Introduction

This case study describes a weather episode, which is typical for eastern and northeastern Europe in summer.

The most common track for the low pressures reaching the Scandinavian and Baltic countries is from the Atlantic, from southwest or west. Yet these countries regularly experience cyclone passages either

• From the northwest (the Norwegian and the Arctic Sea). This area is a typical source for wintertime cyclones bringing snowstorms and heavy winds. (For example, see Case Study 29 - 30 January 1998 ).
• Or from south-southeast (the Balkan region and the Black Sea region). Cyclones generated over this area have characteristic features that are dealt within this case study. Among the meteorological community a common term "Black Sea low" (henceforth referred to as "BS lows" is used) even if the origins of the low pressure might not necessarily be exactly over the the Black Sea, but more generally over the southeastern Europe.

BS lows develop, when the upper trough over Central Europe has an extension to Black Sea and Anatolia peninsula, meanwhile a stationary surface anticyclone strengthens over Northern Russia and Fennoscandia. These cyclones have very meridional trajectories and in some cases are even bend westward in the Baltic region. A meridional jet stream can be found on both edges of the amplified upper level trough. The eastern jet stream is found on the upper troposphere.

BS lows can develop in two ways:

• They can be re-generated low pressures which travel from the Mediterranean countries northeastwards, possibly beginning already to fill up, but then starting to deepen again, taking a turn towards north or northeast. This type of development takes place most often in late summer and early autumn.
• Alternatively the low pressure can be formed along the (stationary) front which extends from southeast Europe towards north or northwest. This stationary front is located on the leading (eastern) side of a extensive upper level low, which may be either at its tear-off or cut-off stage. Sometimes this stationary front can be non-developing for days, with weak waves travelling along the front not deepening.

It has to be kept in mind, however, that the waves being "weak" does not mean that the accompanying weather would be weak. The stationary or semi-stationary front represents a distinct air-mass boundary with very distinct changes in temperature and wind and along which the accumulated precipitation amount due to prolonged rainfall can be very high.

A typical configuration of main conveyor belts, surface fronts and accompanying weather is shown on the images below.

In Scandinavian area the forecasting of the weather pattern of the BS lows generally is more tricky than of the Atlantic low pressures. This can easily be understood by considering the orientation of Cold and Warm Conveyor Belts within a BS low. Typically the conveyor belts associated with a BS low are oriented according to the schematic image above left. It can easily be seen that due to the direction of the conveyor belts (cold conveyor belt stretching southwards from the arctic areas and Warm Conveyor Belt extending northwards from warm south-eastern Europe) the air-masses can have extremely pronounced temperature contrasts. Typically the temperature differences are of the order of tens of degrees. In late spring, when the cold conveyor belt still can bring very cold air from the Arctic areas, the Warm Front precipitation can fall as snow even on relatively low latitudes (Northern Germany, Poland).

It is also evident that even small errors in forecasting the low pressure tracks may lead to significant errors in the actual weather forecast. As the precipitation areas on BS lows are elongated and narrow and they approach the forecast area with their "nose" in front, one forecast point can receive little or no precipitation while the adjacent point gets heavy and prolonged precipitation. This fact makes the correct forecasting of these systems challenging.

The case described below is very similar to the classic situation shown above.

Initial state satellite features and conceptual models

The IR, VIS and WV satellite images of the 14th June 06.00 UTC show the initial state of this case study, with warm air moving from the Black Sea to Finland.

From the IR image (H00) it can be seen that between the warm airmass over Russia and eastern parts of the Ukraine, White-Russia and the colder airmass over Scandinavia and central Europe there is a long structured band of clouds. Over Romania and Poland it looks like a frontal cloud band with cold cloud tops and embedded convective cells. Further to the north (Baltic states and Eastern Finland) the cloud looks more like an anticyclonically curved cloud band. It has lower cloud tops than the frontal clouds, but it is connected with a huge circular MCS around 60N/33E. Over the Czech Republic and Austria the cloud bands resemble an Occlusion of the Cold Conveyor Belt type.

Furthermore there seems to be a cyclonic eddy structure over the Norwegian Sea centered at 65N/03E with extended cyclonically curved cloud bands along the boundaries. These features are indicative of an Upper Level Low (ULL). A distinct cloud band over the North Sea from England to Norway has the structure of a Baroclinic Boundary and might be associated with the ULL. However, in this case study we only concentrate on the clouds over eastern Europe.

All the IR features are recognizable in the VIS image (H01) as well. The grey shade difference on the area of the Occlusion (typical for the CCB type) is not as distinct as in the IR image. The small scale cloud band over Western Finland can be seen better. There was a lot of precipitation within the frontal cloud band (Romania - Baltic States) and thunderstorms within the eastern edge, in southern parts, were observed. The cloud band from the Baltic States to Finland produced no precipitation at this stage. A lot of precipitation is also observed in the southernmost (thickest) part of the cloud over Eastern Austria. The Water Vapor image (H02) clearly shows the thick cellular convective areas of the cloud band from Romania to the Baltic States. The cloud bands extending from the Baltic States to Finland are much broader than the IR bands in this area. There is a distinct boundary between wet and dry air along the west coast of Finland.The CCB Occlusion appears to be well developed and detached from the front in WV. A Comma - like feature in the WV image over Northern Finland cannot be identified in IR or VIS images. As a summary of the initial state we can look at the IR image with the names of conceptual models superimposed based on satellite features and relevant NWP output (H03). With this additional information (not derivable from satellite imagery alone) the frontal cloud band from Romania to Bulgaria can be identified as a Cold Front in Warm Advection. Not superimposed on the picture is the fact, that from Bulgaria northward the frontal clouds can be identified as a Warm Front Band.