Deep-moist convection is the result of three co-existing ingredients : Moisture, Instability and Lift.
Moisture advection was present by the northwesterly flow over the Atlantic Ocean (and later the North Sea Region). QG-Forcing resulted of the position of the cold front ahead of the trough axis and in the left-exit region of the jet:
The 300mb reanalysis, valid for Thursday, 18 January 2007 18z, map shows a strongly curved jetstreak with very high windspeeds in exceed of 190 Kts average winds. The well-defined diffluent jet exit region lies over Germany, with the left exit part over Northern and Eastern Germany where the cold front had shown the greatest convective activity.
Moreover, the present dry intrusion introduced high values of isentropic potential vorticity in the cyclogenesis process which leads to an augment of absolute vorticity due to the decrease of the static stability by synoptic-scale lift. Potential instability was caused by the dry stratospheric air overlapping the relative warm and moist air masses in lower levels. Synoptic-scale lift was sufficient to convert this potential instability into conditional instability released at the leading edge of the cold front.
Well, the conditions for deep convection were present. Another ingredient being generally responsible for severe convective weather : vertical wind shear. As a result, the thunderstorms were strongly sheared downstream, especially in the lower troposphere, and the high average winds in 850mb of 70-90 Kts could be transported downward to the surface. In addition to several microbursts with the cold front as well as in the postfrontal sector, there have been also some tornadoes over Eastern Germany. By yet, it remains still elusive which mechanism had caused these tornadoes - mesocyclonic or non-mesocyclonic ones, the latter as a result of Shear instabilitiesat the leading edge of the squall line.
I do review this cold front in two parts - at first, the possible storm forecast basing upon the lightningwizard charts from Oscar van der Velde related to the cold front activity. Then I will describe chronologically what happened with the cold front passage by composit satellite imagery (Eumetsat, FU Berlin). To understand the used maps from lightningwizard, I refer to the Guide to using Lightningwizardmaps
Map 1 - Thompson Index
Above, the Thompson Index is plotted for the dates of 18 January 2007, 15z, 18z and 21z. The maps show the cold front passage with a strong signal especially over Northern and Eastern Germany as well as significant decrease with southeastward movement of the cold front after 18z. Hence, thunderstorms were very likely where the signal had been strong.
Map 2 - Equilibrium Temperatures
EL Temperatures reaching -30°C up to 18z and -40°C over Poland support the development of deep instability. If the upper-level cold pool had been in rather low geopotential heights, the instability areas in a sounding ascent would have been quite wide. As it could be seen by the model charts in the first part of the analysis, the upper-level cold pool had been in relatively high geopotential heights. Hence, the calculated EL temperatures referred to the upper troposphere and the resulting instability area have been narrow, but deep. Concluding these assumptions, the EL temperatures favoured rather deep convection with summery character.
Map 3 - Moisture Influx
The maps show favourable values in about 80 g/s/m² related to the southeastward propagating cold front, at least to the Thuringian Forest which is confirmed by the observations of thunderstorms.
Map 4 - 2-4km lapse rates
The map indicates a horseshoe-like swath of slightly drier air over Northern and Eastern Germany to West Poland. This is coincident with the run of occlusion and cold front.
Map 5 - Lapse rates between surface and 500m AGL
Respective lapse rates in lower levels were quite weak in the order of 4-5K indicating a relative moist boundary layer. Thus, according to the 2-4km lapse rates, the conditions for potential instability could be matched.
Map 6 - 0-3km Storm-relative Helicity
There are extreme values of helicity in the order of about 400 m²/s², partly up to 700 m²/s². Maximum of helicity and the position of the cold front overlap eachother. Shear values decrease behind the front and increase with the trough line over the Netherlands and Lower Saxony.
Map 7 - 0-6km shear (black), 0-1km shear (coloured)
LLS exceeds 40-50 Kts in total germany and is well-defined in the range of the cold front (black area) as well as alongside the Alpine Region in the warm sector, due to a barrier jet. DLS is somewhat weaker over Northern Germany, with 20-40 Kts, because this region is close to the core of the deep low. To the south, the DLS is very strong, with 70 Kts, and over 100 Kts in Western Germany. Most of the cold front see 30-70 Kts.
The ratio between LLS and DLS yields the following message: There is a rather strong low-level jet while the augment of the wind speed with height is relatively small, except of the extreme Western Germany with 100 Kts DLS. A strong low-level flow generally favours the development of bowing segments with an enhanced potential for severe straight-line winds (Downbursts).
Map 8 - Potential for severe downdrafts.
The map shows in colored style the expected convective gusts. Broad area has 70 Kts Minimum to 90 Kts Maximum, also in the range of the cold front. Observations (80 Kts in Düsseldorf, Berlin, 75 Kts in Magdeburg, Braunschweig and widespread T4-damaged forests (about 110 Kts) confirm this forecast.
Conclusion:
The cold front passage was expected to enforce severe thunderstorms with straight-line winds as primary mode, exceeding 170 km/h. Somewhat risk of tornadoes could not been excluded due to the strongly enhanced vertical wind shear. However, the rapidly southeastward moving cold front bared from a local forecast of short-lived mesocyclones.
In the second part of the analysis, the actual development will be described by satellite imagery from Eumetsat and FU Berlin.
A colored RGB composit image (Eumetsat) will be used, showing air masses in different heights. Pink colors indicate air masses from the stratosphere or upper troposphere, brown colors from the middle troposphere. The brighter the cloudiness, the depper it is.
Satellite image on 18 January 2007, 10 UTC, shows the core of the deep low over Eastern England. The occlusion front is located at the cold side of the jet axis leading to dropping cloud tops compared with the warm sector respectively the warm conveyor belt. The cold front crosses England as a shallow, but compact line. It is completely overlapped by dry stratospheric air, descending to the middle troposphere (with subsequent warming of the cloud top temperatures). Contemporarily, the dry intrusion results into potential instability. So far, the warming of the cloud tops (no thunderstorms have been observed as yet) outweights. Though, shallow convection yields very severe wind gusts over England (> 120km/h).
In the afternoon hours, 16 UTC, the core has reached the Kattegat, yielding a clearly coiled up occlusion front. The distance between the warm conveyor belt and the cold front has grown. The cold front intensified due to released potential instability. Cloud top temperatures respectively sank. The images reveal increasingly a comb pattern whose single comb cells are directed alongside the axis of the upper flow. Typical postfrontal subsidence develops behind the cold front, subsequently trough line is forming nearby the coastal line of Northern Germany.
The most spectacular satellite image at 18 UTC shows a stout cold front extending from Southern Sweden to Central Germany. Its coverage increases like a wedge from Southwest to Northeast. It may be deduced from the strongest forcing over Northeastern Germany as well as the respective upper-level trough with the coldest upper-level air there. The leading edge of the cold front includes some kind of lobe and cleft structure like in association with gravity currents. The reason for this convex bulges should be the strong low-level jet. Subsequent subsidence zone is visible as well and the subsequent trough line possesses scattered, but broad multicells.
There are different theories to explain the strong intensification of the cold front by 14 UTC when first lightnings occured. Initially, the subsidence process of stratospheric air extended to mid-level heights, suppressing deeper convection. Then, stratospheric air went back to higher troposphere levels (around 350mb), allowing for rising cloud tops. Another theory constates enhanced friction over the interior as a reason. Backing winds ahead of the cold front and decelerated flow may have led to enhanced speed and direction convergence alongside the line. Perhaps, both theories are true.
Subsequent IR Imagery from FU Berlin show chronologically the development of the cold front to the west of Ireland to the culmination in the evening hours over the easterly Central Europe.
The severe weather potential of the cold front becomes visible on 18 January 2007 at 1 UTC as the dry intrusion overruns the cold front and lowers the cloud tops. Later on, the differences in the cloud tops between the cold front and the warm conveyor belt as well as the occlusion front with the surge of warmer theta-e becomes more clearly. Though, the cold front is less organized in the morning hours. By 11 UTC, the cold front starts forming as a sharp-edged line again. Towards the evening, the cold front exhibits a strong linear organization with the aformentioned comb-like cell structures, arranged like on a pearl necklet.
Observations show the southeastward propagation of the thunderstorms associated with the cold front. Electrified convective activity started at 13 and 14 UTC nearby the exit region of the English Channel and intensified rapidly further inland. However, the influence of diabatic heating may be neglected since the squall line was embedded in a more continuous precipitation area. The thunderstorms reached Poland, Czech Republic and Slovakia in the late evening. By 2 UTC, the storms traveled to extreme Northern Austria where the forest quarter was struck by widespread damaging winds, too. Then, the upper-level trough rapidly moved eastward whereas the cold front runned out of the area of QG-forcing and hence, the thundery activity stopped.
The most repesentative sounding ascent in the range of the cold front stems from Lindenberg, to the east of Berlin, at 18 UTC:
It reveals extreme vertical wind shear with 15 m/s at surface level and 40 m/s in 850mb whose surface dropped to 1160m in this case. Almost 50 m/s can be found in about 750mb height. The profile is moist-neutrally layered and maybe slightly unstable. In any case, the very strong large-scale forcing as well as the forcing due to the squall line itself could help for the release of at least low-end instability. The equilibrium level (limit of convection or level of neutral buoyancy) could have been situated in about 350mb - rather summery conditions for deep convection. Maximum winds were confined to the lower troposphere and caused the observed bowing segments within the cold front passage.
Source : Wetterspiegel
A more detailed discussion:
The winds in the lower troposphere were much stronger than in the middle and upper troposphere. As a result, the linear convective line exhibited some forward-directed bulges. The well-defined low-level jet led to evaporative cooling and hence, an acceleration of the downdraft winds. Since the layer was moist throughout the troposphere, that effect might have been lesser than with comparatively drier levels. However, the vertical momentum flux of the extreme horizontal winds could have compensated that. After some hours, the squall line gradually passed into the downdraft mode and intensity decreased. Precipitation then became more stratiform (with somewhat convective part, though).
Such squall lines including bowing segments are called line echo wave pattern (LEWP). They bear the risk of large hail, microbursts and tornadoes. Numberous suspicious events are named in the tornado list from Thomas Saevert, with three confirmed F3-rated events as well as scores of forest damages rated as T3 and higher prove the severe convective weather character of the passing LEWP especially over Eastern Germany.
Interestingly, the strongest wind gusts were observed before and during the first minutes of the cold front passage. Then, the wind decreases suddenly.
Matthias Kirsch, Dresden - Wetterzentrale 18.01.2007, assumed that the heavy precipitation with hail and soft hail encouraged the development of a cold pool near the surface. As a consequence, the upper-level winds were decoupled to the surface. The temperatures increased after the front passage while the upper-level temperatures sank with the passing upper-level trough. The developing instability results in the downward transport of the upper winds again, and the cold pool was eroded by them.