WEATHER FORECASTING IN MID-LATITUDE REGIONS

1. WEATHER FORECASTING IN MID-LATITUDE REGIONS Prepared in close collaboration with the “Working Group on Convection” in the frame of the Plan de Formation des Prévisionnistes program of Météo-France. This group, headed by J-Ch Rivrain and with the support of the scientific expertise provided by J-Ph Lafore, is composed of Mrs Canonici, Mercier, Mithieux and Mr Boissel, Bourrianne, Celhay, Jakob, Hagenmuller, Hameau, Lafore, Lavergne, Lecam, Lequen, Mounayar, Rebillout, Rivrain, Rochon, Robin, Sanson, Santurette, Voisin and many others. Proofreading, references by Jean Paul Billerot.

2. Squall Lines MCS and MCC Plan 1- MCS: • Definition • The Squall line: an archetype of MCS 2 - MCS and MCC: synthesis 3 - The V-shaped systems.

3. The storm cells are organized at mesoscale up to the Rossby radius (200-400 km for midlatitudes) This group of storms exhibits a global behavior: internal circulation, organization, propagation, life cycle… Due to their size they impact the large scale circulation and interact with it (two-way) Mesoscale Convective Systems

4. Squall Lines Radar Image of a squall line, 11 June 1997

5. Squall Lines • Favorable conditions for their occurrence • moderate to strong CAPE • dry air at midlevel (Minimum of q’w  600 hPa) • Shear • Eventually Large Scale forcing at lease during the initiation stage • • Signature at the surface • as for an intense DC passage: • wind rotation, gust, temperature drop, pressure jump, intense precipitation…

6. Shear Stratiform Part L Convective Part DC H L H H L La ligne de grains Squall Lines Melting Layer (Bright Band)

7. Squall Lines • Main Characteristics: • A narrow (10-20 km) convective part with intense radar echo • A wide-spread stratiform part with weak trailing precipitation at the rear of the system (possibly up to a 100 km) • The SL leading edge corresponds to the sharp limit of the DC (gust or pseudo cold front) • The rear inflow increasing the shear at the system rear, evaporation and the DC strength • • A self-controled system owing to its internal dynamics: • able to maintain itself in regions of strong CIN (at night for instance) and weaker CAPE

8. Squall Lines

9. Squall Lines

10. Squall Lines

11. Squall Lines

12. 10 40 9 35 8 30 7 25 6 5 20 4 15 3 10 2 5 1 0 0 13:42 13:36 13:30 13:24 13:18 13:12 13:06 13:00 12:54 12:48 12:42 Squall Lines Evolution of ground station parameters at a 6 minutes frequency on the 19/09/00 at Bourges at the SL passage direction/10 température p-970 force Squall line motion

13. Squall Lines Evolution of ground station parameters at a 6 minute frequency, 19 Sept 2000 at Guéret

14. Squall Lines Some records at surface, 19 Sept 2000 Cher Department Bourges: temperature drop of 5,6 °C in 1 h, max. gust wind: 21 m/s or 76 km/h. Precipitation: Léré (NE of Bourges): 24.8 mm from 14 to 15h UTC Sancerre (NE de Bourges): 14.8 mm from 14 to 15h UTC DonAuron (SE Bourges): 18 mm from 13h30 to 14h30 UTC Propagation speed: about 50 km/h from 12h to 13h UTC

15. Mesoscale Convective Systems MCSs are less organized than SL. They share many characteristics with SL, but are organized differently in space - a Convective Part P C • - a Stratiform Part P S • - a Density CurrentC D • - a Rear Inflow NB: Strong modifications of the wind within a MCS

16. MCC = quasi-circular MCS of large size Cloud shield (IR:<-32°C) larger than 100 000 km2( Maddox) Not frequent over Europe More frequent in the Tropics and over US Great Plains Mesoscale Convective Complex

17. With radar in some cases V-Shaped Systems 1 Description V-shaped MCS looking like a smoke plume Detectable from IR channel Villefort 22 Sept 1993 The V points in the direction opposite to the mean tropospheric wind

18. V-Shaped Systems 2 Formation conditions?  Cyclonic, fast flow, divergent in the upper troposphere. Often from the SW sector  Large Scale Forcing • Strong Shear  Warm and Wet Advection at low levels  Low level forcing (convergence line…)  Dry air Advection in the mid troposphere 3 Factors of intensification:  Strong inversion capping the moist sub layer

19. Sometimes downwind, warmer areas Cold area (overshoot) Maximum of convection Downwind, a less cold area V-Shaped Systems Infra-Red image Upper trospospheric wind Flux at 850 hPa Low level wind

20. Close to the V point, Intense echo, Convective towers (overshoot) Downwind side, Weak echo Stratiform part Those echos propagate With the mean tropospheric wind Upper Tropospheric Wind Flux at 850 hPa Surface wind V-Shaped Systems Radar Image Quasi-steady echos

21. V-Shaped Systems The V-shaped system is often quasi-steady. Why ? It is a Multicell System whose cells are triggered at the V point. Then these cells propagate at the mean wind speed (northeastward)  Cell Propagation The processes involved in the new-cell triggering (convergence line, relief, DC…) allow the renewal of cells always in the same region  Discrete Propagation

22. V-Shaped Systems Propagation of cells X Stationary V-shaped system ? Discrete Propagation • Stationary • Extreme rainfall accumulation  Flash flood

23. V-Shaped Systems Villefort, 22 Sept. 1993. 300 mm.

24. V-Shaped Systems Propagation of cells Propagation V-shaped system ? Discrete Propagation The system slowly propagates toward the SW,  less rainfall accumulation

25. V-Shaped Systems The system slowly propagates toward the SW  less rainfall accumulation

26. V-Shaped Systems Cells Propagation Propagation V-shaped system ? Discrete Propagation System propagates toward the East  Less rainfall accumulation

27. V-Shaped Systems System propagates toward the East  Less rainfall accumulation

28. Vaison la Romaine, 22 Sept. 1992 Château neuf du pape 30 juillet 1991 265 mm Nîmes, le 3 octobre 1988 V-Shaped Systems Some flash flood cases 400 mm 8 morts 300 mm. 34 dead, 8 missing