Variations in bora weather type in the north Adriatic Sea,

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1 ~ (1979b) ~ (1979~) ~ (198a) ~ (198b) ~ (198~) Weather Vol. 55 December 2 just winds, temperature and rainfall but, for example, derivatives such as trafficability and parachuting weather. At the start of the century the planners were not too sure just how hot or cold it could be in different parts of the world. As technology progressed, the demand for such information increased and this led to the formation of the most interesting Branch of the Meteorological Ofice that would deal with Special Investigations, in particular military and aviation climatology. Army Air Corps Once upon a time, as an airman meteorologist, I assumed that the only real pilots were RAF types and anyone flying for the army must be a renegade or on a punishment posting. But that was before I met the occasional army flyer who seemed to know which way up an aeroplane should fly. Then, about 197 1, Alan Turner was posted to Middle Wallop as the Senior Meteorological Officer, charged with raising the standard of meteorological training of army flyers. I was in charge at RAF Andover just up the road where a short meteorological refresher course was included for aircrew training to fly light transport aircraft. As neighbours Alan and I compared notes, with the result that, after many discussions and some tricky typing, we produced a Pocket guide to aviation meteorology in In early 1998 the basic text was supplied to the current helicopter training establishment to help them produce a sixth edition of the guide - the Army Air Corps have been taking the weather seriously (if only we had received royalties!). Since the Napoleonic Wars the army has become increasingly mobile and technical, and is arguably more dependent on the weather than ever before. References Anon. (1 9 17) Minutes of the Boceedings of the Meteorological Committee HMSO Cotton, H. (1979a) Memoirs of an army meteorolo- gist. Part 1. Meteorol. Mag., 18, pp Part 2. Meteorol. Mag., 18, pp Part 3. Meteorol. Mag., 18, pp Part 4. Meteorol. Mag., 19, pp Part 5. Meteorol. Mag., 19, pp Part 6. Meteorol. Mag., 19, pp Douglas, C. K. M. (19 16) Weather observation from an aeroplane. J. Scott. Meteorol. SOL., 17, pp (also later versions) Meteorological Office (1925) Annual Report of the Meteorological Office, HMSO Correspondence to: h4r M. E. Crewe, 36 Trevelyan, Bracknell, Berkshire, RGI 2 8YD. Variations in bora weather type in the north Adriatic Sea, A. Menezes and M. Tabeaud Universite Paris I and Unite Mixte de Recherche. France In the north Adriatic Sea, the bora is a wind sonal variations in the western Mediterranean which blows from the north or north-east. The atmospheric circulation. During the cold word bora comes from buro, which is a term season, between October and March, the bora for cold wind in the Slovenian language, or (from the north-east) is the most frequent wind boreas, Latin for a north wind. During any and blows continuously for between two and given year, north and east winds are the most eight days. During the hot season, between prevalent, but their frequency depends on sea- April and September, calm periods increase 452

2 Fig. 1 Location of the meteorological stations used in the study but bora or scirocco (a southerly wind) gales can occur. For the characterisation of the synoptic bora winds in this research, we examine only winds from between 4 and 7" blowing for at least 12 hours because winds from that direction are not necessarily bora winds. Some short-lived winds with a north-easterly component are associated with frontal disturbances. Others are local winds, such as land-breezes. To identify bora synoptic situations in the north Adriatic, we used daily sea-level pressure charts between October 1866 and March 1998, and the daily topography of the 5mbar surface from January 1945 to March To validate this method, the synoptic situations defined from the different meteorological bulletins have been compared with thrice-hourly anemometer data for from Trieste, Tessera (Venice), Falconara (Ancona), Bari, and Leuca (Fig. 1). In the north Adriatic a comparison between days of north-east wind over the period recorded by anemometer at Trieste and Venice produced a good comparison (79 and 92% respectively). The high incidence in the north Adriatic enables us to extrapolate with confidence before the anemometric recording period. Fig. 2 Sea-level (mbar) and 5mbar (dam) charts for (a) anticyclonic (Aa) and (6) anticyclonic (Ab) bora. Heavy solid line indicates the normal pressure at sea-level and the normal height of the 5 mbar pressure level. (From Yoshino 1976.) 453

3 Definition of the bora atmospheric different Italian names - chiara and scura circulations respectively. For the northern Adriatic, Yoshino (1976) recognises four sub-types Yoshino (1976) has suggested distinguishing according to the synoptic pattern (Figs. 2 anticyclonic from cyclonic bora, which have and 3). Fig. 3 Sea-level (mbar) and 5mbar (dam) charts for (a) cyclonic (Ba) and (b) cyclonic (Bb) bora. Heavy solid line indicates rhe normal pressure at sea-level and the normal height of the 5 mbar pressure level. (From Yoshino 1976.) 454 a Jan. Feb. Mar. Apr. May. June July Aug. Sept. Od. Nov. Dec. [ + Trieste + Tessera - Falconara 4- Bari -+ Leuca I Fig. 4 Moirthlyfrequency ofbora at Trieste, Enice (Tessera), Ancona (Falconara), Bari, and Leuca,

4 Anticyclonic situations (A) Table 1 Monthly number of bora synoptic situations during the cold season, North-west Europe and the eastern Atlantic are under the influence of high pressure which sustains north-east winds of 1 to 15kn (5 to 8ms-') from Hungary to Italy. At 5mbar when there is a ridge over the British Isles and a trough over the Balkans, at sea-level the high is over the north of France (Aa, see Fig. 2(a)). When the whole of western Europe lies under a planetary trough, the high extends to central Europe (Ab, see Fig. 2(b)). Cyclonic situations (6) At sea-level there is a col with two areas of high pressure, one over the Atlantic and the other continental, as well as two areas of low pressure (over the Mediterranean Sea and over Iceland). According to the size of the planetary trough over the west of the Mediterranean Sea (Bay see Fig. 3(a)) and its evolution in the cut-off cold pool (Bb, see Fig. 3(b)) the north-east wind in the Adriatic gives cloudier, rainy weather. Month Total Aa+Ab Ba+Bb Oct Nov Dec Jan Feb Mar Total the Northern Hemisphere, it is the month of the most frequent meridional flow in Europe. Cyclonic situations have the same yearly disuibution as the whole of the bora situations, while anticyclonic situations reach their highest point during the month of December (Table 1). Year-on-year variability The average annual number of bora situations is 11.1 (Table 2), but from year to year the variability is very high. Some cold seasons have only.6 days and others 35.2 days (Fig. 6). A synoptic wind of the cold season In the central Adriatic (Falconara), a northeast wind is observed in only 34.4% of the bora synoptic situations and even less at Bari (12.4%) and Leuca (19.5%) in the south. The bora are the dominant winds in the north Adriatic, but their frequency decreases in the central and southern parts of this sea (Fig. 4). The monthly frequency of the wind recordings shows the decrease of the bora during the hot season between April and September at Trieste and Venice (Fig. 4). The shape of the graph remains the same whatever the wind speed. Table 2 Annual frequency of bora situations during the cold season (October-March) Bora Aa+Ab Ba+Bb Mean Standard deviation Since 1866, four periods have a higher than average occurrence of bora: , 198-1, (except 1941), and (Fig. 6). Except for the beginning of the period, the annual variability depends more on cyclonic situations than anticyclonic situations (Fig. 7). There is no distinct trend. Frequency of the bora during the period Monthly distribution during the cold season Over the period we counted 1599 days with bora cyclonic situations (Ba+Bb) and 132 bora anticyclonic situations (Aa+Ab) (see Table 1). The highest frequency is in January, followed by December and February (Fig. 5). As January is the coldest month of the year in Relation between the bora and the variation in atmospheric circulation over the Atlantic Ocean and Europe Numerous researchers have demonstrated that the greatest variation of planetary-scale flow over the Atlantic Ocean and Europe takes place in winter (December-March) (Houghton et al. 1995; Hurrell 1995). The North Atlantic Oscillation (NAO) index is calculated from the 455

5 s 1 f: OSL g osz 4 - : O K OSE

6 I 2 n % 15 - m c. c. 1 5 Fig. 7 Annual total of bora situations (anticyclonic or cyclonic) during the period in the north Adriatic.6 u NAOlBora -A- NAO/anticyclonic Bora ++ NAOlcyclonic Bora 1 Fig. 8 Correlation between North Atlantic Oscillation (NAO) and bora for a 2-year period (step 5 years) 19s and 195s, and negatively correlated (-.55) in the 197s (Fig. 8). The correlation between NAO and anticyclonic bora is negative (-.44) in the 189s and 193s (Fig. 8). An oscillation is present but there is no trend. Hurrell (1995) suggests that the meridional circulation (negative NAO) causes a northerly flow over Europe. But the meridional flow over western Europe is made up not only of polar cold air domes but also southerly hot tropical thrusts (scirocco). Fragmentation of the zonal westerlies during negative NAO periods creates airmasses and temperature contrasts in an east-west direction. A slight shift in the planetary trough can generate either southerly or north to north-easterly winds over the Adriatic 457

7 Sea. This accounts for the correlation between the NAo and bora numbers. Conclusion The synoptic winds called bora have had a high variability during the twentieth century in the north Adriatic Sea. They show a strong correlation with NAO at the beginning and the middle of the century, but overall we cannot see any change. The observed incidence seems to be independent of global warming. As there is not always a significant correlation with NAO, further studies should concentrate on the purely meridional flow which consists of both the bora and the sirocco. References Houghton, J. T., Meira Filho, L. G., Callander, B. A., Harris, N., Kattenberg, A. and Maskell, K. (Eds.) (1995) Climate change 1995: The science of climate change. Contribution of Working Group 1 to the Second Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press Hurrell, J. W. (I 995) Decadal trends in North Atlantic Oscillation: regional temperatures and precipitation. Science, 269, pp Yoshino, M. M. (1976) Bora in Trieste, Italy. Tokyo University Press Correspondence to: Dr A. Menezes and Dr M. Tabeaud, Universite Paris Pantheon Sorbonne, Institut de Geographie, 191 rue de Saint Jacques, 755 Paris, France. World Meteorological Day, Thursday 23 March 2 - a special 5th anniversary occasion David Axford Stanford in the Vale, Oxfordshire As I flew into Geneva Airport for the first time for a couple of years I had a twinge of nostalgia at the sight of Mont Blanc in the distance and the villages of Nyon and La CBte on the borders of Lake Geneva below. I had enjoyed my six years living and working in Switzerland, and now I was invited back to a reunion with old friends to celebrate the 5th anniversary of the World Meteorological Organization (WMO). Every year on 23 March the United Nations (UN), and in particular WMO, which is one of its Specialised Agencies and was established on this day in 195, celebrates World Meteorological Day (WMD). This year WMD was also dedicated to WMO s 5th anniversary with the theme WMO - 5 Years of Service. WMO is the UN System s authoritative voice on the state and behaviour of the earth s atmosphere, its interaction with the oceans, the resulting climate, and the consequent distribution of water resources. WMO has been recognised as the original networker. Weather and climate do not recognise political or economic boundaries, and the strength of WMO is based on the 185 nation states and territories which make up its membership and their unflinching commitment to contribute voluntarily to the scientific and operational work of the Organization. I was fortunate to have been invited by Professor G.. P. Obasi, the Secretary-General (SG) of WMO, as one of the principal guests who had worked with him during his period as SG. I moved from The Met. Office to Geneva in October 1989, and lived there with my wife until the end of May We enjoyed living in the Swiss countryside just outside Geneva, and the experience of working in such a multicultural organisation. On arrival at Geneva Airport I was whisked away to Gex on the edge of the nearby Jura Mountains where I was staying with friends in their newly constructed house. Although I was staying in France there was no problem in entering and returning from Switzerland through the local small frontier post, which is only manned on random occasions during the week. That Wednesday evening I was invited to a party in the home of Professor Obasi and his lovely wife, where I joined many of the other guests for the occasion. Amongst those present were Dr John Zillman (Australia), the current President of WMO, Dr R. L. Kintanar (Philippines), an ex-president from 1979 to 1987, all the five ex-deputy Secretary-Generals (DSGS) 458