a case study of heavy and persistent rainfall: 4-7 ...añarbe, 122 mm in bidania, 120 mm in ibai...

ERAD 2012 - THE SEVENTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY

A case study of heavy and persistent rainfall: 4-7 November 2011

M. Ruiz 1,2 * , J. Egaña 1,2 , S. Gaztelumendi 1,2 , M. Maruri 1,2, I. R. Gelpi 1,2

1 Basque Meteorology Agency (Euskalmet). Vitoria-Gasteiz, Spain. 2 Tecnalia, Meteo Unit. Vitoria-Gasteiz, Spain.

*[email protected] (Dated: 31 May 2012)

Presenting author name: Miriam Ruiz

1. Introduction

During 4-7 November 2011 a Cut-off Low (COL) moves from south of Pyrenees towards Mediterranean Sea and centres near Balearic Islands. In the surface, a low is generated over the northeast of Iberian Peninsula. As this low gets stronger, northerly flow intensifies over the Bay of Biscay and different fronts fully affect the Basque country during several days. The characteristics of the Mediterranean air mass, its posterior passage over Cantabrian Sea, and the front persistence over the Basque Country, cause high efficiency in precipitation leaving extraordinary amounts of precipitation in some parts of northeastern Basque Country, concentrated in two days (November 5 and 6, 2011). The worst affected places were different regions near Donostia-San Sebastian, accumulating more than 300 mm in some places. Because of the persistence and intensity of rainfall, some personal and material losses are produced due to floods.

In this paper, a study of this severe weather event is made analyzing different parameters. The present paper starts with a description of Euskalmet Weather Radar. Then, an analysis of synoptic environment is done. In part three and four, we can find the episode description and Radar analysis and finally there are the damages and a summary of the rainfall and flooding event.

2. Kapildui Radar Description

Euskalmet Agency’s Weather Radar is sited in Kapildui mountain (1174 meters), near Vitoria-Gasteiz, the capital of the Basque Country (see fig. 1). This Radar is a METEOR 1500 Doppler Weather Radar with Dual polarization capabilities, is based on a Klystron transmitter system, operates in C-band frequency and uses the advanced signal processing environment Aspen DRX as digital receiver and signal processor. The radar control processor RCP interfaces to the signal processor and buffers the pre-processed raw data, establishing a control structure for all components in the system and interfaces the radar with the two software packages available for control and products generation Ravis and Rainbow.

Fig. 1 Kapildui Radar and some AWS mention in the text.

Euskalmet Weather Radar performs with a configuration based on four different scans providing meteorological data every 10 min. First volumetric scan takes 120 seconds and is configured for a maximum range of 300 km with 5 elevations from 0º to 2.5º, a range step of 1 km and an angle step of 0.8º. Second volumetric scan takes 363 seconds and is configured for a maximum range of 100 km with 14 elevations from –1.5º to 35º, a range step of 250 m and an angle step of 1º. Third and fourth scans are elevation scans, the first one oriented to the coast (NW-338 º) and the last one oriented to the south (SW-241º). Both elevated scans are oriented to the west due meteorological interest and also to avoid topographical barriers in azimuth close to population areas. See Gaztelumendi et al (2006) for more details about Kapildui radar system configuration and Aranda and Morais (2006) for some details about Radar installation, construction and site selection.


3. Synoptic environment

The upper-level flow (500 hPa) is shown in figure 2. We can see on November 4, 2011 a trough over the northwest of the Iberian Peninsula. This trough becomes a closed circulation and cut off from basic westerly current during November 5. During November 6, the cut-off low moves from south of Pyrenees to the Mediterranean Sea, in the vicinity of Balearic Islands. The air mass in 500 hPa level is cold (temperature around -22 ºC) during the 5th of November and gets warmer on November 6, but there is a moderate instability degree (TTI 50-52) both days.

Fig. 2: Geopotential heights (m) and isotherms ( ºC) in 500hP from 12 UTC November 4 through 00 UTC November 7, 2011.

The cut-off low in the upper-air flow extends down to the surface (see fig. 3). During the 5th of November the formation of a low over the northeast of the Iberian Peninsula is visible in figure 3. The low centres on the Mediterranean Sea along November 6 and becomes stronger generating an intense northerly flow over the Bay of Biscay. This northerly flux provides lower layers of warmer and more humid air. The sea temperature is about 17.5 ºC, whereas the mean temperature for the beginning of November is around 16 ºC, so the air can contain a higher quantity of water vapour. During this weekend, different weather fronts, with characteristics of Mediterranean Sea, cross the North part of the Iberian Peninsula. The fronts are nearly stationary and within northerly meteorological regimes the rainfall is strong, widespread and persistent. In Meteosat images (Fig 3) we can observe different frontal systems affecting the study area during the episode.

Fig. 3. Sea level pressure, Meteosat HRV and IR 10.8 for 12 UTC days 4, 5, 6 and 7 November 2011.

4. Episode Description

Between Friday 4th and Monday 7th, different weather fronts associated with a COL affect the Basque Country generating persistent, and in many places, heavy rain. As we can see in figures 4 and 5, the worst affected place is the northeast region of the Basque Country, with an amount of precipitation about 200 mm. In some regions near Donostia-San Sebastian total amount of precipitation is around 300 mm, the highest rainfall occurred at Belauntza 328.2 mm, Bidania 307.9 mm and Añarbe 298.2 mm.


Fig. 4: Distribution of rainfall (mm) on 4, 5, 6 and 7 November 2011.

Fig. 5: Distribution of rainfall (mm) from 4 November to 7 November 2011 .

Afternoon on November 4, 2011, different storm cells formed in Basque Country area. Later, in the evening, upper-level flow turns from southerly to northerly and from Cantabrian Sea developing convective cells produce heavy precipitation in some places; more than 20 mm within an hour in the west of Basque Country. The maximum total amount of precipitation was 44.6 mm in 6 hours in Mungia weather station (see fig 6). At the end of the day, all the Basque Country is under a widespread rainfall and in some places in the east, the precipitation persist until morning 7 November.

During November 5, 2011, although it is raining everywhere in Basque Country, the heaviest rainfall takes place in the west part, (23.4 mm in one hour in Punta Galea and Mungia weather stations). Along the afternoon, precipitation goes down in the west region of the Basque Country and intensifies in the east, but there are some cells with convective vertical structure in the western part. In the cantabrian slope more than 100 mm are recorded, especially in the eastern part, (125 mm in Añarbe, 122 mm in Bidania, 120 mm in Ibai Eder).

The rainfall is more effective in the east of the Basque Country during the early hours and morning on November 6, 2011; in Añarbe and Belauntza more than 100 mm are measured in just 12 hours. Although at midday rain spreads to Bizkaia, more intense precipitation is recorded in Gipuzkoa, 22.9 mm in one hour in Añarbe and 22.6 mm in one hour in Belauntza. In the evening the rain goes down in the east part of the Basque Country but in some weather stations are measured more than 150 mm in 24 hours, we have to emphasize the amount of precipitation in Belauntza with 211.5 mm in 24 hours (see Table 1).

During November 7, 2011, precipitations disappears progressively, with rain accumulation minor than two days before. Rainfall is few intense and mainly weak. Maximums around 20 mm are registered in the east part of Cantabric slope.

Fig. 6: Six hourly accumulated precipitation at Mungia, Anarbe and Belauntza during the episode.

Station 4 Nov 5 Nov 6 Nov 7 Nov Total 10 min 1 h 24 h 10 min 1 h 24 h 10 min 1 h 24 h 10 min 1 h 24 h

Belauntza 2.9 7.7 12.7 2.9 11.1 104 5.9 22.6 211.5 1.2 4.6 30.4 358.6 Bidania 3.7 12.5 17.5 3.3 14.1 121.9 2.9 13.3 168.5 1.4 4.7 32.3 340.2 Añarbe 0.4 0.5 0.9 5 12.7 125.2 5.6 22.9 172.1 1.1 4 32.7 330.9 Berastegi 1.5 3.9 11.4 2.6 11.1 106.3 3.8 16.3 168.4 0.6 3 28.7 314.8 Mungia 6.2 26.7 44.6 9 23.4 85.4 4.1 17.1 89.5 1 1.6 9.7 229.2 Iurreta 0.5 0.9 1.9 3.9 13.7 78.4 3.2 17.5 122.4 0.8 2.2 11.3 214

Table 1: Precipitation 4-7 November 2011. Maximum 10-minutes precipitation, maximum 1-hour precipitation, maximum daily accumulated and total accumulated (in mm).


5. Radar Analysis

As we can see in Radar images (Max product) (fig. 7), during the afternoon of November 4, 2011 storm cells develop with a limited vertical extension (the maximum reflectivity is below 4 km). Different convective cells, with 50-55 dBZ reflectivities inside, affect some regions of the Basque Country. At 21:20 UTC a convective cell with 50 dBZ arrives at the west of Bilbao and at the same time a shower of 8.1 mm in ten minutes is measured in a nearby weather station. Figure 7 shows stratiform precipitation at the end of the day.

During November 5, 2011, although main precipitation has stratiform structure, with 30-35 dBZ, sometimes convective cells develop. The maximum reflectivities (45-50 dBZ) take place during midday in the west of the Basque Country and although convective cells do not show a deep development, electrical discharges are measured. At night, convective cells develop in the eastern part.

Fig. 7. Some MAX product images from 4 November to 7 November 2011.


The event has similar characteristics during November 6, 2011: Kapildui Weather Radar, in figure 7, shows stratiform

precipitation and some convective cells. The maximum reflectivities, with 50 dBZ, take place during early morning in the eastern part of the Basque Country, but as it has happened during November 4 and 5, 2011, they do not have deep development. The rest of the day, stratiform precipitation shows 30-40 dBZ reflectivities.

During November 7, 2011, rainfall are weak and mainly coming from low clouds, with reflectivities about 10-20 dBz, precipitations are less frequent and occasional.

An analysis of the vertical profile of reflectivity for the event is shown in figure 8. As an example of the event we will take the vertical scan in 339º direction at 15:27 UTC on 5 November 2011. We can see that the event had stratiform and convective structures. Stratiform precipitation is characterized by a pronounced maximum of reflectivity just below the zero degree isotherm, the so-called radar bright band. In the PPI product the “bright band” seems to gradually disappear at longer ranges due to the increased height and averaging volume of the radar at those ranges. Whereas the convective event exhibits much more uniform vertical reflectivity profile. In the RHI product the convective cell shows 6 km vertical extension, with a reflectivity of 45-50 dBZ. The echotop values of 15 dBZ are agree with vertical scan in 339 º direction, they both show quite low clouds (2-4 km), sometime interspersed with convective cells (7-8 km).

Fig. 8 . Range Height Indicator339º (RHI NW), Plan Position Indicator (PPI) and 15 dBZ Echotop products at 15:20-15:30 5 November 2011.

6. Summary and conclusions.

During 4-7 November 2011, a COL moves from the south of Pyrenees toward the Mediterranean Sea, the center of the depression is located near the Balearic Islands. From this location Northerly flux over the Basque Country area is generated and different fronts associated with the depression arrive to the Basque Country. The characteristics of the Mediterranean air mass, its posterior passage over Bay of Biscay and the slow moving of the fronts result in extraordinary amounts of precipitation in the Cantabric slope during the episode.

Even though heavy showers take place during the event (more than 20 mm in one hour and 8-10 mm in ten minutes), the most serious damages are originated by the persistence of the rainfall. The rain began in the last hours on November 4, 2011, and in many regions it did not stop until the night of Sunday, 6 November. The worst affected area was the northeast of the Basque Country. Near Donostia-San Sebastian total amount of precipitation is about 300 mm, the highest rainfall occurred at Belauntza 328.2 mm, Bidania 307.9 mm and Añarbe 298.2 mm. In some areas in the northeast of the Basque Country, accumulated precipitation during this episode represents over two times the average November rainfall.

Analyzing the vertical profile of precipitation, the event shows stratiform and convective structure. The stratiform precipitation, as usual, has a homogeneous reflectivity in the horizontal plane, whereas the vertical shows a maximum of reflectivity (bright band) just below the zero degree isotherm. Sometimes during the event the precipitation also has convective structure, although the vertical development is quite low. Inside the storm cells, the maximum of reflectivity, with 50-55 dBZ, is always below 4 km.

In we focus on damages, the worst affected area was the east of the Basque Country where a person, resident in Irun died. But during the event, a thousand people have been affected by persistent and heavy rainfall. On 5th, Bilbao’s tube was closed


due to flooding in several stations. On the other hand, on November 6, 2011, the east of the Basque Country was badly hit: a hundred people were evacuated, a power cut affected 5400 people, a lot of properties were flooded with significant damage to houses and garages and several roads were closed due to flooding and landslides.

Fig. 10 .Photographs of damages.

Acknowledgment

The authors would like to thank the Emergencies and Meteorology Directorate - Interior Department - Basque Government for public provision of data and operational service financial support. We also would like to thank all our colleagues from EUSKALMET for their daily effort in promoting valuable services for the Basque community.

References

Aranda, J.A, Morais, A, 2006: The new Radar of Basque Meteorology Agency: site selection, construction and installation. ERAD 2006, Barcelona, Spain.

Berne, A., G. Delrieu, H. Andrieu and J. D. Creutin, 2004: Influence of vertical profile of reflectivity on radar estimated rain rates at short time steps. J. hydromet, 5, 296-310.

Donaldson, N., 2001: Combining C-band radars in Canada`s upgraded weather radar network. Proc. 30th AMS Conf on Radar Meteorology, Munich, Jul 2001, 261-263.

Egaña J., Gaztelumendi S,, Gelpi I.R., Otxoa de Alda K. Maruri M. and Hernández R. 2008: Radar Analysis of Different Meteorological Situations in the Basque Country Area, Proceedings 5th ERAD Conference, Helsinki, 4 pp.

Egaña J., Gaztelumendi S., Gelpi I.R., Mugerza I. 2005. Synoptic patterns associated to very heavy precipitation events in the Basque Country. EMS5/ECAM7 Utrech Netherland.

Egaña J., Gaztelumendi S., Pierna D., Gelpi IR, Hernandez R. and Otxoa de Alda K. 2010: Flash-floods on Basque Country at the end of January 2009. 10th Annual Meeting EMS 8th ECAC Zurich, (Switzerland), 13 - 17 September 2010.

Gaztelumendi S., Otxoa de Alda K. and Hernandez R, 2003.: Some aspects on the operative use of the automatic stations network of the Basque Country, ·Preprints 3th ICEAWS Conference, 2003.

Gaztelumendi S., Egaña J., Gelpi I.R., Otxoa de Alda, K., Maruri, M., Hernandez, R. 2006: The new Radar of Basque Meteorology Agency : configuration and some considerations for its operative use. ERAD 2006, Barcelona, Spain.

Gaztelumendi S., Otxoa de Alda K., Egaña J., Gelpi IR., 2006b: Inclusion of Radar information in the surveillance panel of the Basque Meteorology Agency. ERAD 2006. Fourth European Conference on Radar in Meteorology and Hydrology. Barcelona 18–22 Sep 2006.

Germann U., G. Galli, M. Boscacci and M. Bolliger, 2006: Radar precipitation measurement in a mountainous region. Q. J. R. Meteorol. Soc., 132.

Maruri M., Gaztelumendi S., Egaña J., Otxoa de Alda K., Hernández R., and Gelpi I.R., 2008: Product Quality Monitoring of Kapildui Weather Radar During Critical Meteorological Events. Proceedings of the 5th ERAD Conference, 5 pp.

Maruri M., López J., Aranda J.A. and Gaztelumendi S., 2010 : Effects of the Meteorological Conditions Over the Radome of Kapildui Weather Radar and Studies of their Quality Impacts. Proceedings of 6th ERAD Conference.

Marzano F.S., E. Picciotti and G. Vulpiani, 2004: Rain field and reflectivity vertical profile reconstruction from C-band radar volumetric data. IEEE Trasn. Geosci. Rem. Sens., 42, 1033-1046.

Ronald E. Rinehart, Ph.D, 2001: Radar for meteorologist. Third edition. University of North Dakota. Rinehart Publications. United States of America.

a case study of heavy and persistent rainfall: 4-7 ...añarbe, 122 mm in bidania, 120 mm in ibai...

Documents