Analysis of Convective Thunderstorm Split Cells in South-Eastern Romania

e mesoscale con�gurations are analysed associated withthe splitting process of convective cells responsible for severe weather phenomena in the south-eastern part of Romania. e analysis was performed using products from the S-band Doppler weather radar located in Medgidia. e cases studied were chosen to cover various synoptic con�gurations when the cell splitting process occurs. �o detect the presence and intensity of the tropospheric �et, the Doppler velocity �eld and vertical wind pro�les derived from radar algorithms were used.e relative Doppler velocity �eld was used to study relative �ow associated with convective cells. �ra�ectories and rotational characteristics associated with convective cells were obtained from re�ectivity and relative Doppler velocity �elds at various elevations. is analysis highlights the main dynamic features associated with the splitting process of convective cells: the tropospheric �et and vertical moisture �ow associated with the con�guration of the �ow relative to the convective cells for the lower and upper tropospheric layers. ese dynamic characteristics seen in the Doppler based velocity �eld and in the relative Doppler velocity �eld to the storm can indicate further evolution of convective developments, with direct implications to very short range forecast (nowcasting).


Introduction
In south-eastern Romania, in the convective season (from May to September), severe weather phenomena develop frequently and evolve [1][2][3][4][5], sometimes leading to signi�cant damages.Analysis of severe convective events and their structure, at least for certain classes of phenomena (isolated convective cells), taking into account the mesoscale con�guration can improve the nowcasting procedures.Using an S-band Doppler radar (10 cm wavelength), we see that the mesoscale phenomena (supercellular thunderstorms) have certain features during their dynamic evolution.Of great importance in understanding the evolution of supercells is the splitting process (separation of the convective cells in two other ones rotating opposite to one another, namely, cyclonic and anticyclonic), a process that takes place during the evolution of convective developments and is closely linked to the state of mesoscale dynamic con�guration, especially vertical wind shear [6,7].is process has been studied both qualitatively [8][9][10][11][12][13][14][15][16][17][18] and by numeric analysis [19][20][21], highlighting the interaction between horizontal vorticity from the vertical pro�le of horizontal wind and the updra.
Depending on the curvature of the hodograph, the development of convective cells is favoured when the propagation is either to the right, or to the le oroposphericshear; both cells may be favoured as well, when the hodograph is linear.Along with this particular interaction, there may also exist secondary ones [22] that lead to the diminishing of the cell that propagates to the le of shear vector, in the Northern Hemisphere.Observations of the split process when the Doppler radar is used [23][24][25][26][27][28][29][30][31][32] highlight the cyclonic and anticyclonic rotation, within the �eld of Doppler velocity, associated with convective cells compatible with vertical shear.
e above studies also showed the asymmetry between the cells that propagate to the right and those that propagate to the le of the tropospheric shear vector in the Northern Hemisphere, the latter generally has a shorter lifespan than the former, but is associated with severe hail.
In some cases the in�ow at low levels and the out�ow at upper levels [33] to and from the storm were seen with the aid of the Doppler relative velocity �eld� these two �ows are in direct correlation with the intensity of updra.
In this paper, several cases are analysed which featured the splitting process of strongly convective cells (supercells).e basic criteria in choosing these cases are the different mesoscale con�guration (various directions of the tropospheric �ow).e study focused on the analysis from �elds and products of the meteorological Doppler radar that can provide relevant data on the severity of convective phenomena.
is paper is structured as follows.e �rst part contains the radar data that were used during the analysis of splitting processes, and the methodology thereof.At the second part, several case studies are presented that emphasize certain characteristics of the mentioned process.e paper ends with conclusions derived from these case studies, namely, mesoscale dynamic features and some particularities of convective cells trajectories.Since most of the convective cells appearing in the radar �elds featured supercell traits, the common elements that were identi�ed could be used in nowcasting.

Data and Methods
For the dynamic mesoscale analysis and that of the splitting process, the WSR-98D Doppler radar in S band (10 cm) was used, located in Medgidia (south-eastern Romania) (Figure 1).e range of the radar is shown in Figure 1.
e splitting process was analyzed in the �elds of radar re�ectivity, Doppler base velocity, and that of Doppler relative velocity at different scan elevations, in order to see the formation of cyclonic and anticyclonic vortices.e Doppler relative velocity �eld is obtained from the Doppler base velocity �eld minus the average velocity of all detected storms and represents Doppler velocity �eld of the reference storm [34].e Principal Users Processor (PUP) soware (METSTAR WSR-98D PUP version 10.8.R) was used.e study of Doppler velocity �eld at various elevations, in parallel with the Vertical Azimuth Display (VAD) Wind Velocity product resulted in information about the vertical shear of horizontal velocities and tropospheric jets.e VAD Wind Velocity is a Doppler weather radar product which gives the vertical pro�le of horizontal wind vectors above radar station [34][35][36].Strong shear in the wind �eld assumes the existence of tropospheric low-level jet (LLJ) or upperlevel jet (ULJ).Also the Doppler relative velocity �eld gave information about the low and upper relative �ow associated to the storms in radar �eld.e presence of relative lowlevel �ow was analysed (arrow-shaped tube, in the lower part of Figures 2(a For the �rst three analysed cases, vertical vorticity pro�les were made, further to be compared with circular movement associated with supercells (horizontal circles with cyclonic and anticyclonic vorticity, in Figure 2).e evolution of convective cells was observed in the re�ectivity �eld at elevations of 0.� and 1.� degrees, also in the Doppler radial velocities �eld and in the Doppler relative velocity �eld at elevations of 0.�, 1.�, 2.4, and 3.4 degrees.
e evolution in time and space of each convective cell that was characterised by the splitting process was also observed.Identi�cation of splitting was done by tracking both re�ectivity and cyclonic or anticyclone rotation, as seen in Figures 3(a) and 3(b).Figure 3(c) shows how the vertical vorticity of rotational movements was calculated.
us, for each rotational motion, maximum inbound and outbound velocities were identi�ed in the Doppler relative velocity �eld at elevation where rotational movement had maximum intensity.e PUP application was used.For the actual calculation, radar algorithm is dividing the difference between the maximum inbound velocity and the maximum outbound velocity, by the distance between the two (mesocyclone diameter in Figure 3(c)).
Trajectories of convective cells in each case were obtained using polar coordinates, relative to the radar position, of the cells before and aer the splitting.For the three cases studied in detail, the result was a map showing the individual characteristics of convective developments, taking into account the stage before splitting and the aer-splitting stage (which is associated with the cyclonic and anticyclone rotation).Vertical variation of the horizontal velocity (vertical shear) was derived from the VAD Wind Pro�le algorithm and vertical pro�les were then constructed from wind shear at particular moments, representative of the evolution of vertical shear during the life of the convective cell.e vertical domain associated with the -coordinate was separated into a domain with directional shear (in the lower tropospheric layer) and a domain with nondirectional shear (in the middle tropospheric level).is division was possible because the dynamic con�guration near the radar site, at least in the three cases studied, has a nondirectional shear above 1800 m associated with an ULJ, and directional shear below 1800 m.International Journal of Atmospheric Sciences In the �rst three cases, this dynamic con�guration is qualitatively compatible with Doppler velocity �eld at 3.4-degree elevation.
To analyse the presence of the cell splitting process of convective cells in south-eastern Romania and its role in supercell evolution, cases of severe weather were analysed where this process was shown clearly in the radar �elds.For the �rst three cases, in the re�ectivity �eld the convective cells were identi�ed that split, and those that resulted from the split.�sing the Doppler base velocity �eld at 3.4-degree elevation one can tell if there is a quality equivalence between the vertical pro�le of hori�ontal velocity near the radar station and the mesoscale �ow.Trajectories of split cells were rebuilt, to identify some of their peculiarities.e trajectories were compared with the precipitation �eld, as obtained from WSR-98D S-band radar precipitation algorithm (Precipitation Processing System, PPS [37]).
e vertical pro�les associated with hori�ontal vorticity were compared with vertical vorticities speci�c to cyclonic or anticyclonic rotation.In the last two cases, the presence of an upper-level jet was analysed, and the intensity of relative �ow was examined.
International Journal of Atmospheric Sciences   �eld.e vertical wind pro�le was characterized by intense mesoscale �ow from the north west to south east, above 1800 m (6 KFT) (Figure 4(a), in the white box).Below this level, from the �AD �ind Pro�le and base Doppler velocities, one can notice the anticyclonic rotation of velocity vector (Figure 4(a), in the yellow box).
At the 5000 m level, Figure 4(a), the values of velocity reach 35 m s −1 growing to 40-45 m s −1 at 35 KFT (10500 m).e colour for the wind vectors represents the level of con�dence in the �AD �ind �elocity data; the level of con�� dence is larger in the lower troposphere (green, dispersion of 0-2 m s −1 ) and in the upper troposphere (yellow, dispersion of 2-4 m s −1 ).A larger dispersion of velocity is recorded in the lower layers (red, dispersion of 4-6 m s −1 ), which can be associated with different gust fronts caused by convective developments.In the same �gure, both the anticyclonic rotation of the velocity vector in the lower two km and the upper tropospheric jet are consistent with the warm sector of the cyclone located north east of Romania.is vertical con�guration of the horizontal velocity is also associated with increased thermodynamic instability in the troposphere below approximately 500 hPa; the main factor for the increase in conditional instability is moisture advection in the lower layers.However, the presence of a vertically expanded upper tropospheric jet is associated with ageostrophic velocities, which are the result of horizontal variation of the geostrophic velocities that lead to changes in the thermal gradient within the middle troposphere [18].
�ertical wind pro�le con�gurations of the mesoscale �ow can be seen in the Doppler velocity �eld at 3.4�degree elevation (Figure 4 colours, in Figure 4(b)).Near the radar site (in the boundary layer) the isodope is S shaped, compatible with the velocities pro�le (within the yellow bo�, in Figure 4(a)).Going away from the radar (increasing the height), this has a linear shape on the NE-SW direction and is compatible with the upper tropospheric jet direction (Figure 4(a)).e intensity of the jet stream is obtained by measuring the Doppler velocities at 90 degrees from the isodope, having the same height (same distance from the radar in the Doppler velocity �eld).e dynamic �eld of the Doppler velocities was maintained over the interval with the mentioned convective phenomena (a few hours).In Figure 5(a) the re�ectivity �eld is shown at 1500 UTC at 1.5-degree elevation, and in Figure 5(b) the Doppler relative velocity �eld is shown at 2.4-degree elevation at 1500 UTC.e re�ectivity �eld at each elevation has a 1 km 2 resolution with a coverage area of 230 km radius around the radar site and the corresponding product is built every 6 minutes.e angle of the radar azimuth beam is about 0.97 degrees, so that 366 azimuths are needed to cover the entire radar area.At 1.5-degree elevation (Figure 5(a)), as the height of the radar echo increases along with the distance to the site at 230 km from the site the height is 9.6 km.In Figure 5(a), the Doppler velocities �eld relative to the storm is at 2.4degree elevation, and at the 230 km the height is 13.2 km.
In the re�ectivity �eld, si� convective cells can be seen, resulting from three convective cells that had split, and in the relative Doppler velocity �eld the rotation can be seen, associated with resulted convective cells.A fundamental feature is the relative jet at 100-1000 m with velocities of up to 29 m s −1 .is relative jet is the storm input �ow and can only be observed in the storm's reference frame; it may be associated with the moisture in�ow.Above 6000 m there is a relative �ow with velocities up to 11 m s −1 .
Trajectories of convective cells that had split during evolution present symmetry to the mean shear in the 1800-6000 m layer (blue arrow), with cells characterised by cyclonic rotation le of the vector and anticyclone rotation right of it (Figure 6(a)).In Figure 6(a), advection of convective cells by the tropospheric circulation is on the NW-S� direction.In this case, because the �ow is parallel wih the shear vector on the mean troposhere, the convective cells with cyclonic rotation (red) deviate to the right and the ones with anticyclonic rotation (blue) to the le of this direction.Following the cell evolution, the intensity of the cyclonic International Journal of Atmospheric Sciences rotation is larger than that of anticyclonic rotation.e cause for this difference can be associated with directional shear in the 300-1800 m layer and is favoured by advection of vorticity, whose vector is parallel to the wind relative to the storm [17,18].
e splitting process may occur several times in the cells following from a previous split.For multiple splits, cells are favoured whose movement occurs on the le of the mean tropospheric shear.One possible explanation is that these cells draw more vorticity from their surroundings, vorticity which is perpendicular to the relative storm �ow [18].
Cumulative rainfall in the evolution area, associated to the convective cells, is presented in Figure 6(b).e maximums are associated with convective cells, appearing as lines with symmetrical shear.Trajectories associated with more precipitation are to the right of shear in the 1800-6000 m layer.e amount of precipitation is ampli�ed by the intersection of convective cells paths (cells with cyclonic rotation intersect those with anticyclonic rotation); indeed, in Figure 6(a), these intersections are more likely to the right of the mean shear vector.
Vertical wind variation associated with VAD Wind Pro-�le, Figure 4(a), is represented for 6 times in Figure 7.Note the presence of the tropospheric jet in the 1800-9000 m layer, with values of about 40 m s −1 at 9000 m. e vertical pro�le of horizontal vorticity versus vertical vorticity associated with cell rotation is given in Figure 8; here, vorticity was calculated every 305 m on the -axis.One can see several maximum values of vorticity.e highest values are found in the 2000-4000 m layer.e horizontal vorticity of the environment (Figure 8, blue chart) was superimposed over cyclonic or anticyclonic vertical vorticity associated with mesocyclones (Figure 8, vertical lines) in the radar area.e parallel lines with vertical axes signify the vorticity of mesocyclones calculated with the radar algorithm in Figure 3(c).Vorticity was plotted both with a plus and minus sign for symmetry, to be more easily compared with the horizontal vorticity of the environment.June 25, 2009.e presence of a stationary cyclone in the SE of Romania modulated the mesosynoptic con�guration in which severe convective cells developed.ese cells had supercellular attributes and had an SE-NE direction in the S-band WSR-98D radar area (located in �edgidia).In the �rst stage, convection had an isolated character and formed near the radar site.Aerwards, the majority of cells split and became supercells.In the end, the cells with cyclonic rotation formed a convective system.Similarly to the previous case, the vertical velocity pro�le is characterized by an intense mesoscale �ow above 1800 m (6 KFT) (Figure 9(a), the white box).Below this level, between 1113 UTC and 1215 UTC, in the same �gure one can see the rotation of velocity vector along the -axis (this rotation may be associated with directional shear) (the yellow box, in Figure 9(a)).

e Second Studied Case,
Above 1800 m, there is strong intensi�cation of wind, associated with the presence of a ULJ from south east; this ULJ reaches 35 m s −1 at 5000 m. e dynamic �eld that can be viewed in the Doppler velocities, Figure 9(b), retained its structure between 1030 UTC and 1800 UTC.
Radar re�ectivities show four secondary convective cells that are resulted from the splitting of two initial cells (Figure 10(a)).ree of them are supercells, and one is nonsevere.In the �eld of relative �oppler velocities a relative jet is present whose speed is up to 16 m s −1 at 1000 m, while above 5000 m intense circulation is present, with up to 16 m s −1 , (Figure 10(b)).e re�etivity �eld and the relative velocity characteristics are the same as in the pevious case.
Severe storms moving towards the le of mean shear (anticyclonic rotation) were characterized by high intensity and shorter lifespan and were associated with the presence of ree-Body Scatter Spike (TBSS) radar artifact (Figures 10(c) and 10(d)).is artifact is a radar signature in re�ectivity �eld, generally a 10-30 �m long, aligned radially downrange from a strong echo region in re�ectivity �eld (>60 dBZ) associated with a severe storm.It is caused by non-Rayleigh radar microwave scattering or Mie scattering and is asociated with high probability of large hail [38][39][40].
In Figure 11 three vertical velocity pro�les are presented, at certain times representative for this case.Here, the tropospheric ULJ has 35-40 m s −1 at 6000 m. e vorticity pro�le is shown in Figure 12 (blue chart).
e mesocyclone vorticities associated with convective cells that splited are represented by parallel lines with the vertical axes (Figure 12).Note that most values of vorticity lie between −0.005 s −1 to −0.015 s −1 and 0.005 s −1 to 0.015 s −1 .
International Journal of Atmospheric Sciences   ere is also rotation with values of shear around 0.04 s −1 .In a �rst appro�imation, the vorticities of the vertical wind pro�le are comparable with those of the mesocyclone.

International Journal of Atmospheric Sciences
Trajectories of convective cells resulted from the split, present symmetry to the mean shear on the 1800-6000 m layer, with cells characterized by cyclonic rotation to the le of the vector and cells with anticyclonic rotation to the right (Figure 13(a)).
e evolution of these cells shows that the intensity of cyclonic rotation is greater than that of anticyclonic rotation.e precipitation distribution function, by shear in the 1800-5000 m layer is represented in Figures 13(b), 13(c), and 13(d).Grouping in three-hour intervals was made in order to highlight the symmetry of precipitation distributions from shear vector.Large amounts were recorded at the intersection of several trajectories caused by several splits and also to the right of shear.August 11, 2003.In this case, starting at 1300 UTC, in SE Romania and NE Bulgaria, during the fast evolution of a trough which moved from W to E, convective cells have developed with supercell characteristics, visible in the radar-swept area.About this event it can be readily said that, depending on the wind distribution, vertical shear, and time of day, two dynamic behaviors are distinguishable.As such, in the interval 0400 UTC�1115 UTC, the vertical velocity pro�le is characteri�ed by a moderate mesoscale �ow above 1500 m (Figure 14(a)).e radar re�ectivity at 1.5-degree elevation is in echoes with a strip form, perhaps associated with conditional symmetric instability [41,42] or with an internalgravitational wave [18] (Figure 14(b)).Here, no splitting process occurred.

e ird Studied Case,
As it can be seen in Figure 15, wind velocities have a maximum at 20 m s −1 at 2000 m. e vorticity distribution, shown in Figure 16, has a pronounced maximum of 0.04 s −1 at 1000 m.
Between 1328 UTC and 1432 UTC, the mesoscale dynamic state changes, a ULJ being present in the high troposphere, that can be seen in the vertical pro�le of Mesocyclone vorticities associated with convective cells that split are represented by parallel lines with the vertical axes (Figure 19).Note that most of vorticity values lie between −0.001 s −1 and −0.009 s −1 , and between 0.001 s −1 and 0.009 s −1 .e maximum vorticity here had a value around 0.011 s −1 .
e Doppler relative velocities to the elevation of 1.5 degrees, Figures 20(a) and 20(b), show a relative jet in the low levels, whose speed is up to 23 ms −1 at 500 m, while the relative �ow at higher levels (above 6000 m) gets to 18 m s −1 .Spatial distribution of convective cell trajectories before and aer splitting is shown in Figure 21(a).ere is overlapping of the mean tropospheric shear vector on the 1500-6700 m layer.Symmetric trajectories to the mean shear are seen.In the precipitation �eld (Figure 21(b)), cells having higher water amounts are to the right side of the mean shear.September 27, 2004;June 23, 2009).In the �rst case, a �uasi-stationary trough of a big amplitude was present to the west of Romania.e �rst severe convective cells were seen by the radar at 1020 UTC in Bulgaria and were advected in Romania by the tropospheric circulation.e convective cells which later split were initiated nearby the radar site at 1220 UTC.In this case, the convective cells with anticyclonic rotation had a bigger life time and a greater severity as well.

Other Cases (
In the second case, the mesosynoptic con�guration was modulated by the cyclone west of Romania.e circulation due to this cyclone was southern over the south east of Romania in the boundary layer as well as at higher altitude.In the �rst part of the day, the Doppler radar caught a mesoconvective system with supercells.Aer the convective system dissipated, other new convective cells have been initiated and split; the most severe ones of them have cyclonic rotation. As in the cases above, for these two other cases the main feature associated with the mesoscale con�guration was the ULJ, whose speed exceeded 33 m s −1 .In the �rst case (September 27, 2004), intense �ow is visible at lower levels in the �eld of Doppler relative velocity, and in upper layers at approximately 11-15 m s −1 , as visible in Figure 22(b).
e convective cells in Romania that evolved to the le of tropospheric shear were associated with nonnegligible probability of hail as indicated by the presence of ree-Body Scatter Spike (Figure 22(d)).Cyclonic shear at low levels (Figure 22(a)) can be a direct indication of the severity of convective cells that propagate to the le of the shear vector.e cells in Bulgaria show strong propagation to the right of the tropospheric shear and also showing the ree-Body Scatter Spike (Figure 22(c)).For these, the splitting process was not seen.In the second case (June 23, 2009), intense �ow is not present at lower levels in the �eld of Doppler velocity relatively, but a relatively high speed of �ow of about 15 m s −1 is seen at upper levels (Figure 23(b)).e radar re�ectivity �eld at 1.5-degree elevation (Figures 22(c) and 23(c)) highlights the splitting process; the secondary cells evolve to the right (cyclonic rotation) and to the le (anticyclone rotation) of the shear vector S in the 1800-6000 m layer.

Conclusions
is study shows the splitting process in SE Romania, linked to convective developments which can be seen using the Sband Doppler radar.is process is important as a dynamic one, because it can modulate the energy transfer between the synoptic scale and the mesoscale.e main mesosynoptic conditions associated with the generation of this dynamic process can be distinguished, and also its evolution.Information about all of these can be used as an additional tool in nowcasting.
From the analysis of convective cells resulted from the split it can be said that the splitting process of convective cells observed in radar �elds indicates a corresponding severe weather event.As a fundamental condition, splitting of convective cells occurred when an upper level jet (30 m s −1 -40 m s −1 ) was detected in the high troposphere (above 6000 m).In the �rst stages of the convective cells, the presence of the jet stream in the upper troposphere and especially its vertical extent lead them to be strongly advected by the �ow.Also, the presence at lower levels of a directional variation in the horizontal wind vector leads to a stronger relative �ow.In the case of a tropospheric con�guration that favours the splitting process, the lower relative �ow has a high intensity and is opposite to the upper relative �ow; this con�guration is seen in the relative Doppler velocities �eld at different elevations.In all the three detailed cases, below 1800 m the horizontal wind vector had a rotation associated to the asymmetry of the splitting process, and also to the strong relative �ow between 100 and 1000 m, providing a powerful moisture �ow through updra and strong vorticity advection.
All the analysed cases have shown the independence of the splitting process to the atmosphere circulation.is is characteristic to baroclinic waves with a west-to-east evolution, and also to cyclones near Romania: north-west circulation for May 1-28, 2008, south-east circulation for June 2-25, 2009, west circulation for August 3-11, 2003, south-south-east circulation for September 27, 2004, and south circulation for June 23, 2009.e largest amounts of precipitation are related to cell cross-trajectories associated with the splitting process and lie to the right of shear in 1800-6000 m layer (Figures 6, 13, and 21).e splitting process may lead to a mesoscale convective system.If this happens the secondary cells interact constructively aerwards; this will happen to the right side of the mean tropospheric shear.
e local maxima of vertical vorticity (Figures 9, 13, and 21) are most positive (to the right side of the 0,0 coordinate).Keeping the same sign of horizontal vorticity is essential in formation of cyclonic and anticyclonic vortices, con�rmed by Figure 1.e magnitude of the horizontal vorticity associated with vertical wind pro�les and that of vertical vorticity for cyclonic and anticyclonic rotation are comparable; the majority of values is lying in the 0.001 s −1 -0.009 s −1 interval (in a �rst approximation, these magnitudes verify the dynamic process associated to the formation of cyclonic and anticyclonic vortices).
Both the June 25, 2009 and in the September 27, 2004 cases highlight the TBSS associated with convective cells (anticyclonic rotation) with a motion to the le of the shear vector in the mean troposphere, which means that these cells have attributes that indicate the presence of severe hail.An explanation could be the fact that the convective cells with motion le to the mean shear are more isolated than those with motion to the right of it.e former cells are moving to the zone where the values of geopotential and temperature are falling.Here, the thermal gradient is perpendicular to ) and 2(b)) and the relative upper-level �ow (arrow-shaped tube, at top of Figures 2(a) and 2(b)).

F 2 :VR = 19 F 3 :
Tiling of the horizontal vorticity due to updra (a).Negative vorticity maxima (on the le of shear) and positive vorticity maxima (on the right of shear) and (b) the convective cell split in two other cells with opposite vorticity (from Lin 2007, adapted by Klemp 1987, and from Rotuno 1981).m/s, S = 0.005 1/s, R = 122 km, D = 8 km (c) Radar re�ectivity at 1.��degree elevation (a), relative velocities at 1.��degree elevation, (b) the white circles mar� cyclonic and anticyclonic rotation associated with the two cells aer the split, and shear due to the cyclonic and anticyclonic rotation (c).

F 6 :F 7 :
May 28, 2008, the convective cells evolution for 0900 UTC-2000 UTC using the Medgidia Doppler radar in (a).e center of images is the radar position.S vector represents the mean shear at 1800-6000 m altitude.Red stands for cyclonic rotation, blue for anticyclonic, and blac� signi�es cells before split.e distance is measured in �m.e accumulated precipitations (mm), for 0900 UTC-2000 UTC (b).May 28, 2008, vertical velocity size distribution for 1800-9000 m altitude at 1406 UTC-1506 UTC.

3 .F 8 :
Results and Discussions3.1.e First Studied Case,May 28, 2008.A cyclone located in north-east Romania contributed to the instability conditions present in the Doppler radar area of the Medgidia site.Starting at 1130 UTC, on an NW-SE intense tropospheric circulation, convective cells developed in SE Romania that were split and became supercells.e most severe supercells were the ones with cyclonic rotation, such as those having the � shape in re�ectivity �eld.�esides these, multicells systems were present in the radar �eld, with a lower degree of severity.etemporal evolution of mesoscale �ow is represented in Figure4, between 1406 UTC and 1506 UTC, as seen in the �AD Wind �ro�le product and in the base Doppler velocities May 28, 2008, vertical shear distribution for 1800-9000 m layer, at 1406 UTC-1506 UTC (blue chart).Shear is calculated every 305 m.Horizontal shear distribution associated with convective cells (vertical lines).

F 12 :
(b)), and they are qualitatively equivalent with the vertical wind pro�le given by the �AD �ind Pro�le product.is equivalence is the result of the zero isodope analysis [34] in the Doppler velocity �eld.e zero isodope represents the area where the Doppler velocities are zero (gray Velocity (m s −1 ) Velocity (m s −1 ) Velocity (m s −1 ) F 11: June 25, 2009, vertical velocity size distribution for 1800-6000 m, at 1113 UTC-1215 UTC.June 25, 2009, vertical shear distribution for 1800-6000 m, at 1113 UTC-1215 UTC (blue chart).Shear is calculated every 305 m.Horizontal shear distribution associated with convective cells (vertical lines).

F 13 :
June 25, 2009, the convective cells evolution at 1030 UTC-1800 UTC using the Medgidia Doppler radar in (a).e white vector represents the mean shear for 1800-5000 m altitude.Red is the cyclonic rotations and blue is the anticyclonic rotations.e distance is in km.Accumulated precipitations are for every three hours: (b) 1040 UTC-1340 UTC, (c) 1205 UTC-1505 UTC, and (d) 1405 UTC-1705 UTC.

F 20 :F 21 :
August 11, 2003, Doppler relative velocities, at 1432 UTC (a) and 1712 UTC (b) at 1.5-degree elevation.August 11, 2003, (a) the convective cells distribution, between 1126 UTC and 1925 UTC, using the Medgidia Doppler radar.e center of the image marks both the radar site and the origin of coordinates.e vector represents the mean shear for 1500-6700 m.Red stands for cyclonic rotation, blue for anticyclonic, and black signi�es cells before split.e distance is measured in km.(b) Accumulated precipitation in mm, between 1547 UTC and 1847 UTC.

F 23 :
June 23, 2009: (a) Doppler velocities, at 1004 UTC at 3.4-degree elevation; (b) Doppler relative velocities, at 1004 UTC at 2.4-degree elevation; (c) re�ectivity, at 1004 UTC at 1.5-degree elevation.horizontal wind (Figure 17(a)).Between 304 and 1500 m, neither the measurements of wind nor the �AD �ind �ro�le are reliable.�ere, the in�uence of convective phenomena that can generate gust fronts is present.In Figure 18 the presence of the tropospheric jet is shown by velocities of 37 to 40 m s −1 at 6700 m.Due to the tropospheric jet, the re�ectivity �eld has changed, emphasizing now the splitting process both in radar re�ectivities and Doppler relative velocities (Figures 17(b) and 17(c)).