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.