Les fortes précipitations et les crues-éclair

In autumn, western Mediterranean is prone to heavy precipitation and devastating flash floods. Daily precipitation above 200 mm is not rare during this season, reaching in some cases as exceptional values as 700 mm recorded in September 2002 during the Gard (France) catastrophe. Large amounts of precipitation can accumulate over several day-long periods when frontal disturbances are slowed down and strengthened by the relief (e.g. Massif Central and the Alps), but huge rainfall totals can also be recorded in less than a day when a mesoscale convective system (MCS) stays over the same area for several hours.

 Heavy precipitation climatology

Different categories of precipitating systems affect the Mediterranean areas, according to the season, region and mechanisms of formation. They include orographic precipitation, rainy frontal systems, MCSs and isolated thunderstorms. This precipitating system spectrum is also enlarged by the diversity of cyclones encountered over the Mediterranean region : Atlantic cyclones, African cyclones, thermal lows, hurricane-like lows, Middle-East lows, orographic cyclones, etc.

One characteristic of the Mediterranean precipitating systems is their inclination to produce heavy rain. Daily surface rainfall greater than 200 mm is not uncommon for Mediterranean precipitation events. Most of these intense precipitation events occur during the autumn season over the Western Mediterranean region while the peak of precipitation over the Eastern Mediterranean occurs between December and February.

The occurrence of HPEs is related to specific synoptic patterns. Based on automatic classification of the 500 hPa geopotential fields from ERA-40, it was found that HPEs in the Gulf of Lion are found preferentially associated with a trough-ridge system, with low pressure over Spain and high pressure over Central Europe.

Large amount of precipitation can accumulate over several day periods when a single or a succession of several frontal systems are slowed down and enhanced by the relief of the region. In other cases, the large rainfall amount can be recorded in only few hours when a MCS, sometimes in association with an extratropical cyclone, becomes stationary over an area during several hours. The amount of precipitation is related to the characteristics (intensity, duration, organization) of the precipitating systems and in particular to their motion. The quasi-stationary convective systems are powerful flash-flood producing precipitating systems. Frequently, these quasi-stationary MCSs are backward regenerative systems that take a V-shape in the infrared satellite imagery and in the radar reflectivity images. Backward regeneration is obtained by a continuous generation of new cells at the tip of the V, whereas the mature and old cells are transported towards the V branches.

A climatological approach has been developed to characterize the mesoscale environment in which heavy precipitating events (HPEs) develop in the western Mediterranean area.
Aladin analyses for HPE cases provide useful tools for such a study. Different diagnostics can be used to document the time evolution of mesoscale features associated with the HPEs such as low-level jets (intensity, orientation, moisture transport ... ) and other key ingredients (CAPE, precipitable water, moisture convergence ...).

 Factors leading to Heavy precipitating events (HPEs)

HPEs are multiscale atmospheric phenomena that result from a complex interaction of upper-level synoptic flow and local topography. The synoptic and mesoscale environmental ingredients leading to HPEs over mountainous regions are the same as those encountered for HPEs over other mountainous regions of the world :

  • conditionally or potentially unstable air masses,
  • moist low-level jets (LLJ) that impinge the first foothills,
  • steep orography which helps to release the conditional instability associated with the low-level jet, a slowly evolving synoptic pattern that slows down the advance of heavy precipitation
  • systems or maintains the environment favorable to heavy precipitation.

For some cases, upper-level precursors as upper-level Potential Vorticity (PV) streamers or a deep short trough can be found to approach the triggering area of convection. By reducing the static
stability, intensifying low-level jets and upper-level divergence, these upper-level dynamical structures favour the upward motion and consequently convection. However, the orography as well as diabatic processes associated with convection can alter the streamer evolution, so that the relationship between fine scale structures of PV and heavy rainfall events remains to be clarified.
Upslope triggering is not the only process involved in the conditional instability release in this region. For instance, the role of the nearby mountain ridges in enhancing the low-level jet and/or inducing upwind low-level convergence has been pointed out. Indeed a number of convective systems form over the Mediterranean Sea before anchoring inland as examplified by the extreme rainy event that occurred in September 2002 in South-eastern France. The shape and fine-scale structure of the mountain range also play a role in modulating the precipitation. In some cases, cold pool resulting from evaporation/sublimation/melting of the falling precipitation may trigger new cells at its leading edge, far upstream of the mountain.

Further reading :

  • Delrieu, G., V. Ducrocq, É. Gaume, J. Nicol, O. Payrastre, E. Yates, P.-E. Kirstetter, H. Andrieu, P.-A. Ayral, C. Bouvier, J.-D. Creutin, M. Livet, S. Anquetin, M. Lang, L. Neppel, C. Obled, J. Parent du Châtelet, G.-M. Saulnier, A. Walpersdorf, W. Wobrock, 2005 : The catastrophic flash-flood event of 8-9 September 2002 in the Gard region : a first case study for the Cévennes-Vivarais Mediterranean Hydrometeorology Observatory. J. Hydrometeor., 6, 34-52. DOI : 10.1175/JHM-400.1
  • Ducrocq, V., O. Nuissier, D. Ricard, C. Lebeaupin, S. Anquetin, 2008 : A numerical study of three catastrophic precipitating events over southern France. II : Mesoscale triggering and stationarity factors. Quart. J. Roy. Meteor. Soc., 134, 131-145. DOI : 10.1002/qj.199.
  • Ricard, D., 2005 : Modélisation à haute résolution des pluies intenses de la région Cévennes-Vivarais : l’épisode convectif du 13-14 octobre 1995. La Météorologie, 48, 28-38. (in French)

 Moisture origin

The Mediterranean Sea constitutes an important local source of moisture which is transported by low-level flows towards the target region where heavy precipitation occurs. Current research at CNRM-GAME focuses on characterising the origin and the pathways of the moisture feeding heavy precipitation systems. For that, the Meso-NH model is used to simulate HPEs at high resolution. Meso-NH advanced diagnostics such as inline budgets and Lagrangian (backward) trajectories allow to study thoroughly the role and origin of moisture in HPEs.

 Impact of the Mediterranean Sea on severe precipitation events

In autumn, the Mediterranean Sea is generally warm after the long sunshine periods of the summer, whereas, upper cold air, transported from Northern Europe, begins to concern the area ; both factors produce propitious conditions to HPEs occurrence (low static stability, large scale lifting and sustain of moisture). It is well known that a warmer (colder) SST increases (decreases) air-sea surface heat fluxes which in turn moisten (drain) and destabilize (stabilize) the marine atmospheric boundary layer. This results in an increase (decrease) of the available energy and moisture for atmospheric convection and thus precipitation.

The sensitivity to the sea surface temperature (SST) of high-resolution Meso-NH forecasts of HPEs has been investigated. Some dynamical important effects of the SST on the low-level jet and the motion speed of the precipitating systems have been highlighted. If a SST increase (decrease) induces systematically a CAPE increase (decrease), the link between the SST and the displacement speed of the precipitating system is not so univocal. In fact, a SST anomaly can induce different atmospheric responses which seem mainly associated with different types of precipitating systems.

The sensitivity to the sea surface fluxes parameterization has also been investigated of high-resolution Meso-NH forecasts of HPEs. High sensitivity have been found for two different parameterizations on predicted rainfall and on the ocean mixed layer depth of a one-dimensional ocean model driven by the Meso-NH forecasts.

A two-way one-dimensional coupling was also developed to interface Meso-NH with a one-dimensional ocean model. This tool has been used to study the interactions between the atmosphere and the ocean during HPEs, and their effects on the simulation of HPEs.

Further reading :

  • Lebeaupin, C., V. Ducrocq and H. Giordani, 2006 : Sensitivity of Mediterranean torrential rain events to the Sea Surface Temperature based on high-resolution numerical forecasts, J. Geophysical Research, 111,D12110. DOI : 10.1029/2005JD006541.
  • Lebeaupin-Brossier, C., 2007 : Etude du couplage océan-atmosphère associé aux épisodes de pluie intense en région méditerranéenne. Ph.D. Thesis, University Paul Sabatier, Toulouse, 19 December 2007. (in French)
  • Lebeaupin-Brossier, C., V. Ducrocq, H. Giordani, 2008 : Sensitivity of Mediterranean heavy rain events to the sea surface fluxes parameterization in high-resolution numerical modelling. J. Geophys. Res., 113, D21109. DOI : 10.1029/2007JD009613.
  • Lebeaupin-Brossier, C., V. Ducrocq, H. Giordani, 2009 : Effects of the air-sea coupling time frequency on the ocean response during Mediterranean intense events. Ocean Dynamics, 59(4), 539-549. DOI : 10.1007/s10236-009-0198-1.
  • Lebeaupin-Brossier, C., V. Ducrocq, H. Giordani, 2009 : Two-way one-dimensional high-resolution air-sea coupled modelling applied to Mediterranean heavy rain events. Quart. J. Roy. Meteor. Soc., 135, 187-204. DOI : 10.1002/qj.338.

 Modeling and predictability issues

Non-hydrostatic models, like Meso-NH and Arome, employing grid-spacing of about several kilometers have shown substantial success in simulating realistic heavy precipitation systems. However, their success largely depends on the quality of their initial and boundary conditions as well as their physical parameterizations. For instance, much effort has been spent at CNRM-GAME to improve mesoscale initial conditions through the development of mesoscale data assimilation of, e.g., radar and soil observations.

Predictability limits result from the nonlinearity and instability of the dynamics of the atmosphere, together with the lack of a precise knowledge of the atmospheric state at any time and location. The atmospheric predictability depends on flow regime. Synoptic and large mesoscale systems possess more intrinsic predictability than cloud-scale convective systems. However, some factors can increase the predictability at mesoscale, such as surface heating, synoptic-scale disturbance or topography forcing which are often present for Mediterranean HPEs. The management of the risk occurrence of a flash-flood event requires early warnings. Since the dynamical and physical processes associated with HPEs involve small-scale processes that have a low predictability on a long-term, a risk assessment should necessarily be calculated by indirect strategies combining weather regimes and ensemble prediction at various scales. CNRM-GAME is currently developing a convective-scale ensemble prediction system, with a primary focus on short-term HPE forecast. The HyMeX field campaign will provide a test-bed for this convective-scale ensemble prediction system that will allow to evaluate and refine it.

A specific hydrometeorological system has been designed to simulate the response of watersheds to rainfall predictions made by an atmospheric model. This system is based on Arome-Isba-Topmodel and can currently be applied to the Cévennes-Vivarais watersheds (see Flash-flood simulation).
In order to quantify the predictability of the streamflow forecasts, different methods are currently being investigated to generate hydrometeorological ensembles.

Further reading :

  • Vié, B., O. Nuissier, V. Ducrocq, 2011 : Cloud-resolving ensemble simulations of Mediterranean heavy precipitating events : Uncertainty on initial conditions and lateral boundary conditions. Mon. Wea. Rev., 139, 403-423, DOI : 10.1175/2010MWR3487.1.

 Projections of HPEs in climate scenarios

The evolution of the occurrence and severity of HPEs in the frame of climate change remains an open question. To address this issue, a statistical-dynamical downscaling method to apply to climate model outputs has been designed. This statistical-dynamical downscaling is based on a two-step method. First, representative situations are selected among an ensemble of propitious synoptic environments to HPEs for the present climate and for the future one. Large-scale circulation patterns propitious to high-impact weather events have been identified by a statistical method which has been applied to the coupled atmosphere-ocean regional climate model Arpege-Climate / OPAMED simulation considering an enhanced greenhouse climate following IPCC-SRES-A2 scenario. Second, for these situations, simulations are carried out with the Meso-NH model at 2.5-km horizontal resolution, forced by the global climate scenarios.
This kind of procedure allows to study the evolution of the frequency, intensity, location, and patterns of HPEs in the future, considering a given scenario.