1 ARPA-Servizio Meteorologico Regionale, Bologna, Italy2 CNR, Institute of Atmospheric and Oceanic Sciences, Bologna, Italy3 University of Bath, Bath, UK4 University of Essex, Colchester, UK

The 18 June 1997 Companion supercells:Multiparametric Doppler radar analysis

P. P. Alberoni1;2, V. Levizzani2, R. J. Watson3;4, A. R. Holt4, S. Costa1,P. Mezzasalma1, and S. Nanni1

With 17 Figures

Received June 30, 1999Revised February 3, 2000

Summary

On 18 June 1997 two simultaneous supercells 50 km apartswept the Po valley W-NW to E-SE. An exceptional hailfalllasted for more than 3 hours over a strip 200 km wide. Thereare no records of companion supercells over northern Italy, afact that attributes to the present observations a considerablemeteorological interest.

The forcing due to a baroclinic wave disturbance createdthe synoptic conditions favourable for storm development.A closer mesoscale analysis conducted using the LocalAnalysis and Prediction System (LAPS) indicates that theinteraction of the storm systems with the low-level frontalhigh-humidity band is a key aspect of their evolution.

The operational polarimetric Doppler weather radar of S.Pietro Capo®ume was used for a volumetric analysis of thestorm system focusing on the microphysical and dynamicalstructure, and wind patterns. Re¯ectivity and Doppler wind®elds document the transition from multicell to supercellphase for one of the storms. Differential re¯ectivity (ZDR)®elds are examined. Positive ZDR columns are detected inconnection with the storms strong updraft. Flare echoes hintto the presence of substantial hailshafts. The spectrum width®eld is used to investigate internal motions. Limited dualDoppler analysis is carried out and results are discussed interms of storm evolution properties.

1. Introduction

The enhanced observational capabilities usingremote sensing techniques agree to a detailed

description of the microphysics and the dynamicprocesses inside severe convective storms. Theactual generation of multi-polarimetric radardevices is able to discriminate between waterphases and further down to hydrometeor species.Examples of rain/ice-particle characterisation andidenti®cation can be found in, among others,Doviak and Zrnic (1993), HoÈller et al. (1994), andHubbert et al. (1997). Much effort will be devotedto assessing the impact of the improved observa-tional capabilities from an operational point ofview. The outbreak of two separated supercellsoccurring over northern Italy as a result of thefrontal activity on 18 June 1997 is an ideal test-bed to understand the improved radar informa-tion, and assess their impact on operationalactivities.

No records of companion supercell events overnorthern Italy are found in the literature. In thepast studies were conducted on the interactionbetween synoptic forcing and thunderstormdevelopment over the Po valley (Buzzi andAlberoni, 1992). Alberoni et al. (1996) observedby radar a single tornadic supercell storm andCacciamani and Simonini (1988) described theclimatological occurrence of organized thunder-storm activity in the area.

Meteorol. Atmos. Phys. 75, 101±120 (2000)

The ®rst identi®cation of a supercell storm isdue to Browning (1964), while its principalfeatures were discussed by Chisholm and Renick(1972), Lemon and Doswell (1979) and others tofollow. More recent works have extended thetypical supercell characteristics to a wide spec-trum of storm types that transcend the classicalsupercell structure. Some examples are theincipient supercell storm (Brown, 1992), themulticell-supercell hybrid storm (Foote andFrank, 1983; Nelson, 1987; HoÈller et al., 1994)or the low, medium and high precipitatingsupercell storm as classi®ed by Moller et al.(1994).

The present paper draws upon polarimetric andDoppler observations collected over the entirehistory of the 18 June 1997 companion supercellevent from the C-band weather radar of theServizio Meteorologico Regionale (SMR) in S.Pietro Capo®ume. Limited dual-Doppler analysiswas possible using simultaneous data of the C-band weather radar of the Centro Sperimentaleper l'Idrologia e la Meteorologia (CSIM). Anoutline of the characteristics of the two radars isgiven in Table 1.

The complexity and variety of supercell typesis heavily re¯ected in this event. While one stormresembles closely the conceptual model of super-cell, the other matches more the hybrid multicell-supercell storm type. After a discussion ofsynoptic and mesoscale meteorological featuresin Sect. 2, radar observations of storm structureand dynamics are given in Sect. 3, where alsothe occurrence of a positive ZDR columns isdiscussed. The horizontal wind ®eld from dual-

Doppler observations limited to a single crucialinstant of the storms' development is presented inSect. 4.

2. Synoptic and mesoscale meteorology

2.1. Synoptic analysis

Large-scale atmospheric ¯ow patterns are identi-®ed using the European Centre for MediumRange Weather Forecasting (ECMWF) analyses.The most important feature that characterized thetwin supercells event was the coupled effect of acut-off low at 500 hPa over the Iberian Peninsula,a baroclinic disturbance over Central Europe, andan area of high pressure positioned south of Sicilyadvecting warm moist air from the southernMediterranean.

At 0000 UTC on 18 June (Fig. 1a) the 500 hPageopotential ®eld shows a fully developed cut-offand a strong SW ¯ow over Italy; superimposed onthis large scale structure is a short baroclinicwave positioned over Central Europe travellingSE. The temperature gradient at 500 hPa identi®esthe location of the front at this time. At 850 hPa(not shown) the thermal pattern reveals that thefront is split in two separated bands: the ®rst overthe Alps and the other extending from NorthAfrica to Central Italy. The most evident low-level feature in the geopotential ®eld is the localhigh south of Sicily that enforces the warm moistadvection from the Mediterranean Sea. A lesspronounced trough drives the circulation overEurope.

At noon of 18 June the deep trough at 500 hPahas been replaced by a cut-off low that movesSSE and a quasi-zonal ¯ow over Central Europeas shown in Fig. 1b. Meanwhile a fast displace-ment of the baroclinic disturbance is observed:the short wave reaches the Alps increasing thelocal instability. The warm advection is clearlyvisible in the temperature ®eld from the reloca-tion over Sardinia of the ÿ12 �C isotherm (morethan 400 km in 12 hours). Supercells developaround midday and sweep northern Italy in theafternoon. The episode is characterized by anincrease of warm advection and a south drift ofthe baroclinic disturbance. As a consequence, thetemperature gradient over the storm area strength-ens as indicated by the 1800 UTC situation shownin Fig. 1c.

Table 1. Speci®cation of the radars

Location San PietroCapo®ume

Teolo

Operator ARPA-SMR, ARPAV-CSIM,Emilia Romagna Veneto

Manufacturer SMA, Italy Ericsson,Sweden

Site location 44.654 N,11.624 E

45.363 N,11.674 E

Site altitude 11 m (MSL) 472 m (MSL)Beam width(ÿ3 dB)

0.9� 0.9�

Transmitter Klystron MagnetronDoppler range 110 km 120 kmRange gate length 250 m 1000 m

102 P. P. Alberoni et al.

2.2. Mesoscale analysis

In general, when a frontal system approaches Italyfrom the north its structure is heavily modi®ed bythe Alpine orography (see, for example Morgan,

Fig. 1. 18 June 1997. Geopotential height of the 500 hPasurface (thick line, units dam) and temperature ®eld (thinline, units �C). Light grey shading refers to the temperaturerange [ÿ18, ÿ12]. The geographical grid is 5�; a 0000UTC; b 1200 UTC; c 1800 UTC

Fig. 2. 18 June 1997, 0900 UTC. Mesoscale analysis: amean sea-level pressure (thick line, 1 hPa interval), surfacewind barbs (m sÿ1 WMO convention) and relative humidity(60, 70 and 90% RH levels corresponding to light, mediumand dark grey, respectively); b speci®c humidity at 850 hPa(2 g kgÿ1 interval, light grey encloses values greater than8 g kgÿ1; black refers to mountain areas); c surface potentialtemperature ®eld (thick line, 4 �C interval) with super-imposed negative Lifted Index (shadowing, 0, ÿ3 andÿ6 �C levels, light, medium and dark grey, respectively)

The 18 June 1997 Companion supercells 103

1973; Buzzi and Alberoni, 1992). It is notstraightforward to identify the frontal positionand its displacement only from the surfaceSYNOP analysis. The problem was tackled bycreating a high density dataset that puts togethersynoptic and mesonet surface stations and datacoming from remote sensing platforms (C-banddual linear polarisation radar and Meteosat). Thisallows for hourly high-resolution analyses of theevent using the Local Analysis and PredictionSystem (LAPS). A general description of theLAPS architecture is given by Mc Ginley (1989);for more speci®c details the reader is directed toAlbers (1995), Mc Ginley et al. (1991), Albers etal. (1996), and Birkenheuer (1991). LAPS com-bines in a unique way information from a widevariety of data sources, including radar, satelliteand pro®lers. The area of the analysis (see Fig. 2)covers northern and central Italy and part of thebordering countries with a grid mesh of about10 km horizontal and a vertical resolution of50 hPa. LAPS consists of specialized modules thatperform surface analysis, and 3D temperature,humidity, cloud and wind ®eld analysis. Derivedparameters such as for example the Lifted Index(LI) are computed. The latter is the temperatureexcess at 500 hPa of the environment with respectto an air parcel in the `̀ moist'' layer lifted to itslifting condensation level (LCL), and then liftedmoist-adiabatically above the LCL.

At 0900 UTC (Fig. 2a) the mean sea-levelpressure (hereafter mslp) ®eld shows a low overthe Po valley with a high gradient belt along theAlps. The wind ®eld presents the typical circula-tion structure associated with an incoming NWfront: the ¯ow is split in two separate branchesmoving around the Alpine chain. The ®rstcomponent blows NW to S and veers to the gulfof Genoa, increasing the advection of moist airforced by large-scale patterns. The remaining partof the circulation is directed eastward around theAlps and comes back from the Italian easternboundary where a strengthening in the windspeed shows up. A cold advection associated withsuch a ¯ow regime, visible also in the mslp ®eldas a distortion of isobars, enters the valley andplays an important role in lifting the pre-existentwarm air. As shown in the previous sectionmedium-high level ¯ow is quasi-zonal. Thedevelopment of the easterly low level ¯owconsiderably increases the vertical wind shear.

As suggested by Weisman and Klemp (1982) themagnitude of the vertical wind shear over thelowest 6 km can in¯uence the occurrence of astorm type, generally increasing from short-livedstorm to supercell, and it is also of fundamentalimportance for the creation and maintenance oflong-lived and potentially dangerous systems.

The analysis of the surface relative humidityreveals a straight line of high values crossingnorthern Italy in the NE-SW direction andmarking the frontal position at the ground, whichis not clearly evidenced from the other ®elds. Thejoint action of orography and moist advectionover the Gulf of Genoa moistens the atmospherein the lower kilometres over the Po valley asdisplayed in Fig. 2b where the speci®c humidityat 850 hPa is plotted. Note that humidity isadvected downwind where a high-humidity poolis created in the middle of the Po valley and acloud line is detected by radar and satellite. Theinteraction between the advected moist air andthe pre-existent air mass in the Po valley in-creases the thermodynamic instability as indi-cated by the LI (Fig. 2c). Note the wider veryunstable area in the centre of the valley close tothe place where one of the two systems willoriginate.

At 1200 UTC (Fig. 3) both systems are alreadyactive; the circulation patterns over the valley aremore organised with a convergence zone in themiddle driving the convection. The pressurelowers downwind to the Alps in northwesternItaly as a consequence of the foehn that shows upat 850 hPa. At ground level the katabatic wind isuncovered by the decrease of relative humidity inthe area; Meteosat's water vapour image (Fig. 3b)clearly shows the resulting distortion of thefrontal structure. Both systems are arranged alongthe frontal line and the supercells were originatedby the frontal evolution during the day. TheMilano Linate skew-T diagram and wind hodo-graph at 1200 UTC are reported in Fig. 3c and d,respectively. The instability of the atmosphere isalso indicated by the values of the relevantstability parameters of the Milano Linate and S.Pietro Capo®ume stations in Table 2. While atMilano Linate the values of Bulk RichardsonNumber (BRN) are in the range of supercellstorm (Weisman and Klemp, 1982), in S. PietroCapo®ume the high Relative Humidity (RH)value at midday is an index of the available latent

104 P. P. Alberoni et al.

Fig. 3. a Same as in Fig. 2a but at 1200 UTC; b corresponding METEOSAT water vapour image; c skew T-Log P soundingdiagram for Milano-Linate; d corresponding holograph, labels refer to the height of data in hPa

Table 2. Relevant stability parameters from Milano Linate and S. Pietro Capo®ume WMO sounding stations on 18 June 1997.RH is the average relative humidity (%) in the 850, 700 and 500 hPa altitude range; CAPE is the convective available potentialenergy in m2 sÿ2; and BRN is the Bulk Richardson Number

Station name Coordinates Time RH CAPE BRN

Milano Linate 45.26 N, 9.17 E 0000 75 1848 261200 33 1332 23

S. Pietro Capo®ume 44.654 N, 11.624 E 0000 27 1053 71200 54 1230 6

The 18 June 1997 Companion supercells 105

heat for supporting the ongoing convectivedevelopment.

The impact of the passage of both systems onsurface ®elds becomes evident from the hourlymaps generated by LAPS high time and spatialresolution analysis. Figure 4 shows the mslp ®eldand superimposed wind ®eld and 10 dBZ re¯ec-tivity contours at the lowest elevation (0.5�) ofthe SMR radar in S. Pietro Capo®ume from 1200to 1500 UTC. At noon a pressure minimum is

positioned in the centre of the Po valley and bothsystems are fed by the easterly ¯ow. One hourlater the supercells lie within the Doppler radarrange and a general lowering of mslp ®eld accom-panies the incoming front. At 1400 UTC the SMRmesonet surface network detects the effects of thesystems: the low in the central-eastern part of thevalley is split in two main zones with a local highin between. The eastern low is associated with theupdraft region of the eastern cell while the high

Fig. 4. Surface MSL pressure analysis (thick line, every 1 hPa), surface winds and 10 dBZ re¯ectivity patterns (shaded area).The circle delimits the Doppler radar range (110 km); a 1200 UTC; b 1300 UTC; c 1400 UTC; d 1500 UTC

106 P. P. Alberoni et al.

relates to the downdraft area behind the othersystem. The surface wind analysis shows adeepening of the cyclonic circulation and theDoppler radial wind reveals the development of amesocyclone in both cells. At 1500 UTC the lowpressure moves SE ahead of the systems and thetypical pressure kick (Browning and Reynolds,1994) intensi®es more than 2 hPa in one hour atthe previous location of the low. Afterwards thefrontal structure passes over northern Italy and thesupercells survive over the Adriatic Sea as docu-mented by Meteosat imagery (not shown).

3. Radar analysis

Radar data were collected by the C-band dualpolarisation Doppler weather radar in San PietroCapo®ume, near Bologna, in the southeastern

sector of the Po valley. Figure 5a shows the lifehistory of the companion supercells (hereinafterreferred to as S1 and S2) from 1041 UTC to 1711UTC every 15 minutes. Both systems, especiallythe S1, are characterized by a very long lifetimeand exceptional hailfalls. The hatched zonesrepresent areas where hail was reported at theground by farmer associations on the basis ofinformation collected only inside the Emilia-Romagna region (boundaries shown in Fig. 5a).The radar beam was occluded at the time by aplantation of poplar trees between 315� and 20�azimuth: data within this sector were taken at 2.3�elevation. All other data refer to the 1.4� elevationPlan Position Indicator (PPI). The occlusionnevertheless does not affect the overall indicationscontained in the ®gure. An overall view of thesystems is given by the NOAA Advanced VeryHigh Resolution Radiometer (AVHRR) satellite

Fig. 5. a Cumulative re¯ectivity map. Range markers are every25 km. Shaded areas refer to the 40 dBZ levels (dark grey).Time markers (UTC) of supercells S1 and S2 are plotted.Vertical stippled area refers to the shielding effects due tonearby trees. Slanted stippled areas indicate the hail regions.The location of CSIM Doppler radar is marked by T and thegeographical boundary of the Emilia-Romagna region ofnorthern Italy is shown; b NOAA/AVHRR channel 2 image at1234 UTC; c outline of the major objects from the satelliteperspective in b

The 18 June 1997 Companion supercells 107

image in channel 2 at 1234 UTC (Fig. 5b) wherethe cloud systems are easily recognisable; asimple sketch is added for reference in Fig. 5c.

3.1. Western hailfall supercell (S1)

The supercell produced the heaviest and mostextensive hailfall during 1997 in Emilia-Romagna with a recorded hail swath more than200 km long. The ®rst radar signature of S1 isdetected at 1041 UTC with a vertical extension of6 km and reaches a considerable height in a fewminutes. Meanwhile the anvil assumes the typicaldownwind stretched pattern. After one hour S1 isfully developed with the following main features:a column of re¯ectivity greater than 40 dBZextends up to 7 km above ground, the structure istilted along the vertical and a hook echo isrecognisable in the rear right ¯ank of the storm.The storm propagation velocity increases con-siderably after 1200 UTC (Fig. 5a), with amaximum of about 80 km hÿ1 between 1400and 1500 UTC.

3.1.1. Flare echo

The S1 supercell is prone to produce hail astesti®ed by ground damages. The main indirectindication of hail presence is the observation ofthe `̀ ¯are echo''. A ¯are, also known as a `̀ hailspike'', is an elongated re¯ectivity tail behind themain core of the storm opposite to radar. Zrnic(1987) proposed a theory in which re¯ectivity¯ares are attributed to scattering from largehydrometeors; heavy rain could as well be oneof the causes at C-band (Wilson and Reum,1988). Since hydrometeors in the core of theconvective storm mainly consist of large wet hail,whose size is comparable to the radar wave-length, the scattering occurs in the resonanceregion (Mie scattering). The ¯are echo, as des-cribed in Zrnic's theory, is the result of a three-body scattering process involving scattering ofthe transmitted power to the ground by largehydrometeors, backscattering by the ground tothe upwards high re¯ectivity core, and, ®nally,backscattering to the radar.

Starting at 1210 UTC ¯ares are continuouslyobserved until 1504 UTC; the high correspon-dence with ground traces of hail can be seen inFig. 5, while Fig. 6 shows an example of a ¯are

echo during its evolution. The ¯are reaches amaximum tail extension of about 24 km and iswell visible at the upper levels of the S1 supercell.

The Z and ZDR Range Height Indicator (RHI)data (Fig. 7) display the rather typical signature ofthe ¯are; analogous situations are reported amongothers by Caylor and Illingworth (1989), Hubbertand Bringi (1997), and Alberoni et al. (1998). Thehigh ZDR values above 7 km above ground level

Fig. 6. Radar re¯ectivity PPI (dBZ) at 1304 UTC, 2.4�elevation. Axes are km north and east of radar. The positionof the RHI in Fig. 7 is indicated by a thin black line

Fig. 7. Radar RHI at 1304 UTC, 284� azimuth (see Fig. 6):a Z (dBZ); b ZDR (dB)

108 P. P. Alberoni et al.

(AGL) behind the re¯ectivity maximum do notappear to be linked to any physical meteorologi-cal target, but are due to the three body scatteringeffect between hail and the ground.

3.1.2. Positive ZDR column

Classical supercell characteristics are identi®edduring the lifetime of S1. A bounded weak echoregion (BWER) with rotational pattern shows upinside the storm and the vertical cut uncovers apendant echo (Fig. 6 and 7). The two featureslocalize the main updraft of S1. The vertical

column, composed of raindrops (Hall et al.,1984), is also characterized during its timeevolution by high values of differential re¯ectiv-ity (Illingworth et al., 1987; Meischner et al.,1991; Zrnic et al., 1993).

A BWER signature is detectable in the re¯ec-tivity ®eld in Fig. 8 (1318 UTC at 3.2� elevation)and a broad but short ¯are echo appears in therear right ¯ank of S1. ZDR values for a large partof the area covered by the storm are close to zero.The downwind part (closer to the radar) of theBWER is associated to very high ZDR values(sometimes higher than 8 dB) while upwind ZDR

is positive (around 4 dB) with a thin line of valuesclose to zero in between. At the initial stage the¯are is generally characterized by a very highZDR signature followed along the echo by a sharpdecrease to negative values. Similar signaturescan be seen in Fig. 8b in the ¯are echo regionapproximately from ÿ80 to ÿ70 km east andfrom �10 to �20 km north of the radar position.The difference between the ZDR signatures of¯ares and positive ZDR columns is associated totheir relative position with respect to the core ofthe storm. Flares are detected by the radar assignal propagation delays and are thereforelocated in the rear of the storm opposite to theradar. Positive ZDR columns correspond to micro-physical characteristics of the hydrometeors wellin the body of the updraft.

At this time (Fig. 9ab) the ZDR positive columnis observed up to 4.5 km AGL, slightly higherthan the 0 �C level that the 1200 UTC synopticsounding of S. Pietro Capo®ume identi®es atabout 3 km AGL. The major source of ¯atteneddrops inside the column appears to be related tothe fall of ice particles from the overhang. Similarobservations have been presented by Hubbertet al. (1998) who discuss the occurrence of a ZDR

positive column below the storm overhang. Someauthors (Conway and Zrnic, 1993; HoÈller et al.,1994; Brandes et al., 1995; Hubbert et al., 1998)have linked this radar pattern to a seeder-feedermechanism from the upper overhang or from newgrowing cells. Moreover their calculations ofparticle trajectories and in situ observations fromaircraft penetration have uncovered a close linkbetween the ZDR column and the storm updraftregion. The melted particles re-circulate insidethe storm supplying embryos for the growth oflarge hail.

Fig. 8. PPI at 1318 UTC, 3.2� elevation: a Z (dBZ), theposition of the RHI in Figs. 9a, b is indicated by a thinblack line.; b ZDR (dB)

The 18 June 1997 Companion supercells 109

At 1334 UTC the ZDR positive column is stillclearly identi®able, but appears divided in twomain structures (Fig. 9cd). The highest values (upto 6 dB) are observed in the lower part of theoverhang on the northern boundary of the BWER.At the same location another column rises fromthe ground with a tilted path as suggested by theDoppler wind ®eld (not shown). A thin ZDR

column is present all the way up to the upper

boundary of the BWER (Fig. 9d). We believe thatthe upward path trajectories of the hydrometeorsare oriented NE to WSW and this would becon®rmed by the fact that the ¯are echo is muchmore evident on the central-left side of the core ofS1. The presence of large wet hail at upper levelsalong the 277� azimuth can be inferred from the¯are signature.

3.2. Hybrid supercell (S2)

The S2 supercell has displayed during its lifetimea spectrum of storm characteristics ranging frommulticell-like to those typical of classical super-cells. Its evolution is similar to the cases des-cribed by HoÈller et al. (1994), and Hubbert et al.(1998). In the following the main radar featuresassociated with this evolution will be discussed.

3.2.1. Early multicell stage

From the very ®rst radar echoes of S2 the stormexhibits considerable deviations from the struc-ture of its S1 companion. The system developsaround 1045 UTC at ®rst at upper levels along thefront within a thin cloud line bounded between 4and 6 km AGL. This band is probably composedby ¯at, horizontally-oriented ice crystals and ischaracterized by re¯ectivity values lower than10 dBZ and quite high differential re¯ectivityvalues sometimes greater than 4 dB (not shown).Refer to Fig. 5bc for the location, size andorientation of this cloud line.

In 15 minutes the echo lowers to the groundwith a vertical depth of 7 km and the existence ofseveral re¯ectivity cores suggests a multicellstructure. Consecutive high-re¯ectivity coresdevelop on the right ¯ank of the storm, whichare moving NE with the main environmentalupper level wind while the entire system movesSE (right ¯ank propagation). Further microphy-sical cloud properties can be inferred from ZDR

measurements. The S2 storm appears to beconsisting of three different zones (Fig. 10). Thegrowth of new convective elements composed ofsupercooled drops in the SW ¯ank of the storm issubstantiated by very high values of differentialre¯ectivity above the freezing level (up to 7 dB).The present structure seems rather coherent andnot due to well known mismatches between theantenna's H and V radiation patterns that can

Fig. 9. RHI of S1 supercell (see Fig. 8a for positioning at1318 UTC): a 1318 UTC, 280� Z (dBZ); b 1318 UTC, 280�

ZDR (dB); c 1334 UTC, 279� ZDR (dB); d 1334 UTC, 277�

ZDR (dB)

110 P. P. Alberoni et al.

induce ZDR artifacts (Herzegh and Carbone,1984). The central part of S2 is the most activeand rainy; at this elevation scattering fromrandomly-oriented hydrometeors induces lowZDR values. Dissipating cells inside S2 form thedownwind anvil region. Weak precipitation fallsacross the melting layer and, assuming ¯attenedhydrometeor shapes, this explains the moderatelypositive ZDR values.

Between 1100 UTC and noon S2 slowly moveseastward (about 20 km in one hour) and enters a

moister environment (see the high humidity linein Fig. 3). The increase of the instabilityassociated with this mesoscale feature induces amodi®cation of the storm structure and the emer-gence of supercell characteristics; the transitiontakes more than one hour. During this time lapseS2 undergoes the classical steps of the multicellstorm development. Afterwards, the joint actionof high instability and strong shear creates anadequate environment for its explosive growth.

At 1234 UTC 11 PPIs were collected at 9.7�elevation in less than 4 minutes (approximately 1PPI every 20 sec): an extract from this `̀ special''series of re¯ectivity data (result of a fault of theacquisition schedule) is shown in Fig. 11. Theimages refer to the upper part of the system (7 to11 km AGL in the area of the cloud). The ®rstsweep (Fig. 11a) shows at least four differentareas of re¯ectivity greater than 40 dBZ and asmall core in the rear right ¯ank is visible. About100 seconds later (Fig. 11b) the small core growsconsiderably in size and intensity with a max-imum value of 45 dBZ. A vertical cut along thiscell (not shown) associates the observation to thetop of a strong overhang feature and documentsthe existence of a vertical BWER. On the con-trary the north-eastern core decays quite rapidly.The last available sweep (Fig. 11c) displays asubstantially modi®ed situation: the area coveredby S2 has moved SE while the core inside it hasshifted NE. The more active centre is the westernone corresponding to the smallest centre threeminutes before. From these sweeps we infer thatthe main updraft is in the rear right ¯ank of S2;this area is constantly fed and no appreciabledisplacement occurs in the east direction. Thesatellite view of the scene (Fig. 5bc) con®rms thepresence of more than one core inside S2 at thistime.

3.2.2. Quasi-supercell

S2 expands considerably during the next hour. Alarge area with re¯ectivity greater than 50 dBZdevelops together with a BWER and an overhang,but no indication of a single rotating powerfulupdraft exists. At this time S2 shows featuresclose to the classical supercell conceptual model,but the dynamical framework is quite different;for this reason we refer to this cloud structure as`̀ quasi-supercell''.

Fig. 10. Supercell S2. Same as in Fig. 8 but at 1118 UTC,2.4� elevation. The reference frame for b is the same as a.Dashed black lines mark the mean height reached by thecentre of the radar beam

The 18 June 1997 Companion supercells 111

The PPI at 4.0� elevation displayed in Fig. 12identi®es a very strong downdraft in the middle ofS2, which dynamically drives and sustains thisphase; the re¯ectivity structure is stretched and ahook-echo appears. Note that data displayed inFig. 12 are partially corrupted by trees and there¯ectivity data are also affected by attenuation.The thin line of weak re¯ectivity ahead of thestorm close to the hook echo is the ®rst indicationof new growing cells. As discussed in Sect. 2,low- and medium-level winds blow from SE andthe development of the strong downdraft, whichis opposite in direction, forces an updraft rightahead of it that feeds the system. A secondary

updraft raises on the rear ¯ank of S2. It wouldappear that the latter is split from the main streamturning clockwise around the system duringlifting; a simple sketch of the ¯ow ®eld is givenin Fig. 12b.

A relatively complex 3-D dynamics is asso-ciated to the S2 interior. The various sectors ofthe storm show up in the RHIs of Fig. 13 (again,trees corrupt the lowest 2±3 elevations). Apendant echo (typical of supercells) is uncoveredin the re¯ectivity RHI and appears to be linked tothe column of high re¯ectivity values (greaterthan 50 dBZ). A joint analysis of Z and radialDoppler velocity demonstrates that the column is

Fig. 11. PPI re¯ectivity at 1234 UTC, 9.7� elevation (dBZ); a1st sweep; b 6th sweep (approx. after 120 sec); c 11th sweep(approx. after 220 sec.)

112 P. P. Alberoni et al.

associated to the downdraft. This latter is charac-terized by a strong positive velocity out¯ow(more than 12 m s-1) whereas the pendent echo(tilted column at 24 km) is associated with thein¯ow. The spectrum width ®eld con®rms theexistence of these two separated ¯ows. The up-draft is identi®ed by a homogeneous velocity ®eldand a column of low spectrum width values up to9 km AGL. An area of high variability separates

the updraft from the descending current, the lastone characterized by another low spectrum width¯ow in the lowest 4 km. The dynamics and theunstable environment determine a consistent hailproduction by S2. The updraft rapidly lifts up themoist air close to the ground. The condensationprocess starts producing small drops (low re¯ec-tivity values around ÿ10� 20 dBZ) with quasi-spherical shape (ZDR close to 0 dB) as shown in

Fig. 12. 1318 UTC, 4.04� elevation: a Z (dBZ, dashed linesmark the RHI direction of Fig. 13 and 14); b Dopplervelocity (m sÿ1); positive values refer to velocity toward theradar (I) while negative values refer to velocity away fromit (O). The inferred updraft trajectories are indicated byarrows and dashed lines

Fig. 13. RHI at 13:18 UTC taken at 356� azimuth (seepositioning in Fig. 12a): a Z (dBZ); b ZDR (dB); c radialwind V (m sÿ1); d spectrum width (m sÿ1). Positive valuesof the wind refer to the direction toward the radar

The 18 June 1997 Companion supercells 113

Fig. 13. Above the freezing level (around 3 kmAGL) supercooled water is identi®ed. The layerof high differential re¯ectivity can be explainedby the presence of ¯attened raindrops at thisheight as discussed for the case of the `̀ ZDR

column'' in the recent paper of Hubbert et al.(1998). These drops, cooled by the fast upwardmotion, are quickly frozen to produce graupels,aggregates and embryos for large hailstones.

The downdraft turns counterclockwise aroundthe updraft column exiting on its left (looking

from radar) and dragging the radar targets. Inparticular a hail cascade can be detected from thestorm's central upper level to the rear ¯ank upto the hook-like echo. RHIs taken on this side(Fig. 14) reveal a shallow storm structure(bounded to the lowest 5 km) with clear hailsignatures in the ZDR data.

3.2.3. Supercell phase

From 1349 UTC S2 shows a well-developedrotation at all levels with the presence of BWERand hook pendant echo. The 3-D structure isdisplayed in Fig. 15 where the 45 dBZ iso-surfaceof S2 is viewed from SE. The principal featuresare the strong downwind tilting and the evidentoverhanging. Hail is reported at the ground untilthe system reaches the Adriatic Sea. A RHI alongthe storm (Fig. 16) spots the hailshaft. Severeattenuation is experienced in the ZDR measure-ments being differential re¯ectivity much moresensitive to attenuation. A comparison of the twoRHIs in Fig. 16 reveals a relatively large cor-rupted area in the ZDR ®eld whereas the Z ®eld isnot appreciably degraded.

After this time the system evolves as a classicalsupercell and radar echoes are observed until1630 UTC (see Fig. 5a).

4. Dual Doppler wind ®eld

Dual Doppler analysis methods have been usedfor research for more than thirty years (Lhermitte,

Fig. 14. Same as in Fig. 13a,b but at 346� azimuth (seepositioning in Fig. 12a): a Z (dBZ); b ZDR (dB)

Fig. 15. S2 supercell radar re¯ectivity isosur-face of the 45 dBZ level, 1349 UTC

114 P. P. Alberoni et al.

1968). In contrast with most other dual Doppleranalyses available in the literature the radars usedin this study are both fully operational. The use oftwo operational radars implies a number ofconstraints normally not imposed on researchdual Doppler analyses. These include the non-ideal location of the radars and the dif®culty indeviating from their regular scan strategies inorder to optimise the coverage.

In addition to the ARPA-SMR radar data dis-cussed in the previous section, the analysis makesuse of data from the Veneto Region Dopplerweather radar operated by CSIM. The Venetoradar is located in the northeastern part of the Povalley approximately 80 km north of the ARPA-SMR radar.

4.1. Dual Doppler wind ®eld retrieval

This section brie¯y reviews the techniques ofdual Doppler wind ®eld retrieval, and some ofthe associated problems. For more details onmultiple Doppler analysis techniques the reader isreferred to Ray (1990). Using only two radars, itis possible to directly retrieve two of the threevelocity components (u, v, w). Since in this casethe radars measure near horizontal winds (eleva-tion angles � 20�) it is not sensible to attempt a

direct estimate of the vertical component. Insteadthe radar measurements are used to estimateonly the horizontal wind component (u, v). Itcan be shown that the errors in the horizontalvelocity estimates depend on the geometry andthe vertical velocity. In order to estimate thehorizontal components with the minimum var-iance, the radial velocity estimates must beorthogonal, that is that the radar beams mustintersect at 90�. If the beams intersect at anglesgreater or lower than 90�, then the variance of thehorizontal wind velocity components increases.Ray and Sangren (1983) give the analysis of thevariance of the velocity estimates from suchanalyses.

In the dual Doppler case in order to solve forthe vertical velocity using an auxiliary relation-ship (such as mass continuity), boundary condi-tions must be established. A number of differentpossible boundary conditions exist although it isbeyond the scope of this paper to discuss theirrelative merits and errors here. Due to thedif®culty in obtaining adequately valid boundaryconditions, we con®ne ourselves to the retrievalof horizontal winds only. As we are unable toobtain an estimate for the vertical velocity, thehorizontal components cannot be corrected forthe geometric error. However, this error is ex-pected to be small due to the low elevation anglesused. The measurement uncertainty of the radialvelocity estimate is a function of the signal-to-noise ratio (SNR) of the received signal. In orderto ful®l the criteria of good SNR (>10 dB) in theregion selected for analysis, only data with re¯ec-tivity >15 dBZ were selected. For both radars it isestimated that with this constraint the variance ofthe radial velocity estimate in convective stormshere is � 0:6 m sÿ1.

4.2. Considerations for the retrieval of windsfrom the ARPA-SMR and CSIM radars

To perform dual Doppler wind ®eld retrieval it isnecessary that the radars be closely spaced inorder to resolve motion on small spatial scales(Miller and Krop¯i, 1980). In most Europeancountries the radars are spaced too far apart topermit wind ®eld retrieval. In Italy however, theradars are installed and operated on a regionalbasis. Hence the possibility exists for a numberof radars to have large regions of overlapping

Fig. 16. Same as in Fig. 13a,b but at 1349 UTC, 59�

azimuth: a Z (dBZ); b ZDR (dB)

The 18 June 1997 Companion supercells 115

coverage. This is the case for the Emilia-Romagna and Veneto regional radars. A distanceof approximately 80 km separates the two radarsand this implies that motion on a spatial scale of� 1 km can be resolved (Miller and Krop¯i,1980).

A further consideration is the type of terrain inwhich the overlapping coverage exists. In the caseof the CSIM and SMR radars, the bulk of theiroverlapping coverage lies in the ¯at ¯ood plain of

the Po valley basin. A major limitation in thisstudy is the relative altitude difference of the tworadars. The CSIM radar is approximately 470 mabove sea level, whilst the ARPA-SMR radar isonly 11 m. Due to the limited elevation anglesused in the observations (because of scan-timeconsiderations), the use of a zero vertical velocityboundary condition is effectively precluded.Given these constraints, the lowest altitude forwhich satisfactory overlapping coverage exists is

Fig. 17. Earth-relative dual Doppler windretrieval, at 1218 UTC. a 1 km AGL, windvectors, re¯ectivity contouring every 5 dBZ,convergence (10ÿ3 sÿ1, shadings) and positionof storms C1, C2 and C3; b 2 km AGL, windvectors and re¯ectivity every 5 dBZ (shadings);c same as b but at 3 km AGL

116 P. P. Alberoni et al.

0.5 km. A further limitation that precludes the useof a zero-velocity boundary condition at the stormtop arises from the limited maximum elevationangles used by both radars. This generally limitsthe maximum height of useful common coverageto approximately 5.5 km.

Another consideration in the design of a dualDoppler experiment is the co-ordination of theradar's scan strategy. Since both radars are fullyoperational, each with its own objectives for theregion of coverage, it is not possible for theoperators to deviate far from their normaloperational schedule. However, the scan strate-gies have been co-ordinated such that a scansuitable for limited dual Doppler analysis isprovided approximately every 15 minutes, withthe start times of these scans no more that 2±3minutes apart. This reduces as much as possibleerrors due to advection and storm evolution.

4.3. Implementation

The data was received at the University of Essexin the format native to each of the radars. It wasthen converted into Universal Format (UF)(Barnes, 1980). Once converted to UF, the radialvelocity was unfolded ray-by-ray using a simplecontinuity method. The interpolation procedurewas performed using REORDER, a software

package written by the Atmospheric TechnologyDivision at the National Center for AtmosphericResearch (NCAR). The interpolation was per-formed with a grid resolution of 1 km in thehorizontal and 0.5 km in the vertical. The methodof interpolation used was the Cressman ®lter withthe radius of in¯uence set to 1.9 times the gridresolution, chosen in the light of the ®ndings ofGiven and Ray (1994). The core of the analysiswas performed with another software packagewritten at NCAR's Mesoscale and MicroscaleMeteorology Division, CEDRIC. A completedescription of the functionality of CEDRIC isgiven in Mohr et al. (1986).

4.4. Results

Using the volume scan data at 1218 UTC fromthe SMR and CSIM radars the horizontal windswere retrieved. Given the differing altitudes of thesites (shown in Table 1), the lowest level that cansuccessfully be retrieved is 0.5 km. For theselected region the ray intersection angles forthe two radars is 50±90�. For the multicell stormregion this is reduced to 70±90�. For these inter-section angles it is estimated that the variance ofthe horizontal wind estimates due to this geo-metric error is < 2 m sÿ1. The retrieved horizontalwinds at 1, 2 and 3 km AGL are shown in Fig.

Fig. 17 (continued)

The 18 June 1997 Companion supercells 117

17a±c. The SMR radar is located at the origin andthe CSIM radar is located approximately 80 kmdue north. The grey scale shows re¯ectivity fromthe SMR radar in dBZ. For clarity only re¯ec-tivities in excess of 15 dBZ have been shown. Thelength of the wind vectors indicates velocityrelative to the 20 m sÿ1 reference shown in thelower right of each ®gure.

The three cells C1, C2 and C3 can be clearlyseen in the re¯ectivities shown in Fig. 17a. Acloser look to Fig. 17 reveals a sharp wind shear,both in direction and strength, between 1 and3 km AGL. In particular winds reinforce con-siderably around cell C3 and north of cell C1. Atthe same time a clear veering with height happensin the south-eastern quadrant of the system in thearea of the C2 cell. The westerly component ofthe wind above 1 km north of cell C3 and C1 asopposed to the easterly component of the surfacemesoscale winds (Fig. 4a) con®rms the presenceof a general shear as discussed in Sect. 2. Theshear is a pre-requisite for the development ofstrong convective systems.

A well-de®ned area of convergence is detect-able ahead of cell C3, the youngest convectivecore. The updraft is normally located on this sideof the system and in the present case this iscon®rmed by the converging wind ®eld at 1 and2 km AGL.

A second maximum of the convergence ®eld islocated just north of the mature cell C2 and isgenerated by the interaction of the easterly andwesterly component of the wind at 1 and 2 kmAGL; at 3 km AGL the convergence is stillpresent though somewhat weakened. This con-vergence is not associated to the updraft of any ofthe cells: the nearest cell (C2) lies in a generallydivergent sector. An analysis of the time sequenceof the multicell stage (not included) shows thatcell C2 remains more or less stationary with aslight shift to the north between 1218 and 1226UTC. This means that the cell moves in thedirection of the convergence maximum. At 1226UTC the two cells tend to merge and will do so inthe following minutes; the BWER forms and thesystem changes into a supercell.

5. Conclusions

Two storms were tracked on 18 June 1997. Theinterest was twofold: 1) investigate the unique

dynamic and microphysical characteristics of twocompanion severe hailstorms a few km apart, and2) explore the capabilities of an operationalmeteorological environment for an accuratecharacterisation of the storm structure for futureuse in developing nowcasting procedures. Inparticular, the use of a high-resolution meteor-ological data-set in conjunction with data fromtwo operational Doppler weather radars allowedfor a detailed analysis of the supercell event.

The use of high-resolution integrated LAPSanalyses proved to be very useful in monitoringthe atmospheric conditions. The conventionalsynoptic chart analysis of frontal systems overnorthern Italy is normally not enough for theprecise identi®cation and location of the frontsdue to the interaction with the Alps. The presentLAPS mesoscale analysis, on the contrary, clearlyshows the position of the front marked by highrelative humidity signatures. The operational useof such mesoscale products would then greatlybene®t from the enhanced resolution and descrip-tive power of the ®elds for nowcasting and shortrange forecasting.

Radar data were instrumental for the identi®ca-tion of storm patterns during the system lifetimeand related cloud features. The system consists oftwo supercells, one (S1) a canonical supercellfrom its early stages of development, and theother (S2) originally a multicell system thatreorganises itself into a supercell.

S2 originates in a unstable environment overthe western sector of the Po valley characterizedby relatively low humidity values at low levels.The mesoscale analysis shows that the explosioninto a supercell takes place when the stormsystem interacts with the approaching front thatprovides the humidity necessary to support theconvective development.

The dual Doppler analysis of the horizontalwind components was conducted using thevolume scan data at 1218 UTC from the SMRand CSIM radars and detected a number ofimportant features: 1) a sharp wind shear, both indirection and strength, between 1 and 3 km AGLcon®rmed also by the mesoscale analysis; 2) anarea of convergence ahead of the youngestconvective core; 3) a second convergence max-imum north of the more mature cell generated bythe interaction of the easterly and westerly com-ponent of the wind at 1 and 2 km AGL. This

118 P. P. Alberoni et al.

shows that not only the environmental thermo-dynamic parameters contribute to the transitionbetween different storm types but also the storm'sinternal dynamics.

Despite a large number of constraints it hasbeen demonstrated that horizontal dual Dopplerwinds can be successfully retrieved using theSMR and CSIM operational radars. The situationin north eastern Italy is almost unique, having oneof the highest concentrations of Doppler radars inEurope. The dual Doppler work performed in thisanalysis provides a limited look at the operationalpossibilities in the area. However, some aid forthe interpretation of thunderstorm dynamics inthe Po valley clearly derives from an ef®cient useof the available Doppler radar network.

At the same time relevant storm features areidenti®ed using the S. Pietro Capo®ume polari-metric data. Flare echoes and ZDR columns aredetected. The ®rst are a clear sign of hailproduction while the latter are associated to theupdraft and the presence of relatively largeraindrops whose source can be possibly identi®edin a seeder-feeder mechanism from the over-hanging anvil. The cell produces a hailshaft hit-ting the ground for more than 250 km.

The use of re®ned radar observational techni-ques (dual Doppler and polarimetry) togetherwith state-of-the-art mesoscale analysis methods(LAPS) has demonstrated very bene®cial in theanalysis of severe weather events. Since all theabove techniques are available in real time, it isconceivable to introduce them into the opera-tional meteorological practice. Moreover, polari-metric radars will likely be widely installed forthe operational use and polarimetry used forweather surveillance applications (Zrnic andRyzhkov, 1999). In this way technologies andresults that still belong to an advanced researchenvironment will be available to the forecasterwith obvious advantages for the end-user.

Acknowledgements

Part of the authors acknowledge the support of theEuropean Union through contract ENV4-CT96-0261. Oneof the authors (VL) is grateful for support from: ItalianNational Research Council (CNR), Progetto StrategicoMAP and Accordo CNR-CONICET; Italian Space Agency(ASI); Agenzia Regionale Prevenzione e Ambiente (ARPAEmilia-Romagna)-Servizio Meteorologico Regionale, con-tract CR9701; EUMETSAT. The Gruppo Nazionale per la

Difesa dalle Catastro® Idrogeologiche of CNR alsoprovided ®nancial support to the Italian team. One of theauthors (RJW) acknowledges with thanks L. J. Miller andW. D. Anderson of NCAR's MMM division for making theCEDRIC software available. NOAA-Forecasting SystemLaboratory (FSL) is acknowledged for making available theLAPS software. The Centro Sperimentale per l'Idrologia ela Meteorologia (CSIM) kindly provided data from theTeolo weather radar and G. Kuznetsov elaborated Fig. 15.

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Authors' addresses: Dr. Pier Paolo Alberoni, S. Costa, P.Mezzasalma and S. Nanni, ARPA-SMR, viale Silvani 6, I-40122 Bologna, Italy (E-mail: p.alberoni@smr.arpa.emr.it);V. Levizzani, CNR, Institute of Atmospheric and OceanicSciences, Bologna, Italy; R. J. Watson, Univ. of Bath, Bath,UK; A. R. Holt, Univ. of Essex, Colchester, UK

120 P. P. Alberoni et al.: The 18 June 1997 Companion supercells