A severe weather event in Dobrogea: September 30th - October … · 2018, Volume 15, Issue 3 32 A...

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www.ecoterra-online.ro 2018, Volume 15, Issue 3

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A severe weather event in Dobrogea: September 30th - October 02nd, 2013 Angela Crăciun

National Meteorological Administration, Dobrogea Regional Meteorological Centre,

Romania. Corresponding author: A. Crăciun, [email protected]

Abstract. The described event frames within the pattern of cold season storms that occurred at the contact between a cyclone of Mediterranean origin activated in the western basin of the Black Sea and a high-pressure system situated above the Russian Plain. The event was uncommon through not only the intensity of the wind, the precipitation amounts recorded and its magnitude in time but mostly through the time of the year when it occurred, i.e. at the beginning of October. A volume of 30-40 mm of rain fell, which corresponds to the precipitation climate normal in Dobrogea for the whole month of September, whereas the 15-35 mm climate normal for October was exceeded roughly tenfold in the first two days of the month. If in the first and second ten-day intervals of the month the moisture reserve accessible to plants over the 0-20, 0-50 and 0-100 cm depths was at moderate drought and close to optimum supply levels at Adamclisi and Medgidia weather stations, whereas extreme drought was recorded at Tulcea station. The situation turned to moisture excess in the 21st September – 3rd October interval at all of the mentioned weather stations. The unfavourable weather conditions induced by the analysed episode disturbed the social-economic activity; trees were uprooted, basements and households were flooded and entire localities were left without electricity. However, probably the most severe damage, economically speaking, was the complete halting of the harbour activities. Key words: storm, cyclogenesis, potential vortex, pressure, storms.

Introduction. In the worldwide context with a permanent multiplication of extreme weather phenomena, Romania and especially its southern part was not spared from uncommon meteorological events over the September – October interval. Here is what the media was reporting on 1st October 2013: "…Closed harbours, violent storm at sea, trees uprooted by the strong wind, flying roofs, flooded houses, roads covered by water streams, power outages, abundant rainfalls amounting to 97 mm; these are the effects of the orange code for floods that points at Constanţa county." (www.ziare.com/01.10.2013).

Climatologically, the first two months of the autumn are characterized as follows: "September preserves many of the characteristics of the ending summer, the weather keeping warm at least during the first two ten-day periods. Additionally, September is a less unsettled month, even in comparison with August, because of the persistence over most of Europe of a relatively high-pressure belt that unites the Azores High and the East-European maximum pressure system. October is the most stable month of the autumn, when for days on end the weather keeps sunny although nights become ever colder, such that in most of the country's areas hoarfrost occurs as an usual phenomenon. At that time of the year, our country feels more the influence of the East-European High than that of the Azores High ridge, the latter gradually withdrawing towards the west of the continent."

This paper aims at highlighting the exceptional situation in place in the 30th September – 2nd October 2013 spell. It was targeted to detail the context that led to recording large precipitation amounts, hard wind, rough sea, all followed by a sharp air temperature drop, the general source being the cyclogenesis within the western basin of the Black Sea. There are displayed outputs of the ECMWF, ALADIN and COSMO models, compared to the actual fields, the decisions taken as regards the meteorological warnings and the analysis of the data recorded at the weather stations located on the Black Sea coast.

Diagnosis. The weather in Dobrogea had a dull aspect and it rained, record amounts of precipitation being reported over the analysed period (167 mm at Corbu on 29th September through 2nd October – Figure 1).



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Figure 1. Precipitation amounts from 29th September, 06 UTC to 2nd October 2013,

06 UTC (mm/24h).

The wind was also an element that acted violently, as the mean speeds reached up to 21 m/s as values mediated over 10 minutes on the seacoast and in the Danube Delta, and gusts picked up speeds of up to 21 m/s, whereas offshore mean speeds reached 22 m/s and gusts went up to 27 m/s (Figure 2).

Figure 2. Graph of mean wind speeds and gusts at seashore weather stations from 30th

September 2013, 06 UTC to 2nd October 2013, 00 UTC.

In the western basin, the Black Sea had a 5-degree roughness at the shore (waves of up to 4 m high).

As regards the air temperature, the investigated interval marked the passage from a weather thermally close to normal to a very cold one, with temperatures about 10°C below the multiannual values (Figure 3 and Figure 4). Actually 3rd October 2013 set new records for low maximum temperatures: 9.1°C at Constanţa weather station (the



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previous record was 12.1°C) and 8.5°C at Sf. Gheorghe-Deltă station (the previous record was 11.6°C).

Figure 3. Graph of maximum temperatures and climatological norms at few weather

stations.

Figure 4. Graph of minimum temperatures and climatological norms at few weather stations.



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Synoptic situation forecast and reality

Level of 500 hPa. For 3rd September 2013 (Figure 6a), the probable fields supplied by the atmospheric models used at Dobrogea Regional Meteorological Centre were forecasting deepening of the geopotential trough situated over north-eastern and eastern Europe. As a result, Dobrogea was first under a south-western air circulation advection warm air. The eastwards displacement of the trough was hampered, on the probable maps, by the development of a ridge pre-existing over the Black Sea and Asia Minor. On 1st October 2013 (Figure 5; Figure 6b), once the trough deepened the area of interest was engulfed by the cold air mass pertaining to the trough.

a.

b.

c. Figure 5. Geopotential field at 500 hPa on 30th September 2013, 00 UTC ECMWF model

valid on: a) 1st October 2013, 00 UTC, b) 1st October 2013, 12 UTC, and c) 2nd October 2013, 00 UTC.



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a.

Figure 6. Absolute topography at the level of 500 hPa, COSMO limited area model: a) 30th September, 00UTC and b) 1st October, 00UTC.

As it crossed Dobrogea and the western basin of the Black Sea, the geopotential trough from the level of 500 hPa, also remarkable at the level of 300 hPa, superposed the minimum within the low troposphere, the cyclone from ground level receiving support from the upper air, so that it developed and intensified (Figure 7).



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a.

b.

c. Figure 7. Geopotential field at 300 hPa of 1st October 2013, 00 UTC ECMWF model,

valid on a) 1st October 2013, 00 UTC, b) 1st October 2013, 12 UTC, and c) 2nd October 2013, 00 UTC.

Actually, large values of the potential vortex are remarkable over south-western Dobrogea, which pointed at enhanced whirling with associated stratospheric intrusion over the area, ascertained by the climaxing interval of the extreme phenomena (Figure 8) (Georgescu 2003).



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Figure 8. Potential vortex at 200 hPa, ECMWF model of 30th September 2013, 00 UTC,

valid on 1st October 2013, 18 UTC. The scenario proposed by the models was confirmed by the WV6 satellite images and the isotachs at the level of 300 hPa for the 29th September, 12 UTC – 2nd October 2013, 12 UTC (Figure 9) that highlight the evolution of the area with high values of the potential vorticity behind the cold front.

a) b)

c) d) Figure 9. WV6 satellite image and isotachs at the level of 300 hPa of 29th September, 12 UTC (a); 30th September, 12 UTC (b); 1st October, 12 UTC (c) and 2nd October, 12 UTC

(d) (http://eumetrain.org/eport/archive_euro).

Level of 850 hPa. According to the analysis of 30th September 2013, at the level of 850 hPa a minimum geopotential area is remarkable with a nucleus centred over northern Italy and a low-pressure corridor connecting the latter to the cyclone situated over



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Eastern Europe, whereas the Scandinavian Peninsula was under the strong influence of a high geopotential field. The thermal field points at a warm air mass over the Balkans and the south of the Black Sea, simultaneously with the presence of a cold air mass stationing over northern Europe.

For 1st October 2013 (Figure 10) the forecast was that the cyclonic nucleus would move eastwards, deepen and couple with the pressure low from north-eastern Europe, with cold air pumping on the front part of the Scandinavian high pressure system, which would have caused the thermal gradient to enhance.

a)

b)

c) Figure 10. Geopotential and temperature field at 850 hPa of 1st October 2013, 00 UTC– ECMWF model, valid on a) 1st October 2013, 00 UTC, b) 1st October 2013,

12 UTC, and c) 2nd October 2013, 00 UTC.



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Relative topography (TR 500-1000) highlighted a warm air mass over southern Dobrogea and the south of the Black Sea at the beginning of the interval, in contrast to the colder air mass packed over northern Romania. At the middle of the interval, the warm air expanded its area, covering the whole of the Black Sea and inducing wide contrasts with the cold air that had advanced to the western neighbourhood of Dobrogea. Further, until the end of the interval, the cold air mass engulfed Dobrogea and the warm one withdrew at sea (Figure 11).

a)

b)

c) Figure 11. 500 – 1000 hPa relative of 1st October 2013, 00 UTC – ECMWF model, valid

on a) 1st October 2013, 00 UTC, b) 1st October 2013, 12 UTC and c) 2nd October 2013, 00 UTC.



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Ground level. According to the 30th September analysis, the low-pressure field over the Mediterranean Sea covered its northern most part. The cyclonic nucleus was forecast to move eastwards and deepen over the Black Sea, with the pressure gradient becoming intense over Dobrogea at the contact with the north European high-pressure system (Figure 12).

a)

b)

c) Figure 12. Ground pressure field of 30 September 2013, 00 UTC– ECMWF model, valid on a) 1st October 2013, 00 UTC, b) 1st October 2013, 12 UTC and c) 2nd

October 2013, 00 UTC.



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Of interest for our country was the passage of a cyclonic structure of Mediterranean origin (with a high precipitable potential) that rapidly crossed the Balkan Peninsula on a zonal component, reaching southern Romania and the western basin of the Black Sea, further tending to retrograde. On 30th September, the pressure field at ground level was relatively high, especially in southern Romania (Figure 13a), decreasing by about 10 mb in the following 24 hours (Figure 13b).

a.

b. Figure 13. Sea-level pressure, COSMO limited area model a) 30th September, 00UTC, b)

1st October, 00UTC. Under those circumstances, both ECMWF (Figure 14) and COSMO limited area

model (Figure 15) forecast wide areas with wind speeds of up to 20 m/s over Dobrogea and the west of the Black Sea, which was further confirmed as regards both magnitude and duration.



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Figure 14. ECMWF 10-m wind of 2nd October 2013, 00 UTC, valid on 2nd October

2013, 00 UTC, ECMWF model.

Figure 15. COSMO limited area model 10-m wind speed 30th September 2013, 00

UTC, valid on 1st October 2013, 06 UTC.

With regards to precipitation as well, ALADIN and ECMWF models indicated signals for considerable precipitation amounts over our area of interest. ECMWF global model of 30th September 00 UTC (Figure 16) underestimated the accumulated precipitation amounts for the 30th September - 2nd October interval, suggesting no more than 60-70 mm throughout the mentioned spell.



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Figure 16. 6-hr. accumulated precipitation of 30th September 2013, 00 UTC, valid from 30th September 2013, 12 UTC to 2nd October 2013, 00 UTC, ECMWF model.



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At the same time, ALADIN limited area model (Figure 17) correctly estimated the accumulated precipitation amount, i.e. about 175 mm over the 30th September, 06 UTC – 2nd October, 06 UTC intervals.

Figure 17. Latest 12-hr. precipitation valid over the 30th September 2013, 06 UTC –

2nd October 2013, 06 UTC, ALADIN limited area model.

In the displayed situation, it must be remarked that the seawater had temperatures of 14-16°C, comparable to the maxima of 30th September 2013 and the minima of the night that followed, under overcast sky circumstances since before nightfall. Consequently, the thermal contrasts were not due to radiation of the night breeze but to thermal advection and convergence. Initially, the warm advection was significant in the altitude, which verticalized the thermal contrasts area above Dobrogea.

At ground level, the eastern wind direction allowed through enhanced friction in the sea-land contact area the occurrence of a first convergence source through speeds along the coast. This source was transferred in the altitude.

In a further stage, once the wind changed direction the cold advection began. Its area expanded, leading to an increase of the thermal gradients. Thus, the wind direction change produced the classical convergence, which was forced by the warm air barrage until the latter was dislocated. Both types of convergence were markedly cyclogenetic through their speeds and directions.



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Ground level analysis. Towards the end of September 2013 a cyclonic nucleus detached from a trough of the Icelandic Low and evolved over western and central Mediterranean. Thus, on 30 September, the mentioned nucleus moved eastwards while the monitored area was under the influence of the warm sector of the Icelandic low-pressure system. On that day the air pressure had values of 1015-1017 hPa. North of it the north European anticyclone was acting (Figure 18).

Figure 18. Actual processed map of 30th September 00 UTC: ground level pressure

(hPa) – black contoured isolines, geopotential (damgp) at 500 hPa – coloured bands, characteristic isohypse-red (http://192.168.5.18/misu/modele/homem1.html).

The VAD Wind Profile (Figure 19 and Figure 20) and Base Velocity (Figure 21) radar products very well shape the cold advection within the lower troposphere, where the presence of two low level jets and the relatively warm advection in the altitude are to be remarked.

On 1st October (Figure 22), the cyclonic nucleus reached southern Romania and the western basin of the Black Sea and deepened; the thermal and pressure gradients consequently amplified. That was also the interval when the wind speed reached the maximum values in the studied interval and the values of the pressure field decreased by about 10 mb in 24 hours.

On 2nd October (Figure 23), the whole north of the Black Sea experienced intense pressure gradient, owed to both the northeastwards motion of the cyclonic nucleus and to the southwards expansion of the high-pressure ridge, while the pressure values towards the pressure structure centre increased to 1030 hPa. It was the day when the hazardous weather phenomena gradually turned less intense (Corduneanu 2009; Frățilă 2013; Georgescu 2015).



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Figure 19. VAD Wind Profile (VWP 48) of 1st October 2013, 09.50 UTC.

Figure 20. VAD Wind Profile (VWP 48) of 1st October 2013, 17.32 UTC.



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Figure 21. Base Velocity V (27) product of 1st October 2013 – at 17.02 UTC RDMD.

Figure 22. Actual processed map of 1st October, 00 UTC: ground level pressure (hPa) –

black contoured isolines, geopotential (damgp) at 500 hPa – coloured bands, characteristic isohypse-red (http://192.168.5.18/misu/modele/homem1.html)



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Figure 23. Actual processed map of 2nd October, 00 UTC: ground level pressure (hPa) –

black contoured isolines, geopotential (damgp) at 500 hPa – coloured bands.

Warnings issued for intense wind and very rough sea. On the basis of the forecast materials partially displayed above, on 29th September 2013 (Figure 24), the National Meteorological Administration issued an orange code weather warning for precipitation (amounts in excess of 60 mm and sparsely larger than 80 mm), with a special mention for the wind gusts to exceed 80 – 90 km/h over an area that encompassed Dobrogea.

Figure 24. Warning message issued by NMA on 29th September 2013.



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On 1st October 2013, the warning message was updated (Figure 25).

Figure 25. Warning message updated a second time by NMA on 1st October, 2013

On 30 September, at 09.30 (Figure 26), RWFC Dobrogea issued a local warning for heavy storm at sea (no. 27/30 September 2013) valid from 30 September, 18.00 UTC – to 2 October, 15.00 UTC.

Figure 26. Warning message issued by RWFS on 30th September 2013.

This message was updated again on 2 October (Figure 27), valid until 4 October, at 21:00 hrs. The warning targeted the coast area and the west of the Black Sea. The maximum forecast wind force was 6-8 B on the coast and 7-9 B offshore, with gusts that could have reached force 10-11 B, whereas the roughness degree of the sea was forecast to reach 5 – B on the coast and 6 – 8 B offshore.

30th September 2013 Time: 9:30 No: 27 Affected area:

Coastal area and the west of the Black Sea Forecast phenomena: storm, very rough sea From 30th September 2013, 18:00 hrs. to 2nd October 2013, 15:00 hrs. the wind will blow from the E at first, then mainly from NE and N, enhancing, at speeds of 12-21 m/s (force 6-8 B) on the coast and 15-24 m/s (force 7-9 B) offshore, with gusts that may reach 25-32 m/s (force 10-11 B). The roughness degree of the sea will be 5-7 on the coast and 6-8 offshore. Notice: Depending on the evolution and intensity of the phenomena, this message will be updated.



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Figure 27. Warning message updated again by RWFS on 30th September 2013.

Ground level, forecast wind, rough sea. As early as 30th September models forecast strong wind over Dobrogea, to blow from the east at first, then from the north-east and north (Figure 28). Consequently, ECMWF estimated a wave height of 8 metres offshore (Figure 29) (Bondar et al 1972).

Figure 28. 10-m wind forecast of 30th September 2013, valid on 1st October 2013,

COSMO limited area model.

2nd October 2013 Time: 12:15 No: 28 Affected area:

Coastal area and the west of the Black Sea Forecast phenomena: strong gale, very rough sea

From 2nd October, 2013, 15:00 hrs. to 4th October, 2013, 21:00 hrs., the wind will blow from the N, at speeds of 10-14 m/s (force 5-6 B) on the coast and 13-17 m/s

(force 6-7 B) offshore, with gusts that may still reach 18-20 m/s (force 8 B). The roughness degree of the sea will mainly be 4 on the coast and 5-6, sparsely 7, at

the beginning, offshore.



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Figure 29. Wave and wind gust forecast, WAVE model, 30th September 2013, 00 hrs. run. Diagnosis of the warned elements. Maximum wind speed was 21 m/s/10 min and 28 m/s gusting speed, both recorded at Sulina weather station on 1 October, 18:00 UTC and offshore the maximum wind speed was 24 m/s/10 min and 28 m/s gusting speed. The sea roughness degree was 5 at the shore, i.e. up to 4-m high waves, which led to the total discontinuation of harbour activities throughout the duration of the meteorological warnings. Conclusions. The main characteristic of the pressure structures responsible for the occurrence of the extreme weather phenomena in the mentioned interval in Dobrogea was that it caused cyclonic activity of Mediterranean origin expanding as far as the western basin of the Black Sea. The ground level situation was favoured by the synoptic processes within the mean and upper troposphere, cyclogenetic processes being the result of clod ait penetration from the upper layer of the atmosphere.

On 30th September 2013, 30-40 mm of rain fell, which corresponds to the climatological norm for a whole month of September over Dobrogea. As regards October, the monthly climatological norm of 15-35 mm was exceeded approximately ten times in just the first two days of the month.

Whereas in the first and second decade of September the moisture reserve accessible to plants in the 0-20, 0-50 and 0-100 cm depths pointed at moderate drought and close to optimum supply at Adamclisi and Medgidia weather stations and at extreme drought at Tulcea station, all the mentioned stations reported moisture excess in the 21st September – 3rd October 2013 interval.

We may thus draw the conclusion that the fallen precipitation, though abundant and largely in excess of the climatological norm, benefitted the autumn agricultural works.

The unfavourable meteorological conditions induced by the analysed meteorological episode, warned against by the National Meteorological Administration through YELLOW and ORANGE CODE weather alerts disturbed the socio-economic activity; trees fell, basements and households were flooded and entire localities were left without electricity. These are just some of the effects of this severe weather episode.

However, probably the costliest economic effect was the complete interruption of the harbour activity on the duration of the warnings. Fortunately, both the magnitude and the duration of the hazardous phenomena were correctly forecast in the warning messages sent to all the economic entities within the responsibility area of Dobrogea RMC, so that no severe effects were reported.

The described phenomenon frames within the pattern of the cold season storms occurring at the contact between a cyclone of Mediterranean origin activated in the western basin of the Black Sea and a high-pressure system situated in the area of the Russian Plain.



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The phenomenon was exceptional not only through the intensity of the wind, the accumulated precipitation amounts and their time length but mostly through the time of the year when it occurred, i.e. at the beginning of October.

References

Bondar C., Rovenţa V., State L., 1972 [The Black Sea in the area of the Romanian

seacoast]. Hydro technical information and documentation centre of the Institute of Meteorology and Hydrology. pp. 15-29, 35-47, 49-55. [in Romanian]

Cordoneanu E., 2009 [Peculiarities of the air dynamics above Romania]. Publishing house Romania de Maine, Bucuresti, pp. 46-90. [in Romanian]

Frăţilă M., 2013 [Explosive cyclogenesis of 2-3 December 2012], The session of scientific communications, Bucuresti, pp. 3-11. [in Romanian]

Georgescu F., 2003 [Considerations on the use of the vortex and the isentropic potential vortex in the meteorological practice]. Bucuresti, pp. 21-25, 34-36. [in Romanian]

Georgescu F., 2015 [Adaptation of conceptual models of the cyclogenesis in the geographic area of Romania]. Publishing house Printech, Bucuresti, pp. 43-65, 69-90. [in Romanian]

*** www.ziare.com/01.10.2013. [in Romanian] *** http://eumetrain.org/eport/archive_euro Received: 20 July 2018. Accepted: 29 August 2018. Published online: 30 September 2018. Author: Angela Crăciun, National Meteorological Administration, Regional Meteorological Centre Dobrogea, Romania, e-mail: [email protected] This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution and reproduction in any medium, provided the original author and source are credited. How to cite this article: Crăciun A., 2018 A severe weather event in Dobrogea: September 30th - October 02nd, 2013. Ecoterra 15(3):32-53.

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