Fig. 4: VVP(V) Radar product , wind profile at Radar site from 12:42 to 16:52 UTC.

Contact info:

santiago.gaztelumendi@tecnalia.comjoseba.egana@tecnalia.com

TECNALIA

Parque Tecnológico de BizkaiaC/ Geldo Edificio 700E-48160 DERIO (Bizkaia) Spainwww.tecnalia.com

AcknowledgementsThe authors would like to thank the Emergencies and Meteorology Directorate – Security Department - BasqueGovernment for public provision of data and operational service financial support. We also would like to thank all ourcolleagues from EUSKALMET for their daily effort in promoting valuable services for the Basque community.

Analysis of a tornado event in Basque CountryS. Gaztelumendi 1,2, J. Egaña 1,2, A. Etxezarreta 3, M. Maruri 1,2, J.A. Aranda 4

( 1 ) B a s q u e M e t e o r o l o g y A g e n c y ( E U S K A L M E T ) , B a s q u e C o u n t r y , S p a i n , ( 2 ) M e t e o r o l o g y A r e a , E n e r g y a n d E n v i r o n m e n t D i v i s i o n ,

T E C N A L I A R & I , B a s q u e C o u n t r y , S p a i n , ( 3 ) A D A S A , B a s q u e C o u n t r y , S p a i n . , ( 4 ) E m e r g e n c i e s a n d M e t e o r o l o g y D i r e c t o r a t e , S e c u r i t y D e p a r t m e n t , B a s q u e G o v e r n m e n t , B a s q u e C o u n t r y , S p a i n

16th EMS Annual Meeting 11th European Conference on Applied Climatology (EC AC)

12–16 September 2016 , Trieste, Italy

AbstractIn this work we analyze a tornado occurrence in Basque Country during 23 June 2014, considering synoptical, mesoescale and local aspects. This study includes all available information from the area including radar data provided by Basque Meteorology Agency (Euskalmet). In this poster we summarize some relevant questions.

MethodologyThe analysis of the general severe storms environment is addressed based in all the synoptic and mesoescale information available (Egaña et al 2005.2007,2011, Gaztelumendi et al 2009, 2015), focusing on the relevant aspects that favor supercells development (e.g. Brooks et al1994, Doswell et al 2001, 2007, Markowski et al 2010, Romero et al 2007, Trapp 2013).

Local analyses is performed mainly using Radar data. We identify signatures compatible with tornadogenesis (e.g. Brooks et al1994, Doswell et al 1993, Markowski et al 2009, 2002, Bluestein 2007, Traap et al 1999). Highlight that Euskalmet operational Doppler dual-polarization radar (Aranda et al 2006, Gaztelumendi et al 2006) is sited about 10 km north of impact area. where the tornado occurs.

Finally we focus on surface damages produced in an unpopulated relatively small forested area of 24.000 m2,,analyzing the path length and width, and the treefall patterns within the damage area. (e.g. Beck et al 2010, Brooks 2004, Bech et al 2009, Doswell 2003, Blanchard 2012, Frelich 2012, Edwards 2013).

Discussion

General EnvironmentDuring this episode, Basque Country area (see Fig.1 ) is affected by a cut-off low located at the west of Iberian peninsula, traveling to the east and promoting instability. Difluence in high layers are significant, with weak flux at 500 hPa and 300hPa levels, at low levels convergence is present at the South of Basque Country. At surface level a relative thermal low is present with undefined wind and relatively warm and humid air mass affecting the area. These general environment favors a steep mid level lapse rate that become steeper due to diurnal heating and mixing ratios of 10 g/kg advected from the sea, promoting high degree of instability (TTI 54/55, LI -4/-5, CAPE 1400 J/Kg ) and moderate shear values increasing during day due to approaching upper level low (0-6 km shear 10-20 m/s) (more details available on Gaztelumendi et al 2015b)

Storm Environment & Radar dataFrom 13:30 a storm core quickly begins to grow getting a great vertical extent (see Fig. 2 ), with cloud top reaching the tropopause (13-14 km) (see Fig. 3 ). Splitting are produced and right core is activated, being consistent with hodographs that rotates clockwise (see fig 2,3 and 4). This core continues clearly showing the characteristics of a supercell with the generation of a mesocyclone that can be seen in the radial image of wind in the first kilometers (velocity couplet) (see Fig 5 and 6 ), where a cyclonic eddy is present coincident with hook signature in the supercell with own notch from feeder flow at low layers (see Fig 6 and 7 ).

During the collapse of the supercell there is a high probability of tornadogenesis, and then Tornado Vortex Signature (TVS) can be seen in radar images. Unfortunately this is not the case, where radar characteristics are tuned for general.

The area affected by tornado is in the Rear-Flank Downdraft (RFD), The RFD and the main area of updraft and downdraft forward-flank (FFD) are identified in figure 6. If we consider shear present in the first meters and complex topography of the area, transformation and potentiation of horizontal vorticity into vertical vorticity near surface is possible. If a downdraft is involved in the tilting process, vertically vorticity can be advected toward the surface promoting tornadogenesis.

Damages and impact analysisAnalyzed supercell leaves heavy rain and big hail in the region Gaztelumendi et al 2015b). In particular, wind damages are produced in a 3 Hectares of forest near Izki area (mainly pine). More than a hundred uprooted and broken trees at different heights (from 0.5 to 5-6 meters high) are produced. Big branches and trees up to 30-40 cm in diameter are fallen, all over the area, in different directions compatible with plausible tornado path. (see Fig. 8)

Conclusions & RemarksUnless tornado cases in Europe (e.g. Dotzek 2003, Dessens 1999, Antenesku 2015) and Iberian Peninsula (particularly in Mediterranean area :Catalonia, Balearic Islands, and Andalusia) are not unusual (e.g. Gaya 2005, 2011, Homar 2001, Bech 2007), those phenomena are less observed in north of Iberian Peninsula. Particularly, in Basque Country, none documented and confirmed case exist prior to this event.

During the afternoon of 23 june 2014, different severe storms affect southern part of Basque Country, with heavy rain, hail and wind gust. Particularly in Izki area hundreds of trees are removed in a narrow area compatible with a tornado path. Some Radar data, from the Euskalmet C–band Doppler radar sited near the affected area, present different supercell structure and tornadogenesis signatures

Euskalmet Doppler radar can't see the tornado itself, because the radar beam gets too wide few kilometers away from the transmitter to resolve such a small structure as the tornado vortex structure (TVS). But radar data indicates the presence of different signatures from supercell structure and tornadogenesis ingredients as hook echoes and their associated rear-flank downdrafts. Affected surface area are sited in typical tornado location in the RFD.

Even though there are no direct observers to the tornado, the distribution and characteristics of fallen trees (no diffluent damages are observed) are compatible with the effect of a short lived F2 scale tornado.

.

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Fig. 3: MSG HR VIS and CAPPI 2km for 13:30,14:00,14:30 and 15:00

Fig. 2. MAX (dBZ) sequence from 13:42 to 15:22 each 10 minutes1 (Height 1-13km. Hor res 0.062 km/pixel, Vertresol 0.1)

Fig.5. PCAPPI (1, 3 and 5 km (from top to bottom), 14:22, 14:42, 15:02 (from left to right), resolution 0,062 km/pixel ) (wind barbs from HWIND),

Fig. 7 : PPI (dBZ) -0.5 and 0.5 (from top to bottom), 14:22, 14:42, 14:52 and 15:02 ( from left to righ), resoltuion0,062 km/pixeL)

Fig. 6: 14:52 MAX 1 km to 13 km (top) and CAPPI (V) 1km with 0,062 km/pixel (down).

We can appreciate the Hock echo with (a) the Bounded week echo from updraft region (b) the area with enhanced reflectivity from forward flank downdraft (FFD) and (c) the Rear Flank Downdraft (RFD) where tornado are most probable. In the CAPPI we can appreciate the inbound-outbound couplet in radial velocity data typical from supercelular mesociclonicpattern.

Fig.8: Location of Radar and area affected by tornado and observed damages.

10,9 Km

Radar

Tornado area

23.900 m2

AfterBefore

Fig.1: Basque Country and tornado area

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30

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