Topographically Induced Convection: Comparison of Extreme Rainfall Events in Southeastern France from 19 September 2020 and 2 October 2020

Jacob Escobedo

On 19 September 2020, a quasi-stationary MCS formed along the southern foothills of the Massif Central in south-central France. This MCS lasted for nearly thirteen hours before decaying, only to be a replaced by new quasi-stationary MCS near the coast that eventually merged with additional convection and propagated eastward with time. On 2 October 2020, another quasi-stationary convective event occurred in the southern slopes of the Alps in southeastern France. This convective event lasted for roughly sixteen hours before the heaviest rainfall ended. Both events resulted in maximum observed precipitation of more than 500 mm. As is typical of heavy rainfall events in this region, both events resulted in slowly moving synoptic conditions in which the environmental conditions and the forcing for convection persisted. These conditions typically occur when a slow-moving cut-off low is positioned to the west of France and southerly flow from the Mediterranean Sea transports moist low-level air into southeast France, at which point terrain induces ascent and low-level convergence that provides forcing for convection. The maintenance of convection in both cases is similar, in that after convection is initiated, mid- and upper-level winds have a large southerly component that advect hydrometeors away from updrafts and allows the low-level flow to continuously initiate new convective cells along the terrain in nearly the same location. The two cases differ in their later evolution of the quasi-stationary convection, where the Massif Central convection begins to interact with surrounding convection to become cold pool driven and subsequently more progressive, whereas the Alps case, a cold front moves through the region that cuts off southerly flow from the Mediterranean.