1. 1 FORMATION DES PREVISIONNISTES CONVECTION WEATHER FORECASTING IN MID-LATITUDE REGIONS 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. 2 FORMATION DES PREVISIONNISTES CONVECTION Observed Types of Convective Storms Isolated Convective Storms Convective Cell –Region of strong updraft (>10 m/s) –Cross section of 10-100 km² –Extending through most of the troposphere 3 types of organisation The Single-Cell Storm The Multicell Storm The Supercell Storm

3. 3 FORMATION DES PREVISIONNISTES CONVECTION Definition of Wind Shear: –Wind variation on the vertical, in both intensity and direction Shear vector between two levels –vector difference between the wind at upper level and the lower one –divided by the layer depth Hodograph: curve formed by all shear vectors The Wind Shear 1 2 3 4 5 8 9 11 7 12 10 6 Km 270° 180° 1020 m/s Storm motion 2 4 8 10 km 1 6 km Example of hodograph

4. 4 FORMATION DES PREVISIONNISTES CONVECTION The Single-Cell Storm

5. 5 FORMATION DES PREVISIONNISTES CONVECTION The Single-Cell Storm Formation stageDissipation stageMature stage 10 à 15 km WEAK WIND SHEAR 1 2 3 4 5 8 9 11 7 12 10 6 Km 270° 180° 1020 m/s 2 4 6 8 10 km HODOGRAPH Storm motion DC Life Cycle: 3 phases, 30-50 min Propagation at the mean wind of the environment No severe weather (or short-lived and local)

6. 6 FORMATION DES PREVISIONNISTES CONVECTION The Multicell Storm

7. 7 FORMATION DES PREVISIONNISTES CONVECTION The Multicell Storm 1 2 3 4 5 8 9 11 7 12 10 6 Km STRONG UNIDIRECTIONAL WIND SHEAR 270° 180° 10 40 m/s HODOGRAPH 2 10 12 km 2030 6 4 8 1

8. 8 FORMATION DES PREVISIONNISTES CONVECTION The Multicell Storm 1 2 3 4 5 8 9 11 7 12 10 6 Km STRONG UNIDIRECTIONAL WIND SHEAR 270° 180° 10 40 m/s HODOGRAPH 2 10 12 km 2030 6 4 8 1 Propagation DC

9. 9 FORMATION DES PREVISIONNISTES CONVECTION Group of cells at different stages May last a long time Possibility of flooding and severe weather (short-lived and local) The most frequent Importance of the D.C. Cell Motion may differ from the mean wind The Multicell Storm (driven here by a DC) 30 à 50 km 1 2 3 4 5 8 9 11 7 12 10 6 Km STRONG UNIDIRECTIONAL WIND SHEAR 270° 180° 10 40 m/s HODOGRAPH 2 10 12 km 2030 6 4 8 1 DC Propagation

10. 1010 FORMATION DES PREVISIONNISTES CONVECTION Multicell Storm: Storm Motion Storm motion (group velocity) = results of the combination –of the new cells triggering (discrete propagation) –and of the cells motion (mean wind) If the discrete propagation is opposite to the cells motion, the storm is stationary  Possibility of flooding Storm Motion Cells motion Discrete Propagation Mean wind Adapted from Marwitz 1972

11. 11 FORMATION DES PREVISIONNISTES CONVECTION The Supercell Storm

12. 1212 FORMATION DES PREVISIONNISTES CONVECTION 10 Km 0 100 Km STRONG and CURVED WIND SHEAR Storm motion HODOGRAPH 14 km 270° 180° 40 m/s 2 10 2030 6 4 8 1 12 DC 3D CONCEPTUAL MODEL The Supercell Storm A unique and giant cell, with a steady structure in the storm-moving framework Propagation = cell motion Rotating vertical motions DRY and COLD AIR 5 km WET and WARM AIR 1 km 5 km 10 km Adapted from Klemp, 1987 and Browning, 1964

13. 1313 FORMATION DES PREVISIONNISTES CONVECTION HORIZONTAL CROSS SECTION (AB) C D 20 km 30 40 50 20 A B 20 km 1 2 3 4 5 8 9 11 7 12 10 6 Km 15 14 13 20 dBz 3040 50 VERTICAL CROSS SECTION (CD) 60 km The Supercell Storm SIGNATURES RADAR REFLECTIVITY PATTERNS Severe weather (hail, tornadoes) Hook-like structure at low level Motion differs from the mean wind, to the left or (more often) to the right Weak echo regions or overhangs present 270° 180° 40 m/s HODOGRAPH 2 10 14 km 2030 6 4 8 1 12 Storm motion

14. 1414 FORMATION DES PREVISIONNISTES CONVECTION The Supercell Storm STRONG and CURVED WIND SHEAR Storm motion 270° 180° 40 m/s HODOGRAPH 2 10 14 km 2030 6 4 8 1 12 UPWARD MOTION SUBSIDENT MOTION TORNADOES CONCEPTUAL SCHEME AT SURFACE ROTATING ECHOS DC Storm motion. Severe weather (hail, tornadoes). Hook-like structure at low level. Motion differs from the mean wind, to the left or (more often) to the right. Weak echo regions or overhangs present Adapted from Lemon & Doswell, 1979 (quoted by Rotunno & Klemp, 1982)

15. 1515 FORMATION DES PREVISIONNISTES CONVECTION WIND SHEAR: A CRUCIAL PARAMETER Storm motion 270° 180° 40 m/s HODOGRAPH 2 10 14 km 2030 6 4 8 1 12 270° 180° 10 40 m/s HODOGRAPH 2 10 12 km 2030 6 4 8 1 HODOGRAPH 180° 270° 1020 m/s 2 4 6 8 10 km Storm motion WEAK SHEAR STRONG UNIDIRECTIONAL WIND SHEAR STRONG and CURVED WIND SHEAR Possibility of flooding and severe weather (short-lived and local) The most frequent Cell Motion may differ from the mean wind SINGLE-CELL SUPERCELL MULTICELL No severe weather (or short-lived and local) 3 to 10km, 30 to 50 minutes Propagation at the mean wind of the environment Severe weather (hail, tornadoes) Hook-like structure at low level Motion differs from the mean wind, to the left or (more often) to the right

16. 1616 FORMATION DES PREVISIONNISTES CONVECTION REFERENCES Browning, K.A., 1964 : Airflow and precipitation trajectories within severe local storms which travel to the right of the winds. Journal of Atmospheric Sciences, 21, 634-639. KLEMP 1987 : Dynamics of Tornadic Thunderstorms, Ann. Rev. Fluid Mech. 19, 369-402. Figures are reprinted, with permission, from the Annual Review of Fluid Mechanics, Volume 19 ©1987 by Annual Reviews www.annualreviews.orgwww.annualreviews.org Lemon, L.R. & C.A. Doswell 1979 : Severe thunderstorm evolution and mesocyclone structure as related to tornadogenesis. Monthly Weather Review, 107, p 1184-1197 Marwitz, John D., 1972 : The Structure and Motion of Severe Hailstorms. Part II: Multi-Cell Storms. Journal of Applied Meteorology, 11, 1, 180-188.