- Most dangerious convective phenomena in Middle Europe
- Diameter: 20 to 50 kilometres (low levels), over 100 kilometres at the top
- Big areas of clouds extending to the tropopause
- Overshootings (below -60°C)
- Fast upward and spatial growing (i.e., fast decreasing of temperature and increasing of areas with cold cloud tops)
- Life time often 6 hours, partly up to 12 hours
- Often hail, heavy precipitation and strong gusts.
We assume that the synoptic preconditions (vertical wind shear, potential instability, dynamical forcings) are fulfilled. In the following the main features of a super cell will be explained.
Fig 1. Relative streaming within a super cell
The diagram above shows the relative streaming within a super cell (RFD: Rear flank downdraft / FFD: Forward flank downdraft). If the lifetime of super cell is long enough we observe a positive feedback between the production of positive vorticity and horizontal convergence (Divergence term) because the upward streaming air packages are stretched. The cold air comming from the super cell (FFD) spread itself horizontally under the warm air streaming to the frontal sited of the super cell. At the boundary of warm and cold air a direct thermal circulation starts equivalent to the production of horizontal vortcity (diagram: „streamwise vorticity"). In the updraft of the super cell the horizontal vorticity is changed to positive vertical vorticity (twisting term). Additionaly, we observe an extreme accelaration and an stretching. The divergence term of the vorticity equation leads to a cyclonic circulation. Now, the positive feed back between convergence and the development of positive vertical vorticity starts yielding finaly an increase of circulation within the lower parts of the atmosphere. Sometimes a tornado develops.
Fig 2. Schematic cross section through a super cell with hail and relative wind
In the figure above, the super cell is moving from left to right. In the lower levels we oberserve a streaming into the super cell whereas in the higher levels an outward streaming happens. Due to the baroclinic situation the updraft is tilted yielding hail. We observe hail behind the gust clouds and a few kilometres after the passage of the gust front. In some cases hail is also possible several kilometres before the super cell. After the onset of hail the downdraft arrives the location with prevailing moderate to severe rain. At the top of the updraft overshooting is likely.
References
- Jacobs, W.: Nutzen von Satellitendaten für Wetterüberwachung und -prognose. Lecture notes, March 2007.
- Manual of Synoptic Satellite Meteorology