atmosphere

Article Case Study of a Heavy Snowstorm Associated with an Extratropical Cyclone Featuring a Back-Bent Warm Front Structure

Yu Zhao 1,2,3,*, Liang Fu 4, Cheng-Fang Yang 5 and Xiang-Fu Chen 1,2,3

1 Key Laboratory of Meteorological Disaster, Ministry of Education (KLME), Nanjing University of Information Science & Technology, Nanjing 210044, China; [email protected] 2 Joint International Research Laboratory of Climate and Environment Change (ILCEC), Nanjing University of Information Science & Technology, Nanjing 210044, China 3 Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters (CIC-FEMD), Nanjing University of Information Science & Technology, Nanjing 210044, China 4 Weifang Meteorological Bureau, Weifang 261011, China; fl[email protected] 5 Shandong Meteorological Observatory, Jinan 250031, China; [email protected] * Correspondence: [email protected]



Received: 29 October 2020; Accepted: 21 November 2020; Published: 25 November 2020


Abstract: An extreme snowstorm event that occurred over Heilongjiang and Jilin Provinces on 24–26 November 2013 was related to a cyclone characterized by a back-bent occluded front structure. This study investigates the structure of the back-bent occluded front and snowfall mechanism using multiple observations and NCEP/NCAR 1 1 reanalysis data in concert with the HYSPLIT model. ◦ × ◦ The main results show that the extreme event was more synoptically governed by the outbreak of the polar vortex and moisture anomaly of the East Sea. The cyclone occurred just ahead of the 500-hPa merged deep trough, and then developed under the effect of the positive vorticity advection ahead of the 500-hPa trough and intense divergence of the upper-level jet. The south-southwest wind strengthened obviously after the merger of the southern and northern branch troughs, which was the main reason behind the cyclone moving northward. The moisture mainly originated from the Sea of Japan, insofar as that dry and cold air in the lower troposphere over the western mainland moistened obviously as it turned southward and passed over the Bohai Sea and the Sea of Japan, supplying abundant moisture for the snowstorm event. The intensity of moisture transport depended on the location and intensity of the cyclone. When the cyclone developed, the dry air continuously intruded into the cyclone’s center, and made a conveyor belt of warm air wrap around it. The dry air gradually changed from descending to ascending motion as it moved ahead of the westerly trough, while the moist air in the northern part of the cyclone moved to the west and south and incorporated into the south of the cyclone center. Warm and moist air was lifted and arrived in the northwestern part of the cyclone after the occluded front’s formation. Frontogenesis within the comma head was enhanced evidently owing to the rotation and deformation. The convergence between the southeast and northeast winds resulted in intense frontogenesis, leading to the enhancement of the front-scale ascent. Strong ascent formed in the comma head of the cyclone, which resulted in intense snowfall.

Keywords: snowstorm; extratropical cyclone; back-bent warm front; occluded front; frontogenesis

1. Introduction An occluded front is often formed during the development of a stronger cyclone. Operational forecasts show that some extratropical cyclones producing snowstorms over northeastern China in

Atmosphere 2020, 11, 1272; doi:10.3390/atmos11121272 www.mdpi.com/journal/atmosphere Atmosphere 2020, 11, 1272 2 of 21 winter are more likely to occlude. Apart from the traditional occluded front in the Norwegian cyclone model, which is formed as a faster-moving cold front catches up to a slower-moving warm front, some occluded fronts are formed as warm fronts bend backward and cold conveyor belts wrap around the low center of the cyclones [1–3]. An occluded (warm) front that undergoes this type of wrap-up process is called a back-bent occluded (warm) front [4]. Many severe snowstorms are associated with this type of occluded cyclone over northeastern China. For example, a strong snowstorm event that occurred on 3–5 March 2007 stemmed from a cyclone, whose warm front stretched westward and southward and then twined in a spiral around the cyclone’s center to form an occluded front during the period when the cyclone rapidly deepened and moved northward from the Yangtze River Basin. This cyclone resulted in snowstorms and storm surge disasters in eight Provinces over northeastern and northern China, with total accumulative snowfall at 32 stations in Lioaning Province exceeding 25 mm, which was the most severe snowstorm and rare temperature-dropping weather in the same period in history over the past 50 years [5]. Two conceptual models that describe cyclone structure and evolution are accepted generally. The formation of an occluded front in the Norwegian cyclone model was first described by Bjerknes [6] and Bjerknes and Solberg [7], which is characterized by the warm sector continuously narrowing and being lifted during the process of a cold front catching up to a warm front. The cold front is generally stronger than the warm front and the cyclone becomes meridionally elongated. In order to explain a rapidly deepening marine cyclone, Shapiro and Keyser [8] proposed another conceptual model of cyclone evolution, which is characterized by a back-bent warm front, T-shaped frontal structure and warm-core seclusion. The warm front is generally stronger than the cold front and the cyclone becomes zonally elongated. To further emphasize the lengthening of warm fronts and the continued existence of the frontal T bone, Schultz et al. [9] changed the back-bent warm front into a back-bent occluded front in the Shapiro–Keyser cyclone model. A T-shaped frontal structure and back-bent warm front are frequently observed in explosive-deepening cyclones over the temperate ocean in winter [9–11]. Several continental cyclones over Mongolia and northeastern China also exhibit an obvious back-bent warm-front structure [12–14]. Fu et al. [15] surveyed 17 snowstorm cases related to northward-moving cyclones over northeastern China from 2000 to 2016, among which comma- and spiral-shaped cloud belts were frequently observed [2,16], indicating that the development of cyclones was characterized by a back-bent warm front and wrap-up process. Wang et al. [17] investigated a rain–snow event associated with a cyclone that formed over the Hetao area, and proposed that the occurrence and development of the cyclone did not fall under the classical “second type” development mechanism for extratropical cyclones. Some investigations on snowstorms resulting from cyclones over eastern China have been documented, mainly focusing on the mechanism of rapidly deepening cyclones [18,19], dry- and cold-air activities [20], cloud system features [21], and mesoscale gravity waves [22,23]. However, few studies have been devoted to the structure of back-bent occluded fronts and associated effects on snowfall. Severe snowstorm events associated with extratropical cyclones are also frequently observed in the United States and Europe, and so many related studies have been carried out [24–26]. Case studies and climatological analyses show that precipitation in the northwest quadrant of cyclones occurs in an environment conducive to frontogenesis [27–30]. Statistics reported by Novak et al. [30] show that the mesoscale precipitation belt occurs within a 700-hPa trough that appears to be vertical extensions of the surface warm occluded front, and the banded precipitation occurs in the area where the warm advection and potential vorticity advection increase with height. Novak et al. [31] found that the snow band in the mature stage is marked by increasing conditional instability and increased frontogenetical forcing. The snow band dissipates when the frontogenetical forcing weakens and conditional stability increases. They found changes in inertia, symmetry and conditional instability, as well as inertial neutrality, in the different stages of snowfall. Schumacher et al. [32] showed the existence of symmetric, conditional and inertial instability in the banded convective snow band. The northwest quadrant Atmosphere 2020, 11, 1272 3 of 21 of surface cyclones is a juxtaposition of frontogenesis and instability, where it is conducive to the occurrence of intense snowfall [29]. However, understanding is limited with respect to the evolution, frontal structure and associated snowfall mechanism in a cyclone characterized by a back-bent warm front and wrap-up process, which leads to inaccurate fine-scale forecasting of snow. In operational forecasts, the prediction of snowfall amount is often underestimated, and the locations of intense snowfall are inaccurate, especially in quantitative snow intensity with a big bias. An intense snowstorm event hit Heilongjiang and Jilin Provinces on 24–26 November 2013, with accumulated snowfall exceeding 20 mm in the east of these provinces and a maximum snowfall of 60 mm in Shuangyashan. The daily snowfall in nine counties over Heilongjiang Province broke the historic record for November since 1961. This extreme event, induced by a rapidly deepening cyclone that formed in northern Jiangsu Province and moved northeastward, brought strong winds and low visibility and disrupted roads and air traffic, seriously affecting people’s lives [33,34]. Notably, the development of the cyclone featured a back-bent warm front and warm-core seclusion [2], thus raising the following questions: How did the fronts evolve? What was the structure of the back-bent occluded front? And what effect did it have on snowfall? This paper seeks to answer these questions and provide a reference for forecasting this type of snowstorm. The rest of this paper is organized as follows: Section2 describes the data and methods. An overview of the cyclone, snowfall and cloud systems is provided in Section3. The synoptic-scale background, front evolution and mesoscale characteristics of the event are presented in Section4. Section5 explores the snowfall mechanism. And finally, a summary and discussion are presented in Section6.

2. Data and Methods The data used in this paper include conventional surface, upper-air, Jiansanjiang Doppler radar (indicated by the black open circle in Figure1a), and FY-2E satellite observations; and the Final Analysis (FNL) gridded data of the National Centers for Environmental Prediction (NCEP) with a 6 h interval and 1 1 horizontal resolution. The FNL data were obtained from the archive ◦ × ◦ (https://rda.ucar.edu/datasets/ds083.2/). FY-2E is Chinese third operational geostationary meteorological satellite, whose Stretched Visible and Infrared Spin Scan Radiometer (S-VISSR) include four Infrared channels (IR1, IR2, IR3, and IR4) and one Visible channel, with a 5 km and 1.25 km resolution at nadir, respectively. The infrared brightness temperature in spectral range between 6.3 and 7.6 µm (IR3) with a 1 h interval was used in this study. The Jiansanjiang radar is a C-band radar, operating in volume coverage pattern 21 (VCP21), scanning nine elevation angles of 0.5◦, 1.5◦, 2.4◦, 3.4◦, 4.3◦, 6.0◦, 9.9◦, 14.6◦, and 19.5◦ with a volumetric update time of 6 min during this event. The elevation of the radar antenna is 165 m above mean sea level (MSL) and 90 m above ground level (AGL). The maximum unambiguous ranges for radar reflectivity and radar radial velocity are 300 and 150 km, respectively. The Nyquist velocities are from 24.86 to 24.86 m s 1. The Jiansanjing radar’s observation time is from − · − 0100 UTC to 0700 UTC during this event. We used the Air Resources Laboratory Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model, version 4.9 [35,36], developed by the National Oceanic and Atmospheric Administration, to track the moisture transport trajectory and moisture change of the snowstorm event. In this model, the Lagrangian method is applied to calculate the advection and diffusion, and it is commonly used to track the trajectories of air particles, with good results having been achieved [37–40]. Thus, the moisture transport features and relationship with the cyclone were examined using this model. The central snowfall area (46.5◦–47◦ N, 130◦–131.5◦ E, indicated by red rectangle in Figure1a) was selected as the simulation area, and 500 m, 1500 m, and 3000 m were selected as the initial heights in the vertical direction. There are 24 initial points in the simulated track, with 8 initial points at each level. NCEP/GDAS (Global Data Assimilation System) data with a horizontal resolution of 0.5 0.5 ◦ × ◦ and an interval of 6 h were used as the initial fields and 60 h air parcel back trajectories were calculated Atmosphere 2020, 11, 1272 4 of 21 at the time of initial snowfall (0000 UTC 25 November) and intense snowfall (0600 UTC 25 November), respectively. All air parcel trajectories were recorded every 6 h from the model forecast. Atmosphere 2020, 11, x FOR PEER REVIEW 4 of 21

FigureFigure 1. (a) 1. The (a) centralThe central sea sea level level pressure pressure of of the the cyclonecyclone with with positions positions plotted plotted every every 6 h. Also 6 h. Alsoplotted plotted are theare names the names of related of related provinces, provinces, the the city city of of ShuangyashanShuangyashan, and and the the location location of Jiansanjiang of Jiansanjiang radar radar (open circles, labeled as letters “S” and “J”, respectively). The red rectangle area denotes the simulated (open circles, labeled as letters “S” and “J”, respectively). The red rectangle area denotes the simulated region of the HYSPLIT model. (b) Intensity evolution of the minimum central sea level pressure of b regionthe of thecyclone HYSPLIT (unit: hPa). model. ( ) Intensity evolution of the minimum central sea level pressure of the cyclone (unit: hPa). 3. Overview of the Cyclone, Cloud Systems and Snowfall 3. Overview of the Cyclone, Cloud Systems and Snowfall 3.1. Overview of the Cyclone 3.1. Overview of the Cyclone The cyclone initially formed over northern Jiangsu Province at 0000 UTC 24 November, with a Thecentral cyclone pressure initially of 1012 formed hPa (Figure over 1a). northern After that, Jiangsu the cyclone Province moved at northeastward 0000 UTC 24 and November, developed with a centralintensively, pressure of passed 1012 over hPa (Figurethe Yellow1a). Sea After andthat, northe thern cycloneKorean Peninsula, moved northeastward moved on over and the Sea developed of intensively,Japan and passed Stanovoy over Range, the Yellow and entered Sea and the northernSea of Okhotsk Korean by 0000 Peninsula, UTC 27 November. moved on The over cyclone the Sea of Japan anddeveloped Stanovoy at a linear Range, deepening and entered rate in thethe initial Sea of 30 Okhotsk h after formation by 0000 (Figure UTC 27 1b). November. Its central pressure The cyclone was 986 hPa by 0000 UTC 25 November, and it reached its minimum central pressure of 981 hPa at developed at a linear deepening rate in the initial 30 h after formation (Figure1b). Its central pressure 0600 UTC 25 November. During the subsequent 36 h, the central pressure rose slightly and was 986 hPa by 0000 UTC 25 November, and it reached its minimum central pressure of 981 hPa at 0600 maintained at lower values. The central pressure of the cyclone decreased by 26 hPa from 0000 UTC UTC 2524November. to 0000 UTC During25 November the subsequent at a cyclone-deepening 36 h, the central rate of 1.08 pressure Bergeron, rose meeting slightly the and criteria maintained of an at lower values.explosive The cyclone. central pressure of the cyclone decreased by 26 hPa from 0000 UTC 24 to 0000 UTC 25 November at a cyclone-deepening rate of 1.08 Bergeron, meeting the criteria of an explosive cyclone. 3.2. Cloud Systems of the Cyclone 3.2. Cloud Systems of the Cyclone The water vapor cloud images (Figure 2) show the evolution of the cyclone’s cloud systems in Theits waterdevelopment vapor cloudprocess. images At 0600 (Figure UTC 224) showNovemb theer, evolution cloud systems of the had cyclone’s been transformed cloud systems into in its developmentcomma cloud process. from At the 0600 baroclinic UTC leaf 24 November,as the cyclone cloud initially systems formed had (not been shown), transformed and the core into of the comma cyclone was in the area of cold and moist air with cloud top temperatures <−52 °C and 500-hPa cloud from the baroclinic leaf as the cyclone initially formed (not shown), and the core of the cyclone relative humidity >90% (Figure 2a). The dark area was located to the southwest of the cyclone, was in the area of cold and moist air with cloud top temperatures < 52 C and 500-hPa relative corresponding to the adiabatic descending dry and cold air with 500-hPa relative− ◦ humidity <70% and humiditycloud>90% top temperatures (Figure2a). The>−22dark °C. By area 1800 was UTC located 24 November, to the southwest the cyclone of had the deepened cyclone, correspondingobviously, to thewith adiabatic more dry descending air intruding dry into and the cold cyclone air and with the 500-hPa core of the relative cyclone humidity being occupied<70% by and the clouddark top temperaturesarea. Common> 22 C.cyclogenesis By 1800 UTC features 24 November, were evident the in cyclone the cloud had patterns, deepened including obviously, a comma with head more dry − ◦ air intrudingover Jilin into and the Heilongjiang cyclone and Provinces the core and of a the dry cyclone slot near being northern occupied Korean by Peninsula the dark (Figure area. Common2b). cyclogenesisCompared features with Figure were 2a, evident the warm in front the cloud be patterns,lts stretched including westward a and comma southward head around over Jilinthe and cyclone center, with the cold front cloud belts moving eastward. As a result, the northern portion of Heilongjiang Provinces and a dry slot near northern Korean Peninsula (Figure2b). Compared with the cold front weakened and the cold and warm fronts in the center of the cyclone did not connect Figurewith2a, the each warm other, front showing cloud a frontal belts fracture stretched. The westward coverage of and cloud southward top temperatures around <− the52 °C cyclone and 500- center, with thehPa cold relative front humidity cloud belts >70% moving within the eastward. warm fr Asont acloud result, belts the was northern expanded portion to the of north the coldand front weakenednortheast, and theand coldthe warm and warmand moist fronts air on in the the nort centerhwest of side the was cyclone wrapped did around not connect the cyclone with center, each other, showingshowing a frontal that a fracture. back-bent The occluded coverage front was of cloud formed top at this temperatures time (Figure< 2b).52 C and 500-hPa relative − ◦ humidity >70% within the warm front cloud belts was expanded to the north and northeast, and the Atmosphere 2020, 11, 1272 5 of 21 warm and moist air on the northwest side was wrapped around the cyclone center, showing that a back-bent occluded front was formed at this time (Figure2b).

Figure 2. Water vapor satellite images from the cloud top temperature (unit: ◦C) of FY-2E at (a) 0600 UTC 24 November, (b) 1800 UTC 24 November, (c) 0000 UTC 25 November, and (d) 0600 UTC 25 November 2013. Overlaid on the images are the surface low pressure center (L) and the front, the sea level pressure field (black lines; unit: hPa), and the 500-hPa 70% and 90% relative humidity (green lines; unit: %), calculated from NCEP/NCAR reanalysis data with a resolution of 1 1 . The letter ◦ × ◦ “J” represents the radar station position at Jiansanjiang; the yellow lines AB and CD in (d) denote the location of the cross sections used in Figure 9.

By 0000 UTC 25 November, the dry air had intruded into the core and southwest quadrant of the cyclone, with the cyclone’s dry slot evident in coastal areas of eastern Jilin Province and the cloud systems to the north of the occluded front starting to intensify (Figure2c). The warm front cloud belts stretched southwestward and southward evidently, and wrapped around the cyclone’s center (Figure2c). At 0600 UTC 25 November, the cloud systems to the north of the back-bent occluded front intensified significantly, and the comma head elongated with an east–west orientation and narrowed with a more obvious warp-up feature (Figure2d). Six hours later, with the movement of the cyclone, the intense cloud systems to the north of the occluded front moved into Russia (not shown), so the snowfall over Heilongjiang Province weakened. In1 short, the cyclone development was characterized by zonally elongated warm front cloud belts and underwent a wrapping-up process [4,8,9], which is similar to the Shapiro–Keyser cyclone model. That is, the warm front stretched westward and southward as the cyclone developed, and then wrapped around the center of the cyclone. Atmosphere 2020, 11, 1272 6 of 21

3.3. Overview of Snowfall Atmosphere 2020, 11, x FOR PEER REVIEW 6 of 21 Affected by the cyclone, rain first emerged in Liaoning Province at around 0600 UTC 24 November (Figure3),3.3. and Overview snow of Snowfall fell in Heilongjiang, Jilin and northern Liaoning Provinces at 1200 UTC 24 November,Affected with the by snowfall the cyclone, event rain persisting first emerged until in 0000 Liaoning UTC Province 26 November. at around The 0600 distribution UTC 24 of 24 h snowfall showsNovember that (Figure the snowstorm3), and snow fell occurred in Heilongjiang mainly, Jilin in eastern and northern Jilin andLiaoning southeastern Provinces atHeilongjiang 1200 Provinces,UTC with 24 November, a maximum with snowfall the snowfall of 26event mm persisti in Antung until from 0000 0000 UTC UTC26 November. 24 to 0000 The UTCdistribution 25 November (Figure3a).of 24 The h snowfall heavy snowfallshows that moved the snowstorm to central occu andrred easternmainly in Heilongjiang eastern Jilin and Province southeastern (maximum Heilongjiang Provinces, with a maximum snowfall of 26 mm in Antu from 0000 UTC 24 to 0000 UTC snowfall amount reaching 57.4 mm in Shuangyashan) from 0000 UTC 25 to 0000 UTC 26 November 25 November (Figure 3a). The heavy snowfall moved to central and eastern Heilongjiang Province (Figure3b),(maximum with much snowfall of the amount snow reaching occurring 57.4 duringmm in Shuangyashan) the period 0000–1200 from 0000 UTC UTC 2525 to 0000 November UTC 26 and the maximumNovember 6 h snowfall (Figure being 3b), largerwith much than of 20 the mm snow (Figure occurring3e,f). during The observed the period snowfall 0000–1200 in UTC Shuangyashan 25 shows thatNovember the strongest and the snowfall maximum occurred 6 h snowfall during being 0600–0900 larger than UTC 20 mm 25 (Figure November, 3e–f). withThe observed a maximum 3 h snowfall ofsnowfall 12.7mm. in Shuangyashan shows that the strongest snowfall occurred during 0600–0900 UTC 25 November, with a maximum 3 h snowfall of 12.7 mm.

Figure 3. FigureRain gauge3. Rain gauge observed observed precipitation precipitation ((aa)) from 0000 0000 UTC UTC 24 to 24 0000 to UTC 0000 25,( UTCb) from 25, 0000 (b) fromUTC 25 0000 UTC 25 to 0000to UTC 0000 UTC 26 November 26 Nov 2013, 2013,(c) from (c 1200) from UTC 1200 to 1800 UTC UTC to 24, 1800 (d) from UTC 1800 24, UTC (d) 24 from to 0000 1800 UTC UTC 25, (e 24) to 0000 from 0000 UTC to 0600 UTC 25, (f) from 0600 UTC to 1200 UTC 25 Nov 2013 and (g) snowfall (bars) UTC 25, (e) from 0000 UTC to 0600 UTC 25, (f) from 0600 UTC to 1200 UTC 25 November 2013 and (g) snowfall (bars) evolution at Shuangyashan station from 1200 UTC 24 to 0300 UTC 26 November 2013 (unit: mm; the interval is 3 h in the daytime and 10 h in the night (indicated by red stars above the bar). The letters “S”, “A” and “J’ denote the cities of Shuangyashan, Antu and the location of Jiansanjiang radar, respectively. Atmosphere 2020, 11, x FOR PEER REVIEW 7 of 21

Atmosphereevolution2020, 11 ,at 1272 Shuangyashan station from 1200 UTC 24 to 0300 UTC 26 Nov 2013 (unit: mm; the interval 7 of 21 is 3 h in the daytime and 10 h in the night (indicated by red stars above the bar). The letters “S”, “A” and “J’ denote the cities of Shuangyashan, Antu and the location of Jiansanjiang radar, respectively. 4. Synoptic-Scale Background and Mesoscle Features 4. Synoptic-Scale Background and Mesoscle Features 4.1. Atmospheric Circulation Patterns 4.1.The Atmospheric atmospheric Circulation circulation Patterns patterns were favorable for the extreme snowfall event. Figure4a,b show theThe average atmospheric geopotential circulation height patterns at were 300 hPa, favorabl thee average for the extreme vertically snowfall integrated event. moisture Figure 4a,b flux, andshow its anomaliesthe average for geopotential 25 November height 2013, at 300 respectively. hPa, the average The polar vertically vortex integrated was deflected moisture in flux, the eastern and hemisphere,its anomalies with for intense 25 November negative 2013, anomalies respectively of the. The geopotential polar vortex height, was anddeflected a split in low the center eastern was locatedhemisphere, over the with Novosibirsk intense negative Islands anomalies (Figure4 ofa). th Ae blockinggeopotential high height, extended and a from split Lake low center Balkhash was to westernlocated Siberia, over the and Novosibirsk another high-pressure Islands (Figure ridge 4a). existed A blocking in the high Sea of extended Okhotsk. from A deep Lake trough Balkhash between to themwestern stagnated Siberia, over and eastern another China. high-pressure The larger ridge positive existed and in negative the Sea anomaliesof Okhotsk. of A the deep geopotential trough heightbetween corresponded them stagnated with the over high-pressure eastern China. ridges The and larger thelow-pressure positive and system,negative respectively. anomalies of The the split polargeopotential vortex was height favorable corresponded for the cold with air to the move high into-pressure the deep ridges trough and and the affected low-pressure northeastern system, China, whichrespectively. is consistent The split with polar polar vortex vortices was playing favorable an fo importantr the cold roleair to in move extreme into weather the deep events trough [ 41and–43 ]. Thereaffected is also northeastern abundant moistureChina, which over is the consistent East Sea duringwith polar the developmentvortices playing of thean important cyclone (Figure role in4b). Associatedextreme weather with the events intensified [41–43]. southwesterly There is also flow abundant ahead ofmoisture the deep over trough the East and Sea the southeasterlyduring the development of the cyclone (Figure 4b). Associated with the intensified southwesterly flow ahead of flow in the northeast side of the cyclone, above-normal moisture was transported from the Sea of Japan. the deep trough and the southeasterly flow in the northeast side of the cyclone, above-normal Enhanced moisture flux led to moisture convergence over the eastern part of the northeastern China, moisture was transported from the Sea of Japan. Enhanced moisture flux led to moisture convergence resulting in the snowfall. over the eastern part of the northeastern China, resulting in the snowfall.

FigureFigure 4. 4.(a ()a The) The mean mean geopotential geopotential heightheight at 300 300 hP hPaa (contoured (contoured every every 80 80 gpm) gpm) and and its itsanomaly anomaly (shaded), and (b) the mean vertically integrated (surface–300 hPa) moisture flux (vectors; kgm−1 s1−1) 1 (shaded), and (b) the mean vertically integrated (surface–300 hPa) moisture flux (vectors; kg m− s− ) −4 −2 −1 and anomalies of moisture flux (shaded, 104 kg m 2 s 1) averaged over 0000 UTC 25 Nov–0000· UTC· and anomalies of moisture flux (shaded, 10− kg m− s− ) averaged over 0000 UTC 25 November–0000 26 November 2013. The anomalies were computed· rela· tive to the November climatological mean of UTC 26 November 2013. The anomalies were computed relative to the November climatological mean 2000–2019. of 2000–2019.

4.2.4.2. Synoptic-Scale Synoptic-Scale Background Background TheThe heavy heavy snowfall snowfall event event occurred occurred inin anan environmentenvironment that that is is favorable favorable for for cyclogenesis. cyclogenesis. At At0000 0000 UTCUTC 23 23 November, November, a a 500-hPa 500-hPa low low waswas presentpresent over the central central Siberian Siberian Plateau, Plateau, with with a longwave a longwave trough extending southward from the low center into eastern Mongolia (Figure 5a). A middle branch trough extending southward from the low center into eastern Mongolia (Figure5a). A middle branch trough was centered over central Gansu and eastern Qinghai Provinces, with strong cold advection trough was centered over central Gansu and eastern Qinghai Provinces, with strong cold advection behind its axis. In addition, a strong frontal zone at the base of the two troughs can be observed behind its axis. In addition, a strong frontal zone at the base of the two troughs can be observed (Figure 5a). More to the south, a southern branch trough extended from eastern Sichuan Province to (Figure5a). More to the south, a southern branch trough extended from eastern Sichuan Province to Yunnan Province. A 35 m·s−1 northern jet and a 45 m·s−1 southern jet at 300 hPa were found at the base Yunnan Province. A 35 m s 1 northern jet and a 45 m s 1 southern jet at 300 hPa were found at the of the 500-hPa longwave· and− the middle branch troughs,· − respectively (Figure 5b). By 0000 UTC 24 baseNovember, of the 500-hPa the 500-hPa longwave middle and branch the middle and the branch southe troughs,rn branch respectively troughs merged (Figure into5b). a deep By 0000 trough UTC 24moving November, eastward, the 500-hPa with strong middle positive branch vorticity and the advection southern branchahead of troughs the trough merged (Figure into 5c). a deep An trough850- movinghPa vortex eastward, over with Shandong strong positiveProvince vorticity (not shown) advection and aheada surface of thecyclone trough over (Figure northern5c). An Jiangsu 850-hPa vortexProvince over (Figure Shandong 1a) had Province occurred (not just shown) ahead andof this a surface500-hPa cyclonemerged overdeep northerntrough. Furthermore, Jiangsu Province the (Figure 1a) had occurred just ahead of this 500-hPa merged deep trough. Furthermore, the northern longwave trough at 500 hPa, with evident cold advection behind its trough axis, lay slightly behind the merged deep trough (Figure5c). Twelve hours later, the 850-hPa vortex moved over the Yellow Sea, AtmosphereAtmosphere2020, 202011, 1272, 11, x FOR PEER REVIEW 8 of 218 of 21

northern longwave trough at 500 hPa, with evident cold advection behind its trough axis, lay slightly with abehind west–east the merged oriented deep shear trough line (Figure extending 5c). Twelve from thehours low later, center the and850-hPa strong vortex southwesterly moved over the airflow reachingYellow 33 mSea,s 1 withon the a southwest–east side oriented of the shear shear line line (Figure extending6a), transporting from the low moisture center fromand thestrong Yellow · − Sea tosouthwesterly northeastern airflow China. reaching Accompanying 33 m·s−1 on thethe south cold airside sweeping of the shear to line the (Figure south and6a), transporting the transport of warmmoisture and moist from air, the the Yellow baroclinicity Sea to northeastern of the atmosphere China. wasAccompanying intensified, the leading cold air to explosivesweeping deepeningto the of thesouth surface and cyclone the transport (Figure of 2warm). and moist air, the baroclinicity of the atmosphere was intensified, leading to explosive deepening of the surface cyclone (Figure 2).

FigureFigure 5. The 5. The geopotential geopotential height height (black (black solid solid lines;lines; 10 geopotential meters meters (unit: (unit: gpm), gpm), temperature temperature −1 −1 (red dashed lines; unit: °C), wind (full barb = 4 m s 1, pennant = 20 m s ) and1 positive vorticity (red dashed lines; unit: ◦C), wind (full barb = 4 m s− , pennant = 20 m s− ) and positive vorticity advection (shaded; unit: 10−9 s−2 ) at 500 hPa, at (a) ·1200 UTC 23 Nov, (c) 0000· UTC 24 Nov and (e) advection (shaded; unit: 10 9 s 2 ) at 500 hPa, at (a) 1200 UTC 23 November, (c) 0000 UTC 24 November 0000 UTC 25 Nov 2013 (thick− − black solid line denotes the trough line). The 300-hPa wind speeds ≥ 30 and (e) 0000 UTC 25 November 2013 (thick black solid line denotes the trough line). The 300-hPa ms−1 (black contours every 5 m s−1) and divergence >0 (shading; unit : 10−5 s−1) are plotted at (b) 1200 wind speeds 30 m s 1 (black contours every 5 m s 1) and divergence >0 (shading; unit: 10 5 s 1) are UTC 23 Nov,≥ (d)1200· − UTC 24 Nov and (f) 0600 UTC· − 25 Nov 2013. The red dashed lines with an arrow− − plotted at (b) 1200 UTC 23 November, (d) 1200 UTC 24 November and (f) 0600 UTC 25 November 2013.

The red dashed lines with an arrow denotes the upper-level jet axis. The locations of Qinghai (QH), Gansu (GS), Sicuan (SC), Yunnan (YN), Shandong (SD), Anhui (AH), Jiangsu (JS), Heilongjiang (HLJ), Jilin (JL), and Liaoning (LN) Provinces are labeled in Figure5b. Atmosphere 2020, 11, x FOR PEER REVIEW 9 of 21

denotes the upper-level jet axis. The locations of Qinghai (QH), Gansu (GS), Sicuan (SC), Yunnan Atmosphere(YN),2020 Shandong, 11, 1272 (SD), Anhui (AH), Jiangsu (JS), Heilongjiang (HLJ), Jilin (JL), and Liaoning (LN)9 of 21 Provinces are labeled in Figure 5b.

Figure 6. Geopotential height (black solid lines; unit: gpm), gpm), potential potential temperature temperature (red (red dashed lines; unit: °K),K), and and wind wind (units: (units: m·s m−1)s at1 )850-hPa at 850-hPa and andthe divergence the divergence of integrated of integrated moisture moisture flux of flux 1000– of ◦ · − 1000–300-hPa300-hPa (color (colorshaded; shaded; units: 10 units:−4 kg·m 10−2 s4−1)kg at m(a) 21200s 1) UTC at (a 24) 1200 November, UTC 24 (b November,) 0000 UTC 25 (b) November, 0000 UTC − · − · − 25(c) November,0600 UTC 25 (c )November, 0600 UTC 25 and November (d) 1200 andUTC ( d25) 1200November UTC 25 2013. November Overlaid 2013. on Overlaidthe images on are the the images low arecenter the (D) low and center warm (D) core and (W). warm Thic corek solid (W). Thicklines are solid the lines trough are lines. the trough lines.

By 0000 UTC 25 November, the northern longwave trough at 500-hPa and the merged deep trough to to the the south south had had merged merged to toform form a deep a deep trough trough with with a span a spanof more of than more 30° than of latitude 30◦ oflatitude (Figure (Figure5e), which5e), enlarged which enlarged the southerly the southerly component component ahead of ahead the trough of the and trough steered and the steered cyclone the cycloneto move tonorthward. move northward. At 300-hPa, At the 300-hPa, northern the northernand southern and southernjets had merged jets had to merged form toa core form over a core eastern over easternHeilongjiang Heilongjiang Province Province with a wind with aspeed wind of speed 50 m·s of− 501, with m s Jilin1, with and Jilin Heilongjiang and Heilongjiang Provinces Provinces on the · − onright the side right of the side entrance of the entranceof the northern of the jet northern where strong jet where divergence strong divergenceexisted (not existedshown). (not The shown).merger Theof the merger northern of the and northern southern and jets southern occurred jets at occurred 1200 UTC at 24 1200 November UTC 24 November (Figure 5d), (Figure and the5d), northern and the northernjet core intensified jet core intensified obviously obviously after the merger, after the with merger, eastern with Liaoning eastern Province Liaoning being Province on the being right on side the rightof the side entrance of the of entrance the northern of the upper-level northern upper-level jet. At 850-hPa, jet. At the 850-hPa, vortex the deepened vortex deepenedobviously obviously owing to owingthe substantial to the substantial intensification intensification of the positive of the vort positiveicity vorticityadvection advection ahead ofahead the 500-hPa of the 500-hPatrough and trough the andenhancing the enhancing warm advection warm advection (Figure 6b (Figure). The 6southeastb). The southeast winds to windsthe north tothe of the north shear of theline shear increased line increasedevidently, evidently,with an enlarged with an enlargedstrong-wind-speed strong-wind-speed region, transporting region, transporting the moisture the moisture from the from Sea the of SeaJapan of Japanto Heilongjiang to Heilongjiang Province Province (Figure (Figure 6b),6 b),which which supplied supplied abundant abundant water water vapor vapor for thethe snowstorm. The The merger merger of of the the upper-level upper-level jet jet stre streaksaks made made the the cyclone cyclone move move northward northward obviously, obviously, which was similar to the approaching of two upper- upper-levellevel jet streaks in the snowstorm event examined by Tian et al. [[43]43] and Gao et al. [[18].18]. The cyclone deepened rapidly under the joint effect effect of the right side of the entrance of the 300-hPa jet and ahead of the 500-hPa merged deep trough. By 0600 UTC 25 Atmosphere 2020, 11, 1272 10 of 21

November, eastern Heilongjiang Province was located just to the left side of the exit of the 300-hPa southern jet, with a maximum divergence of integrated moisture flux of 14 10 5 s 1 (Figure5f). × − − The intense pumping of the upper-level divergence facilitated the falling of surface pressure and the intensification of ascent, which was then favorable for the deepening of the cyclone and snowfall. In summary, the extreme event was more synoptically governed by the outbreak of the polar vortex and moisture anomaly of the East Sea. The outburst of the polar vortex intensified the trough and was favorable for cyclogenesis. The snowstorm event occurred in the divergence area of the upper-level jets. Cold air from the west merged with the southern trough, and then merged with the cold air from the polar vortex, leading to the explosive deepening of the cyclone. The merger of the troughs resulted in the increase of the southerly wind component ahead of the trough, which was beneficial to the northward movement of the cyclone. The abundant moisture supply and flow convergence resulted in the snowstorm over northeastern China.

4.3. Mesoscale Features of Snowstorm Cloud Systems The water vapor cloud images (Figure2) show that the snowstorm in northeastern China occurred in the intensified cloud belts to the north of the occluded front. Two mesoscale-banded echoes were observed by the Jiansanjiang radar. Thus, the mesoscale features of snowstorm cloud belts are examined from the 6-min resolution reflectivity data. Figure7a shows two pieces of scattered echo of reflectivity >20 dBZ were first present to the south of the radar scan. After about one hour, the scattered echo was highly organized and two nearby west–east-oriented narrow-banded echoes of reflectivity >30 dBZ appeared in the central part of the echo. The eastern portions of the two narrow-banded echoes had merged by 0548 UTC 25 November, with expansion of reflectivity >35 dBZ (Figure7c). By 0612 UTC, the two narrow-banded echoes had evolved into a single intense (>30 dBZ) banded echo, with a length of about 200 km, a width of 20–30 km and a maximum reflectivity exceeding 40 dBZ (Figure7d), showing the formation of a mesoscale banded echo. About 12 min later, the mesoscale banded echo reached the strongest intensity, with a maximum reflectivity of larger than 45 dBZ, and the broadest coverage of reflectivity >40 dBZ during its life span (Figure7e). The mesoscale banded echo propagated slowly northward and intense reflectivity approached the radar station, but weakened slightly at 0712 UTC (Figure7f). The life span of the mesoscale echo was about 2 h, contributing more to the total snowfall. The echo gradually weakened, and no observation of the dissipation stage was obtained owing to missing data. This mesoscale band echo was similar to the single band echo defined by Novak et al. (2004), with a linear reflectivity feature 20–200 km in width and >250 km in length, and a minimum reflectivity intensity >30 dBZ that maintained for at least 2 h. A cross section of the radar reflectivity through the two narrow-banded echoes before the formation of the mesoscale banded echo exhibits two distinct reflectivity cores near the surface, at around 46.3◦ N and 46.6◦ N, with reflectivity >30 dBZ below 3 km (Figure7g), corresponding to the two narrow-banded echoes. The northern narrow-banded echo was a little stronger than that in the southern, with the height of the 20 dBZ contour extending to about 6 km and 5 km, respectively, indicating that the two narrow-banded echoes were in different development stages. At 0624 UTC, the mesoscale banded echo formed, and the cross section (Figure7h) shows that reflectivity had evolved into a single core from multiple cores (Figure7g), with a reflectivity core of >40 dBZ near the surface and the height of the 20 dBZ contour extending no more than 5 km. The evolution shows that the formation of the mesoscale band echo experienced merger and intensification of radar reflectivity. In summary, the intense snowfall to the north of the occluded front was closely related to the mesoscale banded echo of reflectivity >30 dBZ in stratiform clouds, which is different from traditional occluded front cloud systems of uniform stratiform cloud precipitation. The snowfall echo was not high, with reflectivity of larger than 30 dBZ below 3 km and a reflectivity core near the surface. Atmosphere 2020, 11, 1272 11 of 21 Atmosphere 2020, 11, x FOR PEER REVIEW 11 of 21

FigureFigure 7. RadarRadar compositecomposite reflectivity reflectivity (unit: (unit: dBZ) dBZ) at Jiansanjiangat Jiansanjiang station station (a–f) ( anda–f) Vertical and Vertical cross section cross section of radar reflectivity along 132.6° E (g-h) at (a) 0412UTC, (b) and (g) 0518UTC, (c) 0548UTC, (d) of radar reflectivity along 132.6◦ E(g,h) at (a) 0412 UTC, (b,g) 0518 UTC, (c) 0548 UTC, (d) 0612 UTC, 0612UTC,(e,h) 0624 ( UTC,e) and and (h) ( f0624UTC,) 0712 UTC and 25 ( Novemberf) 0712UTC 2013. 25 Nov The 2013. red dotThe above red dot the above letter the “J” letter denotes “J” the denotes radar thestation radar of station Jiansanjiang. of Jiansanjiang. The number The in number the top in left the corner top left is time. corner The is blacktime. The lines black in (b ,linese) represent in (b) and the (locatione) represent of the the cross location section of the used cross in Figure section6g,h. used in Figure 6g,h.

5.5. Mechanism Mechanism of of the the Snowstorm Snowstorm PrecipitationPrecipitation in in the the cool cool season season is is closely closely related related to to moisture, moisture, stability, stability, dynamic lift, snowfall efficiency,efficiency, and temperature [44]. [44]. Among Among these these factor factors,s, a a rich rich moisture moisture supply supply and and strong strong ascent ascent are are thethe necessary necessary conditions conditions for for snowfall. snowfall. This This section section examines examines the the frontal frontal structure structure and and the the two two key key ingredientsingredients for for snowfall: snowfall: moisture moisture and and frontogenetical frontogenetical forcing. forcing.

5.1.5.1. Frontal Frontal Zone Zone Structure Structure and and Its Its Effect Effect on on Snowfall Snowfall FromFrom the the above above analysis, analysis, it itcan can be beseen seen that that the theintense intense snowfall snowfall was closely was closely related related to the toback- the bentback-bent occluded occluded front. How, front. then, How, did then, the didfrontal the zo frontalne evolve? zone evolve?What was What the structure was the structureof the occluded of the front?occluded And front? what Andinfluence what did influence the occluded did the front occluded have front on snowfall? have on snowfall?

5.1.1.5.1.1. Horizontal Horizontal Structure Structure of of the the Frontal Frontal Zone Zone AsAs shown shown in in Figure Figure 66a,a, anan 850-hPa850-hPa cycloniccyclonic circulationcirculation occurredoccurred toto thethe right-handright-hand sideside ofof aa northeast-southwest-orientednortheast-southwest-oriented andand continuouscontinuous frontal frontal zone, zone, with with obvious obvious cold cold and warmand warm frontal frontal zones zonesnear the near cyclone the cyclone center. center. The corresponding The corresponding surface coldsurface and cold warm and fronts warm in thefronts cyclone in the center cyclone connected center connectedwith each otherwith beforeeach other the cyclone before explosivelythe cyclone deepenedexplosively (Figure deepened2a). By (Figure 0000 UTC 2a). 25By November, 0000 UTC the25 November,vortex had movedthe vortex northeastward had moved and northeastward deepened rapidly, and deepened but the temperature rapidly, but gradient the temperature of the cold gradientfrontal zone of the near cold the frontal cyclone zone center near wasthe cyclone weakened center (Figure was7 weakenedb). As a result, (Figure the 7b). initial As a continuous result, the initialfrontal continuous zone was fractured,frontal zone with was a fractured, contracting with and a narrowingcontracting warm and narrowing frontal zone. warm It isfrontal clear thatzone. a Itsurface is clear cold that fronta surface catching cold front up to catching a warm up front to a towarm form front an occluded to form an front occluded did not front occur did insteadnot occur of insteadforming of a forming fractural a front fractural (Figure front2b,c), (Figure where 2b–c), the cold where and the warm cold fronts and warm in the fronts center in of the the center cyclone of didthe cyclonenot connect did not with connect each other. with Thiseach stageother. of This the frontstage evolutionof the front is calledevolution a T-shaped is called front a T-shaped structure front [8], structure [8], in that the east-moving cold front was nearly perpendicular to the back-bent warm front. The westward (backward) bent warm front twined around the cyclone center to form an occluded front (Figure 2c).

Atmosphere 2020, 11, 1272 12 of 21 in that the east-moving cold front was nearly perpendicular to the back-bent warm front. The westward (backward) bent warm front twined around the cyclone center to form an occluded front (Figure2c). At 0600 UTC 25 November, the cold frontal zone continued to move eastward and the warm frontal zone further stretched westward, with the increased frontogenesis appearing in the west (back) portion of the cyclone where the northerly airflows, accompanied by the polar cold air, prevailed (Figure5c). The warm frontal zone contracted and narrowed in the northeast part of the cyclone, whereas the cold front weakened with its northern end extending to about 38◦ N (Figure6c). At 1200 UTC 25 November, the back-bent warm front and polar cold air turned cyclonically and surrounded the low center. A relatively warm center formed around the low center, with a potential temperature of 286 K (Figure6d), and the coverage of the warm center had enlarged by 1800 UTC 25 November (not shown). Although the potential temperature was higher than that outside of the warm center, it was lower than that in the warm sector, indicating a separation between the warm sector air and the low center. The separation of the warm core occurred in the baroclinic polar air, where the relatively cold and moist air originated from the northeast side of the cyclone rather than the warm sector air in the initial location of the cyclone. Accompanying the warm-core formation, the surface cyclone center was surrounded by the warm front, and the cold front further weakened and moved eastward (not shown). The central pressure increased to 982-hPa, indicating that the cyclone had begun to weaken. Next, the relative humidity and vertical velocity are analyzed to further discuss the structure of the front in the upper level. Relative humidity, vertical velocity and winds at 600-hPa can also exhibit the evolution of the dark area in the water vapor cloud images (Figure2). An east–west oriented dry tongue originating from Gansu, Qinghai, to Shanxi Provinces generated from 0000 UTC 24 November (not shown), and continued to move eastward. At 1800 UTC 24 November, a wedge-shaped dry-air area with relative humidity <60% was located in the southwest portion of the cyclone (Figure8a), which originated from the northwesterly airflows with descending motion and corresponded to the dark area that extended to the cyclone center in the water vapor cloud images (Figure2b). The surface cyclone center was located in the area where the relative humidity isolines were dense. The comma head and the cold front cloud areas were in the area of relative humidity >90% that formed by the ascent of warm and moist air, whereas the air to the north of the head of the dry area descended into the cyclone center. At 0000 UTC 25 November (Figure8b), the 600-hPa-low circulation formed, and ascent intensified within the comma head that was associated with the back-bent occluded front, with an ascent center over southeastern Heilongjiang Province, corresponding to the snowfall. Moreover, a small coverage of ascent presented in the head of the dry-air area. By 0600 UTC 25 November, the coverage of ascent in the head of the dry area expanded largely (Figure8c), indicating that the dry air was lifted as it arrived ahead of the westerly trough, and was incorporated into the cyclone center through cyclonic rotation (dry air moved westward) at the end of the cyclone’s deepening. At the same time, the moist air in the north portion of the cyclone moved to the west and south and was wrapped into the south of the cyclone center through descending motion. Ascent intensified and moved to eastern Heilongjiang Province, with a maximum ascent of 3.2 Pa s 1 within the comma head, which corresponded to the − · − mesoscale banded echo and resulted in intense snowfall. At 1200 UTC 25 (Figure8d), the intense ascent moved out of Heilongjiang Province and weakened, leading to a decrease in snowfall. It is clear that four stages existed during the rapid deepening of the cyclone, including the incipient frontal cyclone, frontal fracture, frontal T-bone and back-bent warm front, and warm-core seclusion, which is similar to the explosive deepening marine cyclone model. Furthermore, the temperatures at other levels showed that the warm core was able to extend to 600-hPa, which is consistent with a cyclone over Mongolia studied by Xiong et al. (2013). Atmosphere 2020, 11, 1272 13 of 21

Atmosphere 2020, 11, x FOR PEER REVIEW 13 of 21

Figure 8. Distributions of 600-hPa relative humidity (black line; interval: 10%), upward vertical velocity Figure 8. Distributions of 600-hPa relative humidity (black line; interval: 10%), upward vertical −1 −1 −1 (shaded; unit: 10 Pa·s ) 1and winds1 (unit: m·s ) and low circulation1 (D), at (a) 1800 UTC 24 November, velocity (shaded; unit: 10− Pa s− ) and winds (unit: m s− ) and low circulation (D), at (a) 1800 UTC 24 (b) 0000 UTC 25 November, (·c) 0600 UTC 25 November· and (d) 1200 UTC 25 November 2013. The “L” November, (b) 0000 UTC 25 November, (c) 0600 UTC 25 November and (d) 1200 UTC 25 November 2013. in (a) denotes surface low center and the “D” in (b–d) represent the 600-hPa circulation center. The “L” in (a) denotes surface low center and the “D” in (b–d) represent the 600-hPa circulation center.

5.1.2.5.1.2. Vertical Vertical Structure Structure of of the the Frontal Frontal Zone Zone ToTo further further understand understand the the structure structure ofof thethe back-bentback-bent occluded occluded front front and and its its effect effect on on snowfall, snowfall, zonal (along 46.6° N) and meridional (along 131.5° E) cross sections passing through the snowfall zonal (along 46.6◦ N) and meridional (along 131.5◦ E) cross sections passing through the snowfall center and the occluded frontal zone are analyzed. center and the occluded frontal zone are analyzed. The zonal section along 46.6° N at 0600 UTC 25 November (Figure 9a) shows that deep moist air The zonal section along 46.6 N at 0600 UTC 25 November (Figure9a) shows that deep moist extended to 300-hPa around the back-bent◦ occluded front and intensive ascent between 700 and 450- air extended to 300-hPa around the back-bent occluded front and intensive ascent between 700 and hPa, which resulted in the intense cloud area with lower cloud-top temperature within the comma 450-hPa, which resulted in the intense cloud area with lower cloud-top temperature within the comma head and the heavy snowfall over eastern Heilongjiang Province. Relative humidity <60% above 600- headhPa and between the heavy 132° snowfallE and 136° over E easterncorresponded Heilongjiang to the descending Province. Relativedry and humiditycold air that<60% rotated above 600-hPacyclonically. between A small 132◦ areaE and of descending 136◦ E corresponded motion can tobe theseen descending below 700-hPa dry ar andound cold 135° air E, thatwhere rotated the cyclonically.distribution A of small equivalent area of potential descending temperature motion can shows be seen that below the air 700-hPa was relatively around warm, 135◦ E, which where in the distributionturn shows of that equivalent the head potential of the dry temperature air had changed shows from that descending the air was to relatively ascending warm, motion. which Another in turn showsdeep that and themoist head air oflayer the with dry airascent had was changed situated from to the descending east of the to dry ascending air layer, motion.corresponding Another to the deep andwarm moist frontal air layer zone with in the ascent eastern was part situated of the to occluded the east front. of the dry air layer, corresponding to the warm frontal zone in the eastern part of the occluded front. The longitudinal section along 131.5◦ E at 0600 UTC 25 November (Figure9b) shows that deep and moist air with ascent was situated to the south of 30◦ N, corresponding to cold-front cloud systems. The lines of equivalent potential temperature are downward concave between 43◦ N and 45◦ N below 500-hPa, indicating that warm air was isolated and corresponded to the occluded front. A deep

Atmosphere 2020, 11, 1272 14 of 21

and moist layer between 44◦ N and 54◦ N with intensive ascending motion occurred around 45◦ N, corresponding to the occluded front, resulting in heavy snowfall over Heilongjiang Province. Atmosphere 2020, 11, x FOR PEER REVIEW 14 of 21

Figure 9. Cross sections of potential temperature (black line; unit: K), K), temperat temperatureure (red (red dashed dashed line; line; −1 unit: °C),C), relative humidity (green dashed line; unit:unit: %), andand verticalvertical velocityvelocity (shaded,(shaded, unit:unit: PaPa·ss 1),), ◦ · − along line ABAB ((aa)) andand CE CE ( b(b)) in in Figure Figure2d, 2d, respectively. respectively. Front Front boundary boundary (gray (gray yellow yellow thick thick line) line) ( a) and (a) and(b) at (b 0600) at 0600 UTC UTC 25 November 25 November 2013. 2013.

TheAlthough longitudinal intense section ascent andalong a deep131.5° moist E at layer0600 UTC still existed 25 November near the (Figure occluded 9b) front shows at 1200that deep UTC and25 November, moist air thewith occluded ascent was front situated frontal zoneto the moved south northeastward,of 30° N, corresponding with the maximum to cold-front ascent cloud also systems.moving northwardThe lines of (not equivalent shown), resultingpotential intemp theerature location are of precipitationdownward concave being further between north 43° and N and the 45°intensity N below decreasing 500-hPa, at indicating the end of that the warm cyclone’s air was development. isolated and corresponded to the occluded front. A deepIn summary,and moist the layer strong between ascent 44° within N and the 54° comma N with head intensive was closely ascending related motion to the snowfall. occurred With around the 45°intensification N, corresponding of the 600-hPa-lowto the occluded circulation, front, result theing dry in air heavy was snowfall constantly over wrapped Heilongjiang into the Province. cyclone center,Although while the intense moist ascent air in the and north a deep of themoist cyclone layer movedstill existed to the near west the and occluded south. The front dry at and1200UTC moist 25air November, converged withinthe occluded the comma front headfrontal on zone the northwest moved northeastward, side of the cyclone with the center maximum and resulted ascent in also the movingenhancement northward of the (not frontal shown), zone, whichresulting was in conducive the location to of the precipitation lifting of the being moist further air to the north north and of the intensitycyclone, anddecreasing then the at snowfall the end overof the eastern cyclone’s Heilongjiang development. Province. In summary, the strong ascent within the comma head was closely related to the snowfall. With the5.2. intensification Moisture Condition of the 600-hPa-low circulation, the dry air was constantly wrapped into the cyclone center,The while occurrence the moist of aair snowstorm in the north needs of the an cycl abundantone moved moisture to the supply. west and So, south. what were The dry the featuresand moist of airthe converged moisture transport within the and comma relationship head on with the thenorthwes cyclonet side for thisof the snowstorm cyclone center event? and resulted in the enhancementFigure6 illustrates of the frontal the zone, geopotential which was height, conducive temperature to the andlifting wind of the at moist 850-hPa, air asto the well north as the of thedivergence cyclone, ofand integrated then the snowfall moisture over flux eastern from 1000 Heilongjiang to 300-hPa. Province. A moisture channel extending from eastern coastal China to eastern Liaoning Province can be clearly seen as the low was just forming, 5.2. Moisture Condition with weak southerly airflows over northeastern China (Figure6a). A large amount of moisture was transportedThe occurrence northward of a and snowstorm converged needs ahead an abundant of cold and moisture warm frontal supply. zones, So, what and were the moisture the features flux ofconvergence the moisture areas transport corresponding and relationship to cold and with warm the frontscyclone were for connectedthis snowstorm to each event? other. Moisture was transportedFigure 6 to illustrates Liaoning Provincethe geopotential from the height, Yellow temperature, Sea and the East and China wind Seaat 850-hPa, at this time as well (Figure as 6thea). Atdivergence 0000 UTC of 25integrated November, moisture the southeasterly flux from 1000 airflow to 300-hPa. to the northA moisture of the channel shear line extending strengthened from easternand reached coastal jet China intensity to eastern owing Liaoning to the deepening Province can of the be vortexclearly (Figureseen as 6theb). low Northeastern was just forming, China waswith locatedweak southerly in the northeast airflows quadrant over northeastern of the vortex China and (Figure the southeast 6a). A large airflow amount transported of moisture moisture was transportedtoward northeastern northward China and converged from the Sea ahead of Japan. of cold Becauseand warm the frontal northern zones, portion and the of themoisture cold front flux wasconvergence weakened, areas the corresponding moisture flux to convergence cold and warm areas fronts corresponding were connected to cold to and each warm other. fronts Moisture were wasdisconnected. transported The to northwest–southeast-orientedLiaoning Province from the Yellow moisture Sea and flux the convergence East China within Sea at thethis warm time (Figure frontal 6a).zone At stretched 0000UTC to 25 the November, west, with the two southeasterly relatively intense airflow moisture to the north flux convergence of the shear areasline strengthened over eastern and reached jet intensity owing to the deepening of the vortex (Figure 6b). Northeastern China was located in the northeast quadrant of the vortex and the southeast airflow transported moisture toward northeastern China from the Sea of Japan. Because the northern portion of the cold front was weakened, the moisture flux convergence areas corresponding to cold and warm fronts were disconnected. The northwest–southeast-oriented moisture flux convergence within the warm frontal zone stretched to the west, with two relatively intense moisture flux convergence areas over eastern

Atmosphere 2020, 11, 1272 15 of 21

Atmosphere 2020, 11, x FOR PEER REVIEW 15 of 21 Heilongjiang Province and the Sea of Japan (Figure6b). These two intense moisture flux convergences resultedHeilongjiang from confluences Province and between the Sea of southeast Japan (Figure and 6b). northeast These two winds intense and moisture southwest flux convergences and southeast winds,resulted respectively. from confluences between southeast and northeast winds and southwest and southeast winds,The backwardrespectively. trajectories show that air parcels originated over western China with slightly differentThe moving backward tracks trajectories at different show heights that air (Figure parcel 10s originated). The air parcelsover western at 1500 China m and with 3000 slightly m first moveddifferent east moving or southeast tracks andat different turned heights north as(Figure they reached10). The theair parcels eastern at coastal 1500 m area and of3000 the m Korean first Peninsula,moved east and or then southeast rose to and arrive turned in thenorth snowfall as theyarea. reached The the air eastern parcels coastal at this area two of altitudes the Korean were initiallyPeninsula, descending and then dry rose air to (Figure arrive 10inc,e), the withsnowfall specific area. humidity The air parcels<2 g kg at 1this, and two they altitudes moistened were as · − theyinitially passing descending over the Bohai dry air Sea (Figure and the 10 Seac,e), of with Japan, specific with humidity specific humidity <2g·kg−1, increasingand they moistened (Figure 10 e).as It wasthey shown passing that over the moisture the Bohai at Sea 1500 and m the and Sea 3000 of Japan, m mainly with originated specific humidity from the increasing Bohai Sea (Figure and the 10e). Sea of Japan,It was with shown the southerly that the moisture flows transporting at 1500 m and moisture. 3000 m The mainly moisture originated transport from of the the Bohai event Sea is consistentand the withSea the of workJapan, carried with the out southerly by Sun etflows al. [transporting19]. They pointed moisture. out The that moisture the moisture transport originated of the event primarily is fromconsistent the Yellow with Sea the andwork the carried Sea of out Japan. by Sun Unlike et al. air[19]. parcels They pointed at 1500 out and that 3000 the m, moisture the air parcelsoriginated at an altitudeprimarily of 500 from m firstthe Yellow moved Sea northeast and the to Sea an of area Japan. further Unlike north, air andparcels then at turned1500 and cyclonically 3000 m, the as air they reachedparcels Russia at an onaltitude the east of of500 Heilongjiang m first moved Province. northe Initially,ast to an they area were further moist north, air withand relativelythen turned high cyclonically as they reached Russia on the east of Heilongjiang Province. Initially, they were moist specific humidity owing to passing over the Bohai Sea (Figure 10a,e). air with relatively high specific humidity owing to passing over the Bohai Sea (Figure 10a,e).

Figure 10. The 60-h backward trajectories of air parcels, based on the HYSPLIT model, at (a,c,e) 0000 Figure 10. The 60-h backward trajectories of air parcels, based on the HYSPLIT model, at (a,c,e) 0000 UTC and (b,d,f) 0600 UTC 25 November 2013: (a,b) horizontal distribution; (c,d) vertical distribution; UTC and (b,d,f) 0600 UTC 25 November 2013: (a,b) horizontal distribution; (c,d) vertical distribution; (e,f) specific humidity (g·kg−1). (e,f) specific humidity (g kg 1). · −

Atmosphere 2020, 11, 1272 16 of 21

The vertical distribution of the trajectories (Figure 10c) shows that the air parcels at different heights ascended between 0 h and 12 h, and descended before 12 h. Thus, the air parcels between 0 − − h and 12 h contributed the most to the snowfall, and those before 12 h contributed little. The air − − parcels at 3000 m before 12 h comprised dry air originating from the mainland. They were obviously − moistened, therefore bringing moisture for snowfall when they turned north and passed over the Sea. The air parcels at 500 m and 1500 m were moist during longer period, providing moisture to the snowfall. At 0600 UTC 25 November, the southeast winds to the north of the shear line intensified, and the intense moisture flux convergence maintained over eastern Heilongjiang Province, disconnecting with that over the Sea of Japan (Figure6c). Di fferent from 6 h before, the air parcels at 1500 and 3000 m first moved southeast and then turned northwest as they reached the western coast of Japan, traveling longer distances over the Sea of Japan before they rose to arrive in the snowfall area (Figure 10b). As a result, the moisture transport channel became wider and the moisture transport was significantly enhanced, with the specific humidity increased by about 2 g kg 1 (Figure 10f). Notably, the cyclonic · − rotation of the air parcels at 500 m was more obvious and some air parcels passed over the Sea of Japan instead of the mainland. The moisture transport and moisture flux convergence intensified significantly over the snowfall area (Figure6c). The back trajectories clearly show that the moisture transport was influenced by the cyclone. The air parcels at 1500 m and 3000 m were on the northeast portion of the cyclone, influencing by southeasterly winds, while those at 500 m were on the northwest portion of the cyclone, dominating by northeasterly winds, showing the tilting of the cyclone with height. This can explain why the air parcels at different heights have different moving tracks. The deep cyclone and its baroclinic structure intensified the snowfall. In summary, the main moisture source over northeastern China originated from the Bohai Sea and the Sea of Japan, and the southeasterly low-level jet continuously transported abundant moisture to the snowstorm area. The moisture transport intensity varied significantly at different times, which mainly depended on the location and intensity of the cyclone.

5.3. Analysis of Frontogenesis The above analysis shows that a back-bent occluded front formed during the rapid deepening of a cyclone, with strong moisture flux convergence and ascent in the northern quadrant of the cyclone. So, what was the main mechanism of ascent? Temperatures and winds at 850-hPa (Figure6) show that obvious warm advection was present over northeastern China, which played an important role in generating synoptic-scale ascent. The frontogenetical forcing was also important, so, the frontogenesis function is analyzed below. Petterssen [45] defined the atmospheric frontogenesis function as the Lagrangian rate of change of the horizontal potential temperature gradient. Frontogenesis can induce an ageostrophic vertical circulation. So, the frontogenesis function is often used to diagnose the vertical motion and it is regarded as a good predictor for snowfall [46,47]. In this paper, the two-dimensional surface frontogenesis function defined by Miller [48] is applied to analyze frontogenesis qualitatively: " ! ! # d 1 ∂u ∂θ ∂v ∂θ ∂θ ∂u ∂θ ∂v ∂θ ∂θ F = hθ = − + + + (1) dt ∇ θ ∂x ∂x ∂x ∂y ∂x ∂y ∂x ∂y ∂y ∂y |∇h | where u and v denote the horizontal wind component, and θ represents potential temperature. The frontogenesis function is calculated using formula (1). As shown in Figure 11, divergence and descending motion were present to the south of the low center, and obvious frontogenesis was present to the north of the low center, at 1200 UTC 24 November (Figure 11a). An intense frontogenesis associated with the warm shear line was located over the Korean Peninsula. Another weaker southwest–northeast oriented band of frontogenesis passed through Liaoning, Jilin and Heilongjiang Provinces, with maximum frontogenesis in Liaoning Province, where strong ascent existed. This frontogenesis resulted from airflow convergence between Atmosphere 2020, 11, 1272 17 of 21

southeastAtmosphere and 2020 northeast, 11, x FOR PEER winds REVIEW in the north portion of the cyclone, which was the vertical extension17 of 21 of the surface warm front (not shown). At 1800 UTC 24 November, the dry air intruded into the cyclone centercyclone and acenter frontal and fracture a frontal formed, fracture with theformed, low centerwith inthe the low convergence center in andthe frontogenesisconvergence region.and Thefrontogenesis convergence region. between The convergence southeast and between northeast southeast winds and was northeast over central winds Jilinwas over and central Heilongjiang Jilin and Heilongjiang Provinces, which resulted in frontogenesis, with a maximum frontogenesis2 of >31 × 1 Provinces, which resulted in frontogenesis, with a maximum frontogenesis of >3 10− K km− h− 10−2 K·km−1·h−1 and ascent smaller1 than −0.2 Pa·s−1 in Jilin Province, consistent with× the northward-· · and ascent smaller than 0.2 Pa s− in Jilin Province, consistent with the northward-moving snowfall moving snowfall region− (Figure· 11b). region (Figure 11b).

Figure 11. The Petterssen frontogenesis (black solid line; unit: 10−22 K·km−1·h1 −1), 1convergence (shaded; Figure 11. The Petterssen frontogenesis (black solid line; unit: 10− K km− h− ), convergence (shaded; unit: 105−5 s−11), and wind vector at 850-hPa and vertical velocities (red contours;· · units: 10−1 Pa·s−11) at 700-1 unit: 10− s− ), and wind vector at 850-hPa and vertical velocities (red contours; units: 10− Pa s− ) at hPa, at (a) 1200 UTC 24 November, (b) 1800 UTC 24 November, (c) 0000 UTC 25 November and· (d) 700-hPa, at (a) 1200 UTC 24 November, (b) 1800 UTC 24 November, (c) 0000 UTC 25 November and 0600 UTC 25 November 2013. The pink dashed line denotes the convergence line, and the black (d) 0600 UTC 25 November 2013. The pink dashed line denotes the convergence line and the black dashed line represents warm shear line. The “D” denotes the 850-hPa low center. dashed line represents warm shear line. The “D” denotes the 850-hPa low center. At 0000UTC 25 November, the dry air occupied the cyclone center, with the frontogenesis of the At 0000 UTC 25 November, the dry air occupied the cyclone center, with the frontogenesis low center increasing to 5 × 10−2 K·km−1·h−1. Accompanying the warm shear line moving northward of the low center increasing to 5 10 2 K km 1 h 1. Accompanying the warm shear line moving to 42°–43° N, the southwest and ×southeast− · winds− · −on both sides of the warm shear line intensified, northward to 42 –43 N, the southwest and southeast winds on both sides of the warm shear line resulting in the◦ enhancement◦ of the transport and convergence of the warm and moist air. A large intensified, resulting in the enhancement of the transport and convergence of the warm and moist range of frontogenesis formed over the central–eastern area of northeastern China. The convergence air.between A large southeast range of and frontogenesis northeast winds formed and overthe wi thend central–easternspeed convergence area of ofthe northeastern southeast winds China. Theresulted convergence in another between band southeastof frontogenesis and northeast extending winds northward and the from wind the speed low convergencecenter to eastern of the southeastHeilongjiang winds Province. resulted The in another frontogenesis band ofindicated frontogenesis that the extending ageostrophic northward motion was from enhanced, the low and center the central-eastern area of northeastern China was covered by convergence and ascent. The strong

Atmosphere 2020, 11, 1272 18 of 21 to eastern Heilongjiang Province. The frontogenesis indicated that the ageostrophic motion was enhanced, and the central-eastern area of northeastern China was covered by convergence and ascent. The strong frontogenesis was located at the border regions between Jilin and Heilongjiang Provinces (Figure 11c), and the corresponding snowfall was heavy. At 0600 UTC 25 November, the frontogenesis over central and eastern Heilongjiang Province further enhanced, with an ascent center of < 1.5 Pa s 1 − · − in eastern Heilongjiang Province (Figure 11d), which was consistent with the intense snowfall. With the bending and wrapping up of the warm front towards the backward of the cyclone center, the back-bent occluded front was formed, with a new convergence area between the southeast and northeast winds within the comma head. It is clear that the intense ascent corresponded to the larger frontogenesis at different times, indicating that frontogenesis played an important role in the ascent. In summary, with the dry and cold air intruding into the cyclone center and the intensification of the low-level jet, the warm-front frontogenesis obviously enhanced, with the strongest frontogenesis along a convergence area between the southeast and northeast winds; that is, around the surface inverted trough. The increase in frontogenetic secondary circulation and geostrophic deviation led to the intensification of frontal-scale ascent, which was more favorable for the development of snowstorms. After the occluded front formed, the secondary circulation of frontogenesis was strengthened owing to the warm and moist air being lifted to the northwest of the cyclone, where rotation and deformation increased the secondary circulation of frontogenesis, which led to the formation of mesoscale snow belts in the northwest quadrant of the cyclone, resulting in strong snowfall.

6. Summary and Discussion A heavy snowfall event occurred in northeastern China on 25–26 November 2013, which was associated with a back-bent occluded front. The synoptic-scale circulation, satellite images, radar echo, frontal structure, and snowfall mechanism were examined. The main conclusions are as follows: (1) The extreme event was more synoptically governed by the outbreak of the polar vortex and moisture anomaly of the East Sea. The outburst of the polar vortex intensified the trough and contributed significantly to cyclogenesis. The snowstorm event occurred in the divergence area of the northern and southern jets. Cold air from the west merged with the southern trough, and then merged with the cold air from the polar vortex to form a deep westerly trough, leading to the explosive deepening of the cyclone ahead of the trough. The increase of the southerly wind component ahead of the trough due to the merger of the troughs steered the cyclone to move northward. (2) Heavy snowfall was related to the mesoscale-banded echo within the comma head with reflectivity factor of >30 dBZ in stratiform cloud. The heavy snowfall occurred after the occluded front’s formation, which is different to the uniform stratiform cloud precipitation in the traditional occluded front cloud system. (3) The moisture primarily originated from the Bohai Sea and the Sea of Japan, and the dry air from the mainland rotated cyclonically and moistened to provide a supply moisture for snowstorms after passing over the Sea. The moisture mainly come from the middle and lower troposphere. The intensity of the moisture transport was significantly different at different times, mainly depending on the location and intensity of the cyclone. (4) The dry and relatively warm air was continuously incorporated into the cyclone center, and the warm front stretched towards the rear of the cyclone center. Thus, the dry air gradually changed from descending to ascending motion as it moved ahead of the westerly trough. The moist air in the north of the cyclone moved westward and southward, and descended into the south of the cyclone center. After the occluded front’s formation, the warm and moist airflows rose to arrive in the northwest quadrant of the cyclone and the warm-front frontogenesis intensified obviously owing to the rotation and deformation. The strongest frontogenesis was along the convergence area of the southeast and northeast winds, which led to the enhancement of the front- scale intense ascent within the comma head and resulted in strong snowfall. Atmosphere 2020, 11, 1272 19 of 21

(5) Due to the shortage of high spatial and temporal resolution observation data, the analysis of the snowfall mechanism was limited. The snowfall event occurred in the warm frontal zone with convective stability (∂θ/∂p < 0). The preliminary study showed that it probably existed moist symmetric and inertial instability, which can explain the observed banded snowfall echoes. However, the resolution of the FNL data is too coarse to resolve the instability. So, the stability mechanism of the event needs to be further revealed based on numerical simulation, which will be discussed in another paper. In addition, we have found many snowstorm cases were associated with the S-K conceptual configuration’s cyclones over northeast China, but we can’t get a general result through a case study. We will survey many such type cases and hope to obtain a good result in the future. If the result is positive, it may be helpful in pattern recognition in the operational forecast.

Author Contributions: Conceptualization, Y.Z. and C.-F.Y. Investigation and methodology, Y.Z. and L.F. Software, L.F. and X.-F.C. Formal analysis, Y.Z. and L.F. Writing—Review and editing, Y.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Natural Science Foundation of China (41975055) and the National Key Research and Development Program of China (2017YFC1502002). Acknowledgments: We thank two anonymous reviewers for their constructive comments that have helped us to improve the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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