Basic parameters like surface pressure (height of 1000 hPa) and upper level height (here at 300 hPa) do not reveal the whole physical background of the situation and cannot fully explain the details seen in the cloud features. In the following chapters the case will be further investigated using relevant derived meteorological parameters overlayed on satellite images.
Equivalent Potential Temperature (usually referred to as ThetaE) is a quantity related to the stability of a column of air in the atmosphere. The ThetaE increases with increasing temperature and increasing moisture content. Therefore, areas of relatively high ThetaE (called ThetaE ridges) are often the burst points for thermodynamically induced thunderstorms. ThetaE ridges can often be found in the areas with the greatest warm air advection and moisture advection. In this chapter ThetaE overlayed on Meteosat 8 IR 10.8 µm imagery is presented.
Isotachs at 300 hPa show the distribution of the wind speed and the approximate location of the jet streak. Enhanced convective cloudiness lies within the left exit region of the jet. Superimposed upon the enhanced cloudiness the maximum of PVA at 300 hPa can be found. Isotachs and PVA at 300 hPa are overlayed on Meteosat 8 IR 10.8 µm satellite images and shown in a time sequence at 6-hourly intervals.
Showalter index is a measure of thunderstorm potential and severity. In other words, it gives a good indication where the atmosphere is unstable and where convective development may be expected. Showalter indices between 1 and 3 give the indication that thunderstorms are possible but a strong trigger is still needed. For values between -2 and 1 chance of thunderstorm increases. Values between -3 and -2 are a sign of very unstable atmosphere with a good potential for thunderstorms. Values under -3 are associated to heavy thunderstorms.
In WV 6.2 µm satellite images the intrusion of dry stratospheric air to the lower levels of the atmosphere is represented in the form of the dark stripes. In this chapter the information in the WV images is supported by the field of potential vorticity at the height of 300 hPa.
Investigation of the derived parameters helps us to understand the processes which happened during the intense convective growth
within and ahead of the cold front.
Stability of the atmosphere is studied by NWP fields of the equivalent potential temperature at 850 hPa and the Showalter index
combined with the infrared satellite images. The area of maximum convection fits well with negative values of the Showalter index and
the maxima in the ThetaE field.
The intensification of convective development takes place mainly at the left exit region of the jet streak coming from the north-west,
as well as the one coming from the east and another maximum is connected to the right entrance region of the jet coming from the south.
Downward protruding stratospheric air is documented in the WV 6.2 µm satellite images
in form of dark stripes or darker area. This detection is also supported by model fields of Potential Vorticity at 300 hPa.