24 - 30 SEPTEMBER 1997 - INTRODUCTION AND CONCEPTUAL MODELS

by KNMI


Introduction

Thanks are due to Mr. Riosalido from Instituto Nacional de Meteorologica (I.N.M.) in Madrid in selecting the case and adding important information and advice.

This case study handles the life cycle of an Upper Level Low in chronological order. The system developed over the Atlantic Ocean west of the Iberian Peninsula and reached its mature stage near the entrance of the Strait of Gibraltar on the 28th of September.
From the 26th till the 30th of September 1997 this system was responsible for heavy rainfall especially in the southern part of Spain and the northern part of Marocco. At the east side of the ULL frontogenesis in an unstable environment caused upward motion and the development of cloud bands with embedded Cb's, Cb Clusters and MCSs. The severe weather conditions were at their maximum in the dissolution stage of the Upper Level Low. In the southern and south eastern parts of the peninsula flooding affected many areas and caused a lot of damage to agriculture. In Spain there were five casualties due to the flooding and two as a result of a plane crash in severe weather conditions.

Overview of satellite features and conceptual models

24 September 1997/08.00 UTC - NOAA CH2 image
24 September 1997/08.00 UTC - NOAA CH5 image
The initial situation on 24 September 1997 08.00 UTC. Cloud systems indicating the development of the Upper Level Low, can be seen in South Western Europe:

CF II is approaching the Iberian Peninsula while CF I is in a decaying stage.

To the rear of CF I a trough at upper levels has formed. To the rear of CF II a (small) trough is also developing.

The Trough Stage

24 September 1997/12.00 UTC - Meteosat IR image; green: equivalent thickness 500/850 hPa, red: height contours 500 hPa

The Tear-off Stage

25 September 1997/23.00 UTC - Meteosat IR image; green: equivalent thickness 500/850 hPa, red: height contours 500 hPa

The Cut-off Stage

27 September 1997/00.00 UTC - Meteosat IR image; green: equivalent thickness 500/850 hPa, red: height contours 500 hPa

28 September 1997/00.00 UTC - Meteosat IR image; green: equivalent thickness 500/850 hPa, red: height contours 500 hPa

The Dissolution Stage

29 September 1997/00.00 UTC - Meteosat IR image; green: equivalent thickness 500/850 hPa, red: height contours 500 hPa
30 September 1997/00.00 UTC - Meteosat IR image; green: equivalent thickness 500/850 hPa, red: height contours 500 hPa

SUB-MENU OF 24 - 30 SEPTEMBER 1997
THE UPPER LEVEL TROUGH STAGE