A SEVERE WEATHER EVENT IN ROMANIA DUE TO MEDITERRANEAN CYCLONIC ACTIVITY

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1 A SEVERE WEATHER EVENT IN ROMANIA DUE TO MEDITERRANEAN CYCLONIC ACTIVITY Florinela Georgescu, Gabriela Bancila, Viorica Dima National Meteorological Administration, Bucharest, Romania Abstract Mediterranean cyclones are responsible for the occurrence of most episodes of severe weather (heavy rains or snows, wind gusts) in the south and south-eastern part of Romania. The maximum intensity of this type of phenomena is reached when an anticyclonic field, often generated above the Russian Plain and extended as far as Central Europe, blocks the initial trajectory of a cyclone. The severe weather event of of November was selected to illustrate the above, with a special focus on the interpretation of water vapour satellite imagery (WV channel 5). Different satellite products were used in the nowcasting activity, to determine the aggravation degree in the weather evolution: IR channel 9 enhanced; RGB Composite (Airmass RGB) WV 6.2 WV 7.3, IR 9.7 IR 10.8, WV 6.2i; RGB Composite IR 12.0 IR10.8, IR 10.8 IR 3.9, IR 10.8; RGB Composite IR 12.0 IR10.8, IR 10.8 IR 8.7, IR 10.8 and RGB Composite NIR 1.6, VIS 0.8, VIS 0.6. INTRODUCTION The Mediterranean cyclones activity is a matter of considerable scientific interest and their spatial distributions and general cyclogenesis mechanisms have been analyzed by numerous authors, beginning with Petterssen (1956). Studies performed in the latest years emphasize the high spatial variability of weather conditions over the Mediterranean Sea and its immediate vicinity and the importance of analyzing meteorological events occurring in specific areas. (Trigo et al., 1999 and 2002, Radinovic, 1987, Flocas et al., 1996). The enhancement or re-enhancement of cyclogenesis in central and eastern Mediterranean is often forced by the upper troposphere dynamics (Flocas et al., 1996). According to Hoskins et al. (1985) conceptual model, if an upper level potential vorticity anomaly moves over a pre-existing low-level perturbation, their interaction generates the re-enhancement of low level cyclogenetic processes (Bluestein, 1993). Satellite images and products, mostly those using water vapour channels (WV) are of great help to forecasting and analysing these processes (Santurette and Georgiev, 2005). The event of of November illustrates the typical weather displays induced in the Romanian area by the activity of a Mediterranean cyclone in the cold season. In the mentioned interval, precipitation (in significant amounts especially in the south-west of the country) mostly fell as rain, but there was also short-lived snow, associated with hard wind gusts. The Mediterranean cyclone had been generated in Genoa Golf, and its evolution eastward was sustained by the upper level dynamics. DATA AND METHODS The synoptic and mesoscale analysis of the event was performed, using the ECMWF numerical model, the ALADIN limited area numerical model, observed data and the satellite images and products: WV 6.2, IR channel 9 - enhanced, RGB Composite (Airmass RGB) WV 6.2 WV 7.3, IR 9.7 IR 10.8, WV 1

2 6.2i; RGB Composite IR 12.0 IR10.8, IR 10.8 IR 3.9, IR 10.8; RGB Composite IR 12.0 IR10.8, IR 10.8 IR 8.7, IR 10.8 and RGB Composite NIR 1.6, VIS 0.8, VIS 0.6. SYNOPTIC AND MESOSCALE EVOLUTION The severe weather episode of the November 2007 was caused by the activity of a Mediterranean cyclone in the proximity of Romania, developed along the whole of the tropospheric column (fig.1). Figure 1: Geopotential and temperature at 300 hpa ECMWF numerical model, on , 00 UTC, analysis The events were considered worth analysing from the night of 15/16 November, when the development of the cyclone, generated in the Gulf of Genoa, was almost vertical, suggesting a maturity phase and the nearing of the oclusion phase. Figure 2 : Potential vorticity at 300, ECMWF numerical model, , 00 UTC 2

3 However, the geopotential distribution in the upper troposphere displayed strong gradients and an intense north-easterly circulation, whereas the potential vorticity field at the level of 300 hpa (fig.2) and of the relative vorticity at the level of 500 hpa also pointed at continued advection from the same direction As a result of the temperature and vorticity advective processes, the cyclone continued to develop and evolve, coupled with the ridge of the Azores High, expanding towards Central Europe. The vortex formed by the cloud systems associated to the cyclone is markedly visible in the satellite images. In the water vapour image (fig.3) the vorticity advection is also visible in the upper part of the cyclone (the black band bending cyclonewise in the vortex), whereas in the RGB image (WV6.2 - WV7.3, IR9.7 - IR10.8, WV6.2i), the different characteristics of the air masses are also noticeable: warm air in France, the Iberian Peninsula, northern Africa, delimitating the vast area with cold mass and vorticity penetration in the central-southern part of Europe and over the western Mediterranean basin (fig.4). Figure 3: METEOSAT 9, WV6.2, , 00 UTC Figure 4: METEOSAT 9, RGB (WV6.2 - WV7.3, IR9.7 - IR10.8, WV6.2), , 00 UTC In the morning of the following day ( , fig. 5), the advance of the depression area is clearly noticeable, with the generation of a second nucleus in the low layers, eastwards, so that the vertical axis of the whole system was now inclined to the north-west, which was a sign of the re-enhancement of cyclogenetic processes. The analysis of the water vapour images, compared to the ECMWF model 3

4 outputs allows recognizing significant features of the flow in upper levels. Gradients in the upper troposphere were very strong and the vorticity advection continuous. The wind at 300 hpa was remarkable; it can be assimilated to the jet stream, very intense and bent towards Romania, displaying an undulation in the area where a second cyclonic nucleus had already emerged in the low levels (figs. 6 and 7). At ground level the wind was very intense in the area of the cyclonic nuclei (not shown). Figure 5: Sea-level pressure and temperature at 850 hpa, ECMWF numerical model, , 00 UTC, analysis On the other hand, the ridge of the Azores High went on expanding eastwards, advecting polar continental mass on its front side and generating a blocking circulation. Figure 6: METEOSAT 9, WV6.2, , 00 UTC The jet stream is easily recognizable in the water vapours image, in comparison to the map of the wind at 300 hpa. The jet streak, situated on the descending side of the trough, caused it to enhance in the hours that followed. In the same image, the dark zone from the southern part of Italy corresponds to the warm and dry air, and the anomaly of potential vorticity is situated west of Sicily. The filaments of relative vorticity at 500 hpa point at the marked baroclinicity in the area, over which the potential vorticity anomaly superposed (in agreement with the numerical model, not shown). Taking into account the evolution up to 4

5 that moment of the cyclone generated in the Gulf of Genoa, we may consider that the blocking circulation is a result of the secondary cyclone generation. Figure 7: Wind at 300 hpa, ECMWF numerical model, , 00 UTC, analysis During the latter part of the night of 17/18 November, the two-nucleus cyclonic area was located in the eastern Mediterranean basin, and in the Romanian area circulation had become intensely easterly, at the contact with the high pressure field. Further, the ground-level cyclones found correspondence and support along the whole of the tropospheric column. Figure 8: Relative vorticity at 500 hpa, ECMWF numerical model, , 00 UTC On the ascending side of the trough, cloudiness organized in a cold altitude front, whose position is also indicated by the relative vorticity at 500 hpa (fig.8). The front moved along a somewhat meridianal direction (blocked by the ridge situated over Asia Minor and Black Sea), towards the south of Romania, 5

and the contact with the high relief of the Balkan Peninsula generated deep convection, visible in the IR enhanced (fig.9). Figure9: METEOSAT 9, IR10.8 enhanced, 18.11.

6 and the contact with the high relief of the Balkan Peninsula generated deep convection, visible in the IR enhanced (fig.9). Figure9: METEOSAT 9, IR10.8 enhanced, , 00 UTC The nighttime convection (fig.10), analysed with RGB composite IR 12.0 IR10.8, IR 10.8 IR 3.9, IR 10.8, displayed the presence of very cold Cumulonimbus clouds (with temperatures under -65 C, according to the IR 10.8 enhanced image). Figure 10: METEOSAT 9, RGB (IR 12.0 IR10.8, IR 10.8 IR 3.9, IR 10.8), , 00 UTC: the very cold Cumulonimbus clouds are represented by the yellow-greenish dots (also in southwestern Romania) In the 24 hours that followed, the axis of the altitude trough bent and the cloud band on its ascending side curved in a cyclonewise. Throughout 18 November, analysis of the cloud systems was performed with the RGB composite IR 12.0 IR10.8, IR 10.8 IR 8.7, IR 10.8 and RGB composite NIR 1.6, VIS 0.8, VIS 0.6 products (figs. 10, 11). Vertically developed, compact cloudiness over the south of the country clearly differentiates, in both images, from the high, thin cloudiness in the north. On that same day, circulation 6

7 became prevailingly easterly, at both ground level and in the altitude. In the area of Romania, at ground level, the front took stationary aspect, practically in alignment with the isobars. In the southern part of Romania precipitation started in the evening of 16 November, and during the night that followed the wind became easterly and it gradually strengthened. The maximum amplitude of the phenomena occurred during 18 November and during the following night. The branch of the jet stream in the high levels south of Romania is visible in the WV 6.2 images as a strong humidity gradient (nuances from black to light grey, not shown). During the night of 18/19 November, the jet was maximally close to the southern part of the country. Figure 11: METEOSAT 9, RGB (IR 12.0 IR10.8, IR 10.8 IR 8.7, IR 10.8), , 12 UTC Figure12: METEOSAT 9, RGB (NIR 1.6, VIS 0.8, VIS 0.6), , 12 UTC At ground level the wind took speeds of m/s, especially in southern Walachia (Bucharest included). Cross sections performed with the ALADIN limited area model show the presence of the low tropospheric jet, a structure characteristic to winter storm situations (figs. 12). 7

Figure 12: Vertical cross section for wind, centred on Bucharest, ALADIN numerical model, analysis, 18.11.

The abundant precipitation and the sustained wind gusts claimed attention from the meteorologists, especially in the nowcasting activity.

8 Figure 12: Vertical cross section for wind, centred on Bucharest, ALADIN numerical model, analysis, , 06 UTC CONCLUSIONS The November episode is one of the most severe of the 2007/2008 cold season in Romania. The abundant precipitation and the sustained wind gusts claimed attention from the meteorologists, especially in the nowcasting activity. Using simultaneous analysis of satellite images and numerical models, the authors were able to capture additional elements towards understanding the dynamic processes occurred during the event. It could thus be explained how a cyclone, although undergoing its maturity phase in the Central Mediterranean, was revived from upper levels forcing, the processes thus initiated generating the expansion of the cyclonic activity towards the east of the Mediterranean. The presence of the surface low in the proximity of the strengthening high-pressure level generated the emergence of strong pressure gradients, which made the wind to enhance, with the formation of the low level jet. Although Romania does not lie in an important cyclogenetic area, the analysed episode proves that the Mediterranean cyclonic activity, supported by the upper troposphere dynamics, is capable of generating severe weather episodes, especially in the southern part of the country. Satellite images play a determining role in monitoring and further analysing such events. BIBLIOGRAPHY Bluestein, H., (1993) Synoptic- Dynamic Meteorology in Midlatitudes, Oxford University Press, pp Flocas, H. A., Karacostas, T. S., (1996) Cyclogenesis over the Aegean Sea: Identifications and synoptic categories. Meteor. Appl, 3, pp Hoskins, B.J., McIntyre, M.E., Robertson, A.W. (1985) On the use and significance of isentropic potential vorticity maps. Quart. J. Roy. Meteor. Soc., 111, pp Santurette, P., Georgiev, C., (2005) Weather Analysis and Forecasting. Applying Satellite Water Vapor Imagery and Potential Vorticity Analysis. Elsevier Academic Press, pp Radinovic D., (1987) Mediterranean Cyclones and Their Influence on the Weather and Climate. PSMP Report Series, No. 24, WMO, 131 pp Trigo IF, Davies TD, Bigg GR (1999) Objective climatology of cyclones in the Mediterranean region. J Clim 12, pp Trigo, I.F., G.R. Bigg, Davies, T.D., (2002) Climatology of Cyclogenesis Mechanisms in the Mediterranean, Mon. Wea. Rev., 130, pp

THE LOW-LEVEL JET FOR BUCHAREST S AIRPORTS - A STUDY OF ITS CHARACTERISTICS IN WINTER SEASON BETWEEN 1959 AND 1982

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