Summary
The case study describes the development of severe convection over Germany on 27 July 2006, which took place in the forefield of a cold front and an upper air trough. With the trough's approach the potential unstable warm and moist air over Central Europe was lifted. Supported by solar insolation, single cells and multi cells as well as super cells developed over eastern France and Germany, finally forming a north-south oriented squall line. Heavy precipitation of up to 30 mm/h and gusts of up to Beaufort 10 were registered.
Following conclusions can be summarised:
- The convective development took place in the area between warm, moist subtropical air and dry air.
- The air over Germany was characterized by high potential instability as seen in the KO-index and the evaluation of a single radiosounding (Bergen). The radiosounding of Bergen shows warm air advection below 3 km and small advection above, yielding an intensification of potential instability. The pseudopotential temperature and precipitable water also indicate humid air.
- The two conceptual models "Enhancement of convection by PV" and "Convective cloud features at the leading edge of frontal cloud bands" could be well applied to this case study. The WV image (in relation to the model parameter IPV) showed this in a classical way. In comparison to the WV image the airmass RGB shows information about middle or low clouds, too. The Airmass RGB also reveals IPV maxima in relation to different cloud patterns more clearly, which leads to a better monitoring for this area. The airmass RGB is very powerful for indentifying areas with a higher risk of convection's onset during the next few hours. First convection can be detected whereas the WV images reveal only the processes in the middle and upper troposphere.
- From the IR-enhanced images, a rapid vertically and horizontally growing of cells can be seen, leading to the development of multi cells and super cells over Germany. The transformation of cloud structures was visible as follows: Super cells could be identified by IR-enhanced because the lowest cloud top temperature was near the centre of a big cirrus shield. High dBz values in radar images and lightning detection near the cell's centre indicate the development of super cells, too.
- The IR enhanced reveals best the conceptual model "cell splitting" or "severe right moving" in a super cell over northern Germany.
- The RGB "Convective Storms" gives results about the ice particles' size that is directly connected to the updraft's intensity and to the possibly cell development during the next minutes (yellow and rounded shape). In contradiction to the airmass RGB "Convective Storms" is very useful for detailed diagnosis of already existing convective cells.
- Due to the high horizontal resolution overshootings were best detectable in the highly resolved visible channel (HRV) or the composite (HRV/HRV/IR) during daylight. During night IR-enhanced is most useful for investigating this phenomenon. It was not possible in this case study to consider differences (e.g., WV minus IR). More can be found in MSG interpretation guide.
- The Nowcasting-product "Lifted index" gives for nowcasting purposes in for each cloudless pixel and every 15 minutes valuable hints to potential instability. However, 12h-forecasts with the ECMWF exhibit a better quality. In any case information about forcings (e.g., upward motion) are required, too.