1. High-resolution Non-hydrostatic Numerical Weather Prediction of Mediterranean torrential rain events Véronique DUCROCQ, Cindy LEBEAUPIN, Olivier NUISSIER, Didier RICARD, Hervé GIORDANI CNRM/GAME Météo-France & CNRS World Weather Research Program Symposium on Nowcasting and Very Short Range Forecasting, Toulouse, 5-9 Sept. 2005

2. Algiers, 10 Nov. 2001 ~260 mm in less than 24 hours >700 deaths, 4 billion € damages (Hamadache et al, 2002) Gandia, 3 Nov. 1987 ~800 mm in 24 hours (Fernandez et al, 1995) Vaison La Romaine, 22 Nov. 1992 ~300 mm in 24 hours >50 deaths, 1 billion € damages (Sénési et al, 1996) Piedmont, 4-5 Nov. 1994 ~300 mm in 36 hours > 60 deaths, 12 billion € damages (Buzzi et al, 1998) Spain Italy France Mediterranean Sea Western Mediterranean region is prone to torrential rain events © Midi- Libre

3. Algiers, 10 Nov. 2001 ~260 mm in less than 24 hours >700 deaths, 4 billion € damages (Hamadache et al, 2002) Gandia, 3 Nov. 1987 ~800 mm in 24 hours (Fernandez et al, 1995) Vaison La Romaine, 22 Nov. 1992 ~300 mm in 24 hours >50 deaths, 1 billion € damages (Sénési et al, 1996) Piedmont, 4-5 Nov. 1994 ~300 mm in 36 hours > 60 deaths, 12 billion € damages (Buzzi et al, 1998) Spain Italy France Mediterranean Sea Western Mediterranean region is prone to torrential rain events © Midi- Libre

4. Piedmont, 4-5 Nov. 1994 ~300 mm in 36 hours > 60 deaths, 12 billion € (Buzzi et al, 1998) Spain Italy France Mediterranean Sea For France, the southeastern France (Massif Central) is a threatened region Number of days with daily precipitation > 200 mm from 1958 to 2000 for Southern France Massif Central Alps Pyrenees Toulouse

5. Gardon d’Anduze Watershed (545 km 2 ) Discharges hourly raigauge and radar rainfall depths over the watershed Hérault Vidourle Gard GardonsCèze Ardèche Chassezac Eyrieux Rhône  A lot of rivers have their sources in the Massif Central  Steep slopes of the rivers and small catchments increasing the speed of the streamflow *Anduze Rainfall peak LTHE Flow peak ~6 h The heavy precipitation touch areas that have a high-level flash-flood risk due to orography : small catchments (500km 2 -2000 km 2 ) with fast responses to rain showers  Need for very short range model forecasts to provide longer lead times to prepare for flash-flooding and secure people.

6. High Conditional Convective instability A very moist Low-Level Jet Steep orography to trigger/enhance the convection Slow progressing synoptic patterns Quasi- stationary high- geopotentials over Central Europe Mesoscale PV anomalies Synoptic forcing and topography forcing (reliefs + Mediterranean sea) increase the predictability of flash- flood driven thunderstorms. Synoptic conditions propitious to torrential rain events In most cases, the forecast of propitious conditions at synoptic scale allows to forecast the occurrence of an heavy precipitation event over the region, however it is still difficult to forecast : - the magnitude (normal or extreme heavy precipitation event) - the precise location (at a scale of less than 100 km). 100km ? Vigilance map

7. High Conditional Convective instability A very moist Low-Level Jet Steep orography to trigger/enhance the convection Slow progressing synoptic patterns Quasi- stationary high- geopotentials over Central Europe Mesoscale PV anomalies Synoptic forcing and topography forcing (reliefs + Mediterranean sea) increase the predictability of flash- flood driven thunderstorms. Synoptic conditions propitious to torrential rain events In most cases, the forecast of propitious conditions at synoptic scale allows to forecast the occurrence of an heavy precipitation event over the region, however it is still difficult to forecast : - the magnitude (normal or extreme heavy precipitation event) - the precise location (at a scale of less than 100 km). The aim of the study is to examine the capability of a high-resolution non-hydrostatic model to forecast torrential rain events at very short range (in view of the next generation operational model AROME).  emphasis on the sensitivity to initial conditions (atmosphere and surface)

8. Date TypeMaximum of 24-h rainfall 13-14/10/1995quasi-stationary MCS~ 250 mm 3-4 /10/1995quasi-stationary MCS~ 200 mm 12-13/11/1999 The Aude extreme flash-flood event ~30deaths, 3 millions € damages quasi-stationary MCS~ 550 mm 6-7/10/2001quasi-stationary MCS~350 mm 8-9/09/2002 The Gard extreme flash-flood event ~20 deaths, 1.2 billion € damages quasi-stationary MCS and front~ 700 mm 3/12/2003 The 3 rd day of a major Rhône flooding quasi-stationary frontal system with embedded convection ~ 180 mm including cases of - quasi-stationary back-building MCS - quasi-stationary frontal systems The case studies Gard flash-flood, 2002 Rhône flooding 3 Dec.2003 Torrential rain events over Southeastern France

9. Performed with the non-hydrostatic MESO-NH model (Lafore et al, 1998) Two-way grid-nesting (Stein et al, 2000) : Domain 1 ~ 10 km Domain 2 ~ 2.5 km (centred on the convective event) Characteristics of the high-resolution simulations Almost the same physical package as in AROME, including a bulk microphysic parameterization (Pinty et Jabouille, 1998, Caniaux, 1994) with 6 prognostic water variables : water vapour, cloud water rainwater, primary ice, graupel and snow The convection is parameterized following the Kain and Fritsch parameterization for the 10-km domain whereas convection is explicitly resolved for the 2.5 km domain (no deep convection scheme). 10-km domain 2.5-km domain ~500-600 km Initial conditions provided either by a large scale analysis (ARPEGE/IFS) or by a mesoscale analysis following Ducrocq et al, 2000 (objective analysis of mesonet surface observations, q v, q r and q s bogussing based on radar and satellite data)

10. >100 mm >75 mm >50 mm >30 mm >20 mm >10 mm >5 mm >2 mm >1 mm Rhône flood event (3 December 2003) 9-h accumulated precipitation from 03 UTC to 12 UTC, 3 Dec. 2003 Initial conditions : large scale ARPEGE analysis at 00 UTC, 3 Dec. 2003 Raingauge data 2.4 km MESO-NH forecast 100 km

11. + Nîmes + Observations Nîmes radar Raingauges Same Initial Conditions (ARPEGE analysis 12UTC, 08/09/02) and same boundary conditions (3-hourly ARPEGE forecast) + 12-h accumulated precipitation from 12 UTC, 8 Sept to 00 UTC, 9 Sept 2002 Gard flash-flood (8-9 Sept.2002) Aladin (9.4 km) operational forecast MESO-NH (2.5km) 100 km

12. + Nîmes + Observations Nîmes radar Raingauges Initial Conditions : ARPEGE analysis 12UTC, 08/09/02 + MESO-NH (2.5km) 12-h accumulated precipitation from 12 UTC, 8 Sept to 00 UTC, 9 Sept 2002 Initial Conditions : Ducrocq et al (2000) Initialisation 12UTC, 08/09/02 100 km + Gard flash-flood (8-9 Sept.2002)

13. SST for the Aude flash-flood case : SST=17.8°CSST=17.4°C - In-situ buoys and ships SST data analysis (ARPEGE) - High-resolution satellite SST (AVHRR/NOAA ) Sensitivity to the Sea Surface Temperature of the Mediterranean Sea - Warming [Cooling] of the ARPEGE analysis by 1.5°C (SST analysis error range) Several SST fields tested :

14. Initial cond.: ARPEGE analysis, 12 UTC, 12 Nov. 1999 SST ARPEGE SST AVHRR SST ARPEGE -1.5°CSST ARPEGE +1.5°C 18-hour accumulated precipitation from 12 UTC, 12 Nov. 1999 to 06 UTC, 13 Nov. 1999 Max:303mm Max:296mm Max obs.: 485mm Max:368mmMax:274mm 100 km (mm)

15. Several cases of flash-flood have been simulated with the high-resolution non- hydrostatic MESO-NH model  get an insight into the capabilities of the next-generation operational high- resolution NWP systems, including the future Météo-France AROME model, to forecast Mediterranean torrential rain events. In all the cases, the 2.5 km simulations improve the very short range QPF with respect to the current operational models. Forecast rainfall amounts are close to the observed ones for normal flash-flood event, but still underestimated for extreme events. The improvement in location and in intensity is however in most cases a tribute to the mesoscale initial conditions. Dynamical adaptation from large scale analyses in most cases is not sufficient : mesoscale data assimilation improves rainfall forecast even in cases with strong synoptic forcing. Primary importance of the assimilation of observations in the boundary layer (mesoscale surface observations) : cold pool, low-level convergence line, low-level moisture flow, The SST acts to modulate the magnitude and location of the convective system : A significant dependency on the mean value of the SST over the Western Mediterranean basin, but a weak one to the mesoscale patterns of the SST field. also a role of the parameterization of the sea-atmosphere fluxes (not shown here). Errors in location of the same order than the size of the small/medium watersheds plead in favor of a probabilistic post-processing of the high-resolution forecasts for hydrological applications. Conclusion and outlook

16. Thank you for your attention

17. ARPEGE+1.5°ARPEGE ARPEGE+3° ARPEGE-1.5° ARPEGE-3° AVHRR Aude flash-flood (12-13 Nov. 1999) Synthetic infrared METEOSAT radiances at 00 UTC, 13 Nov. 1999 from 12-h MESO-NH forecasts using various initial SST fields Initial conditions : large scale ARPEGE analysis at 12 UTC, 12 Nov. 1999