Sensitivity of torrential rain events to the sea surface temperature based on high‐resolution numerical forecasts

Abstract

[1] Torrential rains often occur in the western Mediterranean region during the fall season when the Mediterranean Sea is still warm. The Mediterranean Sea acts in moistening and warming the low level of the atmosphere. Then, the southerly to easterly flow that prevails before and during torrential rainfall events, transports the conditionally unstable air toward the coasts where the convection can develop often triggered by a flow interaction with orography. This study examines the sensitivity to the sea surface temperature (SST) of very short range (18-24 hours) high-resolution (2.4 km) forecasts of heavy precipitation events. Three torrential rainfall events were selected as representative of extreme rainfall events that occurred over southern France: two cases of quasistationary mesoscale convective systems and one other case characterized by a slow moving frontal perturbation. For each case, a number of runs is performed with the MESO-NH research model using several SST fields differing in their resolution or/and their average value over the Mediterranean basin. Results show that an increase (a decrease) of SST by several degrees, on average, intensifies (weakens) the convection. The convection can even be stopped with strong decrease of SST. Impacts on the stationary behavior of the systems have also been pointed out. A more detailed SST field influences the mesoscale patterns of the sea surface heat fluxes but have almost no significant effect on the convection and the low-level jets forecast. Eventually, the SST has been allowed to evolve during the runs through the action of the air-sea interface fluxes, resulting in local effects such as significant cooling of the SST beneath the low-level jet but almost no impact on the very short range forecasts of heavy precipitation.Citation: Lebeaupin, C., V. Ducrocq, and H. Giordani (2006), Sensitivity of torrential rain events to the sea surface temperature based on high-resolution numerical forecasts,

Coupled atmosphere ocean climate model simulations in the Mediterranean region: effect of a high-resolution marine model on cyclones and precipitation

“…It results in an increase of the available energy and moisture for the atmospheric convection and consequently for precipitation. Lebeaupin et al (2006) highlighted that an SST increase induces systematically a CAPE (convective available potential energy) increase. Giordani and Caniaux (2001) have shown that the differential surface heating/moistening and thus the spatial variability of surface fluxes can be a significant source of ageostrophy, vertical velocity and vorticity for the atmosphere that plays a fundamental role in the cyclone development.…”

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

“…To study the coupled processes, the system is run either in the full interactive coupled mode or in the more traditional way in which only the ocean model is driven by the imposed atmospheric forcing. It is applied to the same three Mediterranean heavy precipitation events as those simulated by Lebeaupin et al (2006) and Lebeaupin Brossier et al (2008) (Figure 1(b)), which enables comparison with those studies. Additional sensitivity experiments allow us to see the impact of the different components of the intense atmospheric forcing (precipitation, low-level winds and air-sea fluxes) on the Ocean Mixed Layer (OML).…”

“…The Mediterranean Sea contributes to fuelling these heavy precipitating systems in heat and moisture (Millan et al, 1995;Romero et al, 1997;Pastor et al, 2001;Homar et al 2003;Lebeaupin et al, 2006;Lebeaupin Brossier et al, 2008) and could also play a part in amplifying floods when the swell and waves perturb the river runoff. Based on convective scale numerical simulations of three torrential rainfall events over southeastern France, Lebeaupin et al (2006) and Lebeaupin Brossier et al (2008) have shown that the atmospheric convection intensity and consequently the surface rainfall totals are notably sensitive to the energy available in the atmospheric low levels supplied by the sea through the surface heat fluxes.…”

“…Based on convective scale numerical simulations of three torrential rainfall events over southeastern France, Lebeaupin et al (2006) and Lebeaupin Brossier et al (2008) have shown that the atmospheric convection intensity and consequently the surface rainfall totals are notably sensitive to the energy available in the atmospheric low levels supplied by the sea through the surface heat fluxes. These last strongly depend on the intensity of the low-level jet as well as on the mean seasurface temperature (SST) over the sub-basin.…”

“…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).…”