Gueiko, Ludmila (State Committee for Hydrometeorology, Kiev, Ukraine)
INTRODUCTION.
The weather conditions of the Ukrainian territory have been depended on many circulation processes over the Europe and Asia. The geographical position of the country and atmospheric processes have caused a grate variability of climatic conditions and sufficiently grate frequency of hazardous weather phenomena. Some times these phenomena (heavy rainfall, snowdrift, strong winds, etc.) become as catastrophe and have produced serious damage.
As a rule the hazardous weather phenomena in Ukraine are conditioned by travel of southern or south-western cyclones on its territory. So every fourth case of the hazardous weather phenomenon and every second case of the storm wind call forth by Mediterranean cyclones.
DATA AND RESULTS
The present research is devoted to problems of the main signs of travel of Mediterranean cyclones on the Ukrainian territory. The analysis of difference hydrometeorological information is very useful for such research.
That's why the annual technical reviews of hazardous weather phenomena, the meteorological monthly data, the climatological reference books, the contour and thickness charts, significant weather charts have been used for this research. It is necessary to take into consideration the following factors for successful forecast of Mediterranean cyclone entrance to the Ukrainian territory. The Mediterranean cyclones travel to the south-eastern part of the Europe when the upper-level barical field becomes meridional. The majority of Mediterranean cyclones travel along the jet stream axis that oriented from Adriatic Sea to Ukraine. The cyclones that formation over the northern part of Italy travel to the north-east across the Western Carpathians when the divergent flow takes place over the Middle-Danube low-land. The western jet stream with flow divergence spreads to Balkan Peninsula in the middle troposphere over the Low-Danube low-land before the cyclone entrance to the Black Sea. The centers of cyclones situate under eastern part of upper-level trough in 59% cases, on the axis of trough in 22% and in 4% - under its western part day before their entrance to the Ukraine or the Black Sea.
The decrease of pressure over the Adriatic Sea and Balkan Peninsula and sharpening of latitudinal frontal zone over the Italy is the sign of cyclone travel to the Black Sea. The contrast of air temperature of Europe-Africa more then 5-7 C is the indicator of front activity.
As a rule the isohypses orientation of contour chart 700 hPa give the possibility to determine the direction of cyclone travel. It is possible to distinguish five main types of synoptic situations of Mediterranean cyclones travel to Ukraine that depend on upper-level trough location:
- across the Danube mouth to the north; - across Carpathians to the north; - across Aegean Sea and straits to the north-east; - across Ionic and Aegean Seas, Turkey to the east of the Black Sea; - stationary of cyclone over the eastern part of the Black Sea on the south-western periphery of the blocking continental anticyclone.
Majority of the Mediterranean cyclones move with a grate traveling speed - 40-65 km/h. They slow off the travel near their trajectory turning-point. These cyclones travel over the Ukrainian territory during 18 hours in average. The intensity of the cold invasion to the cyclone rear, the spreads of cold to its southern periphery determine the degree of distortion and bend of cyclone trajectory to the north or north-east. The geographical position of the cold advection area, the trajectory turning-point and the active warmth curries out have determined the future evolution of cyclones. The synoptic situation over the eastern part of Europe does not give exactly the essential signs of approach Mediterranean cyclone day before its entrance. The meteorological data have shown the signs of wind intensification connected with cyclone entrance no earlier than 12 hours before.
The atmospheric pressure gradients in this time over the Ukraine increase in two times in average compare to gradients day before. The change of difference of surface pressure between observational points Synop-Odessa and Kerch-Istanbul can be the indicator of the cyclone entrance. These differences exceed 5 hPa, some time they increase to 20-24 hPa before cyclone entrance.
The data of radiosonde observations in the southern regions of Ukraine give the possibility to determine travel of cyclone to the north:
- the strong southern flow in the low troposphere is the sign of cyclone trajectory; - the south-western flow over the southern regions of Ukraine slows down the cyclone travel that entry to the
Black Sea and maintain the storm weather over the south of Ukraine.
Only Mediterranean cyclones have property to be stationary over the Ukrainian territory in the warm period of year. As a rule the beginning of their stationary is observed at the period of their maximum development.
CONCLUSIONS
The Mediterranean cyclones are pretty difficult for forecasting. In some case they get the development and travel to the Ukrainian territory but in the other case they end the existence and stay nearly on the same place when the thermodynamic conditions are the identical. Therefore it is very necessary to make careful analysis every case of these cyclones and to elaborate the definite recommendations, to bring to light the essential factors that determine of the cyclones evolution before and after their entrance to Ukrainian territory. The above mention recommendations can promote to increase the accuracy of cyclone warning and weather advisory in Ukraine.
Reference
1. The hazardous weather phenomena in Ukraine and Moldavia. Under edition V.Babichenko.Leningrad.
Hydrometeoizdat. 1991. 224 p. 2. N.Belskaya. The southern cyclones and their traveling on the territory of the USSR. Leningrad.
Hydrometeoizdat. 1949. P. 64-113.