IR 10.8, surface pressure tendencies, time sequence, 3h: 29 February to 01 March 2008

In this chapter we will consider surface pressure tendencies during the last 3 hours (isolines) in combination with mean wind (barbs) and gusts (numbers). For a better background, the IR 10.8 image will be added. The surface pressure tendencies are given in 1/10 hPa. The contour interval is 10, i.e., 1 hPa/3h. Negative values (decreasing pressure) are in red, increasing pressure in blue. Pressure tendencies of about 0 hPa are in green.

Several images reveal unrealistic surface pressure tendencies (extreme gradient). This could not be taken out. Whereas so-called 333-gusts of at least 25 knots are being reported in Germany (and some other countries) other countries show also reports of gusts less than 25 knots (for example France).

This image reveals in the forefield of the approaching fronts negative surface pressure tendencies up to 8 hPa/3h.

Strong negative surface pressure tendencies can be seen over North Ireland and Scotland.

Gusts between about 30 and 47 knots (Scotland, Ireland) are measured and pressure tendencies up to -10 hPa/3h. This strong pressure decrease fits well with the dry slot that hit Northwest Scotland. The corresponding image with airmass-RGB in combination with the IPV 310 K reveals a split front over Northwest Scotland that was situated in front of the approaching PV-maximum, i.e., influenced by positive vorticity advection.

The area with rapid surface pressure fall (up to -11 hPa/3h) moved to the east. Strong winds of 40 knots and gusts of 56 knots were observed in North Ireland.

The area of negative surface pressure tendencies continued moving. At the north coast of Ireland gusts of 76 knots were reported. During the last 3 hours the axis of a short wave crossed this station (29-02-12UTC, 29-02-15UTC).

Also this image reveals severe gusts (UK, North Sea up to 58 knots). Whereas the pressure decreased over the North Sea by up to 11 hPa/3h (in relation to the approaching PV-maximum) we observe increasing pressure northwest of Ireland and Scotland. The zero line marks the axis of the upper air trough. During the last 3 hours the wind direction at the North Irish coast turned from southwest to west-northwest.

The strongest surface pressure's decrease occured near Denmark. The image reveals now a dipole of rising and decreasing surface pressure over Ireland and West of Scotland and further to the east (North Sea and Denmark). Again gusts up to 56 knots were measured in Ireland.

The areas of decreasing and increasing surface pressure moved fast to the east. A second relative extreme of negative surface pressure tendencies developed over Northwest Germany. The reason for this can be derived from the airmass-RGB in combination with the geopotential in 300 hPa: Between 21 UTC and 00 UTC a second trough developed. In its forefield a separated centre with negative pressure tendencies established and led to the development of the convective line (Geop 300 hPa, airmass-RGB from 29-02-21UTC and 01-03-00UTC).

The corresponding images of the airmass-RGB in combination with the PV in 310 K reveal a second PV maximum north of the convective line and the second extreme of falling surface pressure. Because this PV-maximum moved to east-southeast PVA occured yielding the negative pressure tendency (PV 310 K, airmass-RGB from 29-02-21UTC and 01-03-00UTC).

Heavy gusts and high mean wind speeds (in higher elevations between 50 and 70 knots) were measured. The trough line crossed the British Islands. The convective line can be found east of its axis (01-03, 03UTC). Southeast of the convective line the most intense surface pressure's decreased occured. The corresponding image of airmass PV in 310 K reveals a PV-maximum superimposing the convective line and purple colour. We see again the high potential of the airmass-RGB because it delivers both, the relevant cloud structures and the dynamic processes.

For completion the corresponding image with airmass and wind speed in 300 hPa shows the convective line at the left exit of the jet that is equivalent to positive vorticiy advection.

This image reveals the maximum intensity of the "dipole" at the convective line (in its frontal part up to -8 hPa/3h, in its rear up to +5 hPa/3h). This led to a so-called isallobaric wind and faster movement of the convective line. Further to the North we find values of 11 hPa/3h.

Especially, in the frontal part of the convective line strong gusts occured (up to 58 knots in flat areas, on Brocken (height about 1100 m), Germany). This image shows a strong wind turn from southwest to northwest.

All structures continued moving fast to Southeast. The dipol structure of surface pressure continued (-5 hPa in the forefield and up to 6 hPa in the rear of the convective line).

All structures moved fast to the Southeast. This image reveals decreasing pressure east of the convective line (up to -5 hPa/3h) and intense rising pressure in its rear (up to 8 hPa/3h). Severe gusts were registered over Germany (62 knots in flat areas, in mountains 88 knots (Feldberg, blackforrest)).

This image shows that the cold front/occlusion and succeeding convective line were superimposed by rising pressure. Over entire Middle Europe the pressure increased, too. However, we still find heavy gusts (German coast: up to 60 knots, Alps 95 knots). The distribution of the surface pressure tendencies shows the Foehn-effect over North Italy. In the lee of the Alps we still have decreasing pressure. However, further to the east the pressure increased already.

The corresponding images of the airmass-RGB in combination with the geopotential in 300 hPa shows that the trough's axis has crossed the Alps and that the convective line came to its rear. The wind in 300 hPa exhibits that the convective line was not longer in the jet's left exit (equivalent to less positive vorticity advection). The PV maximum northwest of the convective line became weaker. These images indicate that the convective line and the related weather phenomena will become weaker.

Big parts of the image reveal rising surface pressure. This indicates the end of the critical situation of Middle Europe.

With exception of the Baltic coast of Germany and some stations in the Alps the wind speed decreased. The surface pressure tendencies became generally smaller.

During the last 3 hours the surface pressure tendencies continued weakening over Middle Europe. However, some heavy gusts were still observed.