Genesis, Maintenance and Demise of a Simulated Tornado and the Evolution of Its Preceding Descending Reﬂectivity Core (DRC)

atmosphere

Article Genesis, Maintenance and Demise of a Simulated Tornado and the Evolution of Its Preceding Descending Reﬂectivity Core (DRC)

Dan Yao 1,2,3 , Zhiyong Meng 2,* and Ming Xue 3 1 State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing 100081, China; [email protected] 2 Laboratory for Climate and Ocean-Atmosphere Studies, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing 100871, China 3 Center for Analysis and Prediction of Storms, and School of Meteorology, University of Oklahoma, Norman, OK 73072, USA; [email protected] * Correspondence: [email protected]

Received: 12 March 2019; Accepted: 18 April 2019; Published: 1 May 2019

Abstract: This study demonstrates the capability of a cloud model in simulating a real-world tornado using observed radiosonde data that define a homogeneous background. A reasonable simulation of a tornado event in Beijing, China, on 21 July 2012 is obtained. The simulation reveals the evolution of a descending reflectivity core (DRC) that has commonalities with radar observations, which retracts upward right before tornadogenesis. Tornadogenesis can be divided into three steps: the downward development of mesocyclone vortex, the upward development of tornado vortex, and the eventual downward development of condensation funnel cloud. This bottom-up development provides a numerical evidence for the growing support for a bottom-up, rapid tornadogenesis process as revealed by the state-of-the-art mobile X-band phase-array radar observations. The evolution of the simulated tornado features two replacement processes of three near-surface vortices coupled with the same midlevel updraft. The first replacement occurs during the intensification of the tornado before its maturity. The second replacement occurs during the tornado’s demise, when the connection between the midlevel mesocyclone and the near-surface vortex is cut off by a strong downdraft. This work shows the potential of idealized tornado simulations and three-dimensional illustrations in investigating the spiral nature and evolution of tornadoes.

Keywords: tornado; supercell; DRC; genesis; maintenance; demise

1. Introduction Meteorologists are obtaining progressively more detailed information on the structures of tornadoes and their environment, particularly for tornadogenesis with improved observational instruments, especially mobile fast-scan polarimetric Doppler radars [1,2]. However, the whole life cycle of a tornado from genesis to maintenance and demise, most notably in terms of the near-surface features, is hard to capture with observational data [3,4]. Relative to observations, numerical modeling can provide more thermodynamic information close to the tornado scale with higher spatial and temporal resolutions [5,6]. Numerical simulations of real-world tornadoes in a full-scale numerical weather prediction (NWP) model are diﬃcult. To successfully capture tornado-scale vortices in terms of both timing and location, the model needs to simulate the atmospheric features and ﬂows on multiple scales, including the parent storms, which is by itself still a challenging task [7]. So far, only few studies have documented NWP-model simulations of real-world tornadoes at tornado-resolving resolutions [8–11]. A common

Atmosphere 2019, 10, 236; doi:10.3390/atmos10050236 www.mdpi.com/journal/atmosphere Atmosphere 2019, 10, 236 2 of 18 practice in tornado modeling is to use an idealized cloud model, where the parent storm is typically initiated with a warm bubble in a uniform background based on a radiosonde profile (e.g., [6,12,13]). Such idealized experiments have long been used in storm-scale studies. Most early studies focused on the environment of tornadic storms and their effects on the rotational features of the simulated storm, rather than tornado-scale features. For example, Wilhelmson and Klemp [14] studied the 3 April 1964 storm that hit Oklahoma using a cloud model based on a composite sounding obtained from nearby rawinsonde observations at two observation times, initiated with a 4-K warm bubble. The grid spacing was 2 km horizontally and 0.75 km vertically. Their focus was to study the splitting process of the convective systems, which showed a resemblance between the simulation and the radar and surface observations. Klemp et al. [15] simulated the 20 May 1977 Del City tornadic storm using the same model initiated with a composite of two special sounding observations from the event, with a grid spacing of 1 km horizontally and 500 m vertically, and further investigated the generation of large low-level vertical vorticity inside the tornadic region [16]. In addition to the limited grid spacing, the microphysical scheme used in these studies was the Kessler warm-rain parameterization scheme, which does not address ice-phase processes. Due to the insufficient model resolution to resolve tornado-scale features, these studies were unable to capture the detailed structure and evolution process of the tornado. More recently, using higher resolution and more realistic physical parameterization schemes, Orf et al. [17] successfully reproduced a long-track enhanced Fujita scale 5 (EF5) tornado based on a sounding taken from the rapid update cycle (RUC) forecast. The simulated supercell had a life span, wall cloud and reflectivity pattern similar to the observations. The simulated tornado was also similar to the observed tornado in its peak intensity, life span, track and length. However, to the best of our knowledge, there has not been any idealized simulation study that has used a single real radiosonde observation as the uniform background field to investigate an event’s tornado-scale features and evolution. The observed rawinsonde sounding has not been directly used much in idealized storm simulations for a variety of reasons, including inappropriate sampling of the pre-storm environment due to the sounding location relative to the storm, and/or mismatch of sounding and storm times. Additionally, most observed soundings contain discontinuities that are not well-handled by the numerical models. A question we want to ask is: can a real sounding be used to successfully simulate tornadic features if it is available at appropriate time and location relative to the storm? The latter is true for a tornado case that is the focus of this study. On 21 July 2012, an EF3 supercell tornado occurred in Zhangjiawan town, Beijing, China, and caused two fatalities. Through a detailed damage survey, radar and rawinsonde analyses, Meng and Yao [18] (hereafter referred to as MY14) documented some of the fine-scale features of this tornado and its parent supercell, such as the evolution of the mesocyclone, the descending reflectivity core (DRC), the tornadic vortex signature (TVS), and the tornado debris signature (TDS). This paper examines how well a cloud model can successfully simulate the observed features of this tornado case using a real radiosonde and, furthermore, how well the numerical modeling results can help improve our knowledge of tornado evolution from the formation to demise, including the relationship between DRC and tornadogenesis, the bottom-up versus top-down development of the tornado vortex, and detailed tornado vortex cycling processes, etc. The remainder of this paper is organized as follows: The model configuration and data are introduced in Section2. An overview of the simulated supercell and tornado and their commonalities with observations are presented in Section3. A detailed analysis of the important features in the simulated tornado life span is presented in Section4. Some insights from the simulation and a summary are given in Section5.

2. Methodology The Bryan cloud model, version 1 (CM1, [19]), release 17, is used in this study. This model is capable of producing a large-eddy simulation and has been widely used in recent supercell and resolution. Specifically, it conserves mass and energy better than other modern cloud models (http://www2.mmm.ucar.edu/people/bryan/cm1/faq.html). The limitations of CM1 mainly come from idealized convection initiation and horizontally homogeneous environment as far as the simulation realism is concerned. Our study is based on an idealized simulation using an observed proximity sounding to define the base state and a warm bubble for initiation.

AtmosphereThe rawinsonde2019, 10, 236 profile used for the homogeneous initial background was representative of3 ofthe 18 storm environment in which the tornadic supercell occurred. It was a sounding with 60 layers from the Beijing Meteorological Observatory at 1400 local standard time (LST) (Figure 1), which was initiallytornado located research 20 (e.g.km west [12,17 of]). the As tornado a non-hydrostatic damage path numerical (Figure 1a model). It happened designed to particularlybe located in for a gapdeep between precipitating two convection convection, cells, the and aim thus for CM1was not was contaminated to do very-large by convect domainion simulations. The time of using the soundinghigh resolution. was quite Specifically, close to the it conservesestimated masstornado and lifetime energy of better ~1340 than−1400 other LST. modern The radiosonde cloud models was around(http://www2.mmm.ucar.edu the storm (Figure 1a) and/people thus/bryan providing/cm1/faq.html representative). The limitationsdepicting of of the CM1 storm mainly enviro comenment. from Thidealizedis sounding convection in close initiation proximity and of horizontally the tornadic homogeneous storm, both spatially environment and temporally, as far as the makes simulation the Beijingrealism tornado is concerned. case a Ourvaluable study one is basedfor idealized on an idealized simulation simulation studies. To using better an observedrepresent proximitythe near- surfacesounding conditions to define, the the base surface state data and of a warm the Beijing bubble radiosonde for initiation. were modified with the automatic weatherThe station rawinsonde observations profile from used the for same the homogeneouslocation at the same initial time. background The lowest was level representative of the original of radiosondethe storm environment observation is in about which 30 the m tornadicabove ground supercell level occurred. (AGL), while It was the a soundingsurface observation with 60 layers is 2 mfrom AGL. the Beijing Meteorological Observatory at 1400 local standard time (LST) (Figure1), which was initiallyThe located background 20 km environment west of the tornadoindicated damage by the pathproxim (Figureity sounding1a). It happened was favorable to be located for severe in a convectigap betweenve storm two development convection cells, and tornadogenesis. and thus was not The contaminated winds veered by with convection. height (Figure The time 1b) with of the a 0sounding–6-km vertical was quite shear close of to 23.4 the m estimated s−1 and tornado a 0–1-km lifetime vertical of ~1340 shear of1400 7.1 LST. m s The−1, which radiosonde provides was − environmentalaround the storm low (Figure-level 1horizontala) and thus vorticityproviding required representative for at depicting least the of initial the storm development environment. of mesocyclones.This sounding inMoreover, close proximity convection of the tornadicis easy storm, to initiate both spatially and maintain and temporally, with a makeshigh instabilitythe Beijing characterizedtornado case aby valuable surface- onebased for (which idealized was simulation also the most studies. unstable) To better convective represent available the near-surface potential energyconditions, (CAPE) the surfaceof 2737 data J kg of−1 theand Beijing a convection radiosonde inhibition were modified (CIN) of with 0 J kg the−1 automatic(Figure 1c). weather It should station be notedobservations that near from-surface the samedifference location betwee at then temperature same time. Theand lowestdew point level temperature of the original is very radiosonde small, whichobservation gives rise is about to a low 30 m lifting above condensation ground level level (AGL), (LCL) while of the230 surfacem. observation is 2 m AGL.

Figure 1. Beijing weather station rawinsonde in close proximity to the Beijing tornado at 0600 UTC Figure 1. Beijing weather station rawinsonde in close proximity to the Beijing tornado at 0600 UTC (1400 LST, which was launched at 1315 LST) on 21 July 2012. (a) Spatial proximity of the rawinsonde (1400 LST, which was launched at 1315 LST) on 21 July 2012. (a) Spatial proximity of the rawinsonde trace (magenta curve) to the supercell (denoted by the contour of 45 dBZ at 2.4◦ elevation angle at trace (magenta curve) to the supercell (denoted by the contour of 45 dBZ at 2.4° elevation angle at 1350 LST) and the tornado track (in red, adapted from MY14). The blue dots show the seven aﬀected 3 villages. The grey curve is the eastern border of Beijing. The Beijing weather station is denoted by the

grey dot and the distance scale is given by concentric grey dashed circles. (b) Hodograph of the Beijing rawinsonde, with the altitudes (km) of diﬀerent levels given alongside the curve. (c) Vertical proﬁles of temperature, dew point and wind from the rawinsonde in a Skew-T diagram. Also shown are the values of CAPE, CIN and LCL. Atmosphere 2019, 10, 236 4 of 18

The background environment indicated by the proximity sounding was favorable for severe convective storm development and tornadogenesis. The winds veered with height (Figure1b) with 1 1 a 0–6-km vertical shear of 23.4 m s− and a 0–1-km vertical shear of 7.1 m s− , which provides environmental low-level horizontal vorticity required for at least the initial development of mesocyclones. Moreover, convection is easy to initiate and maintain with a high instability characterized by surface-based (which was also the most unstable) convective available potential energy (CAPE) of 1 1 2737 J kg− and a convection inhibition (CIN) of 0 J kg− (Figure1c). It should be noted that near-surface difference between temperature and dew point temperature is very small, which gives rise to a low lifting condensation level (LCL) of 230 m. The horizontal grid spacing used in the simulations was 100 m within the central 20 km 20 km × region, and stretched gradually to 3.9 km at the outer boundary (as in [12]) of the 100 km 100 km × domain. The domain moved with the average motion of the simulated supercell, at a constant speed of 1 1 2.5 m s− eastward and 10 m s− northward. The vertical grid spacing was 20 m within the lowest 1 km AGL, and stretched gradually to 500 m at the upper boundary 18 km AGL. A higher resolution was used at lower levels for a better representation of the processes around and below the cloud base. A symmetric ellipsoid warm bubble 20 km wide and 2.8 km deep was added in the initial field, as widely used in previous studies employing idealized simulations. The temperature perturbation was 7 K at its center, located at 1.4 km AGL, and decreasing gradually to zero at its outer edge. The large time step was set as 1 s. A fifth-order advection approximation was used both horizontally and vertically. The Klemp–Wilhelmson time-splitting for acoustic modes was used for the pressure solver, with a vertically implicit solver and horizontally explicit calculations, as in MM5, ARPS and WRF. No topography was considered in this simulation. The explicit moisture scheme used was the Morrison double-moment microphysics scheme with both hail and graupel, which is more realistic in supercell simulation than single-moment schemes. The lateral boundary conditions were set as open radiative, while the bottom boundary condition was free-slip (no friction). The simulation was run for 2 h, with the output saved every 30 s. The simulations were sensitive to the horizontal and vertical grid spacing. The intensity of the mesocyclone and near-surface vortex decreased with the increase in the horizontal grid spacing from 100 m to 200 m. Increasing the grid spacing to 50 m substantially increased the intensity, but the three-dimensional distribution of the vorticity became complex and lost the key characteristics captured in the 100-m simulation that were close to the observations at both the supercell and tornado scales. The impact of changing the vertical grid spacing was smaller than changing the horizontal grid spacing, but it changed the height of the lowest level and thus the features of the near-surface processes. The configuration used in this study produced a supercell with DRC evolution and tornado intensity variation closest to the observations.

3. Overview of the Simulated Storm The simulation captured the main features of the supercell structures obtained from radar observations and the tornado characteristics deduced from the damage survey. To compare the simulation and observations more easily, a reference time with respect to tornadogenesis was used. Tornadogenesis was deﬁned as the earliest time when vertical vorticity at the lowest level (10 m AGL) 1 reached 0.1 s− and extended from the surface to the cloud base associated with a negative pressure perturbation, as used in the vorticity analysis of a tornado life cycle in Wicker and Wilhelmson [13]. The simulated tornado formed at t = 79 min from the model initiation. This time, along with the estimated time of tornadogenesis in the observations (1340 LST) obtained by MY14, was deﬁned as t0, and other times are denoted relative to t0. The location of the Beijing radar in the model was determined based on the relative distance of the Beijing radar from the supercell at t0 + 4 min of the 2.4◦ elevation angle scan, to facilitate the visualization of the plan-position-indicator (PPI) view and the calculation of the simulated radial velocities. Atmosphere 2019, 10, 236 5 of 18

The simulated supercell captured the key features of the observed supercell well in its shape and motion at 2.4◦ PPI, albeit a smaller size and weaker intensity (Figure2). The hook echo and weak echo regions observed by radar are also seen in the simulated supercell, which develop and are accompanied by a mesocyclone with a maximum radial velocity somewhat lower than the observed 30 m s 1 (Figure3). tornadogenesis, a thin, discrete pillar-shaped DRC was produced with similarity to− the radar tornadogenesisThe evolution of, a a DRC thin, similar discrete to pillar the observations-shaped DRC was was also produced captured. with Before similarity tornadogenesis, to the aradar thin, observation. Since the evolution of the DRC using radar observation is hard to examine due to the observation.discrete pillar-shaped Since the DRCevolution was produced of the DRC with using similarity radar toobservation the radar observation. is hard to examine Since the due evolution to the limitations in the spatial and temporal resolutions, our simulation provides a valuable dataset for limitof theations DRC usingin the radarspatial observation and temporal is hard resolution to examines, our due simulation to the limitations provide ins the a valuable spatial and dataset temporal for examining how the DRC evolves. examiningresolutions, how our the simulation DRC evolves. provides a valuable dataset for examining how the DRC evolves.

Figure 2. Two-dimensional comparison between observed (upper row) and simulated (lower row) FigureFigure 2. TwoTwo-dimensional-dimensional comparison comparison between between observed observed (upper (upper row) row) and and simulated simulated (lower (lower row) row) supercells in terms of basic reflectivity (R) at 2.4  elevation angle contours at five scanning times. The supercellssupercells in in terms terms of of basic basic reflectivity reﬂectivity (R) (R) at 2.4 at 2.4 elevation◦ elevation angle angle contours contours at five at ﬁvescanning scanning times. times. The ranges at 10, 20 and 30 km are given by concentric circles centered on the radar site. rangesThe ranges at 10, at 20 10, and 20 30 and km 30 are km given are given by concentric by concentric circles circles centered centered on the on radar the radar site. site.

FigureFigure 3. 3. AAss inin in Figure Figure 2 but for radial velocity (V (Vr).r).).

In addition to the supercell, the simulationimulation also also show shows tornadotornado-scale features with commonalities toto thethe composite informationinformation from from damage survey and radar observation.. The simulated tornado lifespan was 24 min long, similar to the estimated duration of the observed tornadotornado ofof ~20~20 min.min. Withith compositecomposite investigationinvestigation based on Doppler radar and damage survey, MY14 5

Atmosphere 2019, 10, 236 6 of 18

In addition to the supercell, the simulation also shows tornado-scale features with commonalities to the composite information from damage survey and radar observation. The simulated tornado lifespan was 24 min long, similar to the estimated duration of the observed tornado of ~20 min. With composite investigation based on Doppler radar and damage survey, MY14 concluded that thisconcluded tornado that event this could tornado be dividedevent could into be ﬁve divided stages: into the five formation stages: ofthe the formation DRC about of the 16 minDRC before about tornadogenesis,16 min before tornadogenesis the retraction, ofthe DRC retraction and the of DRC genesis and of the tornado, genesis the of intensiﬁcation,tornado, the intensification mature and, demisemature stagesand demise of the stage tornado.s of the This tornado. is also the This case is also in the the simulation case in the (Figure simulation4). (Figure 4).

FigureFigure 4.4. Three-dimensionalThree-dimensional perspective of the simulated tornado and cumulonimbus cloud at the (a)) tornadogenesis,tornadogenesis, ((b)) intensiﬁcation,intensification, ((c)) maturity and ((d)) demise stages. The isosurfaces in grey, cyan, 1 1 blueblue andand purplepurple denotedenote thethe cloudcloud waterwatervapor vapor mixingmixing ratioratio (0.1 (0.1 g g kg kg−−1),), verticalvertical velocityvelocity (15(15 mm s s−−1),), 1 verticalvertical vorticity vorticity (0.1 (0.1 s s−−1) and potential potential temperature temperature perturbation perturbation ( (−11 K, showing showing the the temperature temperature − propertiesproperties ofof thethe coldcold pool),pool), respectively.respectively. TheThe perspectiveperspective isis fromfrom thethe north.north.

The above analyses showed that the cloud model simulated the supercell and tornado reasonably well by using a real radiosonde background. This simulation was therefore used to investigate the evolution of the simulated supercell and tornado, as reported in the next section.

4. Life Cycle of the Simulated Tornado The model took 27 min to form the supercell from the initial warm bubble. The formation of a supercell was defined as the earliest time when the 2–5-km integrated updraft helicity reached 900 m2 s−2, as introduced by Naylor and Gilmore [6]. After meeting the supercell criteria, the value of 2– 5-km integrated updraft helicity oscillated around 900 m2 s−2 for about 15 min and remained greater than 900 m2 s−2 from t = 40 min for the rest of the integration, except for a 10-min oscillation 6

Atmosphere 2019, 10, 236 7 of 18

4. Life Cycle of the Simulated Tornado The model took 27 min to form the supercell from the initial warm bubble. The formation of a supercell was defined as the earliest time when the 2–5-km integrated updraft helicity reached 2 2 900 m s− , as introduced by Naylor and Gilmore [6]. After meeting the supercell criteria, the value 2 2 of 2–5-km integrated updraft helicity oscillated around 900 m s− for about 15 min and remained 2 2 greater than 900 m s− from t = 40 min for the rest of the integration, except for a 10-min oscillation immediately before tornadogenesis. A DRC started to develop at t = 60 min, or 19 min before the tornadogenesis (t 19 min), and lasted for 17 min. Following the dissipation of the DRC, the tornado 0 − was generated at t = 79 min. The tornadogenesis, maintenance (including intensification and maturity) and demise took 4, 13 and 7 min, respectively. These stages are defined by the vertical continuity of the 1 0.1 s− vertical vorticity isosurface columns. There was no sign of more tornadoes after the demise of the tornado. The tornado lasted for 24 min, as measured by the period during which the near-surface 1 vortex column of the 0.1 s− vertical vorticity isosurface was sustained, which is similar to the tornado lifetime of about 20 min estimated by MY14.

4.1. DRC Formation (t 19 min t < t min) 0 − ≤ 0 At t 19 min, an overhanging reflectivity core was first perceivable in terms of the 40-dBZ 0 − isosurface (brown in Figure5a), and later became evident at t 16 min in terms of the 50-dBZ 0 − isosurface at a height of 1.5 km (red in Figure5b,c). It then expanded while growing rapidly downward and reached the ground at t 14 min (Figure5d,f), showing the characteristics of a type I DRC as 0 − generalized by Byko et al. [20], which refers to those forming as a result of midlevel flow ‘stagnation’, rather than resulting from precipitation that forms within a new updraft or descended down the axis of an amplifying low-level vertical vorticity maximum. After reaching the ground, the DRC grew wider in the shape of a wall (Figure5g), then separated into two parts (Figure5h), gradually forming a pillar of higher reflectivity at its far end (Figure5i,l), showing the characteristics of a process resembling the 27 May 1997 tornadic supercell case [20], although the latter was not categorized into a typical type as it was not ‘descending’. The high reflectivity pillar became separated from the main body of the supercell and had a diameter of about 500 m, as characterized by the 50-dBZ reflectivity isosurface, extending all the way up to 3 km AGL. Unlike the 50-dBZ isosurface, the 40-dBZ isosurface remained a wall-shaped pattern until t = t 2.5 min. 0 − Two minutes after the separation of the DRC from the rear flank downdraft, a weak vertical vorticity column formed near the DRC close to the ground (blue in Figure5j,k) and attained a strength 1 of 0.1 s− . This vortex quickly lost contact with the ground and diminished (Figure5l). The tornadic vortex was not very stable at the earlier stage, with a transient feature. Figure6 shows that V0 developed at low-levels from t 9 min to t 5 min, which, however, was detached from the main 0 − 0 − updraft center and hence was not stretched. Later, V1 developed with a better collocation to the strengthening updraft center throughout the lowest 1-km layers, and hence developed significantly and evolved to become the tornadic vortex. Atmosphere 2019, 10, 236 8 of 18

FigureFigure 5.5. Simulated three-dimensionalthree-dimensional supercell structurestructure inin terms of reﬂectivity,reflectivity, verticalvertical velocityvelocity andand

verticalvertical vorticityvorticity fromfrom ((aa)) tt00 –– 1919 minmin toto (r()r)t 0t0+ + 3.53.5 minmin atat speciﬁcspecific timestimesas as denoted. denoted. TheThe tornadictornadic 1−1 supercellsupercell isis duringduring thethe periodperiod ofof DRCDRC formation formation and and tornadogenesis. tornadogenesis.The The isosurfacesisosurfaces of of 0.1 0.1 s −s andand −11 −11 0.150.15 ss− vertical vorticity are given inin darkdark blue.blue. TheThe updraftupdraft ofof thethe mesocyclonemesocyclone ofof 15 15 m m s −s isis shownshown inin cyan.cyan. TheThe isosurfacesisosurfaces ofof 4040 andand 5050 dBZdBZ reﬂectivityreflectivity areare givengiven inin brownbrown andand red, red, respectively. respectively.

Two minutes after the separation of the DRC from the rear flank downdraft, a weak vertical vorticity column formed near the DRC close to the ground (blue in Figure 5j,k) and attained a strength of 0.1 s−1. This vortex quickly lost contact with the ground and diminished (Figure 5l). The tornadic vortex was not very stable at the earlier stage, with a transient feature. Figure 6 shows that V0 developed at low-levels from t0 − 9 min to t0 − 5 min, which, however, was detached from the main

8 updraft center and hence was not stretched. Later, V1 developed with a better collocation to the strengtheningAtmosphere 2019, 10updraft, 236 center throughout the lowest 1-km layers, and hence developed significantly9 of 18 and evolved to become the tornadic vortex.

FigureFigure 6. 6. HorizontalHorizontal cross cross-section-section of ofthe the simulated simulated supercell supercell and and tornado tornado during during DRC DRC formation formation and and tornadogenesis at the height of (a c) 990 m, (d f) 490 m and (g i) 190 m AGL at (left column) tornadogenesis at the height of (a−c) 990− m, (d−f) 490− m and (g−i) 190− m AGL at (left column) t0 − 5 t 5 min, (middle column) t 1 min and (right column) t + 3 min. Vertical velocity is given by min,0 − (middle column) t0 − 1 min0 − and (right column) t0 + 3 min.0 Vertical velocity is given by shaded coshadedntours. contours. Reflectivity Reﬂectivity of 40 (50) of dBZ 40 (50)is given dBZ by is givenorange by (red) orange contours. (red) contours. Vertical vorticity Vertical vorticityof 0.05, 0.1 of 1 and0.05, 0.15 0.1 s and−1 is 0.15given s− byis blue given contours. by blue Pressure contours. perturbation Pressure perturbation of -1.5 and of-2 -1.5hPa andis given -2 hPa by isdark given grey by dark grey contours. V denotes the vanishing vortex associated with the DRC, while V denotes the contours. V0 denotes the0 vanishing vortex associated with the DRC, while V1 denotes the1 downward developingdownward tornadic developing vortex. tornadic vortex.

4.2. Tornadogenesis (t0 min t < t0 + 4 min) 4.2. Tornadogenesis (t0 min ≤≤ t < t0 + 4 min) Tornadogenesis was characterized by the newly developed near-surface vortex V1 at t0. It happened after the separation of the 40-dBZ isosurface wall of the DRC at t 2 min, when the 0 − previous transient vortex V diminished (Figure5m). A weak echo hole formed at 500 m AGL and grew 0 9 larger immediately afterwards, accompanied by a narrowing of the isolated pillar of the DRC in terms of Tornadogenesis was characterized by the newly developed near-surface vortex V1 at t0. It happened after the separation of the 40-dBZ isosurface wall of the DRC at t0 − 2 min, when the Atmosphereprevious2019 transient, 10, 236 vortex V0 diminished (Figure 5m). A weak echo hole formed at 500 m AGL10 ofand 18 grew larger immediately afterwards, accompanied by a narrowing of the isolated pillar of the DRC in terms of both the 50- and 40-dBZ isosurfaces, with a diameter of about 300 m and 1 km, respectively, both the 50- and 40-dBZ isosurfaces, with a diameter of about 300 m and 1 km, respectively, at t0 1 min at t0 − 1 min (Figure 5n). Both isosurfaces became thinner and retracted upward approaching− (Figure5n). Both isosurfaces became thinner and retracted upward approaching tornadogenesis at tornadogenesis at t0 and thereafter (Figure 5o–r), and a persistent, vertically upright vortex column t and thereafter (Figure5o–r), and a persistent, vertically upright vortex column near the ground 0near the ground became evident. This retraction of DRC before tornadogenesis was also observed in became evident. This retraction of DRC before tornadogenesis was also observed in the radar analysis the radar analysis of MY14. A similar process has also been recorded in the pre-tornadic phase of the of MY14. A similar process has also been recorded in the pre-tornadic phase of the 5 June 2009 Goshen 5 June 2009 Goshen County supercell [3]. County supercell [3]. A weak and wide vertical vorticity column, which was likely associated with the mesocyclone, A weak and wide vertical vorticity column, which was likely associated with the mesocyclone, developed downward starting at t0 – 3 min (denoted by V1 in Figure 7). About 1 min after the weak developed downward starting at t0 – 3 min (denoted by V1 in Figure7). About 1 min after the weak and wide vertical vorticity column touched the ground at t0 − 1 min, the 0.1 s−1 vorticity appeared and wide vertical vorticity column touched the ground at t 1 min, the 0.1 s 1 vorticity appeared inside the column near the ground and rapidly developed 0upward.− These analyses− showed that the inside the column near the ground and rapidly developed upward. These analyses showed that the vertical vorticity column was growing in a top-down direction at first, and a stronger vorticity formed vertical vorticity column was growing in a top-down direction at ﬁrst, and a stronger vorticity formed inside the column when it was very close to or reached the ground. inside the column when it was very close to or reached the ground.

FigureFigure 7. 7.Evolution Evolution ofof verticalvertical vorticity vorticity column column near near the the tornadogenesis tornadogenesis period period from from ( a()at)0 t0 −3 3 min min to to 1 − (f()f)t 0t0+ + 22 minmin atat anan intervalinterval ofof 11 min.min. CyanCyan andand lightlight cyancyan denotedenote thethe15 15 and and 10 10 m m s −s−1 vertical velocity,velocity, 1 respectively.respectively. The The dark dark blue blue to to transparent transparent dark dark blue blue denote denote the the 0.15, 0.15, 0.1 0.1 and and 0.05 0.05 s −s−1 verticalvertical vorticity,vorticity,

respectively.respectively. The The distance distance scales scales in in the the vertical vertical and and horizontal horizontal directions directions are givenare given in (a ).inV (a0).denotes V0 denotes the vanishingthe vanishing vortex vortex associated associated with thewith DRC, the whileDRC, Vwhile1 denotes V1 denotes the downward the downward developing developing tornadic tornadic vortex. vortex. The tornadic vortex ﬁrst formed immediately next to the ground, with a diameter of about 100 m and a height of several hundred meters, connected to the cloud base (Figure4a). It grew drastically 1 thicker, taller and stronger within 1 min, as indicated10 by the appearance of 0.15 s− vertical vorticity

(Figure8a), which was also connected to the ground at birth. At t0 + 1.5 min, the DRC pillar disappeared while the tornadic vortex intensiﬁed (Figure5p). During this process, the midlevel updraft also grew The tornadic vortex first formed immediately next to the ground, with a diameter of about 100 m and a height of several hundred meters, connected to the cloud base (Figure 4a). It grew drastically thicker, taller and stronger within 1 min, as indicated by the appearance of 0.15 s−1 vertical vorticity

(AtmosphereFigure 82019a), which, 10, 236 was also connected to the ground at birth. At t0 + 1.5 min, the DRC 11 pillar of 18 disappeared while the tornadic vortex intensified (Figure 5p). During this process, the midlevel updraft also grew lower toward the low-level vortex column (For convenience in describing the results,lower toward in terms the of low-level the size of vortex the storm, column we (Foruse ‘midlevel’ convenience to indenote describing levels thebetween results, 1 and in terms 3 km ofAGL the andsize ‘low of the-level’ storm, for we those use ‘midlevel’below 1 km to denoteAGL.), levelsforming between a vorticity 1 and column 3 km AGL between and ‘low-level’ 1 and 2 km for AGL, those immediatelybelow 1 km AGL.), insideforming the strong a vorticity updraft column core at between t0 + 2 min 1 and (Figure 2 km AGL,8a). Fro immediatelym t0 + 2 min inside onwards, the strong the intensificationupdraft core at oft 0the+ 2tornadic min (Figure vortex8a). was From also tmanifested0 + 2 min onwards, in the intensification the intensification of horizontal of the tornadicvelocity nearvortex the was surface. also It manifested reached 15 in m the s−1 near intensification the bottom of of horizontal the low-level velocity vortex near at t0 the+ 2 min surface. and expanded It reached 1 in15 both m s− horizontalnear the bottomand vertical of the scale. low-level The s vortexequence at oft0 +mesocyclone,2 min and expanded mesocyclone in both updraft horizontal and near and- surfacevertical tornadic scale. The vortex sequence intensification of mesocyclone, is also mesocycloneobserved in the updraft idealized and simulations near-surface of tornadic Roberts vortex et al. [intensification21]. is also observed in the idealized simulations of Roberts et al. [21].

FigureFigure 8 8.. StructureStructure and and development development of of rotational rotational column column (upper (upper row) row) versus versus pressure pressure perturbation perturbation corecore (lower (lower row) row) during during stages stages of of ( (aa)) tornadogenesis tornadogenesis,, ( (bb)) intensification intensification and and ( (cc)) demise. demise. The The isosurface isosurface −1 1 ofof 0.1 0.1 s s− verticalvertical vorticity vorticity is given is given in dark in dark blue, blue, embedded embedded with with a darker a darker blue blueisosurface isosurface denoting denoting 0.15 s0.15−1. The s 1 updraft. The updraft of the of mesocyclone the mesocyclone of 15 ofm 15s−1 m is sshown1 is shown in cyan, in cyan, while while the downdraft the downdraft of −6 of m s6−1 m is sin1 − − − − brown.is in brown. The dark The grey dark and grey black and blackisosurfaces isosurfaces in the in lower the lower row depict row depict the pressure the pressure perturbation perturbation of −1.5 of and1.5 −2 and hPa,2 respectively. hPa, respectively. The distance The distance scales scalesin the vertical in the vertical and horizont and horizontalal directions directions are given are in given (a). − − in (a). The formation and intensification of the tornadic vortex column was found to be associated with a columnThe formationof negative and pressure intensification perturbation, of the which tornadic resembled vortex a columnfunnel cloud was found in appearance. to be associated At t0 − 0.5with min, a column a pressure of negativeperturbati pressureon of −1.5 perturbation, hPa was accompanied which resembled by a region a funnel of 15 cloud m s−1in updraft, appearance. both 1 confinedAt t0 0.5 to min, layers a pressureabove 1 perturbationkm AGL. With of the1.5 occurrence hPa was accompanied of the 0.1 s−1by vertical a region vorticity of 15 m column s− updraft, next − − 1 toboth the confinedground, the to layers negative above pressure 1 km AGL.perturbation With the started occurrence to develop of the downward, 0.1 s− vertical and vorticity extended column to the groundnext to theat t0 ground, + 2 min the(Figure negative 8a, lower pressure row perturbation). A stronger started perturbation to develop of −2 downward, hPa was embedded and extended inside to thethe −1.5 ground hPa atisosurface,t0 + 2 min which (Figure was8a, between lower row). 1 and A stronger2.5 km AGL perturbation at t0 + 0.5 of min,2 hPaand wasthen embedded extended − downwardinside the to1.5 the hPa ground isosurface,. Opposite which to was the between growth 1of and the 2.5tornadic km AGL vortex at t0 column,+ 0.5 min, which and thenwas extendedfrom the − bottom,downward the pressure to the ground. perturbation Opposite isosurface to the growth developed of the downward tornadic vortex from column,the top, whichin the same was frompattern the asbottom, a tornado the pressurefunnel cloud. perturbation This suggests isosurface that developedtornadogenesis downward process from herein the may top, inbe thefurther same divided pattern intoas a three tornado steps: funnel the cloud.downward This suggestsdevelopment that tornadogenesisof mesocyclone process vortex, herein the upward may be development further divided of tornadointo three vortex, steps: and the the downward eventual developmentdownward development of mesocyclone of pressure vortex, theperturbation upward developmentand associated of cotornadondensation vortex, of water and the vapor eventual into cloud downward water developmentto form the funnel of pressure cloud. perturbation and associated condensationThe tornado of water formation vapor first into near cloud the water ground to form followed the funnel by bottom cloud.-up development is consistent with Thestate tornado-of-the- formationart mobile first X- nearband the phase ground-array followed radar by observations bottom-up development (e.g., [22]) isand consistent numerical with sstate-of-the-artimulations (e.g., mobile [21,23 X-band]), although phase-array much earlier radar observation observationss of (e.g., TVS [based22]) and on numericalS-band Doppler simulations radar suggest(e.g., [21 that,23]), the although portion of much descending earlier and observations non-descending of TVS tornado based on development S-band Doppler is half radar and half suggest [24]. that the portion of descending and non-descending11 tornado development is half and half [24]. In our simulation, the downward growth of the vorticity column lasts only about 2 min and thus is likely to be missed by the S-band radar data collected at ~5-minute intervals. Atmosphere 2019, 10, 236 12 of 18

4.3. Tornado Intensification and Maturity (t + 4 min t < t + 17 min) 0 ≤ 0 After genesis, the simulated tornadic vortex column experienced a separation and reconnection 1 between the upper and lower vortex column. The 0.1 s− vorticity column separated at around 1 km AGL at t0 + 4 min (Figure8a). The lower part kept moving outward away from the main supercell while the upper part maintained the same storm-relative location together with the updraft, and shrunk upward. The low-level vortex column was maintained for 10 min (t0 to t0 + 10 min). In contrast to the upward retracting dissipation of the weak, thin and short-lived vortex accompanying the DRC formation (Figure5j–l), this vortex dissipated by retracting downward and staying in contact with the 1 ground, in terms of both the 0.1 and 0.15 s− vertical vorticity isosurfaces (Figure8b). The separation of the primitive vortex column was not the end of the whole tornado event. Instead, the lower part of the vortex column was replaced by a newly-generated near-surface tornadic vortex (Figure8b). During this process, the low-level circulation of the tornado vortex decayed while the midlevel vortex remained intact. Then, a new near-surface vortex developed more in line with the midlevel vortex, forming a new surface-based tornado that went on to become stronger. The new vortex was initiated near the ground under the upper part of the vortex column and updraft at t0 + 9 min (denoted by the magenta dotted ring), and connected to the upper part of the vortex column at t0 + 12 min. The separation and re-connection process in the variation of the vorticity column also manifested in the variation of the pressure perturbation column, especially in terms of the 2 hPa − isosurface (Figure8b, lower row). The separation of the pressure perturbation isosurface was generally collocated with the separation of the vortex column. After the replacement of the near-surface tornadic vortex as described above, the tornado reached 1 its maturity. The tornadic vortex became much stronger, such that even the isosurface of 0.15 s− vertical 1 vorticity was connected throughout the lowest 2 km depth from t0 + 14.5 min onwards. The 0.1 s− isosurface column expanded to as large as 500 m in diameter. During the mature stage, the top of the whole vorticity column grew gradually higher (Figure8b), from 2 km AGL to its maximum of over 2.5 km AGL at t0 + 16.5 min. In terms of the pressure perturbation core, the lower part grew much thicker, with a diameter comparable to its upper part. Coincidentally, the strongest wind of this real tornado event at the ground was produced in this stage, which corresponded to the occurrence of the two fatalities. The wider pressure perturbation and vorticity column might be indicative of the wider damage swath in the Daxinzhuang village, as reported in MY14. This replacement of the near-surface vortex V1 by its newly-generated counterpart V2 was also seen in the horizontal cross-section (Figure9). Results show that while V1 weakened due to the detachment from the updraft center, V2 intensified with the stretching by the strong updraft center. The three-dimensional isosurface helps to depict the general structure of the spiral nature of the tornadic vortices, while the two-dimensional contour plots provide more precise details. This indicates that a combination of these two illustration methods may be more suitable for tornado structure analysis. The vortex replacement process is similar to the cyclic mesocyclogenesis in numerical simulation by Adlerman et al. [25] and tornadogenesis in radar observation by Dowell and Bluestein [26]. The key difference is that the replacement in our simulation happened mainly for the near-surface vortices with the midlevel mesocyclone remaining the same. The former near-surface tornadic vortex was detached from the updraft center while the newly-generated vortex was connected to and stretched by the same midlevel mesocyclone. However, in Adlerman et al. [25] the replacement happened mainly to the midlevel mesocyclones rather than the near-surface vortices. Atmosphere 2019, 10, 236 13 of 18

Figure 9. Horizontal cross-section of the simulated supercell and tornado in the first near-surface Figure 9. Horizontal cross-section of the simulated supercell and tornado in the first near-surface vorticity replacement during the tornado intensification stage at the height of (a c) 990 m, (d f) 490 m vorticity replacement during the tornado intensification stage at the height of −(a−c) 990 m, −(d−f) 490 and (g i) 190 m AGL at (left column) t0 + 9 min, (middle column) t0 + 10 min and (right column) m and −(g−i) 190 m AGL at (left column) t0 + 9 min, (middle column) t0 + 10 min and (right column) t0 t0 + 11 min. Vertical velocity is given by shaded contours. Reflectivity of 40 (50) dBZ is given by orange + 11 min. Vertical velocity is given by shaded contours.1 Reflectivity of 40 (50) dBZ is given by orange (red) contours. Vertical vorticity of 0.05, 0.1 and 0.15 s− is given by blue contours. Pressure perturbation −1 (red)of 1.5 contours. and 2 hPa Vertical is given vorticity by dark of grey 0.05, contours. 0.1 and V 0.15and s V isdenote given the by first blue and contours. second Pressuretornadic − − 1 2 perturbationvortices, respectively. of−1.5 and −2 hPa is given by dark grey contours. V1 and V2 denote the first and second tornadic vortices, respectively. 4.4. Tornado’s Demise (t + 17 min t t + 24 min) 0 ≤ ≤ 0 The vortex replacement process is similar to the cyclic mesocyclogenesis in numerical simulation The tornado’s demise occurred in association with another separation in the vorticity and pressure by Adlerman et al. [25] and tornadogenesis in radar observation by Dowell and Bluestein [26]. The perturbation columns. Early on at t + 15 min, a new surface-based vorticity column developed key difference is that the replacement0 in our simulation happened mainly for the near-surface near the bottom of the main one (Figure8b), similar to the previous separation and reconnection vortices with the midlevel mesocyclone remaining the same. The former near-surface tornadic vortex process. However, the evolution of the vorticity and pressure perturbation associated with the vortex was detached from the updraft center while the newly-generated vortex was connected to and was different from the previous cycle. Instead of a sharp separation, the columns bent around the stretched by the same midlevel mesocyclone. However, in Adlerman et al. [25] the replacement 1.5 km AGL layer (t + 20 min, Figure8c) while becoming thinner. The final separation occurred at happened mainly to0 the midlevel mesocyclones rather than the near-surface vortices. about t0 + 20 min below 500 m AGL. Though the new tornadic vortex column grew thicker, it was not able to connect to the upper part of the storm. No strong horizontal wind accompanied it either. 4.4. Tornado’s Demise (t0 + 17 min ≤ t ≤ t0 + 24 min) The near-surface vortex column died out rapidly, with a height confined to no more than 500 m. The upperThe tornado’s part separated demise into occurred several in parts association and died with out another as well, separation when it bent in outward the vorticity and wasand pressuredetached fromperturbation the rotational columns. updraft. Early The on whole at t0process + 15 min, looked a new quite surface similar- tobased the disappearance vorticity column of a developedfunnel cloud near (sometimes the bottom in a of rope the shape). main one (Figure 8b), similar to the previous separation and reconnection process. However, the evolution of the vorticity and pressure perturbation associated

13 with the vortex was different from the previous cycle. Instead of a sharp separation, the columns bent around the 1.5 km AGL layer (t0 + 20 min, Figure 8c) while becoming thinner. The final separation occurred at about t0 + 20 min below 500 m AGL. Though the new tornadic vortex column grew thicker, it was not able to connect to the upper part of the storm. No strong horizontal wind accompanied it either. The near-surface vortex column died out rapidly, with a height confined to no more than 500 m.Atmosphere The upper2019, 10part, 236 separated into several parts and died out as well, when it bent outward and14 was of 18 detached from the rotational updraft. The whole process looked quite similar to the disappearance of a funnel cloud (sometimes in a rope shape). The bending of the upper vortex and the failure of the reconnection between the new near-surface The bending of the upper vortex and the failure of the reconnection between the new nearvortex and midlevel mesocyclone were associated with a developing downdraft (brown in Figure8c). surface vortex and midlevel mesocyclone were associated with a developing downdraft (brown in At this stage, updraft at low levels was greatly distorted by the intrusion of downdraft (Figure 10). Figure 8c). At this stage, updraft at low levels was greatly distorted by the intrusion of downdraft Although another newly-generated near-surface vortex V3 strengthened at around t0 + 20 min, (Figure 10). Although another newly-generated near-surface vortex V3 strengthened at around t0 + 20 neither V3 nor V2 was steadily stretched by the updraft, which was maintained above a 1 km AGL layer min, neither V3 nor V2 was steadily stretched by the updraft, which was maintained above a 1 km (Figure 10a–c), but weakened significantly at low levels (Figure 10d–i). From vertical cross-section, AGL layer (Figure 10a–c), but weakened significantly at low levels (Figure 10d–i). From vertical cross- the low-level vortex was ‘capped’ by strong downward wind field (Figure 11d), prohibiting the section, the low-level vortex was ‘capped’ by strong downward wind field (Figure 11d), prohibiting stretching of the low-level rotation from midlevel updraft. By contrast, the near-surface vortices the stretching of the low-level rotation from midlevel updraft. By contrast, the near-surface vortices were accompanied by steady continuous updrafts at early stages (Figure 11a–c). There is also a were accompanied by steady continuous updrafts at early stages (Figure 11a–c). There is also a downdraft presented at midlevel associated with the short-term weakening during the intensification downdraft presented at midlevel associated with the short-term weakening during the intensification stage, which might have been the cause for the first replacement of the near-surface tornado vortex. stage, which might have been the cause for the first replacement of the near-surface tornado vortex. The formation mechanism of these downdrafts is beyond the scope of this paper, and will be The formation mechanism of these downdrafts is beyond the scope of this paper, and will be studied studied separately. separately.

Figure 10. Horizontal cross-section of the simulated supercell and tornado in the second near-surface vorticity replacement during the tornado demise stage at the height of (a c) 990 m, (d f) 490 m and 14 − − (g i) 190 m AGL at (left column) t + 18 min, (middle column) t + 20 min and (right column) t + 22 min. − 0 0 0 Vertical velocity is given by shaded contours. Reﬂectivity of 40 (50) dBZ is given by orange (red) 1 contours. Vertical vorticity of 0.05, 0.1 and 0.15 s− is given by blue contours. Pressure perturbation of 1.5 and 2 hPa is given by dark grey contours. V and V denote the second and third tornadic − − 2 3 vortices, respectively. Figure 10. Horizontal cross-section of the simulated supercell and tornado in the second near-surface vorticity replacement during the tornado demise stage at the height of (a−c) 990 m, (d−f) 490 m and (g−i) 190 m AGL at (left column) t0 + 18 min, (middle column) t0 + 20 min and (right column) t0 + 22 min. Vertical velocity is given by shaded contours. Reflectivity of 40 (50) dBZ is given by orange (red) contours. Vertical vorticity of 0.05, 0.1 and 0.15 s−1 is given by blue contours. Pressure perturbation of Atmosphere 2019, 10, 236 15 of 18 −1.5 and −2 hPa is given by dark grey contours. V2 and V3 denote the second and third tornadic vortices, respectively. The supercell weakened after the tornado’s demise. At the end of simulation (t = 120 min, The supercell weakened after the tornado’s demise. At the end of simulation (t = 120 min, or t0 + or t0 + 41 min, not shown), the storm remained a supercell, with a 2–5-km updraft helicity of 41 min,2 not2 shown), the storm remained a supercell, with a 2–5-km updraft helicity of ~1300 m2 s−2 ~1300 m s− and a bounded weak echo region. and a bounded weak echo region.

Figure 11. Vertical cross-section of the simulated supercell and tornado during the tornado lifecycle at Figure 11. Vertical cross-section of the simulated supercell and tornado during the tornado lifecycle range indicated by boxes in Figure4, at the ( a) tornadogenesis, (b) intensification, (c) maturity and (d) at range indicated by boxes in Figure 4, at the (a) tornadogenesis, (b) intensification, (c) maturity and demise stages. Vertical velocity is given by shaded contours. Vertical vorticity of 0.05, 0.1 and 0.15 s 1 (d) demise stages. Vertical velocity is given by shaded contours. Vertical vorticity of 0.05, 0.1 and− 0.15 is given by blue contours. Wind (vorticity) field is given by magenta (black) vectors. s−1 is given by blue contours. Wind (vorticity) field is given by magenta (black) vectors. 5. Summary and Discussion 5. Summary and Discussion Based on an idealized cloud model simulation using an observed proximity sounding of the 21 July 2012 BeijingBased tornadic on an idealized supercell andcloud warm model bubble simulation storm initiations, using an observed the simulated proxim supercellity sounding and tornado of the 21 reasonablyJuly 2012 captured Beijing tornadic the main supercell features and observed warm bubble by the Beijingstorm initiation operationals, the S-band simulated Doppler supercell radar. and Fromtornado the simulation, reasonably detailed captured evolution the main of features the storm observ and embeddeded by the Beijing tornado operational from their S formation-band Doppler to demiseradar was. From examined. the simulation, detailed evolution of the storm and embedded tornado from their formationThe details to demise of the DRC was andexamined the sequence. of events leading up to tornadogenesis and throughout the evolution of tornado were examined, including downward15 development of mesocyclone vortex, the upward development of tornado vortex, and the eventual downward development of condensation funnel cloud. The simulation revealed a DRC evolution that had commonalities with the radar observation of this tornado event. The DRC formed about 17 min before the tornadogenesis, which retracted upward immediately before tornadogenesis, similar to radar observations. The tornado The details of the DRC and the sequence of events leading up to tornadogenesis and throughout the evolution of tornado were examined, including downward development of mesocyclone vortex, the upward development of tornado vortex, and the eventual downward development of Atmospherecondensation2019, 10 funnel, 236 cloud. The simulation revealed a DRC evolution that had commonalities16 with of 18 the radar observation of this tornado event. The DRC formed about 17 min before the tornadogenesis, which retracted upward immediately before tornadogenesis, similar to radar observations. The formedtornado throughformed through downward downward development development of the of mesocyclone the mesocyclone towards towards the ground, the ground, followed followed by theby the rapid rapid upward upward growth growth of of the the tornado-scale tornado-scale vortex vortex underneath underneath the the mesocyclonemesocyclone (Figure(Figure 1212a). ThisThis bottom-upbottom-up development development provides provides a a numerical numerical evidence evidence for for the the growing growing support support for for a bottom-up, a bottom- rapidup, rapid tornadogenesis tornadogenesis process process as revealedas revealed by by the the state-of-the-art state-of-the-art mobile mobile X-band X-band phase-array phase-array radarradar observationsobservations andand isis alsoalso consistentconsistent withwith mostmost recentrecent simulationsimulation studies.studies. DuringDuring the the tornado tornado intensification intensification stage,stage, the the modeled modeled tornadic tornadic vortex vortex experienced experienced a a verticalvertical separationseparation andand aa reconnectionreconnection betweenbetween thethe upperupper vortexvortex andand aa newlynewly developeddeveloped near-surfacenear-surface vortexvortex (Figure(Figure 1212b).b). The tornado’stornado’s demise camecame aboutabout whenwhen anotheranother roundround ofof vortexvortex columncolumn separationseparation occurred.occurred. EvenEven thoughthough aa newnew near-surfacenear-surface vortexvortex appearedappeared underneathunderneath thethe midlevelmidlevel mesocyclone,mesocyclone, stretchingstretching waswas prohibitedprohibited byby thethe developmentdevelopment ofof downdraftdowndraft overover thethe near-surfacenear-surface vorticity,vorticity, andand thethe tornadotornado eventuallyeventually collapsedcollapsed (Figure(Figure 1212c).c). TheThe keykey processprocess leadingleading toto thethe maintenancemaintenance andand demisedemise processesprocesses hereinherein callscalls forfor properproper alignmentalignment betweenbetween midlevelmidlevel mesocyclonemesocyclone andand low-levellow-level tornadictornadic vortices,vortices, whichwhich generally agrees with with suggestions suggestions from from previous previous studies studies [12 [12,27,27].]. One One uniqueness uniqueness of ofthis this result result is that is that thethe tornado tornado vortex vortex cycling cycling could could be a bereplacement a replacement of the of near the-surface near-surface vortices vortices while whilethe midlevel the midlevel mesocyclone mesocyclone remains remains the the same. same. Moreover, Moreover, our our analyses analyses here here emphasize emphasize the the role of of downdraftdowndraft intrudingintruding onon thethe tornadotornado vortexvortex asas aa causecause ofof tornadotornado demise.demise.

FigureFigure 12.12. SchematicSchematic diagramsdiagrams of thethe ((aa)) genesis,genesis, ((bb)) maintenancemaintenance andand ((cc)) demisedemise processesprocesses ofof thethe BeijingBeijing tornadotornado eventevent basedbased onon thethe idealizedidealized simulation.simulation. TheThe suctionsuction andand circulationcirculation ofof thethe midlevelmidlevel mesocyclonemesocyclone are are denoted denoted by by the the cyan cyan arrow arrow and and greengreen circle, circle, respectively. respectively. The The circulation circulation of of the the enhancedenhanced moderate rotation rotation of of the the midlevel midlevel mesocyclone mesocyclone is denoted is denoted in purple, in purple, which which develop developss top- top-downdown toward toward the the ground. ground. The The stronger stronger rotation rotation at atthe the ground ground is is denoted denoted by by red red circles. Blue arrows denotedenote the the stretching stretching of of the the near-surface near-surface tornadic tornadic vortices. vortices. The grey The column grey column denotes denotes the tornado the tornado funnel cloud.funnel Thecloud. dashed The dashed lines in lines (b) and in ( (bc)) and denote (c) denote the weakened the weakened former former tornado tornado column. column. The brown The brown arrow inarrow (c) denotes in (c) denotes the strengthened the strengthened downdraft downdraft that cuts o thatﬀ the cuts connection off the conn betweenection the between newly developed the newly near-surfacedeveloped near tornadic-surface vortex tornadic and thevortex suction and ofthe the suction midlevel of the mesocyclone. midlevel mesocyclone.

InIn orderorder toto obtainobtain thethe simulationsimulation reportedreported inin thisthis studystudy thatthat sharesshares manymany commonalitiescommonalities withwith observations,observations, we we performed performed many many sensitivity sensitivity experiments experiment beforehand.s beforehand We found. We thatfound tornadic that tornadicfeatures werefeatures highly were sensitive highly sensitive to model to conﬁgurations. model configurations. Furthermore, Furthermore, the large the warm large bubble warm bubble strength strength of 7 K neededof 7 K toneeded initiate to a sustainedinitiate a tornadic sustained storm tornadic suggests storm that suggests some features that some of the features observed of sounding, the observed such as the relatively warm mid-levels, of this case may not be most favorable for most intense supercell storm. Even higher spatial resolutions, the inclusion of mesoscale16 forcing in horizontally inhomogeneous initial conditions, as well as the inclusion of surface friction may be needed to obtain even more realistic simulations. The absence of surface friction as another source of near-surface vorticity may be Atmosphere 2019, 10, 236 17 of 18 an important reason of the weaker than observed tornado vortices obtained, as advocated by recent studies (e.g., [11,21,28]). These can be topics for future studies.

Author Contributions: Conceptualization, Z.M. and D.Y.; methodology, D.Y.; software, D.Y.; validation, Z.M. and D.Y.; formal analysis, Z.M. and D.Y.; investigation, D.Y.; resources, Z.M. and M.X.; data curation, D.Y.; writing—original draft preparation, D.Y.; writing—review and editing, D.Y. and Z.M.; visualization, D.Y.; supervision, Z.M. and M.X.; project administration, Z.M., D.Y. and M.X.; funding acquisition, Z.M., D.Y. and M.X. Funding: This research was jointly funded by National Natural Science Foundation of China (grant number 41425018, 41875051, 41705028, 41375048, 41730965) and China Railway Research Project (K2018T007). Acknowledgments: The authors are grateful to the precious suggestions from Paul Markowski and Yvette Richardson. We wish to acknowledge China Meteorological Administration for providing radar, radiosonde and surface observations. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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