High resolution numerical study of the Algiers 2001 flash flood: sensitivity to the upper-level potential vorticity anomaly

From 9 to 11 November 2001, intense cyclogenesis affected the northern coasts of Africa and more particularly the densely populated city of Algiers. During the morning of 10 November, more than 130 mm of precipitation was recorded at Bouzareah and resulted in mudslides which devastated the Bab-el-Oued district. This disaster caused more than 700 casualties and catastrophic damage. Like many other heavy rainstorms in the western Mediterranean, this event was associated with the presence of an upper-level trough materialized by a deep stratospheric intrusion and characterized by high potential vorticity values. In this study, the impact of this synoptic structure on the localization and intensity of the precipitation which affected Algiers is investigated using a potential vorticity (PV) inversion method coupled for the first time with the French non-hydrostatic MESO-NH model. A set of perturbed synoptic environments was designed by slightly modifying the extent and the intensity of the coherent potential vorticity structures in the operational ARPEGE analysis. It is shown that such modifications may have a strong impact on the fine-scale precipitation forecast in the Algiers region, thereby demonstrating the fundamental role played by the potential vorticity anomaly during this exceptional meteorological event

Quantum cascade laser frequency stabilisation at the sub-Hz level

Quantum Cascade Lasers (QCL) are increasingly being used to probe the mid-infrared "molecular fingerprint" region. This prompted efforts towards improving their spectral performance, in order to reach ever-higher resolution and precision. Here, we report the stabilisation of a QCL onto an optical frequency comb. We demonstrate a relative stability and accuracy of 2x10-15 and 10-14, respectively. The comb is stabilised to a remote near-infrared ultra-stable laser referenced to frequency primary standards, whose signal is transferred via an optical fibre link. The stability and frequency traceability of our QCL exceed those demonstrated so far by two orders of magnitude. As a demonstration of its capability, we then use it to perform high-resolution molecular spectroscopy. We measure absorption frequencies with an 8x10-13 relative uncertainty. This confirms the potential of this setup for ultra-high precision measurements with molecules, such as our ongoing effort towards testing the parity symmetry by probing chiral species

Preliminary study of an intense rainfall episode in Corsica, 14 September 2006

International audienceA preliminary study of the 14 September 2006 heavy rainfall episode in Corsica is presented. Two sets of initial and coupling fields were used to run the French non-hydrostatic mesoscale model, MESO-NH. Three interactive 2-way nested domains were used, having 150×120, 150×150 and 125×200 grid points and 50 km, 10 km and 2 km mesh sizes. 50 vertical levels were defined. From ground level to 2000 m, the vertical resolution was 60 m, whereas it was 600 m above. Convection was parameterized for the coarser and intermediate domains and assumed to be explicitly resolved for the 2 km grid. At large scale, the situation is well represented by both simulations but, at the fine scale, they present significant differences. Comparison with the observations did not allow the two simulations to be discriminated. Even though the main high-level dynamical ingredients usually associated with rain storms were present in both simulations, their fine scale evolution was not reproduced well. Near the surface, there were significant differences between the two simulations. Vertical motion fields associated with convective phenomena were different in the innermost domain. A cold pool acting as a warm front off the east coast was associated with the main rainfall area over the sea in one simulation whereas in the second one, it seemed that the Corsican mountains acted as a barrier, leading to inland rainfall. This preliminary study gives perspectives for future sensitivity tests. For example, an investigation is planned of the influence of the high level dynamics, the SST and a better description of the orography

Study of an intense rainfall episode in Corsica

The Mediterranean basin is known to present one of the highest concentration of cyclones in the world. The Genoa region is the area where the concentration of cyclones is maximal. The continental topography around the Mediterranean Sea forces many cyclonic developments. Corsica is a mountain surrounded by the sea at the middle of the occidental Mediterranean basin. It is 2710 metres at its highest point. It has twenty other mountains of over two thousand metres and the average altitude is 568 m. Corsica has the highest mountains and the most rivers of any Mediterranean island. On September 14 2006, a heavy rainfall episode reached the eastern part of the Island with accumulated precipitations higher than 400 mm which caused impressive material damages. This rainfall episode was quite well forecasted at large scale but neither the intensity nor the fine scale location of the prevision was good enough. In this study, two initial and coupling fields set (ECMWF and ARPEGE analyses) are used to run the French Mesonh model to investigate the case study. Three interactively nested model domains are used. The finest domain is centred on Corsica; its horizontal mesh size is 2 km. If the increase of the simulation resolution and a better description of the synoptic environment of the situation allow a better forecast of the rainfall quantity, the localization seems related to the precision of the description of the craggy relief. This work lies within the scope of the HyMEX (Hydrological cycle in the Mediterranean EXperiment) project.[no pdf

Intense rainfall episode in Corsica : sensitivity study using a mesoscale model.

10-13 septembre 2007

Mountain meteorology in the middle of the western mediterranean basin : Study of an intense rainfall episode in Corsica, 14 september 2006.

3-8 juin 200

Summer convection in the Vosges-Black Forest region: pre COPS investigations

International audienceAs a preliminary investigation for the COPS experiment, a series of 10 rainy days from summer 2006 was studied with the Meso-NH model, run at very high-resolution (2 km), initialized and forced with the ECMWF and ARPEGE analyses. In most cases, the results were not very sensitive to the type of analysis and the model succeeded reasonably well in reproducing the observed precipitation. In the few cases for which the sensitivity to the analysis was significant, the model performance was quite poor. It seems that a discrepancy between the ECMWF- and ARPEGE-driven simulations could be an indicator of low predictability. Furthermore, model skill appears weaker for the precipitation occurring over the Black Forest than for the precipitation affecting the Vosges

Meso-NH forecasts during COPS. Preliminary results.

(décembre 2007