Prévisibilité de précipitations intenses en Méditerranée : impact des conditions initiales et application aux inondations d'Alger de novembre 2001

Abstract : Despite continuous improvements of operational numerical forecasting systems, some rare events still remain hardly predictable even at very short range. In these cases, the skill of the numerical prediction is mainly limited by the quality of initial conditions. In this context, it appears to be essential to improve existing methodologies or develop new ones able to account for the uncertainties inherent in the initial state, in particular for forecasting severe meteorological events such as heavy rainfall in the Mediterranean. The impact of initial condition uncertainties on the numerical prediction of the so-called Algiers' 2001 Superstorm was examined through cloud-resolving atmospheric simulations conducted with the Meso-NH model. This event was characterized by a deep upper-level trough associated with strong surface cyclogenesis. It was shown that minor initial perturbations in the depth of the upper-level trough (and the intensity of the associated positive PV anomaly) at the initial stage of cyclogenesis could propagate and intensify during numerical simulations, strongly impacting the surface cyclone forecast. A closer look at the development of deep convection in each experiment showed that small-scale differences observed on the precipitation fields were directly related to the cyclone forecast and thus to the propagation of the initial perturbations. Slight changes in the surface cyclone forecast led to drastic modifications of the low-level flow and impacted the generation of deep convective clouds and their associated precipitation. In the second part of this work, the influence of potential vorticity (PV) modifications on numerical simulations of this deep Mediterranean cyclone was investigated. Using the French operational numerical weather prediction model ARPEGE, we showed that the forecast starting from the operational analysis of 9th November at 1200 UTC was not able to accurately reproduce the life cycle and the deepening of the surface depression. It appears that local corrections of the upper-level PV field guided by METEOSAT7 water vapour observations could lead to a substantial improvement of the forecast in terms of surface pressure and accumulated precipitation. Using two very high resolution (2 km) Meso-NH simulations and the so-called model-to-satellite approach, we showed that the impact of initial PV corrections was positive, especially during the periods of strong convective activity preceding heavy precipitation.
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Submitted on : Thursday, January 28, 2010 - 9:56:53 AM
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Sébastien Argence. Prévisibilité de précipitations intenses en Méditerranée : impact des conditions initiales et application aux inondations d'Alger de novembre 2001. Océan, Atmosphère. Université Paul Sabatier - Toulouse III, 2008. Français. ⟨tel-00451051⟩

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