Divergence and Convergence, appearance in cross sections
Cross sections for the determination of the divergence pattern and variations of ?e along a line from Oslo through Kattegat to Sont and Poland are constructed and divided into three parts. One of them is used for the presentation of the divergence pattern over the Sont, In fig 15 the reference points for a detailed cross section are depicted.
Figure 15: Reference points for cross section
From fig 16 below at the entrance of the Sont the expected convergence is displayed in the cross section with little divergence between 900 and 700 hPa. From the central part of the Sont (55.5N) downwind to the exit region divergence is forecast near the surface due to the stronger wind over sea compared to the Danish countryside. The effect of Coastal Convergence, which has a close relation to Convergence Lines, must not be overlooked. In this case it might be the most important process for the intensification of the line of showers.
Figure 16: Cross section through the Sont from entrance 56N to 55N to exit at 10 November 2007 0600UTC. Dotted lines in purple = divergence; black lines: Theta-e
Above the minor divergence near 55N little convergence is forecast between 800 and 700 hPa which establish slight descending air. Generaly speaking divergence and convergence patterns are rather weak displayed in the cross section through the Sont, becoming better identifiable in fig 17, a cross section from the exit of the Sont down south to the north west coast of Poland.
Figure 17: Cross section exit Sont region to northwest coast of Poland, 10 November 2007 0600UTC
The upper air divergence pattern at 500hPa does coincide with the position of the Jet streak, the left exit region (see fig 20,the 500hPa analysis). Over the west part of the Baltic Sea (around 54N) and the coastal regions (53N) low level convergence is forecast up to about 950 or 900hPa with a divergence pattern above up to 600hPa, slowly weakening down south. At higher altitudes convergence is still present between 600 and 500hPa, which is not in favour for intensifying convection. A combination of these effects did contribute to the intensification of the line of showers, originally developed as a cloudstreet after a certain fetch off the southeast coast of Norway. But this is about to change.
Figure 18: Cross section Divergence : Oslo-Sont-Szczecin (Poland) 10 November 2007 1200UTC
In fig 18, a long distance cross section almost from Oslo down to Szczecin though the Sont and valid for 1200UTC that day, the later development, between 0600 and 1200UTC are depicted. At the surface, south of 56N, still little divergence is forecast. At the same time increased convergence at 850 hPa together with stronger divergence at 500 hPa above. This will of course result in the intensification of convectivity which is nicely depicted in this cross section and in the satellite image of fig 22: south of the exit of the Sont (55.5N/13E) an intensifying line of showers over the west part of the Baltic Sea. But at the same time colder upper air is advected over the area. Although the temperature at 700 hPa did not change much, at 500 hPa much colder air (falling from -32 to -38 °C) arrived. This changing upper air pattern is displayed in fig 19, the 500hPa analyses and in fig 20, the 300hPa analyses, both from 1200UTC that day.
Figure 19: 500hPa analysis, 10 November 2007 1200UTC (Meteocentre Toulouse)
Figure 20: 300hPa analyses 10 November 2007 1200UTC, (Meteocentre Toulouse)
From the 300 hPa analyses it is clear the intensifying line of showers is in the area of the left exit region of the Jetstream. Not only the line of showers, extending from the Sont down to the Baltic Sea gains in intensity. It did have an effect on the intensification of the line of showers, which developed earlier down south of the island of Fünen, as well. This is discussed in the next chapter about the weather events.