10TH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY & HYDROLOGY

ERAD 2018 Abstract ID 000 1 hidde.leijnse@knmi.nl

1 Introduction

In warm season (from May to September) storms generated by deep convection are frequent in the Basque Country (see Fig.1), although not in all cases storms produce severe conditions. Impacts are usually caused by intense showers exceeding 30 mm in 1 hour or 10 mm in 10 minutes (see Gaztelumendi et al, 2009 and Egaña et al, 2007a,b ), especially if the showers affect urban areas, and by hail and wind gusts associated with storms (see Gaztelumendi et al, 2010). In these situations surface pressure patterns are not very defined, with barometric swamp or relative thermal lows. In high levels, southwest wind usually predominates and there is often a trough configuration with its axe over the west of the Iberian Peninsula. This configuration generates positive vorticity advection (see Egaña et al, 2007). Besides, in this period, warmth accumulated during the day favours atmospheric thermal instability. When instability degree is high, storms might be very active and vertically quite developed and the top of the clouds could reach the tropopause. In many cases, storms are created in the south or southwest, outside the Basque Country, although they can also be generated within. This depends on the convergence areas in low levels and instability degree above the Basque Country. In this region, storms usually move from southwest to northeast, as a consequence of frequent southwest high levels flow. In some circumstances, storms deviation might be relevant in relation to the main flow. Severe storms dynamic is complex and many factors should be taken into account, including instability index and other parameters, such as wind shear or storm relative helicity (e.g. Wesiman and Rotunno, 2000, Doswell 2001).

Figure 1: The Basque Country located in the north of Iberian Peninsula.

2 Synoptic Environment

A depression isolated in high levels of the atmosphere affects the days before the west part of the Iberian Peninsula. During August 30, in 500 hPa and 300 hPa levels (see Fig. 2), a deep trough formed in Atlantic Ocean traverse the Iberian Peninsula causing dynamic instability. The temperature is relatively low in the 500 hPa layer, around -13 ºC, and in 850 hPa level the air mass it is not very hot (approximately 13 ºC), therefore the thermal instability it is not very pronounced.

In surface, the Iberian Peninsula is in a barometric swamp during the first part of the day. The anticyclone of Azores lengthens through the Atlantic Ocean until the British Isles latitude and moves towards the east during the day reaching the Gulf of Biscay and at the end of the day it extends as a wedge along the Cantabrian Sea. In addition an occluded front crosses the Gulf of Biscay towards the Mediterranean Sea.

On the other hand, instability indices are not very high, Total Totals index (TTI) is around 50 ºC and Lift Index around -2 ºC, however those values indicate certain instability and are enough to have events with heavy rainfall in the Basque Country (Egaña et al., 2017). Precipitable water values are significant for this area, around 25-30 kg/m2.

This situation and especially the dynamic instability allow the development of storms which moves from southwest to northeast during the second part of the day.

Using Euskalmet radar for analysis of a heavy storms event in Basque Country during summer: the 30 August 2017 case. Joseba Egaña1,2 , Santiago Gaztelumendi1,2 , Olatz Principe1,2 , Virginia Palacio1,2 and Mercedes Maruri1,2

1Basque Meteorology Agency, Basque Country, Spain. 2Meteorology Area, Energy and Environmet Division,Tecnalia, Basque Country, Spain.

(Dated: 17 April 2018)

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Figure 2: Jet stream and geopotential in 300 hPa, geopotential and isotherms in 500 hPa and 850 hPa, and sea level pressure at 06, 12, 18 and 24, on 30/08/2017.

3 Episode description and radar analysis

The morning is sunny and the temperature is soft in the coast and higher in the interior of the region (maximum around 28-29 ºC in the south and the east). Around midday clouds tend to develop in the west of the area and first heavy storms (>15mm/hour) take place. It is noteworthy the register of Ordunte automatic weather station (AWS) at 15:30 local hour, with 13.5 mm/10-min and 15.1 mm/h (see Fig. 3 and Table 1). After, they extend to Bilbao, the most populated city of the Basque Country and at 16:30 local hour 10 mm are registered in 10 minutes (17 mm/h) (see Fig. 3 and Table 1). The duration of these showers is quite short and the electrical activity is high (see Fig. 11 at 14:50 UTC). In addition showers are accompanied by hail of approximately 2 cm. Z-based Hail Detection (ZHAIL) radar product shows a high probability of hail (see Fig. 6) above 80 %. Waldvogel criterion is used to make the calculus and it studies the echotop 45 dBz above 1.4 km over 0 ºC isotherm (see Waldvogel et al., 1979)

Figure 3. Hourly precipitation in Ordunte and Abusu (Bilbao) AWS on 30/08/2017.

After, storms develop also in east and in northeast of the region. They do not lose intensity, on the contrary they are more intense than those of the west. As a result, rainfall are heavy and very heavy (>30mm/hour) in the east part of the region, in the interior and also in the coast between 18:20 and 19:10 local hour. In Ereñozu, Oiartzun and Behobia stations exceed 30 mm/h and 10 mm/10-min (in Oiartzun 33.2 mm/h and 12.9 mm/10-min, and in Behobia 32.6 mm/h and 12.8 mm/10-min), and the maximum of precipitation is recorded in Ereñozu with 44 mm/h and 15.4 mm/10-min (see Table 1).

10TH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY & HYDROLOGY

ERAD 2018 Abstract ID 000 3

Figure 4. Hourly precipitation in Ereñozu, Behobia and Oiartzun AWS on 30/08/2017.

Convective and electrical activity starts in the west part of the Basque Country and extends to the east and northeast as can be seen in the images of MAX (2-10 km) with lightning (see Fig. 5). Crosses represent cloud-ground strikes and points represent the cloud-cloud discharges. MAX product of the radar, ranging from 2 km to 10 km (see Fig. 6) shows maximum reflectivities up to 55-60 dBz in the heaviest storms in the west and also in those of the east part of the region. They development reaches around 11-12 km as Echotop product reveal (see Fig. 6), which represents the maximum height at which reflectivity is greater than 15 dBz.

Figure 5. MAX (2-10 km) images with lightning activity from 14:10 to 14:40 and from 15:20 to 15:50 UTC every 10 minutes.

Figure 6. MAX (2-10 km), ZHAIL and Echotop-15 dBz products at 14:10 and 15:20 UTC on 30/08/2017.

In late afternoon storms lose intensity, although it continues raining quite heavily in the east, as a result of which the precipitation accumulated in 24 hours exceeds 60 l/m2 in Oiartzun and Ereñozu AWS (see Fig. 4). In many other places precipitation is very abundant (>30 mm/24h) (see Table 1).

The spatial and temporal distribution of this precipitation can be seen in Kriging maps of maximum rainfall in 10 minutes, hourly and total in 24 hours (see Fig. 8 and Table 1). It is consistent with PAC product of Kapildui radar which gathers the precipitation accumulated hourly or in 24 hours not only with radar data, but also with values registered by AWS.

Table 1: Maximum accumulated precipitation in one hour, in 10 minutes and in 24 hours.

Station Hourly precipitation

(mm)

Time

UTC

Precipitation in 10 min.

(mm)

Time

UTC

Precipitation in 24 hours

(mm)

Ereñozu 44 16:40 15.4 16:00 71.6

Oiartzun 33.2 17:00 12.9 16:30 64.9

Behobia 32.6 17:10 12.8 16:50 57.8

Zarautz 29.7 16:20 8.3 15:40 42.5

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Lasarte 26.6 16:30 11 16:10 49

Abusu 17 15:10 10 14:30 25.5

Ordunte 15.1 14:10 13.5 13:30 20.6

Figure 7. Location of some AWS and Kapildui radar in the Basque Country.

Figure 8. Precipitation in 10 minutes, in one hour and in 24 hours on 30/08/2017.

Figure 9. PAC 1h product: hourly precipitation during very heavy storms, at 16:20, 16:40, 17:00 and 17:20 UTC.

Figure 10. PAC 24 h product: precipitation in 24 hours on 30/08/2017.

The amount of precipitation accumulated causes some temporary problems in urban roads (see Fig. 13, Fig. 14 and Fig. 15) and flooding of certain rivers as Jaizubia in Bidasoa basin that reaches yellow warning level. Associated to certain storms very strong wind gusts are registered in some places as Zarautz (73 km/h). Nevertheless, comparing with other similar events, wind gusts are not very significant.

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Figure 11. Spatial distribution and intensity of CG (cloud-ground) and IC (intracloud) strokes during 90 minutes between 14:50 and

16:40 UTC on 30/08/2017.

Figure 12. Hourly distribution of CG (cloud-ground) strikes on 30/08/2017.

The higher electrical activity during the event it produces during 5 hours. In this episode, 1378 lightning strokes are registered, of which 330 are positive and 1048 negative, and they are recorded in the first part of the evening together with the heaviest thunderstorms happen. The most intense positive strike is 115.2 kA and the negative one -64 kA.

4 Summary and conclusions

The convective cells trajectory is eastward and northeastward, a common movement in storm events in the Basque Country due to a deep trough coming from Atlantic Ocean. In studied episode the major factor that leads to the development of heavy storms is the dynamic instability caused by the trough crossing the Basque Country from west to east. Instability indexes are not very high, but enough to originate heavy and very heavy rainfall.

The radar products and data are essential to tracking storms, not only to do nowcasting following in real-time the movement and development of the storms, but also to quantify the heavy rainfall that take place in zones where there is not an AWS. Products as PAC that combine radar and AWS data, allows to estimate the QPE (quantitative precipitacitation estimation) in any place of the region in order to know the impact .

The damages generated by heavy rain are not too important. The most populated city affected is Donostia-San Sebastián, in the northeast of the region. Big pools are formed in many streets making driving complicated and drains overflow. Other locations are also affected, with power cutoffs, dropped trees and small floods. In addition some rivers grow fast and reach warning yellow level. Although the rainfall is heavy also in the west of the region, impact is not so relevant.

Figure 13. Hourly precipitation in Andoain AWS from 12 to 24 UTC and impact in the town with pools in roads and dropped trees on

30/08/2017.

10TH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY & HYDROLOGY

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Figure 14. Hourly precipitation in Donostia-San Sebastián from 12 to 24 UTC and heavy rainfall in Usurbil (Lasarte AWS) on 30/08/2017.

Figure 15. Precipitation from 12 to 24 UTC in Zarautz AWS and floods on 30/08/2017.

Acknowledgement

The authors would like to thank the Emergencies and Meteorology Directorate - Security Department – Basque Government for public provision of data and operational service financial support. We also would like to thank all our colleagues from TECNALIA and EUSKALMET for their daily effort in promoting valuable services for the Basque community. Finally, we would also like to thank the Free Software Movement and all institutions and people that maintain and support availability of free data and tools for scientific community.

References

Doswell, C. A. III, 2001. Severe Convective Storms, Meteor. Monogr., No. 50, Amer. Meteor. Soc, doi: 10.1175/0065-9401-28.50.433. Egaña, J., Gaztelumendi, S., Gelpi, I.R., Otxoa de Alda, K, 2007a. A preliminary analysis of summer severe storms in the Basque Country area: synoptic characteristics”. ECSS 2007. Fourth European Conference on Severe Storms. Trieste (Italy). Egaña, J., Gaztelumendi, S., Gelpi, I.R., Otxoa de Alda, K., 2007b. Synoptic patterns associated to severe storms in the Basque Country. 7th EMS 8th ECAM. Egaña J., Gaztelumendi S., Pierna D., Príncipe O., 2017c. A study of convective severe storms in Basque Country during XXI century. 9th European Conference on Severe Storms (ECSS), 2017. Pula, Croatia. Gaztelumendi, S., Egaña, J., Gelpi, I.R., Otxoa de Alda, K., 2007. Study of a case: the 4th july 2006 storm over the city of Bilbao. 4th European Conference on Severe Storms (ECSS).

Gaztelumendi, S., Otxoa de Alda, K., Egaña, J., Gelpi, I.R., Pierna, D., Carreño, S.,2009. Summer showers characterization in the Basque Country. 5th ECSS 2009.

Gaztelumendi, S., Egaña, J., Gelpi, I.R., Otxoa de Alda, K., Maruri M., Hernández R., 2010. Using Euskalmet Radar data for analysis on July 1st Hailstone storms in Vitoria city. 6th ERAD 2010.

Waldvogel, A., Federer, B., Grimm, P., 1979. Criteria for the detection of hail cells. Atmospheric Physics. ETH, 8093 Zurich, Switzerland Weisman, M. L., and R. Rotunno, 2000. The use of vertical wind shear versus helicity in interpreting supercell dynamics. J. Atmos. Sci., 57, 1452–1472.