Heavy rainfall in Catalonia, October 1994

Dennis Wheeler Geography Department, Sunderland University

In October 1994 the Catalan region of Spain suffered one of its most persistently wet months on record with heavy rainfall extending even into the normally much drier, land-locked, province of Lerida. As summer sea surface temperatures (SSTs) in the western Mediter- ranean had reached some of their highest- recorded levels - off the French coast an unsubstantiated figure of 29 “C has been quoted - a wet and tempestuous conclusion to the summer had been foreseen by local cli- matologists. Many authors (for example, Wheeler 1988, 1989; Wheeler and Tout 1990; Martin-Vide 1989) have already discussed how the autumn rainfall regime in the Spanish Mediterranean coastal region responds to the combination of atmospheric instability, the locally abrupt relief, and the summer’s legacy of high SSTs. Of these factors, atmospheric instability is arguably the most critical. A strong northerly airflow in the upper westerlies can introduce the winter’s first outbreak of cold air aloft, which when overlaid on a warm and humid lower atmosphere can create chronically unstable conditions. Cut-off cold pools of the type described by Boyden (1963) are the best examples of this activity which when located over Iberia can have dramatic consequences with sometimes as much as one-third of the annual rainfall total falling in just one day. Such events, following the perennially dry summer, characterise the onset of the Mediterranean winter. Autumn, if such a season can be said to exist in the Mediterranean context, is an unpre- dictable few weeks that mark the transition from the summer to the winter regime. The protracted British autumn or the polychromatic New England ‘fall’ have no counterpart in these latitudes.

The principal feature of October 1994 in Catalonia (Fig. 1) was the rainfall of the 9th and 10th. Whilst that period was the month’s

wettest there were also other clearly defined spells of less severe precipitation. Each of them displayed slightly different synoptic characteris- tics and they provide an illustration of the variety of factors that control rainfall in the region. It is, however, important to remember that, to a degree almost unknown in areas dominated by mid-latitude cyclones, surface synoptic states often give little indication of the atmosphere’s rain-bearing potential. An exam- ination of conditions throughout the depth of the troposphere is indispensable in determining this.

Rainfall during early October

Following an unsettled conclusion to Septem- ber, when many areas received daily totals in excess of 50mm, the month started cloudy in Catalonia with modest rainfalls in the Pyrenees (up to 9mm) and to the north of Tarragona (8mm). The surface chart showed high pres- sure extending from the Azores towards France. There were, however, some weakly developed thermal lows within that system. In some parts of Catalonia the 1st was the month’s warmest day, and temperatures reached 27°C in the south and 24°C closer to the French border. In that respect the climatic picture at the surface was identifiably one of the Mediter- ranean’s summer regime. More significantly, and a herald of changes later in the month, the thickness charts for the 2nd revealed a cold pool located over the region though the accom- panying trough was well defined only from 300mbar upwards. This combination of sur- face and upper-air conditions was sufficiently unstable to produce isolated showers.

The second period of rainfall on the 4th and 5th was more productive. Large areas of central Catalonia registered over 30mm with volumes declining inland and towards the Pyrenees. The

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Fig. 1 Locations of sites mentioned in the text

wettest locations were towards the south of the region where Igualada and Torrelles de Foix recorded 61 and 51mm respectively on the 4th. On this occasion there was a clearly defined trough at all levels above 700 mbar with an axis to the east of Iberia (Fig. 2(b)). A ridge lay to the west extending to Ireland and sustained a strong north-westerly airflow. Rain was asso- ciated with a front aligned east-west that marked the division between warmer, stagnant Mediterranean air to the south and cooler air to the north circulating around the anticyclone (Fig. 2(a)) and with the small upper vortex and trough over Iberia. This front crossed slowly southwards over the region bringing the cooler air in its wake and the winter's first widespread frosts on higher ground.

Rainfall for 9-12 October

The third, and most notable spell of rainfall was that of the 9th to 12th. The combination of circumstances which brought it into being ad- mirably exemplifies the origin of all such in- tense autumn activity in the region. From the 6th onwards high pressure dominated to the north-east of Iberia encouraging an easterly

airflow towards Spain. Surface pressure gra- dients were, however, slack and on the 8th the airflow assumed a more south-easterly charac- ter which was critical for later events as the now long fetch over the warm waters of the Mediter- ranean gave ample opportunity for slowly mov- ing low-level air to gain warmth and moisture before reaching the Iberian coast (Fig. 2(c)). Meanwhile, a quite different system was oper- ating at levels from 700mbar upwards where a deep cold pool had developed over the north- ern Adriatic with an equally extensive but less marked pool over the Gulf of Cadiz. In Cata- lonia the heaviest rains came on the 9th and 10th as a marked cut-off upper vortex de- veloped to the immediate west of Iberia. In this location it could direct cold air (-14°C at 500mbar) from more northerly latitudes around its southern limits and northwards to- wards Catalonia.

By the 10th the latter feature had disap- peared although the trough to the west was again showing more marked cut-off charac- teristics (Fig. 2(d)). Meanwhile, at low levels moist and humid south-easterly air was being forced to rise along the extensive Catalan Ranges that back the whole of the region's

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Fig. 2 (a) S u d u ~ c clzurr (Firbar) for 0000c;Afl 011 5 October 1994, (b) 300mbar chart (dam) for 12ooGMT on 5 October 1994, (c) sutjace chart (nibar) for 0000tiiCt~ on 10 October 1994, and (d) 300mbar chart (dam) for 12ooGMT on 10 October 1994

coastline to an altitude of over 1OOOm. The resulting realisation of the inherently unstable character of the atmosphere produced heavy rainfalls in all parts of the region. These ex- tended, unusually, westwards to the inland province of Lirrida, but were typically heaviest nearer the coast where altitude is greatest and relief most abrupt. The synoptic conditions were at this time typical of such storms in which there exists a seeming dislocation between low- level air, which retains the temperature charac-

teristics of the summer, and middle- and up- per-level conditions where troughs and cool air are more commonly associated with the winter regime.

Figure 3 shows the geographical distribution of the rainfall over the period from 9 to 12 October. The three sites for which the highest totals were recorded were all in the north-east of the region: Alforja, with 426.2mm, La Bisbal d’Emporda (303.0mm) and L‘Estartit (244.2mm). The latter site also recorded the

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Fig. 3 Distribution of rainfall (mm) over the period 9-12 October 1994 in Catalonia

region’s highest daily total with 217.8mm on the 10th; La Bisbal d’Emporda recorded 176.8mm on the same day. The disruption caused by this storm was widespread. Six people died as a direct consequence of the conditions. The main railway lines to Madrid and Valencia were impassable. The A-19 and A-18 motonvays were damaged, several minor roads were entirely closed and in the village of La Riba (Tarragona) two of the three bridges were destroyed. In all parts of the region, stream beds that for months beforehand had been dry stony channels were converted within hours into raging torrents. In the city of Tar- ragona, the Serrallo district was evacuated of its 900 inhabitants as the Francoli River threat- ened to destroy the embankments recently con- structed to divert its course as part~of the port developments of the city.

The rainfall intensities of this period are unquestionably notable, but within the setting of the region they are not altogether unprece- dented. For example, it has been estimated (Castillo and Beltran 1979) that close to the sites of maximum rainfall at Rosas the 30-year 24-hour average rainfall is 204mm, at Cada- ques 146mm and at Ampurias 159mm - rain- fall amounts which broadly agree with those cited above and for which a similar return period might be inferred.

Rainfall during late October

By the 13th the dominant upper trough had weakened and moved towards the west, and rainfall declined thereafter. The next period of heavy rain was on the 18th and 19th when over 50mm fell principally on coastal districts in Gerona province and around the city of Bar- celona where it was the wettest phase of the month with 69mm at the airport on the 19th and 58.7 mm in the city on the same day. Those locations recording the highest totals were again in the north where LlanCa registered 140mm and Rosas 130mm, both on the 18th. The dominant rain-making conditions were, however, different from those of the 9th-12th period. A well developed depression lay to the immediate west of Ireland with an associated front extending southwards to about 30 “N over western Iberia and north Africa (Fig. 4(a)). This low extended throughout the tro- posphere, while to the east of Iberia a weak upper ridge was present. Winds on the 18th were light and the midday chart showed iso- lated thermal lows over north Africa and the western Mediterranean leading to occasional instability showers. The rain of the 19th, con- versely, was associated with the slowly advanc- ing front noted above which crossed the region on the 20th. Although the upper flow displayed a degree of meridionality at this time (Fig. 4(b)) there is no evidence of cold pool developments of the type that characterised the period of the 9th to 12th. This was one of those less common occasions when surface synoptic states gave a relatively clear impression of rainfall potential in the region.

The month’s final spell of rainfall was on the 28th and 29th. Again, the most notable falls were along the northern coastal regions around L‘Estartit which recorded 89.9 mm, while nearby La Bisbal d’Emporda noted 84.8mm. An east-west aligned front stretched the length of the Mediterranean basin marking the bound- ary between warmer Mediterranean air to the south and cooler maritime air to the north (Fig. 4(c)). There was weak cyclogenesis along this front with developments over Italy on the 27th and Iberia on the 28th. At higher levels a trough lay with an axis extending from over the North Sea southwards towards Iberia (Fig.

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Fig. 4 (u) SinrjaLc churl (inbur) f o r 0000~hfl' oil 19 October 1994, (b) 300nzbar chart (dam) for 1200GMT on 19 October 1994, (c) surjaate chart (Irzbar) for 0000c;Al7 on 28 October 1994, and (d) 300mbar chart (dam) for 12ooGMT on 28 October 1994

4(d)). This combination of circumstances was sufficient for instability storms to be set off and, in addition to the rain, thunderstorms were widespread.

Conclusion

The final monthly totals are listed in Table 1. In a region where annual averages vary between 800mm in the far north coastal districts to less than 500mm in the south, such data exemplify

the occasional significance of autumn storms for the annual totals. The effect on some an- nual totals, for example that of L'Estartit, scar- cely requires elaboration. We should not, however, read too much into such figures. The inherent variability of annual totals, depending as they often do on the fitful appearance of storms such as those described above, renders the long-term mean a less useful summary statistic than it is in areas of more assured rainfall. October 1994 illustrates this point bet-

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Table 1 Rainfall data for selected Catalan stations during October 1994

Location (and province) Monthly total Daily maximum Date of maximum (mm) (mm)

L’Estartit (Gerona) La Bisbal d’Emporda (Gerona) St. Feliu de Guixols (Gerona) Barcelona Airport (Barcelona) Tarragona (Tarragona) Tortosa (Tarragona) Nuria (Gerona) Llavorsi (Lkrida) Lkrida (Lkrida) Balaguer (Lkrida)

ter than most but it was also a month when, curiously, more than one period of heavy rain- fall was active. The major rainfalls of the 9th- 12th are a model for such activity but the others, coming as they did within a short inter- val of time, emphasised the variety of factors that may determine precipitation in this region. It is ironic to note in conclusion that this unwelcome volume of rainfall came following a summer when Spanish farmers took to the streets of Madrid to protest over the lack of provision of irrigation water for their crops, when arrangements were in hand for fresh water to be taken by tanker to the beleaguered citizens of Majorca, and only a few weeks after some of the worst woodland fires of the century had been extinguished.

Acknowledgement

The author acknowledges with thanks the pro-

601.3 465.9 250.9 158.3 164.5 248.7 231.3

84.0 87.2 95.6

217.8 176.8 103.5 69.0 44.9

110.3 72.0 28.0 51.9 42.5

10 10 11 19 5

10 9

27 9 9

vision of data from the Centre Meteorologic de Catalunya in Barcelona.

References Boyden, C. J. (1963) Development of the jet stream and

cut-off circulations. Meteorol. Mag., 92, pp. 287-295 Castillo, F. E. and Beltran, L. R (1979) Precipitaciones

maximas en Espaiia: estimaciones basadas en metodos estadisticos. Ministerio a2 Agricultura, Mono- graph No. 21, Madrid, 545 pp.

Martin-Vide, X. (1989) Pluges i inundacwnes a la Medi- munia. Kestres, Barcelona, 132 pp.

Wheeler, D. A. (1988) The Barcelona storm: I-5th October 1987. 3. Meteorol. (UK), 13, pp. 78-85 - (1989) Majorca’s severe storms of September

1989: A reminder of Mediterranean uncertainty. Weather, 46, pp. 21-26

Wheeler, D. A. and Tout, D. (1990) The early autumn storms of 1989 in eastern Spain. 3. Meteorol. (UK), 15, pp. 238-248

Raindrops keep falling on my head J. J. Holden, S. E. Belcher, A. Horvath and 1. Pytharoulis Department of Meteorology, University of Reading

Rainfall is the cause of many problems, not least flooding, the cause of widespread destruc- tion over northern Europe earlier this year. To most people, though, having to run that final lOOm from the bus stop to the house to avoid

being soaked by pouring rain is often of more immediate concern. But does running in the rain actually reduce how wet we get? This problem is one that many people will have thought about at some time, and quite recently

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