Storm Catastrope Atlantic and Western Europe - Case Study 25 - 28 December 1999

• I. Introduction
• II. Overview
• Summary of typical cloud features
• Lothar and Lothar successor
• Martin
• III. Physical Background
• Baroclinic zone and surface wave
• Upper level jet streak and left exit region
• Relation between upper level jet streak and surface wave
• The role of Potential Vorticity (PV)
• IV. Weather
• V. Numerical Model forecast problems
• VI. References

Summary of typical cloud features

The main features in satellite imagery indicating the processes of a "Rapid Cyclogenesis" are (for more information compare manual Conceptual Models: Rapid Cyclogenesis ):

• During the initial stage of "rapid cyclogenesis" the following typical cloud structures can be identified:
  • An (often zonally oriented) multilayered frontal cloud band associated with a baroclinic zone;
  • A Warm Front Shield towards the front of the frontal cloud band;
  • A cloud bulge protruding from below the frontal cloud band into the cold air consisting of lower clouds; this is indicative of an existing Wave;
  • A Dark Stripe in WV imagery, along the cold side of the frontal cloud band, indicating sinking dry air with high values of PV;
• Indications for an imminent start of rapid deepening can be:
  • A rapidly increasing Dark Stripe in IR and WV to the rear of the low cloud head;
  • Strong negative values in motion corrected difference images (see Basics: Satelllite Channels - Artificial and Combination Channels ) indicating the drying of descending air as well as a decay in cloudiness.