Convergence Lines in Denmark and Sweden - 10 November 2007

Introduction

The effect of the advection of dry air over Skagerrak and Kattegat towards Sont is clearly displayed in the VIS-SAT image of fig 2 and fig 4. Transported over the warmer seawater and a fetch of more then 200km off the Norwegian southeast coast convection starts into a remarkable cloudstreet or convergence line in general with a width of no more then 40km over the Sont, spreading downwind towards the Polish and German coast. At the entrance of the Sont convergence at lower levels triggers convection helping the CU-clouds from the cloudstreet to develop into the first CB-clouds. Convection is visible as well south of the Danish isles Fünen and Sealand in fig 4. After a much shorter fetch off the coast these cloud streets develop. With hardly any change in instability (no change in upper air temperature at 700 hPa compared to the east part of Denmark) the higher humidity (remember f.i. the higher dew points at 700hPa, fig 6) is important for the earlier start. And again, due to convergence at low levels in coastal regions (fig 4), convection is intensifying and smaller CU grow into larger CB clouds when reaching the north coast of Germany.

The Open Cell Convection, clearly noticeable in fig 4, over the North Sea and German Bight will not be discussed in this study.

Divergence and Convergence, angle of 10m wind with coastline

Divergence and convergence patterns induce descending and ascending motions. In the CM "Coastal Convergence" 6 basic types are mentioned. A combination of type 1 and 6 (onshore winds, deceleration and backing at coast / onshore flow and increased backing of wind) contribute to convergence in the lowest levels. In the detailed surface chart (fig. 8) convergence will occur at the narrow entrance of the Sont: 340/25 kt at the coast. Inside the slowly widening strait the windspeed will not change very much. Just downstream the entrance windspeed might be even higher due to Venturi effects. Comparing to the 300/25 kt at the exit of the strait an estimated, average wind of 280-300 / 20-25 kt inside the strait is very likely. This changing windpattern might produce a small scale divergence - convergence effect inside the strait itself and along the southwestcoast of Sweden when the surface wind becomes weaker more inland.

Finally it is obvious divergence will exist at the exit of the strait into the west part of the Baltic Sea. With not many wind reports available in the synoptic chart the model - forecast can issue more detailed information to the changing wind field. In fig 10 the HIRLAM wind forecast at 10m (HR run 10-11-2007 00+12) is plotted with a better, detailed information, especially over the area around the Sont.

Figure 10: HIRLAM forecast 10m surface wind 10 November 2007 0600UTC

Comparing this chart to the synoptic chart (fig 8) the actual reported wind over land is in general more backed and weaker, over sea and in coastal regions somewhat stronger. Along the north coast of Germany almost the same changing wind pattern can be observed. Again low level convergence (decreasing wind over land) is likely over the northern coast of Germany and Poland and divergence (downstream increasing wind) south of the Danish isles into the west part of the Baltic Sea. This convergence - divergence pattern is reproduced in fig 11 (the HIRLAM forecast 10-11-2007 0600UTC). At best the convergence is depicted over Kattegat, the northcoast of the island of Sealand and the entrance of the Sont. The exit has the expected divergence pattern, changing slowly into a new convergence pattern down south towards the northwest coast of Poland. Divergence is found over Fünen and the extreme west part of the Baltic Sea mainly due to lower pressure gradient, but it changes into convergence over the northeast coast of Germany as expected.

Figure 11: Divergence (blue), Convergence (red) at 10m ; HIRLAM 10 November 2007 0600 UTC

Convergence at lower levels induce upward motions, divergence in the upper air enhances this motion. In fig 12 the divergence pattern at 850hPa is clearly visible over Denmark, Sweden and Germany. The convergence at this level near Malmo is a result of the already ongoing development of showers (coastal convergence), see fig 14.

Figure 12: Divergence 850hPa HIRLAM 10 November 2007 0600UTC

In fig. 13 below the vertical velocity at 850 hPa (in hPa/hr) does in general coincide with the divergence pattern. Unfortunately the 925 hPa, better applicable for displaying the level of non divergence and strongest vertical velocity in the lower levels, is not available for presentation. In red small updrafts with onshore wind at the entrance of the Sont, the northwest part of Sealand, the north coast of Germany and quite strong over the northcoast of Poland due to stronger divergence in this area at higher levels. The weak downward motion over the southern part of Sweden, in particular over the Malmö area, does fit with the convergence.

Figure 13: Vertical velocity 850hPa (blue = descending); HIRLAM 10 November 2007 0600UTC

The question remains whether enough moisture could be available for development of CB and snow showers.

Fortunately fig 14, the associated HIRLAM forecast 850 hPa height (blue), temperature (red) and humidity (green), has a clear indication for the increasing humidity downstream at a long distance off the Norwegian coast through the Sont. Together with the calculated vertical velocity, a result of divergence pattern, the model presents an excellent first guess forecast for convection to start after this long distance fetch and showers to occur over Kattegat, Sont and further down to Poland.

Figure 14: RH(green), Isohypse(blue); 850 hPa HIRLAM 10 November 2007 0600UTC

In general the developing first showers north of the Sont is explained by the theory on cloudstreets and the convergence over sea. Coastal convergence, together with cold air advection in the upper air, which we will see later, contribute to new developments and intensification of the original line of showers.

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.