A boundary between between very moist air and very dry air triggered thunderstorms on 3 July.
By Jochen Kerkmann
On the Meteosat-10 HRV image, 3 July 04:30 UTC (Figure 1), due to the low sun long shadows of the high Cumulonimbus(Cb) clouds could be seen on the HRV image. The shadow length of the thunderstorms in southern Germany (Ulm–Augsburg area), was about 88 km.
The image's time flag is 04.30 UTC. This is the start time for the scan which starts at 04.30 and ends at 04.42 UTC. The Cb clouds in southern Germany were scanned at about 04.40 UTC (line number 3347 out of 3712 lines) and positioned 48.8 ° N and 9.9 ° E. For this position and this time, the sun angles are: azimuth = 66.5 ° and zenith 10.2 °.
Using the formula: tan(zenith angle) = Height of Cb/Shadow Length one can estimate the height of the Cb to be about 15.8 km. This sounds reasonable if you compare to the height of the tropopause (as given by the GFS model, source: Estofex) or the Stuttgart radiosounding (source Univeristy of Wyoming).
The Meteosat-9 Severe Convection RGB rapid scan loop, 3 July, 05:04–16:04 UTC (MP4, 5 MB), shows how long (more than 10 hours) the storm system over southern Germany continued, moving from Germany to Austria and northern Italy. The yellow areas show the formation of small ice particles, during the system's development.
Natural Colour RGB
Dust RGB with Total Precipitable Water (TPW) field overlaid (Source: EUMeTrain)
The Natural Colour RGB (Figure 2), shows that the air to the west of the thunderstorm line is much more hazy (moist) than the air east of it (e.g. over Poland). In the end, the thunderstorms developed on the leading edge of the moist air that was pushing in from France towards Germany.
This moisture boundary can also be seen in the Dust RGB product (Figure 3), here displayed together with the Total Precipitable Water (TPW) field from the ECMWF model.
Like a severe storm over Hamburg in 2004, the storm on 3 July was related to the leading edge of a warm, sub-tropical airmass (warm front) with upper-level flow from a northwesterly directions.
It is not well known that severe thunderstorms can develop on warm frontal boundaries with moisture convergence; strong shear in low layers; a jet streak aloft with high-level divergence, and an arriving moist unstable airmass.