14 April 2020, influenced by strong winds associated with a southeastward-moving Mongolian cyclone, a severe transmission line galloping occurred in Baiyin City, which caused a tripping incident of the transmission line.This cyclone was a key influencing weather system for this event.Based on an area-average vorticity budget, the formation of the Mongolian cyclone was investigated.It was found that the convergence-related vertical stretching (i.e., STR) and vertical transport of cyclonic vorticity (i.e., VAV) dominated the increase in cyclonic vorticity within the central region of the cyclone.This contributed to the cyclone’s formation.In contrast, the tilting effect (i.e., TIL) and horizontal transport (i.e., HAV) mainly reduced the cyclonic vorticity within this region, which decelerated the formation of the cyclone.

A KE budget of the production of strong winds associated with the Mongolian cyclone was calculated.Results showed that the fastest KE enhancement associated with the cyclone mainly appeared in its western section and the region around Baiyin City (southwest of the cyclone).For both, wind production rather than transport governed the KE enhancement.The work on rotational wind done by the pressure gradient force(i.e., term K1) was the most favorable factor for the wind enhancement;whereas, the export of KE by divergent wind was the most detrimental factor.In this event, downward momentum transport was not an important factor.The vertical transport of KE had a slightly favorable effect on the increase in KE.This is significantly different from some typical strong wind events in Xinjiang ( Li et al., 2012 ; Lu et al., 2014 ; Fu et al.,2020 ).

It should be noted that, although this study provides some useful information for understanding the formation of strong winds in Gansu, as a case study, it may have limitations in representing the general features of this type of event.Therefore, more cases should be investigated in future, which will help to enhance our understanding of the formation of strong winds in Northwest China.

Funding

This research was supported by the National Key R&D Program of China [grant number 2018YFC0809400], the science and technology foundation of State Grid Corporation of China [grant number 5200–202016243A-0–0-00 ], and the Innovation Fund of China Electric Power Research Institute [grant number NY83–19–002].