Abstract : E.4
Orographic influence on quasi-stationary intense convection in three dimensions

Silvio Davolio, Andrea Buzzi, Piero Malguzzi

Some of the most intense local flood events are due to quasi-stationary convection that may insist on the same place for many hours, producing high values of accumulated precipitation. One of the elements that favour the anchoring of the convective system (MCS) is the orography.

In one of the most severe floods (Gard basin in Southern France, 8-9 Sept. 2002), the orography of the Massif Central played a rather unusual role, favouring the onset and maintenance of the MCS at some distance upstream of the main orographic slope. The low level upstream convergence induced by the Massif Central seemed to have played a role in the triggering of the convection, while the convective downdraft(s) and associated low level cold outflow appeared to directly interact with the orography in the mature stage of the MCS, influencing the its subsequent evolution of the system.

In this study, the initial atmospheric conditions of the Gard case have been largely idealized, taking horizontally uniform conditions for wind, temperature and humidity profiles, and a simplified isolated orography has been considered. Convection has been simulated using the non hydrostatic model MOLOCH. A convective systems is initiated embedded in a quasi-stationary solution of flow over the orography, obtained numerically.

The numerical experiments are aimed at understanding the complex mechanisms of upstream orographic influence on the convective life cycle, as a function of the environmental profiles of wind, temperature and humidity. It is shown that the triggering of convection occurs in the convergence zone upstream of the orographic obstacle. The modifications induced on the subsequent trajectory and evolution of the simulated MCS's are analysed. Although the results obtained in such simplified conditions can be applied with caution to the real events, the results of this study contribute to a better understanding of the problem of interaction between strong convection and orography.