Meteorological and Physical Background

MCSs frequently develop at the leading edge of frontal cloud bands. This is most likely the situation where there is a specific vertical distribution of air mass and conveyor belts: a warm conveyor belt at low levels in front of the front and a dry intrusion in the layers above.

Such a situation develops when a jet streak crosses the front; dry air on the cyclonic side of the jet axis overruns the frontal cloud band and reaches its leading edge. Whilst the relative streams of the dry intrusion are sinking behind the frontal cloud band, they change to rising during the crossing of the front.

The troposphere under such conditions is characterized by (see Key Parameters):

  • frontal conditions indicated by TFP, a zone of high thickness gradients, a downward inclined gradient of isentropes, a frontal circulation created through convergence and upward vertical motion, etc.
  • a rising warm conveyor belt in low to middle layers along the leading edge of the frontal cloud band
  • a (rising) dry intrusion in middle to high layers along the same leading edge of the frontal cloud band
  • the effect of both conveyor belts lying on top of each other leads to the unstable stratification of the troposphere

30 May 2005/00.00 UTC - Meteosat 8 IR10.8 image; blue: thermal front parameter 850 hPa, red: temperature advection 700 hPa
30 May 2005/00.00 UTC - Vertical cross section; black: isentropes (ThetaE), orange thin: IR pixel values, orange thick: WV pixel values
30 May 2005/00.00 UTC - Meteosat 8 IR10.8 image; magenta: relative streams 322K - system velocity: 221° 13 m/s, yellow: isobars 322K. Position of cross section indicated
30 May 2005/00.00 UTC - Meteosat 8 IR10.8 image; magenta: relative streams 326K - system velocity: 221° 13 m/s, yellow: isobars 326K. Position of cross section indicated

The satellite image shows a frontal cloud band extending from Morocco across the Mediterranean Sea, Spain and France and further north over the Netherlands and Denmark. Along the leading edge of the cloud band several MCSs have developed, especially over the Elzas and Germany. The first image (top left) shows the thermal front parameter with the temperature advection field superimposed. The MCSs have developed ahead of the surface cold front, indicated by the maximum values of thermal front parameter. The cross section (top right) crosses the Cold Front over the North Sea and the Netherlands and the convective system situated over Germany. The relative streams on the isentropic surface of 322K (bottom left) show that the MCS is under the influence of the moist air in the warm conveyor belt. On the higher isentropic surface at 326K (bottom right) the whole area of the MCS is under the influence of a relative stream of dry intrusion. This vertical distribution of conveyor belts, together with upward motion, leads to the development of a conditionally unstable stratified troposphere.

29 May 2005/18.00 UTC - Meteosat 8 WV6.2 image; yellow: isotachs 300 hPa, red: vorticity advection 300 hPa
29 May 2005/18.00 UTC - Meteosat 8 WV6.2 image
30 May 2005/00.00 UTC - Meteosat 8 WV6.2 image; yellow: isotachs 300 hPa, red: vorticity advection 300 hPa
30 May 2005/00.00 UTC - Meteosat 8 WV6.2 image

The satellite image (top left and right) shows the development of MCSs over Germany and the Elzas.
The MCS development is situated within the area of the left exit region of the jet streak which extends from the south coast of Ireland (approximately 51N/09W), across the Bay of Biscay into Normandy (top left). The left exit region is accompanied by a pronounced PVA maximum at 300 hPa.
The WV image clearly shows the jet axis (dry air along the cyclonic side and moist air along the anticyclonic side of the jet axis) from the Atlantic, South of Ireland and England. The MCS cloudiness itself is characterized in WV by a black area around each MCS which is typical for such systems (see Appearance in Satellite Data).

Six hours later, the MCSs within the area of the left exit can be found over Western part of Germany and the Elzas. Other MCSs have developed downstream above Southern France and in Southern Spain (Andalusia). The WV image again very clearly shows the jet axis (bottom right). The jet axis can still be found just south of Ireland with an extensive dark area over South-East England and Normandy. The cloud tops along the cyclonic side are continuously increasing downstream which can be explained by the supply of more humid air in upper levels. The surroundings of the MCS cloudiness are again characterized by this typical black (dry) area around of the cell (see Appearance in Satellite Data).

MCSs develop in connection with warm air, therefore they can be found in front of or within the leading parts of Cold Fronts. Convective systems which develop in different synoptic environments and are described in chapters Cumulonimbus Cluster, Enhanced Cumulus and Comma. (see Cumulonimbus Cluster , Enhanced Cumulus and Comma ).