Summary
The following conclusions for practical work can be summarised:
- Satellite images enables us to study both the large scale situation (e.g., cyclones, fronts) and regional structures (e.g., thunderstorms, orographic effects). For a better understanding of the dynamical processes they should be combined with model data. We used in this case study the potential vorticity at 310 K, wind in 300 hPa (about jet niveau) and the geopotential in 300 hPa.
- For a complete synoptic overview also surface data, radiosoundings and other remote sensing data like radar and lightning detections should be used. This leads to relations between cloud structures and relevant weather situations and a better weather monitoring.
- Radar data give a better insight in small scale structures of intensity and types of precipitation than satellite images.
- IR-images deliver cloud top temperatures and the shape of cloud structures. The cloud top tempertures yields hints to the precipitation's intensity and in combination with the shape to the intensity of convection. In this case study we saw that during winter and early spring even severe thunderstorms occure although the cloud top temperatures (CTT) are "only about -45°C". The rule "CTT<-55°C: hail possible" and "CTT<-60°C: hail almost certain" is only valid during summer convection in Middle Europe.
- The airmass-RGB delivers not only information about the cloud structures but also about dynamical processes (in this case study, PV-maxima, left exit of jet streams, split front).
- The RGB "Convective storms" is useful for distinguishing different particles' sizes and phases, however, only during daytime. Relevant for convection are bright yellow spots which are hints to a convective cell of high potential to intensify with strong updrafts. However, this RGB is only useful during summer convection. Because the convection of this case study was of winter-type this RGB yielded only information of limited use.
- In order to investigate small structures, the RGB (HRV-HRV-IR10.8) is most suitable because the horizontal resolution of the HRV is even in Middle Europe 2 km. During summer time overshootings can be detected yieding most active parts of thunderstorms. However, they are more common during summer convection. In this case study orographic effects could be investigated (Foehn, Lee effects with lee clouds).
The following synoptic and dynamic processes will be pointed out:
- Emma-storm was a typical example of line convection with downbursts during a southeast movement of a storm field over the North Sea. (Emma's centre moved along the area south of Iceland over South Norway to the east.)
- The average velocity of the convective line was 110 km/h, sometimes 150 km/h.
- The surface analysis revealed a strong surface pressure gradient.
- Behind the cold front dry atmospheric air (equivalent to high Potential Vorticity (PV)) could be recognised in the airmass RGB. However, corresponding radiosoundings (partly from NWP) reveal more humid air in the lower levels (below 600 hPa) which is typical for potential unstable air. This air was lifted due to positive vorticity advection.
- A strong vertical wind shear was observed (mean wind speeds near the surface: 15 to 20 knots, in 850 hPa (about 1300 metres above NN) between 50 and 70 knots). This is a good indication for the development of convection within a cold front (Bader et al., 1995).
- At the rear of the split front a strong flow existed. This constellation is also a hint to the developmment of a convective line.
- The strong downbursts developed due to the vertical exchange of impulse yielding gusts similar to the wind speed in 850 hPa.
- If the forecaster is pre-warned by corresponding NWP-fields he/she should monitor the airmass-RGB and look for dry intrusion (split front?), the position and orientation of the jet (left exit of a jet?), overruning dry air (high PV over frontal cloud band (pinkish to brownish cloud structures)?). In combination with strong windshear the forecaster should be aware of line convection and strong downbursts. During weather monitoring additional data like radiosundings, observations, radar and lightning detection should be used.