FLOOD CATASTROPHE 06 - 08 AUGUST 2002 - INITIAL WEATHER SITUATION

by ZAMG

Cloud systems in the initial weather situation

05 August 2002/18.00 UTC - Meteosat IR image
05 August 2002/17.00 UTC - Meteosat VIS image
05 August 2002/18.00 UTC - Meteosat WV image
The above Meteosat images, in the three spectral channels, show the cloud systems at the initial stage of the first flood event in August 2002:

There are two synoptic scale cloud bands:

  1. The northern one extends from N. Spain across France and Switzerland to Austria. Many MCSs are embedded along its southern boundary; they are very well developed and have a high vertical extent. The future development of the flood situation takes place largely as a result of this cloud band, which represents a warm and highly unstable air mass.
  2. The southern band extends from S. Spain across the western Mediterranean and Sardinia to mid Italy.
Both cloud bands are separated by a broad Dark Stripe in the WV image.

The images also show a distinct cloud spiral to the north:

  1. The main spiral can be found over Belgium and Germany
  2. A secondary spiral shows up as a Dark Stripe in WV imagery over Brittany; it is well recognisable and shows features which indicate further development.

Synoptic Situation and air mass characteristics

05 August 2002/18.00 UTC - Meteosat IR image; magenta: height contours 1000 hPa, cyan: height contours 500 hPa
05 August 2002/18.00 UTC - Meteosat IR image; red: temperature advection 700 hPa
05 August 2002/18.00 UTC - Meteosat IR image; blue: thermal front parameter (TFP) 500/850 hPa, green: equivalent thickness 500/850 hPa
05 August 2002/18.00 UTC - Meteosat IR image; magenta, yellow: Showalter index
While there are only very weak pressure gradient at surface levels (1000 hPa), an intensive Upper Level Low (ULL) dominates at a height of 500 hPa with its centre over Belgium and an upper level trough extending from Brittany to N. Spain.

Two baroclinic zones - represented by a high gradient of thickness and a TFP, but only weak temperature advection - are associated with the two cloud bands.

In the image including Showalter index, values of < 3 indicate an unstable air mass, but it is even more unstable near the northern Baroclinic Boundary which contains many MCSs and is accompanied by a Showalter index < 0.

Upper Level Features

05 August 2002/18.00 UTC - Meteosat IR image; red: positive vorticity advection (PVA) 300 hPa, green: height contours 300 hPa
05 August 2002/018.00 UTC - Meteosat WV image; magenta: height of PV = 2 units
Pronounced upper level features are involved in the weather situation; they are indicative of developments triggered by processes taking place in the middle and upper troposphere:
  1. PVA max at 500 and 300 hPa (not shown here) located over central N. Spain and Brittany indicate the approach of upper level troughs
  2. Distinct PV anomalies in the same areas indicate that stratospheric air has penetrated downward to approx. 450 hPa. As in well developed cases, there is a strong relationship between the PV anomalies and black areas/vortices in the WV image.
SUB-MENU OF FLOOD CATASTROPHE 06 - 13 AUGUST 2002
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