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OBSERVATIONAL ANALYSIS OF THE 18 OCTOBER 2017 VALLS SEVERE

WEATHER THUNDERSTORM

Acknowledgment: Spanish Ministry MINECO grants CGL2015-65627-C3-2-R (MINECO/FEDER) & CGL2016-81828-REDT

Joan Bech(1), Oriol Rodriguez(1), Patricia Altube(2), Tomeu Rigo(2), Nicolau Pineda(2), Salvador Castán(3), Joan Arús(4) , Joan Montanyà(5)

(1) University of Barcelona, Spain (2) Servei Meteorològic de Catalunya (SMC), Spain (3) Agencia Pericial, Spain (4) Meteorological State Agency, AEMET, Spain (5) Polytechnic University of Catalonia (UPC), Spain

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• Motivation previous related studies

•Synoptic framework large scale mechanisms

• Damage assessment high impact weather event

• Thunderstorm overview storm morphology

• Doppler radar observations signatures

• Final remarks

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OUTLINE

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MOTIVATION

Tornado density contours (cases per 0.5° × 0.5°) in the period 1976–2009, considering 338 tornadoes. Mostly F0 (35%), F1 (46%), F2 (17%), F3 (2%). (Adapted from Gayà, 2011)

Spain Catalonia

Tornado data updated from Rodriguez & Bech (2018)

x

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2017/10/18 12Z

2017/10/18

Upper Level trough passage, relatively small cut-off (552 gpdm) low at 500 hPa, over the Iberian Peninsula. Low Levels: Cold front passage, with southern warm advection – in phase with solar daily cycle

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2017/10/18

BCN 18/10/2017 12 UTC (Valls case) SBCAPE: 1889 J/kg WS01: 14.4 m/s

WS03: 15.9 m/s SRH03: 168 m2/s2 EHI02: 2.0 STP: 0.5 SCP: 8.4 UTI: 0.46

In red thresholds exceeded specified in Rodriguez & Bech (2018)

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2017/10/18

SMC 00-24Z QPE (4 C-band Doppler radars + 190 automatic raingauges) + Cloud-to-ground Lightning (MaxPP=113.3 mm)

Valls EF1 T

+BARCELONA

16Z MSG IR 10.8µm

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2017/10/18 Valls Damage Survey Time: 16:00 ± 0:05 Z Area affected: Valls (E), Industrial Area (N) (rural, urban, periurban) Max. intensity: EF1 Damage Path Length: 6.23 km Av. width: 215 m Max. width: 420 m

-13 injured people (4 in a car accident due to low visibility involving two cars)

- 100 traffic signs, 280 trees, 30 lamp posts - Large hail

1 km

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2017/10/18 Parent-storm radar characteristics (Valls) SMC storm tracking system Rigo et al (2010), del Moral (2018)

MCS with leading convective line and trailing stratiform region, oriented perpendincularly to propagation vector - difficult to track specific embeded parent storm characteristics

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2017/10/18 SMC Z & corrected Doppler data Altube et al (2017, 2018)

No evident parent storm embeded as in EF0 Ivars d’Urgell case (Bech et al 2015) - important radial shear (convective line) Tornado likely linked to well organized storm gust front interaction with low level terrain induced circulation.

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2017/10/18 SMC Z & corrected Doppler data Altube et al (2017, 2018)

No evident parent storm embeded as in EF0 Ivars d’Urgell case (Bech et al 2015) - important radial shear (convective line) Tornado likely linked to well organized storm gust front interaction with low level terrain induced circulation.

Tight Z gradient Gust front

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2017/10/18 SMC Z & corrected Doppler data Altube et al (2017, 2018)

No evident parent storm embeded as in EF0 Ivars d’Urgell case (Bech et al 2015) - important radial shear (convective line) Tornado likely linked to well organized storm gust front interaction with low level terrain induced circulation.

Tight Z gradient Gust front

Strong radial shear & no clear rotation signs

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CASE 2: 2018/01/07

Tight Z gradient, hook echo pattern –collocated with strong wind shear, velocity couplet (cyclonic mesocyclone), similar to 2008 Miralcamp case (Bech et al 2011)

Parent-storm radar characteristics (Cardona)

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CASE 2: 2018/01/07 Parent-storm radar characteristics (Empordà)

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FINAL REMARKS

Analysis benefited from detailed damage survey radar-based storm tracking system recent Doppler radar corrections

Case 2017/10/18 – typical time of day (diurnal forcing) & season Upper level trough & cold front passage EF1 tornado, 6km damage path, 13 injured (urban/periurban) MCS with leading convective with trailing stratiform region (PP>110mm) Well organized gust front, likely interaction with induced terrain flow Hard to examine parent storm characteristics (no obvious SP features) in contrast with the 2018/01/07 case

Work in progress (radar observations with satellite and lightning data)

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REFERENCES

Altube P, Bech J, Argemí O, Rigo T, Pineda N, Collis S, Helmus J, 2017: Correction of dual-PRF Doppler velocity outliers in the presence of aliasing. Journal of Atmospheric and Oceanic Technology, 34, 1529–1543. DOI:10.1175/JTECH-D-16-0065.1 Altube P, Rigo T, Argemí O, Pineda N, Bech J, Collis S, Helmus J, 2018: OPERATIVE CORRECTION OF DUAL-PRF VELOCITY OUTLIERS. ERAD 2018 (this conference) poster 61, abstract 149. Bech J, Arús J, Castán S, Pineda N, Rigo T, Montanya J, van der Velde O, 2015: A study of the 21 March 2012 tornadic quasi linear convective system in Catalonia. Atmospheric Research, 158–159, 192–209. DOI:10.1016/j.atmosres.2014.08.009 Bech J, Pineda N, Rigo T, Aran M, Amaro J, Gaya M, Arus J, Montanya J, van der Velde O, 2011: A Mediterranean nocturnal heavy rainfall and tornadic event. Part I. Atmospheric Research, 100, 621-637. DOI: 10.1016/j.atmosres.2010.12.024 del Moral A, Rigo T, Llasat MC, 2018: PERFORMANCE OF A NEW ALGORITHM FOR NOWCASTING ANOMALOUS TRAJECTORIES. abstract 161 Gayà M, 2011: Tornadoes and severe storms in Spain. Atmospheric research, 100(4), 334-343. DOI:10.1016/j.atmosres.2010.10.019 Rigo T, Pineda N, Bech J, 2010: Analysis of warm season thunderstorms using an object-oriented tracking method based on radar and total lightning data. Natural Hazards and Earth System Science 10, 1881-1893. DOI:10.5194/nhess-10-1881-2010 Rodriguez O, Bech J, 2018: Sounding-derived parameters associated with tornadic storms in Catalonia. International Journal of Climatology, 38, 2400–2414. DOI:10.1002/joc.5343