Conceptual model of a super cell

The following characteristics of a super cell can be found:

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 coming 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: „stream wise vorticity"). In the updraft of the super cell the horizontal vorticity is changed to positive vertical vorticity (twisting term). Additionally, we observe an extreme acceleration 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 over shooting is likely.

The following diagram's show different structures which we sometimes observe during strong convection (squall lines, super cells). Bow echoes are often connected to heavy rain and/or hail and heavy gusts. That is especially the case at the southern tip of a bow echo. In this case study several radar images and satellite images exhibit bow structures.

Just for completion, Hook echoes are connected to even more intense convection. This is a hint to rotating super cells and a risk to tornado formation.

Fig. 3 Different echo forms during intense summer convection