The development of a powerful Mongolian cyclone on 14—15 March 2021:Eddy energy analysis

Cholw Bueh ,Anrn Zhuge ,Zuowei Xie ,Mei Yong ,Gomboluuev Purevjv

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

b College of Geographical Science Inner Mongolia Normal University, Hohhot, China

c Inner Mongolia Key Laboratory of Remote Sensing & Geography Information System, Inner Mongolia Normal University, Hohhot, China

d Information and Research Institute of Meteorology, Hydrology and Environment, National Agency for Meteorology and Environmental Monitoring, Juulchinhy gudamj-5, Ulaanbaatar, Mongolia

Keywords:Mongolian cyclone Dust weather Kinetic energy Available potential energy Frontal cyclone wave

ABsTRACT Intense and extensive dust,caused by a strong Mongolian cyclone,hit Mongolia and northern China on 14—15 March 2021.In this study,the development process of this cyclone is analysed from the perspective of highfrequency eddy energetics.During the low-frequency circulation field of early March of 2021,an amplified polar vortex intruding towards central Asia and a ridge straddling eastern and northeastern Asia worked in concert to comprise a strong baroclinic zone from central Asia to Lake Baikal.Under these favourable conditions,on 13 March,a migratory trough triggered the Mongolian cyclone by crossing over the Sayan Mountains.The downwards transfer of kinetic energy from the eddy at 850 hPa played a key role in the intensification and mature stage of the cyclone.This mechanism was primarily completed by the cold air sinking behind the cold front.The frontal cyclone wave mechanism became crucial once the cyclone started to rapidly develop.The authors emphasize that the anomalously large growth of high-frequency available potential energy,which characterized this super strong cyclone,was obtained by extracting energy first from the time-mean available potential energy and then from the low-frequency available potential energy.The interannual temperature anomaly pattern of “north cold south warm ”facilitated the additional time-mean available potential energy,and the temperature anomaly pattern of “northwest cold southeast warm ”conditioned the extra low-frequency available potential energy.The analysis results suggest that the interaction between high-and low-frequency waves was also important in the development of the intense cyclone.

1.Introduction

From 14—15 March 2021,most parts of northern China,such as South Xinjiang,Inner Mongolia,northern Hebei,Beijing,and Tianjin,were affected by severe dust weather.Sandstorms of various strength also occurred in additional parts of these regions.This dust weather was the most intense and extensive in China during the last decade.The dust area reached 0.45 million square kilometres (Duan et al.,2021 ;Yin et al.,2022).On 15 March 2021,the concentration of PM10in most areas of Beijing exceeded 2000 μg m-3,with some stations reaching 9000 μg m-3.In fact,dust first occurred in southern and eastern Mongolia,approximately six hours before its arrival.

The occurrence of strong dust weather requires three basic conditions: specific surface conditions with an exposed dust source,dynamic conditions with a strong surface wind and atmospheric stratification conditions that are favourable for dust lifting (Kurosaki and Mikami,2005 ;Bao et al.,2021).By March 2021,a broad dust source area formed in the Gobi Desert in southern Mongolia and Northwest China,provided very favourable surface conditions for strong dust weather to occur.During 14—15 March,a strong wind was observed over Northwest and North China,allowing for the dynamic conditions need to form a dust storm.On 15 March,wind gust speed even reached 40 m s-1in central Inner Mongolia,which are speeds almost equivalent to that of a grade 12 typhoon.Meanwhile,reduced stratification stability of the mid-and lower troposphere was observed over North China,which served a favourable stratification condition for the dust storm weather.

As in many strong dust weather cases,this dust weather process was driven by a powerful Mongolian cyclone and its accompanying cold high pressure (Liu et al.,2003 ;Yun et al.,2013).Sandstorms and strong sandstorms also appeared in the areas of large pressure gradients behind the cold front during movement of the cyclone.The central amplitude of the Mongolian cyclone reached 979 hPa,and its 24-hour pressure drop reached 28 hPa,exhibiting an explosive cyclone intensity.

During the 1940s,the baroclinic instability theory was proposed and used to explain the growth of synoptic-scale disturbances,including frontal cyclones (Charney,1947;Eady,1949).In this dynamic framework,the synoptic disturbances grow by extracting available potential energy from the background flow.The baroclinic instability theory was later interpreted by Hoskins et al.(1985) from the perspective of potential vorticity thinking.Since the end of the 1960s,the Chicago three-dimensional conceptual model of fronts and cyclones was built up based on quasi-geostrophic theory (Palmen and Newton,1969;Tao et al.,2014).On the other hand,from the viewpoint of kinetic energy,Pettersen and Smebye (1971) identified a type of cyclone development,which was often initiated by the downwards transfer of kinetic energy instead of the low-level frontal perturbance.They referred to this type of cyclone as a Type B cyclone.Orlanski and Katzfey (1991) first derived the eddy (relative to the time-mean state) kinetic and available energy budget equations and applied them to diagnose cyclone development.

As has been noted,the intense development of the Mongolian cyclone was the most key cause for the dust weather in Mongolia and northern China.Once spring commences,East Asian baroclinic wave becomes highly active,which is in sharp contrast to the phenomenon of"midwinter suppression" during winter (Nakamura,1992).Therefore,the cyclone activity in East Asia in March is considered a normal phenomenon.Although it is frequently observed that the Mongolian cyclone is triggered by the eastward migrating upstream trough,the reason why cyclones have sudden and intense development in inland areas such as the Mongolian Plateau has thus far not been revealed.Because the terrain remains unchanged,topography has been shown to not be the key source of this development;however,the Mongolian cyclone intensity differs from time to time.Diabatic heating,which is the primary mechanism of explosive cyclones over seas,is not the key reason for the rapid intensification of the Mongolian cyclone.Then,the following questions arise naturally: What were the distinct features of the powerful Mongolian cyclone that occurred on 14—15 March 2021? What kinds of background circulation conditions are favoured to allow for the Mongolian cyclone to develop into such a cyclone?

To answer the above questions,a simplified version of the highfrequency eddy (kinetic and available potential) energy budget equation,with time mean and low-frequency circulation backgrounds,is derived in the present study.On this basis,the intense development of the Mongolian cyclone on 14—15 March 2021 is analysed from the perspective of eddy energetics.

2.Data and methods

2.1.Data

In this study,we use 6-hourly atmospheric data from the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis version 5 (ERA5) dataset (Hersbach et al.,2020).The variables used here include geopotential height,horizontal winds,pressure vertical velocity,and air temperature at 37 pressure levels (1000 hPa to 1 hPa).All data are on a regular latitude and longitude grid with a horizontal resolution of 1 ?！? 。.

2.2.Eddy energy Equations

The eddy energy equations (Eqs) were derived in Orlanski and Katzfey (1991).In an analogous way,high-frequency eddy energy Eqs can be derived by decomposing the eddy component into high-and lowfrequency components.The tendency equation of high-frequency eddy kinetic energy (KEH) can be written as:

where overbar and prime represent time-mean and transient eddy components,respectively,and subscripts H and L refer to high-and lowfrequency components.On the right hand side (rhs) of Eq.(1),the first term refers to the conversion of KE H from the high-frequency eddy available potential energy (AP EH),the second term the downwards transfer of eddy kinetic energy due to the high-frequency eddies,the third term the conversion of KE H due to the interaction between high-and lowfrequency eddies,the fourth term the advection of KEHby the timemean flow,the fifth term the ageostrophic geopotential flux divergence,and the sixth and seventh terms the barotropic conversion of KEHfrom the time-mean flow and the frictional dissipation of KEH,respectively.The first three terms on the rhs of Eq.(1) are important generation terms of the KEH.All other symbols in Eqs.(1—5) are standard.The definitions of these symbols and the derivation of Eq.(1) in detail are given in the supplementary material.

Similarly,the AP EHtendency Eq is expressed as:

Fig.1.Daily evolutions of (a—d) 500 hPa and (e—h) 850 hPa geopotential height fields (contours,units: gpm) from 12—15 March 2021,with superimpositions of the corresponding low-frequency geopotential height anomalies (colour shading,units: gpm).The 850 hPa wind fields (arrows,units: m s -1) are superimposed in (e—h).The contour intervals are 40 gpm in (a—h).The green line in (e—h) marks the topography greater than 1500 m.

whereσis the static stability parameter in the pressure coordinate system,the first term in the rhs of Eq.(6) refers to the baroclinic conversion of AP E H from the time-mean APE,the second term is the baroclinic conversion of AP EHdue to the interaction between high-and low-frequency eddies,the third term is the transfer between AP EHand KEH,the fourth term is the nonlinear interaction among high-frequency eddies,and the fifth and sixth terms are the advection of AP EHby the time-mean flow and the nonconservative sources and sinks of AP EHassociated with diabatic processes,respectively.The first two terms on the rhs of Eq.(6) are important generation terms of the AP EH.Other details are given in the supplementary material.The dynamic significance of the terms on the rhs of Eqs.(1) and (6) were also discussed in Chang and Orlanski (1993).

In the next section,we analyse the energetics of the Mongolian cyclone on 14—15 March 2021 based on Eqs.(1) and (6).The time mean of each variable is defined as the temporal average from 1 March to 10 April 2021 (early spring),and the eddy is taken as the departure from this mean.The high-frequency (low-frequency) component of each variable is obtained by performing high-pass (low-pass) filtering with an 8-day cut-offperiod to the eddy component (Bueh et al.,2022).

3.Results

3.1.Daily synoptic evolution

Fig.2.(a—d): Six-hourly evolution of the 850 hPa KEH (colour shading,units: m 2 s -2) and the corresponding high-frequency wind anomalies (arrows,units: m s -1)from 0600 UTC 14 to 0000 UTC 15 March 2021;(e—h): As in (a—d),but for the 850 hPa KEH tendency (colour shading,units: 10 -2 m 2 s -3).The green line marks the topography greater than 1500 m.

Fig.1 shows the 500 hPa and 850 hPa geopotential height fields during 12—15 March 2021.During this period,the low-frequency circulation field exhibited a distinct feature as indicated by the shaded areas in Fig.1.A large-scale tilted trough prevailed from central Asia to subpolar Asia due to the strong polar vortex extending towards central Asia and the midlatitude low-frequency wave.Meanwhile,an elongated warm ridge formed from East Asia to northeastern Asia.Accordingly,a strong baroclinic zone was present from central Asia to the east of Lake Baikal.On 12 March 2021,the tilted trough was located to the north of Lake Balkhash at 500 hPa and 850 hPa (Fig.1 (a,e)).On 13 March,the tilted trough became a typical migratory trough and then moved southeastward until it arrived at the southern edge of the west Siberian plain (Fig.1 (b)).At this time,its counterpart at 850 hPa moved to Lake Baikal (Fig.1 (f)).On 14 March,at 850 hPa,the trough rapidly intensified and developed into a strong Mongolian cyclone to the southeast of the Sayan Mountains (Fig.1 (g)),giving rise to the aforementioned severe dust weather in Mongolia and China.On 15 March,the trough around Lake Baikal (Fig.1 (c)) now moved eastward and developed into a cut-offlow (Fig.1 (d)) as the low-level Mongolian cyclone approached Northeast China (Fig.1 (h)).

3.2.Energy budget

The high-frequency eddy energy budget is analysed here in terms of Eqs.(1) and (6),and the results are given every six hours from 0600 UTC 14 to 0000 UTC 15 March 2021.It was between 1200 UTC and 1800 UTC on 14 March that the local gust in central Inner Mongolia reached their strongest speeds.

Fig.3.(a—d): Six-hourly evolutions of the 850 hPa CKAP (colour shading,units: 10 -2 m 2 s -3) and (contours,units: 0.1 Pa s -1) from 0600 UTC 14 to 0000 UTC 15 March 2021.Solid (dashed) contours indicate positive (negative) anomalies with intervals of 0.1 Pa s -1,and zero lines are omitted;(e—h): As in (a—d),but for the 850 hPa CPBCH (colour shading,units: 10 -2 m 2 s -3) and high-frequency wind anomalies (arrows,units: m s -1).The green line marks the topography greater than 1500 m.

Fig.2 displays the distributions of KEHand the corresponding tendencies.With the advent of the Mongolian cyclone,at 0600 UTC 14 March,the strongest KEHincrease was observed in southern Mongolia and eastern Inner Mongolia (Fig.2 (e)),although at this time,the KEHhad not reached the peak amplitude (Fig.2 (a)).At 1200 UTC 14 March,the KEHover the Gobi Desert in southern Mongolia reached the strongest,and the KE H over central and eastern Inner Mongolia became remarkably strong (Fig.2 (b)).Correspondingly,strong northerly anomalies,up to 20 m s-1,prevailed near the China-Mongolian border,causing the accompanying dust storm weather.At this time,considerably strong KE H increases were observed in eastern and southeastern Mongolia and Northeast China (Fig.2 (f)).The cyclone moved slightly southeastward from 1200 UTC to 1800 UTC 14 March.Correspondingly,at 1800 UTC 14 March,the KEHtendency became negative in southern Mongolia and eastern Inner Mongolia,while it still maintained weak positive values in most parts of Northwest and North China (Fig.2 (g)).Thus,the area with strong northeasterly and northwesterly anomalies exceeding 20 ms-1became extensive,ranging from Northwest China to North China (Fig.2 (c)),which is consistent with the extensive dust weather occurrence at this time.At 0000 UTC 15 March,although the KEHtendency became negative in most areas of Mongolia and northern China (Fig.2 (h)),the strong wind,which reached speeds as high as 20 m s-1,occurred in the Beijing-Tianjin-Hebei region as the cyclone moved further southeastward (Fig.2 (d)).

The Mongolian cyclone development during 14—15 March 2021 showed mixed features of the Type A and Type B cyclones proposed by Pettersen and Smebye (1971),which was initiated by an upstream upper level trough and then noted by an apparent release of low-level baroclinicity.In line with this,we first discuss the conversion of KEHfrom AP EH,as indicated by the first term on the rhs of Eq.(1),i.e.,CK AP.The temporal evolution of CK AP from 0600 UTC 14 to 0000 UTC 15 March 2021 is displayed in Fig.3(a—d).The distribution of CKAP generally resembled those of the KEH tendency.In addition,the magnitude of CK AP was larger than that of the KE H tendency by a factor of 1.5—2.5 (Figs.2 (e—h) and 3(a—d)).Therefore,the CKAPcan explain thestrong KEHincrease over Mongolia and northern China during cyclone development (Fig.2 (e—h)),particularly for the period from 0600 UTC to 1200 UTC 14 March (Figs.2 (e,f) and 3 (a,b)).Such a strong CKAPwas primarily contributed by the warm air rising ahead of the cold front during the developing stage of the cyclone (Fig.3 (a,b)) and by the cold air sinking behind the cold front during the mature and decaying stages of the cyclone (Fig.3 (c,d)).Although this conversion was more apparent during the eastward migration of the cyclone from 1800 UTC 14 to 0000 UTC 15 March,the KE H tendency became negative (Figs.2 (c,d) and 3 (c,d)),which could be attributed to advection and dispersion processes (figure not shown).

Fig.4.(a,b): The 850 hPa CPBCHC (colour shading,units: 10- 2 m2 s- 3) and (contours,units: 。C) at 1200 UTC and 1800 UTC on 14 March 2021;(c,d): As in (a,b),but for the CPBCHA (colour shading,units: 10 -2 m 2 s -3) and (contours,units: 。C);and (e,f): As in (a,b),but for the CPLH (colour shading,units: 10 -2 m 2 s -3)and low-frequency temperature anomalies (contours,units: 。C).In (a—f),solid (dotted) contours indicate positive (negative) anomalies,and zero contours are given in bold solid lines.The contour intervals in (a,b) are 4 。C,in (c,d) are 1 。C,and in (e,f) are 2 。C.The purple boxes in (c,d) cover the domain (46°—57°N,103°—130°E)and in (f) the domain (39°—52°N,100°—117°E).The green line marks the topography greater than 1500 m.

Now,we examine the conversion of AP E H from the time-mean available potential energy,as represented by the first term on the rhs of Eq.(6),i.e.,CP BCH.It is clearly seen in Fig.3 that the distribution of CPBCHagreed well with that of CKAP,but with broader extension and larger amplitude,suggesting the major contributor of CPBCHto CKAPduring the cyclone development process.Since strong warm heat fluxes appeared earlier over Northeast China,the CPBCHover the same region was relatively strong from 0600 UTC to 1200 UTC 14 March (Fig.3 (e,f)).From 1800 UTC 14 to 0000 UTC 15 March,the cold heat flux became dominant,and hence,the CPBCHover southern and eastern Mongolia and Northwest and North China gradually strengthened (Fig.3 (g,h)).During this period,the CPBCHover Northeast China slowly weakened due to the reduction in warm heat flux.The abnormally strong CPBCH,therefore,was an important contributor to the strong eddy kinetic energy during the severe dust weather over Mongolia and northern China.

Why was there such a strong CPBCHover Mongolia and northern China during 14—15 March 2021? Now,we discuss this important issue from the perspective of interannual variation background and low-frequency circulation background.After decomposingˉTintois the climate mean andis the corresponding departure) and if other variables remain the same,CPBCHcan also be expressed as:

Fig.5.(a,b) The 850 hPa CKDTH (colour shading,units: 10 -2 m 2 s -3) and high-frequency wind anomalies (arrows,units: m s -1) at 1200 UTC and 1800 UTC 14 March 2021;(c,d) As in (a,b),but for the CKLH (colour shading,units: 10 -2 m 2 s -3).The green line marks the topography greater than 1500 m.

As such,CP BCHA may be considered the interannual anomaly component of CPBCH.Fig.4(a—d) shows the fields of CPBCHCand CPBCHAfrom 1200 UTC to 1800 UTC 14 March,by this time the Mongolian cyclone developed into a strong and mature cyclone.The distribution of CPBCHCmimicked the CP BCH,and the former naturally accounted for most of the latter (Fig.4(a,b)).However,the CPBCHAassociated with the interannual variation in 850 hPa temperature was positive and apparently large over Mongolia and northern China.In the domain (46°—57°N,103°—130°E;purple boxes in Fig.4 (c,d),the averaged CPBCHAaccounted for～40% of the CPBCHCin both Fig.4 (c,d).Considering its accumulative effect,the CPBCHAdid play a very important role in the strong development of the cyclone by transferring it to the KE H.As clearly seen in Fig.4 (c,d),the climatological southward temperature gradient at 850 hPa was amplified during early spring 2021 due to the warm temperature anomalies over Mongolia and northern China and the negative temperature anomalies to the north of the Mongolian Plateau.This was indeed a preferred interannual anomaly condition for the super strong Mongolian cyclone on 14—15 March 2021.Yin et al.(2022) attributed such climate anomalies to the autumn and winter sea ice anomalies in the Barents and Kara Seas and the winter sea surface temperature anomalies in the east Pacific and northwest Atlantic.

CP LH,the second term on the rhs of Eq.(6),represents the AP E H tendency due to the interaction between high-and low-frequency transient eddies.By assessing each term of Eq.(9),we found that the CPLHwas mainly contributed byi.e.,the baroclinic conversion of AP EHfrom the low-frequency available potential energy.At 1200 UTC and 1800 UTC 14 March (Fig.4 (e,f)),this term was negative over Northeast China,acting to offset the CPBCHC.This negative AP E H resulted from the synergy of the high-frequency warm heat fluxes (Fig.3 (f,g)) and low-frequency northwards temperature gradient(Fig.4 (e,f)) over Northeast China (see Eq.(9)).The most striking effect of the CP LH,however,was found at 1800 UTC 14 March in the domain of Mongolia and Northwest/North China (39°—52°N,100°—117°E;purple box in Fig.4 (f)),where the mean CP LH was equivalent to～38% of the mean CPBCHC.Over Northwest China,the CPLHwas equivalent to～65% of the CPBCHC.Analogously,the combined roles played by the high-frequency cold heat fluxes (Fig.3 (g)) and low-frequency southward temperature gradient (Fig.4 (f)) led to this result.This fact suggests that the sandstorm weather in Northwest and North China on the morning of 15 March (Beijing time) was partly due to the baroclinic conversion of AP EHfrom the low-frequency available potential energy (see also Fig.3 (g)).

CK DTH,the second term on the rhs of Eq.(1),primarily manifests the downwards transfer of KEHby the high-frequency eddies (see the first two terms of Eq.(4)) and represents the key feature of the so-called Type B cyclone (Pettersen and Smebye,1971).As clearly seen in Fig.5 (a,b),the downwards transfer mechanism started to dominate behind the cold front when the Mongolian cyclone was in its rapidly developing stage.At this stage,the CKDTHwas considerably strong over southern Mongolia and Northwest China,up to 2 ×10-2m2s-3.Thus,CK DTH worked as a large term in Eq.(1) and was comparable to CKAP.After maturing the cyclone (from 0060 UTC 15 March onwards),the CK DTH began to weaken (not shown).In contrast,the CKLHhad an advecting effect,acting to displace the KEHnortheastward (Fig.5 (c,d)).

4.Discussion

In the present study,the evolution process of a powerful Mongolian cyclone on 14—15 March 2021 is analysed from the perspective of highfrequency eddy energetics based on 6-hour ERA5 reanalysis data.

Starting in early March 2021,the low-frequency circulation background in the mid-and lower troposphere has been distinct and favourable for the development of the Mongolian cyclone.A remarkably strong baroclinic zone was built up from central Asia to the east of Lake Baikal.On 12—13 March,the tilted trough on the northwestern side of Lake Balkash became a migratory trough and then moved southeastward.This trough triggered the Mongolian cyclone after crossing over the Sayan Mountains.The strong intensification of the Mongolian cyclone led to severe dust weather in Mongolia and northern China.

The strong downwards transfer of eddy kinetic energy was an important mechanism in cyclone intensification.On the other hand,the cyclone development exhibited an obvious feature of the frontal cyclone wave,with a strong KEHconversion from the AP EH.This conversion is consistent with the strong baroclinicity of the mid-and upper tropospheric circulations associated with this cyclone.The strong AP E H resulted from the strong CP BCH,and the latter was contributed both by the interannual temperature anomaly pattern of “north cold south warm ”and by the low-frequency temperature anomaly pattern of“northwest cold southeast warm ”.In both the eddy kinetic and available potential energy budgets,the interaction between high-frequency and low-frequency waves was important,further highlighting the importance of low-frequency circulation background for the development of the Mongolian cyclone.

As clearly seen in Figs.1 (h,g) and 2 (a,b),the cyclonic circulation experienced intense development when it passed through the western Mongolian topography,including the Sayan Mountains.This phenomenon can be explained from the perspectives of potential vorticity(PV) conservation and eddy energetics.In an isentropic coordinate system,as Hoskins et al.(1985) addressed,PV=-g(f+k·?θ×v) ·wheregis gravity acceleration,fis coriolis parameter,kis a unit vertical vector,θis potential temperature,vis the horizontal wind on the isentropic surface,andpis pressure.Thus,an inverse relationship between vorticity and static stability holds if PV is conserved.When an air parcel passes across a mountain,the lower isentropes show a terrain-following feature in the leeward slope of the mountain,where it corresponds to reduced static stability (Martin,2006).Consequently,the relative vorticity increases in the leeward slope of the mountain.For this reason,cyclone genesis and cyclone development often occur on the eastern slope of the western Mongolian topography (Huang et al.,2016).Orlanski and Gross (1994) examined the influence of an east—west oriented mountain,such as the Sayan Mountains,on cyclone development.When a baroclinic wave passes through such an east—west oriented mountain ridge,a relatively intense cyclone forms on the south side of the mountain ridge,where the topography slopes in the same direction as the isentropes.They explained this type of cyclone development as gaining available potential energy from the orographically induced background flow with a strong meridional temperature contrast.This mechanism is somewhat similar to that involved in the CP BCH in Eq.(6).On the other hand,when an air parcel passes across a mountain and slides down along the isentropic surfaces,an extra sinking motion occurs and thus facilitates the possibility of strong downwards transfer of KE H,as the reviewer suggested.We will reveal this issue in future investigations.

Funding

This work is supported by the National Natural Science Foundation of China [Grant No.41630424 ].

Acknowledgments

The authors thank two anonymous reviewers for their constructive and helpful comments.The first and second authors convey their sincere thanks to Prof.Jingbei Peng for her constructive discussions and comments on this work.

Supplementary materials

Supplementary material associated with this article can be found,in the online version,at doi: 10.1016/j.aosl.2022.100259.