Mechanisms governing the evolution of a long-lived northwest vortex that caused a series of disasters in Northwest China

Shung-Long Jin , , , Bo Wng , , Shung-Lei Feng , , Xio-Lin Liu , , Zong-Peng Song , ,Ju Hu , , Zheng Li

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, China

Keywords:Northwest vortex Torrential rainfall Transmission line faults Moistrue budget Circulation budget

ABSTRACT The northwest vortex (NWV) is a type of mesoscale vortex that appears with a relatively high frequency in Northwest China. To further the understanding of the NWV’s evolution, in this study, the moisture and circulation budgets of a long-lived NWV ( ～132 h) that appeared in early August 2019 were calculated. This vortex induced a series of torrential rainfall events in Northwest China and Mongolia, which caused severe transmission line faults and urban waterlogging. Synoptic analyses indicate that the NWV was generated in a favorable background environment characterized by notable upper-level divergence and strong mid-level warm advection. The moisture budget shows that the East China Sea and Bohai Sea acted as the main moisture sources for the NWV-associated precipitation, and the water vapor was transported into the rainfall regions mainly by easterly and southeasterly winds. The circulation budget indicates that, during the developing stage, convergence-related vertical stretching was a dominant factor for the NWV’s development; whereas, the vortex’s displacement from regions with stronger cyclonic vorticity to those with weaker cyclonic vorticity mainly decelerated its development. In the decaying stage, divergence-related vertical shrinking and the net export of cyclonic vorticity due to the eddy flow’s transport resulted in the NWV’s dissipation.

1. Introduction

A mesoscale vortex is a kind of vortex that has a typical horizontal scale of 2—2000 km ( http://glossary.ametsoc.org/wiki/Vortex ;Orlanski, 1975 ; Fu et al., 2020 ). This type of system can be found all over the world and can induce almost all kinds of disastrous weather( Fu et al., 2015 ; Fierro and Mansell, 2018 ). As such, mesoscale vortices have long been a research focus.

Every year, China suffers great economic losses because of mesoscale vortices ( Tao, 1980 ; Fu et al., 2019 ), such as Tibetan Plateau vortices( Tao 1980 ; Curio et al., 2019 ), southwest vortices ( Zhang et al., 2019 ;Feng et al., 2019 ), and Dabie vortices ( Zhang et al., 2015 ; Fu et al.,2016 ). In addition to these, northwest vortices (NWVs), which mainly form around southwest Gansu, should also be paid special attention( Tao, 1980 ; Ding and Lu, 1993 ; Wang et al., 2005 ; Wu et al., 2015 ). Although NWVs occur less frequently than Tibetan Plateau vortices, southwest vortices, and Dabie vortices, their destructive power is comparable( Ding and Lu, 1993 ; Wu et al., 2015 ). Moreover, as most NWVs show a north/northeast trajectory, they often cause torrential rainfall in the arid and semiarid regions of China ( Shi et al., 2019 ). Usually, torrential rainfall of similar intensity tends to induce more severe disasters in arid areas (as vegetation is scarce there) than in humid areas. Therefore, it is of great importance to further our understanding of NWVs. However,compared to Tibetan Plateau vortices, southwest vortices, and Dabie vortices, NWVs have been much less investigated. As a result, to reach a more comprehensive understanding of the mechanisms governing the evolution of NWVs, much effort is still needed.

In early August 2019, a strong NWV formed around the southeastern section of Shaanxi Province ( Fig. 1 (a)). The vortex was mainly located in the middle and lower troposphere, with its top and bottom levels situated around 350 hPa and 850 hPa, respectively. During its life span,it caused strong winds ( Fig. 1 (b—e)) and torrential rainfall events ( Fig. 2 ).These caused severe transmission line faults and urban waterlogging in Gansu and Shaanxi Province, which brought serious economic losses to Northwest China. There were two key features during this event: the first was that the NWV was strong and long-lived; and the second was that precipitation was heavy and persistent. Correspondingly, the main purpose of this paper is to investigate (i) how the NWV in this event was maintained, and (ii) where the moisture of the torrential rainfall associated with the NWV came from.

Fig. 1. (a) Track of the NWV (blue solid line with blue dots), where the open circle and open rectangle mark the formation and dissipation locations of the vortex,respectively. (b—e) The stream field (black solid lines with arrows), cyclonic vorticity (shading; units: 10 ? 5 s ? 1 ), and wind above 8 m s ? 1 (a full bar is 4 m s ? 1 ) at 700 hPa, where the gray shading shows the terrain above 3000 m and purple dashed boxes represent the key region of the vortex. KT = key time.

2. Data and methods

2.1. Data

The ERA5 reanalysis dataset ( Hersbach et al., 2020 ) provided by the European Centre for Medium-Range Weather Forecasts, which has an hourly temporal resolution and 0.25° horizontal resolution, was used in this study for analyses and calculations. This dataset has a total of 37 vertical levels. The gridded precipitation dataset provided by the National Oceanic and Atmospheric Administration, which is produced by using the US Climate Prediction Center morphing technique (CMORPH;Joyce et al., 2004 ) was employed in this study to investigate the precipitation features during this event. The CMORPH precipitation data have a temporal resolution of 30 min and a horizontal resolution of 8 km. The evaluation conducted by Shen et al. (2010) showed that the CMORPH precipitation data perform credibly over China.

2.2. Methods

As circulation is an effective measure to represent the rotation for a finite area ( Holton, 2004 ), this study used the circulation budget( Davis and Galarneau, 2009 ; Fu and Sun, 2012 ) to analyze its variational mechanisms. Its expression is as follows:

whereC= ∮Vhdldenotes the circulation along a vortex’s boundary line;+Mhh) represents the quasi-Lagrangian variation (tis time,Mhrepresents the horizontal moving speed vector of the vortex,and the subscript ‘h’ denotes the horizontal component.Mhwas calculated by using the vortex’s displacement fromt? 1 tot+ 1); the overbar stands for the average around the perimeter of a vortex, and the prime represents the perturbation relative to this mean value.i,j,andkare unit vectors pointing to the east, north, and zenith, respectively.jis the gradient operator;Vh=ui+vjstands for the horizontal wind vector;ηdenotes the absolute vorticity;?δrepresents the vortex-area averaged divergence;Astands for the horizontal area of the vortex;pis the pressure;ωis the vertical velocity under the pressure coordinate; andnis the unit vector normal to the boundary line of the vortex.

Term 1 (TM1) stands for the quasi-Lagrangian variation of the vortex’s circulation, which is an effective indicator for the vortex’s intensity; TM2 represents the effect mainly due to a vortex’s displacement; TM3 denotes the vertical stretching/shrinking effect due to the vortex’s convergence/divergence; TM4 represents the transport of perturbation absolute vorticity by the mean flow; TM5 denotes the transport of perturbation absolute vorticity by the eddy flow;TM6 stands for the tilting effect; a Total (TOT) term was defined as TOT = TM2 + TM3 + TM4 + TM5 + TM6; and RES represents the effects mainly due to friction, subgrid processes, and calculation errors.In order to calculate the circulation budget, we determined a key region for the NWV (purple dashed boxes in Fig. 1 (b—e)) according to the mean size of the vortex during its life span. Sensitivity tests showed that relatively small ( ± 0.5°) changes to its size did not lead to obvious changes to the calculation results (not shown).

The moisture flux (MF) is defined as( Holton, 2004 ),whereqis the specific humidity. In this study, we used the integrated MF (IMF) from the surface to 200 hPa (moisture above 200 hPa can be ignored) to analyze the water vapor transport and to determine the moisture source for the precipitation associated with the vortex. Its expression is:

3. Overview of the event

3.1. Evolution of the NWV and its precipitation

At 2100 UTC 2 August 2019, an NWV formed around the southeast of Gansu ( Fig. 1 (b)). In addition to the formation and dissipation time,three key times (KTs), i.e., KT1 (0000 UTC 4 August), KT2 (0600 UTC 5 August), and KT3 (0600 UTC 7 August), were defined to classify the NWV’s life span ( Table 1 ). Of these, KT1 was the time before which the vortex experienced monotonic development; KT2 was the time when the NWV reached its maximum intensity; and KT3 was the time when the vortex began to dissipate.

Table 1 Classification of the NWV’s life span, and associated features.

After formation, the vortex first moved northeastward from 2100 UTC 2 August to KT1 ( Fig. 1 (a)). This period was defined as Stage I,during which the vortex developed rapidly (cf., Fig. 1 (b,c)). In Stage I,precipitation associated with the vortex was heavy (mainly appearing in the central and eastern sections of the NWV), with a 27-h accumulated rainfall center of more than 200 mm appearing in the northern section of Shaanxi Province ( Fig. 2 (a)). Analyses of the IMF indicate that moisture for the precipitation mainly originated from the East China Sea ( Table 1 )and was transported to the rainfall regions mainly via the easterly winds.

From 0000 UTC 4 August to KT2, the NWV mainly showed a northward displacement ( Fig. 1 (a)). This period was defined as Stage II, during which the vortex first weakened in intensity (from KT1 to 0600 UTC 4 August), then developed again and reached its maximum intensity at KT2 ( Fig. 1 (d)). In Stage II, rainfall associated with the vortex weakened in intensity ( Fig. 2 (a, b)), with torrential rainfall mainly appearing in the northern and southern sections of the NWV. IMF analyses indicated that the Bohai Sea and East China Sea acted as the main moisture sources for the rainfall ( Table 1 ), and the water vapor transport was mainly transported by southeasterly winds. Compared to that of Stage I, the moisture transport and convergence were weaker in Stage II (not shown). This was the main reason for the contrast in precipitation in these two stages.

From KT2 to KT3, the NWV mainly moved in the east-northeast direction ( Fig. 1 (a)). This period was defined as Stage III, during which the vortex changed slowly in intensity ( Fig. 1 (d, e)); whereas, its associated rainfall enhanced again (cf., Fig. 2 (b, c)). Intense precipitation mainly appeared in the eastern section of the NWV and the regions east of the vortex. IMF analyses indicated that moisture for the precipitation was mainly transported into the rainfall regions from the Bohai Sea by the southerly winds ( Table 1 ). As the moisture transport and convergence were stronger than those in Stage II (not shown), precipitation during Stage III was heavier.

From KT3 on, the NWV showed a quasi-stationary behavior( Fig. 1 (a)), and after 0900 UTC 8 August the vortex dissipated. This period was defined as Stage IV, during which the vortex weakened rapidly in intensity, and its associated rainfall also decreased ( Fig. 2 (d)). The weakening in intensity of both the NWV and moisture transport was the key reason for the decrease in precipitation (not shown). The Bohai Sea was the main moisture source for the rainfall and the moisture was mainly transported by southerly winds ( Table 1 ).

3.2. Synoptic analyses

The formation location of the vortex was situated within a notable upper-level divergence region ( Fig. 3 (a)) northeast of the South Asian high. The upper-level divergence contributed to ascending motions due to the continuity of fluid. This was favorable for the NWV’s formation and its associated precipitation. Condensation-related latent heating was also conducive to ascending motions, which enhanced upper-level divergence and lower-level convergence ( Fu et al., 2017 ). The upper-level jet (35°—45°N) was relatively weak and broke into several parts, which meant it played a secondary role in the vortex’s generation. At middle levels, there was a shortwave trough that moved out from the Tibetan Plateau ( Fig. 3 (d)). The NWV was mainly situated in the central regions of the trough where warm advection was strong. According to quasigeostrophic theory, the warm advection contributed to ascending motions ( Holton, 2004 ), which favored the NWV’s formation.

In Stage II when the NWV developed rapidly, the upper-level divergence ( Fig. 3 (b)) and mid-level warm advection ( Fig. 3 (e)) were strong around the vortex, which contributed to its development. The upperlevel jet was weak and relatively far from the vortex, which meant it was only of secondary importance to the vortex’s development. During Stage IV, obvious upper-level convergence appeared in the upper troposphere around the vortex ( Fig. 3 (c)) and the mid-level warm advection around the vortex weakened ( Fig. 3 (f)). This meant that favorable background environmental conditions for the NWV’s maintenance began to disappear, which corresponded to the vortex’s decay.

4. Circulation budget

Before detailed analysis, we first evaluated the balance of Eq. (1) and found that the ratio of TOT to TM1 mainly ranged from 0.83 to 1.18.This means that, after neglecting term RES, Eq. (1) was still in a good state of balance. The key-region averaged vorticity ( Fig. 4 (a)) showed consistent variations to that discussed in Section 3.1 , meaning that the circulation (area-averaged vorticity equivalent to circulation according to Green’s theorem) budget is effective to investigate the mechanisms governing the NWV’s evolution.

During Stage I, the NWV developed rapidly, which can be reflected by the increasing key-region averaged vorticity ( Fig. 4 (a)). TOT was positive in this stage ( Fig. 4 (c)), implying that cyclonic circulation of the vortex enhanced. Comparing TM2—TM6, it can be found that the convergence-related ( Fig. 4 (b)) vertical stretching ( Fig. 4 (e); TM3) acted as the most favorable factor, and the tilting ( Fig. 4 (h); TM6) was the second most dominant factor. Transport of vorticity by the mean/eddy flow (TM4/TM5) showed different effects in this stage, which rendered an overall import of cyclonic vorticity into the key region ( Fig. 4 (f, g)).This contributed to the NWV’s development. In Stage I, the NWV mainly moved from regions with larger cyclonic vorticity to those with smaller vorticity (not shown), which resulted in a negative TM2 ( Fig. 4 (d);Fu and Sun, 2012 ). This was the most detrimental factor for the vortex’s development.

Fig. 3. (a—c) The divergence (shading; units: 10 ? 5 s ? 1 ), geopotential height (black solid, units: gpm), temperature (red contours, units:°C), and wind above 30 m s ? 1 at 200 hPa. (d—f) The warm advection (shading; units: 10 ? 5 s ? 1 ), geopotential height (black solid; units: gpm), and temperature (red contours; units:°C) at 500 hPa.Small green boxes mark the centers of the vortex; thick gray solid lines outline the terrain of 3000 m; and the thick brown lines are the trough lines.

During Stage II, the NWV first weakened, which can be reflected by the decreasing cyclonic vorticity ( Fig. 4 (a)) and negative TOT ( Fig. 4 (c)),and then enhanced rapidly again, which can be reflected by the strong positive TOT. Comparisons among TM2—TM6 showed that the effects due to the NWV’s displacement were collectively the main reason for the weakening in the earlier period ( Fig. 4 (d)). In contrast, the transport of cyclonic vorticity by the mean flow ( Fig. 4 (f)) and convergence-related vertical stretching ( Fig. 4 (b, e)) mainly acted as resistance to the vortex’s weakening. The rapid development in the latter period was mainly due to the NWV’s displacement ( Fig. 4 (d)), vertical stretching ( Fig. 4 (e)),and transport of vorticity by eddy flow ( Fig. 4 (g)); whereas, the mean transport was the most detrimental factor for this process ( Fig. 4 (f)).

During Stage III, cyclonic vorticity within the NWV’s key region decreased gradually with time ( Fig. 4 (a)), which corresponded to the weak negative TOT ( Fig. 4 (c)). The divergence-related ( Fig. 4 (b)) vertical shrinking ( Fig. 4 (e)) and tilting ( Fig. 4 (h)) dominated the negative TOT. In contrast, the transport of vorticity by mean flow ( Fig. 4 (f))was the only factor that contributed to the NWV’s maintenance. In Stage IV, cyclonic vorticity within the NWV’s key region decreased rapidly ( Fig. 4 (a)), which corresponded to the vortex’s dissipation. The divergence-related ( Fig. 4 (b)) vertical shrinking ( Fig. 4 (e)) and transport of vorticity by eddy flow ( Fig. 4 (g)) governed the decay of the vortex,whereas the transport of vorticity by mean flow mainly acted in an opposite manner.

Fig. 4. (a) The key-region averaged vorticity (shading; units: 10 ? 5 s ? 1 ) during the NWV’s life span. (b) As in (a) but for divergence (shading; units: 10 ? 6 s ? 1 ). (c—h)As in (a) but for the budget terms TOT, TM2, TM3, TM4, TM5, and TM6, respectively (shading; units: 10 ? 10 s ? 2 ). KT = key time.

5. Conclusion

To further our understanding of NWVs is of great importance, as they appear with a relatively high frequency and often induce severe disasters. During early August 2019, a long-lived NWV lasted for ～132 h and induced a series of torrential rainfall events that caused severe transmission line faults and urban waterlogging in Northwest China. In this study, detailed analyses of the NVW were conducted to further our understanding of this type of system. It was found that the NWV generated in a favorable background environment characterized by notable upperlevel divergence and strong mid-level warm advection. The vortex was favored by a mid-tropospheric shortwave trough, whereas its relationship with the upper-level jet was not obvious. The moisture budget indicated that the intensity of moisture transport and convergence were consistent with the variation in precipitation associated with the vortex. The East China Sea and Bohai Sea were the main moisture sources for the rainfall of the NWV, and the water vapor was transported into the precipitation regions mainly by easterly and southeasterly winds( Table 1 ). According to the NWV’s variation, its lifespan was divided into four stages. In Stage I, the vortex experienced rapid development.Convergence-related vertical stretching and convection-related tilting dominated its development ( Table 1 ), whereas the vortex’s displacement from the regions with larger to smaller cyclonic vorticity mainly decelerated this process ( Fu and Sun, 2012 ). In the former period of Stage II,the NWV mainly weakened in intensity, and the key reason for this was that the vortex moved from the regions with larger to smaller cyclonic vorticity. In the latter period, the NWV developed again, mainly due to its displacement (from the regions with smaller to larger cyclonic vorticity) and eddy transport of vorticity. During Stage III, the NWV weakened gradually due to divergence-related vertical shrinking and tilting.Mean flow caused a net import of cyclonic vorticity into the vortex’s key region, which was favorable for its sustainment. During Stage IV,the NWV weakened rapidly and finally dissipated. The enhancement of divergence within the vortex’s key region caused a notable vertical shrinking, which acted as a dominant factor in the vortex’s decay. In addition, the eddy flow caused a net export of cyclonic vorticity from the key region, which served as another dominant factor for the NWV’s dissipation.

Funding

This research was supported by the Science and Technology Foundation of the State Grid Corporation of China [grant number 5200-202016243A-0-0-00] and the Innovation Fund of the China Electric Power Research Institute [grant number NY83-20-003].