Karpova, Olga (Department for Intergovernmental Cooperation, State Committee of Ukraine for Hydrometeorology, Kiev, Ukraine)
The study of southern cyclones is to represent a great interest for forecasters. The main features of macrosynoptic processes related to southern cyclones penetrating into the territory of Ukraine are being considered. Analysis of surface and contour charts for 15-years period and annual review on severe weather has been done. Some particular attention has been paid to hazardous weather phenomena occuring with southern cyclones.
Cyclones,that occurred on the Polar Front in subtropical latitudes between 30 - 45 latitudes then moving eastward or north-eastward and penetrating into middle or high latitudes, called as southern.
Three main groups of southern cyclones depending on a direction of their trajectories are distinguished.
Cyclones that relocated from west of the Mediterranean Sea through Hungarian lowland, the Danube delta, west part of the Black Sea and further move to north or north-east, belong to the first group. Cyclones that relocated from central part of the Mediterranean Sea move north-eastward over the eastern part of the Black Sea, especially as spring advances, belong to the second group. Cyclones, which are filled in over the Black Sea not entering the territory of Ukraine or having transferred through the Caucasus mountain ridge belong to the third group. It should be mentioned that such division is quite conditional. Only such southern cyclones which typical for synoptic processes in conformity with abovementioned definition are being considered.
In cases, when a penetration of southern cyclones into the territory of Ukraine is observed, sharply expressed meridional air flow over Europe is marked. Warm ridge of high pressure above the north-eastern part of Atlantic ocean and cold upper trough over the central part of Europe is available. Axis of high-altitude warm ridge is directed lengthways of 20 W longtitude. Second, less developed ridge locates above south of central region with an axis taking place along 50 E longtitude. Between these high-altitude ridges, precisely expressed trough with an axis, tends to be located between longtitudes 10 - 20 E. In these cases planetary high-altitude frontal zone differs sharply from it average long-term location. Western ridge is expressed more weakly and located easterly at the map of average long-term values. Eastern ridge is not evident. Trough between them is expressed weakly. Availability of powerful high-altitude ridge, located over the eastern part of Ukraine is a distinguish feature of penetration of southern cyclones into the territory of Ukraine.
In such situations an increasing of pressure over eastern part of Ukraine and fixing of low-pressure area above the Black Sea is observed. Sometimes, penetration of southern cyclones into the territory of Ukraine and anticyclogenesis above central part of Eastern Europe is occurred simultaneously. Cyclone above the Black Sea and southern part of Ukraine tends to remain as stationary. Easterly wind velocity increases considerably.
Meridional fronts leads to originating of such cyclones have a powerful cumulonimbus cloud system, associated with squalls occurring. Thermal advection over eastern part of Ukraine and invasion of cold air through Europe to west part of the Black Sea and the Dardanelles are to be a precondition that cyclone has an anomalous track.
Cyclone becomes a high-level but near the surface, cyclone center moves slowly along Carpathian to north-west.
Southern cyclones are associated with transferring humid, unstable and stratified air. Southern cyclones are moving from the Black Sea eastward by western periphery of continental anticyclone and occurred across the territory of Ukraine to make a major contribution to the snowfalls, storm wind, sleet, blizzard conditions in winter, heavy rain in autumn, dust storms in spring.
Penetration of southern cyclone on 29 - 30 September 1995 and related to it dangerous weather phenomena occurred in Ukraine are described below.
Upper-level cyclone became more active (P = 1005 hPa) above Greece northerly Athens on 29 September 1995 at 00 UTC. Cyclone located under active frontal zone, which passed through the Mediterranean Sea, Greece then Ukraine central part of Russia and further to Northern Urals. Contrast of temperature was 8-12 /500 km at 850 hPa chart. Central cyclone at 500 hPa located northerly Kola Peninsula, axis of low was oriented meridionally. Active advection of cold air from north started above the Adriatic Sea on the night of 30 September. Air temperature decreased from 11-12 C up to 4 -5 C at 850 hPa above Italy and the Balkan Peninsula. From this moment on cyclone started to move northerly lengthways upper frontal zone through Bulgaria to south of Ukraine. In 30 September from 02 to 15 UTC the cyclone began to deepen and pressure in the center had dropped up to 10 hPa. Velocity of cyclone increased up to 80 km/h. Cyclone moved to the central part of Ukraine. Then it velocity got down up to 40-50 km/h. In the evening of 30 September at 21 UTC cyclone moved to the territory of Belorus. At the moment of the least distance between isallobaric pair (from 11 UTC to 14 UTC), the pressure gradient increased sharply from 1 to 8 hPa/111 km along the line Uman - Kiev.
At the satellite picture on 29 September 1995 (03 UTC), the active atmospheric vortex was observed above southern part of Balkan peninsula and Turkey. Cumulonimbus, cumuli, stratocumulus and small amount of cirriform clouds were visible. More bright and powerful cloudiness was observed over Greece. There were no found cirriform clouds extended to north or north-east. However, clearly identified at the picture cold clear-cut front indicated that cyclone tends to move northeastwards. Further cloud masses joined to frontal band extended from south of Italy through Ukraine to Moscow. The front system of southern cyclone, in zone of cold front and occlusion front especially, moving from south-west across the territory of Ukraine, brought very heavy rain over much of northern and central part Ukraine, where totals of 50 to 70 mm in the 24 hours on 30 September were reported, wind speed exceeded 15-20 m per sec, in southern part up to 25-30 m per sec.
Heavy rain and strong wind hit a number regions of Ukraine on 30 September, downing powerlines, causing considerable damage to more than 200 buildings, roads, large areas of agricultural crops and trees.
References:
1. T.Busyan, N. Gavrilenko: Some data on cyclones, passing through the territory of Ukraine, Proceedings of UkrHMRI,1976,p.136-143 2. M.Leonov: Thermobaric field and vertical current in southern cyclones, Proceedings of UkrHMRI,1959,p.58-69 3. L.Prokh: Investigation of hurricans,storms, tornadoes,squalls in Ukraine,Proceedings of VNIIHMI MCD, 1979 4. Annual Review on severe weather in Ukraine, UkrHydromet,1995