A numerical study of three catastrophic precipitating events over southern France. I: Numerical framework and synoptic ingredients

Abstract

This study examines the simulation of three torrential rain events observed on 13-14 October 1995 (the Cévennes case), 12-13 November 1999 (the Aude case) and 8-9 September 2002 (the Gard case) over the southeastern part of France using the Meso-NH non-hydrostatic mesoscale numerical model. These cases were associated with extreme Heavy Precipitation Events (HPEs) with significant precipitation amounts exceeding 500 mm in less than 24 hours. Several sets of numerical experiments were performed with 10 km and 2.5 km horizontal resolutions. In part I of this study, special attention is paid to the experimental design for obtaining realistic simulations of HPEs with the Meso-NH model, as well as the evolution of the synoptic patterns in which the rainfall events are embedded.The best 2.5 km numerical simulations show the ability of the Meso-NH model to reproduce significant quasi-stationary rainfall events. Moreover, the model fairly reproduces the low-level mesoscale environments associated with the three HPEs. The HPEs formed in a slow-evolving synoptic environment favourable for the development of convective systems (diffluent upper-level southerly flow, PV anomalies, etc.). At lower levels, a southerly to easterly moderate to intense flow provided conditionally unstable and moist air as it moved over the relatively warm Mediterranean Sea, typical for this time of the year (late summer and autumn). The two extreme cases (Gard and Aude) differ from the more classical event (Cévennes) in terms of larger low-level moisture fluxes. Weaker values of conditional convective instability, as in the Aude case, is counterbalanced by a stronger warm and moist low-level jet. The mesoscale triggering and/or sustaining ingredients for deep convection and the physical mechanisms leading to the stationarity of these rainfall events are presented and discussed in a companion paper.

Idealized numerical simulations of quasi‐stationary convective systems over the Northwestern Mediterranean complex terrain

“…Nuissier et al (2011) found that the synoptic-scale pattern propitious to HPEs over the French Mediterranean region consist of an upper-level trough located over the west of France and an upper-level ridge over central Europe. Nuissier et al (2008) underlined the slow evolution of this synoptic pattern, which is an essential condition to maintain favourable low-level mesoscale conditions for a long enough time.…”

“…Meso-NH has already shown its capability to simulate HPEs in either a real or idealized framework (Ricard, 2005;Nuissier et al, 2008;Ducrocq et al, 2008;BRD09). For this study, the domain is centred over the southeastern regions of France and has a horizontal grid spacing of about 2.4 km (Figure 1).…”

“…Usually, the heaviest precipitation is located along the mountain range foothills facing the moist low-level sea winds. For the French Mediterranean region, a typical location is the Massif Central foothills (see figure 2 of Nuissier et al, 2008). However, in some cases the quasi-stationary MCS anchors further upstream, over the plains or even over the sea.…”

Two‐way one‐dimensional high‐resolution air–sea coupled modelling applied to Mediterranean heavy rain events

“…Heavy rainfall events are characterized by mesoscale systems generating large precipitation totals that could persist for several days. The synoptic patterns leading to these events are generally well known, but the intensity and the stationarity of these systems are mainly controlled by various mesoscale factors, including the Mediterranean Sea, the surrounding mountains, cooling beneath the convection due to precipitation evaporation, low-level wind convergence and moisture transport Lionello et al, 2006;Nuissier et al, 2008). Most often, a moderate to intense low-level offshore jet, which is a governing-factor in the air-sea energy exchanges, characterizes the meteorological environment of these events.…”

“…The results of these previous studies advocate using in our study an atmospheric model able to represent Mediterranean heavy precipitation events in a realistic way, including the intensity of the heavy rainfall and the low-level jet associated with these events. The MESO-NH model (Lafore et al, 1998) used at a kilometric-scale resolution has proven its ability to capture these events quite well Lebeaupin et al, 2006;Nuissier et al, 2008) and has therefore been selected for the atmospheric component of our air-sea coupled system. In our study, MESO-NH is coupled to a one-dimensional ocean model (Gaspar et al, 1990) that has been shown to represent relatively well the vertical mixed layer structure and the SST evolution (Blanke and Delecluse, 1993;Alexander et al, 2000).…”