Synoptic case study of the supercell event on 23 August 2007 in Saxony
1. Introduction
On 23 August 2007, a severe supercell event struck wide parts of the Saxony Highlands and produced intense flashflooding, large hail up to 8cm and severe wind gusts. The supercell storm was originated between Plauen and Zwickau in Western Saxony at 09.30-09.45 UTC. Then, the storm has become rapidly stronger and reached soon very high radar reflectivity (about 55 dBZ in low-level doppler radar imagery). The developing high-precipitation supercell storm travelled to the northeast while the steering flow of the residuary storms was directed to the north. As a result, a markedly defined right moving could have been observed indicating a possible mesocyclone. The entire duration of the storm that brushed past Dresden at 12.10 UTC was about five hours from Western Saxony to the Western Poland. Its path had been continuously marked by right moving with 45-90° derivation of the steering flow and numberous large hail and flashflooding events. At, 11.35 UTC, respective dopper wind imagery (not shown) disclosed a strong dipole structure, the corresponding low-level radar imagery had shown a classic to HP supercell structure with a well-defined hook echo. Maximum reflectivity in radar imagery have been in exceed of 65 dBZ.2. Which conditions do favour supercell storms?
Whereas ordinary storms are able to form when moisture, instability and lift is present, supercell storms usually require another ingredient, the wind shear. Strongly veering and increasing winds with height allow for set up of storm-relative helicity that can be ingested by the storm's inflow thereafter. Strong deep layer shear in the order of at least 10-20m/s enables the possibility of separated up- and downdrafts and therefore long-lived convective storms. If sufficient storm-relative helicity is present, the storm may develop a rotating updraft by tilting the pre-existing environmental horizontal vorticity into vertical vorticity. A mid-level mesocyclone results of this process. If strong low-level shear is present, too, a low-level mesocyclone respectively a tornado can evolve. (The complete process of developing mesocyclones and tornadoes goes, however, beyond the scope of this study. I refer to Estofex.org where several papers about the accurate mechanisms can be gleaned from, e.g. Dahl J., Supercells - Their Dynamics and Prediction M.Sc. thesis (2006)).There is an exhaustive manual about forecasting severe convective storms calling "Warning Decision Training Branch". The threshold values for HP supercells are given with below 18 m/s and in exceed of 30 m/s for LP-Supercells. Upper-level Winds below 18 m/s favour hydrometeors to recycle within the main updraft and to grow substantially whereas winds above 30 m/s make the hydrometeors blowing away from the main updraft (perhaps leading to isolated large hailstones falling in the rain-free area of the storm). However, it should be emphasized that strictly following thresholds is not the way forecasters should go to anticipate the character of supercell storms.
The following chapter will verify whether the synoptic-scale and mesoscale conditions have been sufficient for HP supercell storms.
3. Synoptic environment of the supercell storm
The subsequent reanalysis maps stemming from wetter3.de appear to be representative of the storm's large- and mesoscale environment.Fig.1 500mb Geopotential heights (gpdm), Temperature (°C) at Thursday, 23-08-2007, 12 UTC
The 500mb map shows a broad upper-level low system extending from the Iberian Peninsula to the North and Southern Baltic Sea. The core is centered over Belgium. A moderately curved shortwave trough is located with its trough axis over Central Italy embedded in the eastern periphery of the upper system. The packed isolines of geopotential heights indicate a jetstreak stretching from Central Italy, Austria, Czech Republic to Southern Poland. The left exit region of the jet ranges from the Western Czech Republic to Saxony, Brandenburg and Western Poland allowing for quasi-geostrophic forcing in the area of interest. The borderline between upper warm air and upper cold air runs alongside the Erz Mountains.
Fig.2 850mb Geopotential heights (gpdm), Temperature (°C) at Thursday, 23-08-2007, 12 UTC
In lower levels, moderate warm air masses are present over Southeastern Germany, with temperatures of about 12-13°C in 850mb. The thinly-packed isolines provide for a weak southerly to southsouthwesterly flow in the boundary layer. The horizontal temperature gradients are weak and the frontogentic forcing is weak, too (not shown). However, comparing the location of the upper-level cold pool with the low-level warm air masses, there is a superposition present (-18 in 500mb, +12 in 850mb) leading to steep lapse rates in the 500/850mb layer. Though, a capping inversion could have been existed hampering air parcels to rise to the level of free convection. We will come back to this point in the next chapter.
Fig.3 850mb equivalent potential temperature (°C), surface pressure (hPa) at Thursday, 23-08-2007, 12 UTC
A surge of enhanced theta-e-values captured Saxony during the noon and afternoon hours. Flat pressure fields are present in lower levels, centered over Czechia. The northern quadrant of the low covers the area of interest, leading to easterly to northeasterly surface winds.
Fig.4 KO-Index [K] Isolines, Extrema L < -6, S > +6 and 500mb vertical motion (hPa/h), shaded, at Thursday, 23-08-2007, 12 UTC
Strongly negative values of the KO-Index indicate the presence of a potential unstable layer between 500mb and 1000mb (as already supposed due to the lapse rate in that layer, see above) extending over Southern Saxony with values of about -12K. As depicted, a field of strong upward motion (about -16 hPa/h) overlapped the area with potential instability. That is, the potential instability could have been converted to conditional instability and subsequently released given an air parcel reaching the level of free convection.
Fig.5 Wind 10m AGL (Kts), surface pressure (hPa) at Thursday, 23-08-2007, 12 UTC
The last map shows two flat lows over Eastern Germany and Czechia, with easterly winds over Saxony.
Concluding this synoptic review, the maps above show favourable conditions for convective storms, with a strong southwesterly flow in mid-levels allowing for strong deep layer shear. Furthermore, the left exit region offered quasi-geostrophic lift over Saxony converting and (later) releasing the available potential instability into conditional instability. The respective dewpoints (not shown) ranging from 16-19°C led to a small dewpoint spread at the surface (at the same time 23-25°C air temperatures) and sufficient low-level moisture was also present. The most important advantage has been the strong veering in the boundary layer from easterly winds at the surface to southerly winds in 850mb. At all, a typical environment for supercell storms
4. Ambient sounding ascents and conclusions
Unfortunetaley, there is no sounding ascent close to the path of the supercell storm, so we have to resort to the soundings some hundred kilometres away from the storm. However, forming a "mixture" of these soundings, we will obtain a good image of which conditions have been prevailed.I will examine these soundings with respect to...
- moisture layering, instability values
- lapse rates between 700mb and 900mb
- veering winds and deep layer shear
- the strength of the capping inversion
2. Wroclaw I, Southwestern Poland
3. Praha, Central Czechia
4. Kuemmersbruck, Northeastern Bavaria
| Parameters | Lindenberg | Wroclaw | Praha | Kuemmersbruck |
|---|---|---|---|---|
| Low-level moisture/spread | 23,4 | 17,4 | 22,6 | 18,5 | 25,0 | 15,0 | 19,4 | 13,4 |
| LWC above freezing level | moderate | very low | low | low |
| MLCAPE [in J/kg] | 845 J/kg | 266 J/kg | 150 J/kg | 16 J/kg |
| Lapse rates 900/700mb | nearly adiabatic | adiabatic, then moist | nearly adiabatic | nearly adiabatic |
| Veering (cBL) | Northeast to south | Eastnortheast to southsouthwest | Southwest to south (backing!) | north to west, southwest |
| DLS (500mb winds) | 17 Kts | 58 Kts | 60 Kts | 45 Kts |
| CIN (in J/kg) | -5 | -80 | -120 | -170 |
| Wet-bulb zero | ca. 3000m | above 3200m | ca. 3100m | ca. 2900m |
Results of the sounding ascent analysis
Rather small spreads have been present in the afternoon in the area of interest allowing for rich boundary layer moisture. Due to rather dry layers above, potential instability formed possibly leading to a higher release of instability than the MLCAPE values let assume. The veering was strong in all places, save Praha situated further to east where backing winds have been observed (and a strong capping layer). The deep-layer shear reached values in exceed of 45 Kts due to the position of the sounding stations within the jet core. Lindenberg lying further to the west reveals a pronounced decrease of the upper-level winds.The capping was smallest in Lindenberg and Wroclaw and greatest further to the south. Taking the (estimated!) liquid water content (LWC) above freezing level, the mid-level lapse rates and the wet-bulb zero into account, the best chances for large hail events prevailed in Lindenberg, with a high LWC, a low wet-bulb zero level and nearly adiabatic lapse rates (with corresponding dry layer allowing for low wet-bulb zero).
The factors favouring large hail are taken from Marco Kaschuba's article as well as from the Estofex guide to forecast severe convective storms.
Summarizing, the thermodynamic and kinematic conditions were favourable for supercell storms. The storm moved into an area with moderate to strong deep layer shear (between 15-25m/s) suited for classic or high-precipitation supercells (if we use the threshold values above). Due to enhanced liquid water content above freezing level, possibly strong potential instability release and low wet-bulb zero, the environment have been conducive to the growth of large hail. The imbedded dry layers may have caused enhanced evaporative cooling conducive to severe wind gusts.