EUMeTrain: A long heavy rain period in the Baltic States and Finland 9-11 august 2005

Meteorological Physical Background

The case described here deals with several interesting phenomena. In frontal scale, two major frontal features have been under special investigation: the Cold Front (Finland) and the Occlusion front (Lithuania). The frontal features are a part of the classical Polar Front theory, which is also briefly presented here.

The system also can been seen as a "Black Sea low" type of depression, which is a common name for low pressures approaching Scandinavia and the Baltic States from the south-east.

In-depth analysis of frontal theories connected with these phenomena is not possible, but a short summary of each is presented in the following sub-chapters. For further reading, please refer to Manual of Synoptic Satellite Meteorology.

  1. Classical polar front theory
  2. Black Sea lows
  3. Cold front types
  4. Occluded front

Ad 1. Classical polar front theory

The development of the low pressure in this case follows quite closely to the classical development: the wave developed within the south-easterly flow and both warm and cold fronts were rather well developed. The occlusion process was distinctly clear and intensive. Noteworthy here is to repeat the importance of the system velocity slow-down: as the low matured and slowed down over the Baltic Sea, certain areas received continuous rain because of almost a stationary occlusion front.


Ad 2. Black Sea lows

This low pressure case clearly follows the "Type B" development: a wave development in the baroclinic boundary over the western parts of Ukraine was triggered by an approaching upper level trough and PVA. The most remarkable deepening took place very early during the process, and quite quickly (as the PVA decreased) the surface low deepening stopped. This was also connected with the fact that the axis of the system was quite upright from the beginning of the process, and not as "dynamic" as for example would be the case in Rapid Cyclogenesis. The track of the low pressure follows well the classical track of the Black Sea lows.


Ad 3. Cold front types

The cold front in this case rather seems to be a Kata than an Ana Cold front. The reasons for this are the following:

There are some additional features associated with the Cold front, obviously pre-frontal convergence that could be seen in the vertical cross sections, enabled the development of MCS formation later in the evening of 9 August. This was enhanced by Cold advection at middle layers, which further de-stabilised the atmosphere ahead of the Cold front.


Ad 4. Occluded front

The occluded front in this case shows that the cloud spiral was formed of the Warm and Cold front cloudiness that were drawn into cyclonic circulation. No low-level cloud can be seen protruding from below the Cold and Warm front cloudiness.

The vertical cross sections show the classical features of a well-developed occlusion in this case.