Orographic triggering of long-lived convection in three dimensions

A significant fraction of the occurrences of intense flash floods is due to quasi-stationary or long-lived convection that may insist on the same place for many hours, producing high values of accumulated precipitation. One of the elements that favour the initiation and anchoring of the convective system (MCS) is the orography. In one of the most severe floods (Gard basin in southern France, 8-9 September 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. In the present work the initial atmospheric conditions of this event have been largely idealized, taking horizontally uniform values for wind, temperature and humidity profiles, and a simplified isolated orography representing the sole Massif Central. A convective system is initiated in the non-hydrostatic simulations, embedded in a quasi-stationary solution of flow over the orography. It is shown that the triggering of convection occurs in the convergence zone immediately upstream of the orographic obstacle, at an altitude comparable with the mountain height. The subsequent growth of the mesoscale convective system is associated with a slow eastward drift, with the intense precipitation located upstream of the mountain and with the formation of a gust front that propagates against the incoming basic flow. Sensitivity experiments show that the development of convection critically depends on mountain height and moisture content. Although the results obtained in such idealized conditions do not reflect all the observed characteristics of the real event, they contribute to clarify the role of the orography in triggering and maintaining strong convection.