HYDROPTIMET - Interreg IIIB

Snapshot of a simulation with MOLOCH (Gard case)






Case Studies


Gard Flood: 8-9 September 2002

During the afternoon and the night between 8 and 9 September 2002, a quasi-stationary mesoscale convective system (MCS) affected the Gard region, southern France and it was responsible of a terrible flood (more than 20 deaths). The system developed over the Mediterranean Sea ahead of a cold front slowly approaching the region from west. Then, it progressed northward over the Gard region and persisted over the same area for several hours. Later on, during the morning of 9 September, it moves eastward together with the cold front.
The observed precipitation, as measured by the raingauge network, reached values greater than 600 mm in 24 hours, and a large part of the region recorded more than 150 mm in 24 hours.
High resolution simulation (hindcast) performed with Moloch (horizontal resolution 2.2 km) shows an intense area of precipitation over the Gard region (more than 400 mm in 24 hours). The convection seems to be triggered by the orography, therefore the area of precipitation is slightly displaced northward.

24 h accumulated precipitation. Left: Moloch forecast. Right: observation (rain gauges network, S. Anquetin, LTHE, Grenoble) interpolated on model grid.

Click to view the animation (gif): 6-h accumulated precipitation and clouds.


Montserrat Flood: 9-10 June 2000

Total 24-h accumulated rainfall (from 21 UTC 9 Jun, to 21 UTC 10 Jun 2000).
During the early morning of 10 June 2000 a heavy rainfall event affected Catalonia (NE of Spain) and caused severe material damages and 5 fatalities. Rainfall in excess of 200 mm in 24 hours was recorded, with accumulated hourly quantities of over 100 mm and rainfall exceeding 150 mm in three hours (M.C. Llasat et al.). While a cold front was moving over Spain, a mesoscale cyclone developed over the Mediterranean Sea, between the Balearic Islands and Spanish coast, leading to convergence of water vapour at low levels over the area affected by heavy precipitation. Moreover strong wind, rotating from E-SE at low levels to SSW above, generated a favourable condition for MCS development. Finally, the convection was triggered by orography.
High resolution simulation (hindcast) performed with Moloch (horizontal resolution 2.2 km) shows a signal of intense precipitation almost in the correct area. However, the coarse resolution simulation (BOLAM) employed as initial and boundary conditions, is affected by an error concerning with the development of the mesoscale cyclone, which moves away too fast. Because of this error, the convective system does not persist enough over the flooded basins. Therefore, forecast precipitation is lower than the rainfall reported by observations.

24h accumulated precipitation. Left: Bolam 36h forecast at 6.5 km horizontal resolution). Right: Moloch 36h forecast.

Click to view the animation (gif): 6-h accumulated precipitation and wind (at 850 hPa isobaric surface)

Piedmont case study: 24-26 November 2002

Between 24 and 25 November 2002, a deep trough progressively extended his influence over the Mediterranenan basin. The flow became southerly and moist air impinged over Alps and Appennine, producing relevant precipitations. A further deepening of the trough and a cut off process took place between 25 and 26 November. A deep menimum isolated south of Sardinia and produced wet south-easterly flow from the Tyrrenian Sea towards western Alps and Appenines.

6h accumulated precipitation at 12 UTC, 24 Nov. 2002. Left: Moloch 12h forecast. Right: raingauges data (Regione Piemonte)


6h accumulated precipitation at 00 UTC, 24 Nov. 2002. Left: Moloch 24h forecast. Right: raingauges data (Regione Piemonte)

Click to view the animation (gif): 6-h accumulated precipitation and wind (at 850 hPa isobaric surface) (24-25 November 2002)
Click to view the animation (gif): 6-h accumulated precipitation and wind (at 850 hPa isobaric surface) (25-26 November 2002)