18. January 2007 -- Deep low Kyrill in Satellite Imagery

At first, I'd like to review Kyrill chronologically with IR-Satellite imagery from the Free University of Berlin, then different satellite images will follow belonging to different states of the low-pressure system.

The formation of the intense low-pressure system bases upon a secondary cyclogenesis at the triple point. These secondary developments can be detected in model charts and satellite imagery by the following factors...

Previous synoptic analysis showed that the position of the surface low perpendicular to the upper low caused the stationary state of Kyrill 1. In contrast, the triple point forming Kyrill 2 lied clearly ahead of the upper low under (D)CVA as well as nearer to the jet axis favouring rapidly propagating with deepening core pressure.

IR-Satellite Imagery of Free University of Berlin - Chronology of the development of "Kyrill 2"

17. January 2007 - 12 UTC

Initially, the low-pressure system over the North Sea dominates the weather pattern in Western Europe. Kyrill lies to the west of Ireland with a sharp-edged baroclinic leaf, possessing the typically bounded cloudiness at its northern side being frayed to the south. The baroclinic leaf manifests itself as warm conveyor belt ascending and condensing in the midlevel and upperlevel troposphere (600-300mb). Cloud tops are rather high - opposed to the occlusion front with less high cloud tops forming by a second warm conveyor belt. It ascends from the low-level and midlevel troposphere (900-700mb) and is also called cold conveyor belt due to its location on the cold side of the frontal zone.
The occlusion front lies parallel to the frontal cloud band, but is back bent to the west (back-bent-occlusion) and little coiled up. The low pressure centre is situated clearly far from the triple point in an almost cloud-free area of the dryslot where stratospheric air masses descend in the midlevel and lowlevel tropospere upstream from the trough axis.

By comparison, the North Sea low centre is located immediately to the southwest of the triple point, according dry intrusion streches from the entrance of the English Channel across England to the North Sea.

17. January 2007 - 18 UTC

Six hours later, the Dry Intrusion intensified distinctively and produced a rather dark, cloudfree low pressure centre that belongs to Kyrill 1. Dry Intrusion's leading edge extends to the triple point and runs in the rear of the cold front. Relation of warm and cold front to eachother shows a typical warm sector low, with a broad warm sector which is piled with deep, precipitation-bearing clouds by the warm conveyor belt. The cirrus clouds to the south of the baroclinic reaf running perpendicular to the upper low are rather striking features in this state of cyclogenesis. One can assume extraordinarily high vertical wind shear there. Upper winds in 500mb reach about 150 Kts in average - rather infrequent in this region.

18. January 2007, 00 UTC

Splitting process took place after midnight, leading to Kyrill 2. The cold front ist characterized by a narrow, deep and broken cloud band while the warm front continues to be associated with a deep and compact cloud shield. The dryslot extends from the low pressure center of Kyrill 1 to close to the triple point. Kyrill 1 is already clearly coiled up, with weakening tendency. This hypothesis can be confirmed by model charts (the location of Kyrill 1 behind the trough axis supports it).
To the west of the triple point, the occlusion front shows another weak cyclonic curvature hinting the position of the secondary low pressure centre. Respective 10m wind maps corroborate this analysis.

18. Januar 2007, 06 UTC

A quite important development starts between midnight and the early morning hours in regard to the further propagation of the system, the cold front activity and the maximum wind gusts in the warm sector of the deep low....

The former surface low with its occlusion front detaches itself from the new surface low over the Irish Sea. Comparing its position six hours before, there are no doubts anymore having two cyclones. If one considers the coverage of Kyrill 2, it may be amazing to see its size in relation to total cloudiness. Most parts of the warm sector and southward are result of an ample, warm conveyor belt yielding also rich amounts of precipitation about 50mm in 24h.

The virtual deep low took up only a small portion of the whole cloudiness. Additionally, the new occlusion front is narrower, shorter and more rapidly coiled up than the primary occlusion front. Subsequently, simple physics has to be applied to the propagation of the deep low: The moment of inertia of a small whirl is lesser than the one of a greater whirl. As a result, the relatively small deep low Kyrill 2 moves faster downstream than if it had been a great whirl.

Indeed, the decrease of the moment of inertia results in an augment of the angular velocity. The superposition of the strong storm-relative flow (upper-level jetstreak) and the increasing angular velocity causes a well-defined high-velocity wind field to the south of the low pressure center, in the warm sector, where widespread 850mb winds of 35-40 m/s, partly about 45 m/s have been observed. Unusually, this high-velocity wind field was also connected to the cold front what had to lead to the presumption that at least 30 m/s could occur with convectively-induced wind gusts in the lower countries.

During the second half of the night, the dry stratospheric air of the dryslot overflows the warmer and relatively moist air masses in the lower and mid-level troposphere alongside the cold front. As a consequence, the cloud tops dropped strikingly. The superposition of the dry midlevel air and the moister lowlevel air mass yields significant amounts of potential instability released by strong upward motions in front of the trough axis creating a fair number of showers and thunderstorms alongside the cold front.

18. Januar 2007, 12 UTC

By noon of the 18 January 2007, the extent of the deep low looks exiguous compared with the mightful cloud shield of the warm conveyor belt. The cold front is crossing Southern England and London with hurricane-force winds. A short period later, the first thunderstorms are observed alongside the cold front over the southern North Sea. Interestingly, the overflow of the stratospheric air reaches quite far into the warm sector of the low. Again, a narrow cloudfree region ahead of the cold front along the coastal line can be disclosed.

18. Januar 2007, 19 UTC

The deep low reaches its culmination in the early evening hours as far as the cold front activity over Central Europe is concerned. Right now, the core of the deep low lies over the western Baltic Sea. Convex structures can be seen at the leading edge of the cold front, but also to the rear. Thereunder, a line echo wave pattern (LEWP) is hidden. A short postfrontal subsidence area follows behind the cold front, before a trough line approaches from the North Sea, accompanied by severe convective storms with further hurricane-force winds.

Intensity of the cold front decreases to its southwestern edge where convectively-enforced precipitation is located associated with an oncoming wave development caused by upstream warm air advection. The run of the strongly curved jetstream is well seen by the narrow cirrus stripes, also called jet fibres. The left exit regions respectively areas with upward motion may be displaced with curved jetstreams alternatively the total exit region is underneath dividing stream lines with upward motions. In this case, the jet configuration was not responsible for the strong forcing, but the strong curvature ahead of the trough axis allowing for the strongest specification of the cold front over Eastern Germany, also visible in satellite imagery.

19. Januar 2007, 00 UTC

By midnight, the cold front crossed Germany almost completely reaching Northern and Eastern Austria. The cold front passage causes especially in the Wood Quarter (Lower Austria) severe hurricane-force winds with severe damage to the woods. Northern Germany is influenced by further showers associated with a trough line. Most parts of Southwestern and Southern Germany are affected by a weak wave coming in from Northwest. The strong northwesterly directed flow causes damming clouds alongside the northern alpine region. A short period later, the northern foehn winds will enter the Inn Valley to the west of Innsbruck while this had already been the case to the south of the main crest of the Alps where descending air led to dissolving cloudiness in the Po Valley.

19. Januar 2007, 06 UTC

Friday Forenoon, the 19 January 2007.... the cold front has reached Southeastern Europe. The deep low gradually weakens and turns to the northeast. Subsequent wave causes partly rich precipitation over Western and Southern Germany. Northern foehn winds push the maximum temperature values in Northern Italy up to summery 25-27°C. At 9 UTC, the northern foehn reaches Innsbruck with maximum temperature values about 16°C - more than previously occured with southern foehn winds. However, the formidable hurricane-force winds will fail to appear (a very similar situation five years before, with the famous deep low Jeanett, had led to 35 m/s maximum wind gusts).

Further upstream to the west of Ireland, another deep cyclogenesis will be in store... along the former cold front of Kyrill 1. This deep low will be called Lancelot and will lead to severe wind gusts especially to the north of a line Hunsrück- Erzgebirge.

Concluding this chronological digression, two satellite images from Dundee and NOAA display the deep low in a higher resolution:

18. Januar 2007 - 12 UTC - DUNDEE

The visual satellite image from EUMETSAT shows Kyrill 2 with its center over the North Sea. The occlusion front coils up nearby the coastal line of Eastern England. The warmfront runs from Denmark and Baltic Sea eastward. The cold front streches from Southern England to the Southern North Sea being deeply convectively-defined. Cloud tops rising to the south - in the range of the warm conveyor belt where rain fall is quite strong. In contrast, the cold front is already overflown completely by the stratospheric air extending - as mentioned above - far into the warm sector area. The stratospheric air dissolves the clouds behind the cold front totally.

18. Januar 2007 - 16 UTC - NOAA

Satellite Imagery from NOAA was taken in a quarter of an hour in the afternoon hours. The enframed area is totally flooded by the dry intrusion. The cold front reveals some pronounced comb-like structures in a row, with a northwest-southeast orientated axis being coincident to the upper flow direction. Each of these comb cells yielded heavy precipitation, hail and several microbursts. Further to the south, the wind was slightly weaker though widespread severe wind gusts about 100km/h and some hurricane winds occured, too. However, these winds never reached the broad intensity of the cold front why the damage to the forests in Southern Germany were kept within a limit.

These ribbed comb structures were likely to be caused by gravity waves. There are several theories about the formation of gravity waves in association with frontal systems. In 1972, Hoskins and Bretherton formulated the theory that non-geostrophic and non-hydrostatic accelerations in a region with increasing temperature contrasts may create gravity waves whose amplitudes in a great distance to the front may be comparable with oscillations in the surface pressure field ahead of fronts.

Another theory states that trapped waves underneath the front produce traveling gravity waves by breaking in higher levels, see for example Gravity waves generated during frontogenesis (Gall, Williams, Clark, 1988) im Journal of Atmospheric Sciences [16 Seiten, pdf], for a more exhaustive insight to this topic.

There seems to be a connection between high vertical wind shear as well as low static stability in the left exit region and developing gravity waves there. I found this article in the Journal of Atmospheric Science: Generation of mesoscale gravity waves in the upper-tropospheric jet-front-systems (Wang und Zhang) , [19 Seiten, pdf]. As yet, however, the role of deep convection with this major gravity waves remains still elusive to me. It is well-known that deep convection may cause gravity waves itself, but which effects may result of gravity waves to deep convection? Is there an effect?

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© Felix Welzenbach