WEATHER FORECASTING
IN MID-LATITUDE REGIONS
C O N V E C T IO N
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.
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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
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The Single-Cell Storm
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The Single-Cell Storm
Km
12
11
10
9
8
7
6
5
4
3
2
1
10 à 15 km
Formation stage
Mature stage
10 km
6
4
Dissipation stage
WEAK WIND SHEAR
180°
2
DC
8
10
270°
•
•
20 m/s
Storm motion
HODOGRAPH
•
Life Cycle: 3 phases, 30-50 min
Propagation at the mean wind of the
environment
No severe weather (or short-lived and local)
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The Multicell Storm
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The Multicell Storm
Km
12
11
10
9
8
7
6
5
4
3
2
1
STRONG UNIDIRECTIONAL WIND SHEAR
180°
8
6
1
10
12 km
4
2
270°
10
20
30
40 m/s
HODOGRAPH
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The Multicell Storm
Km
Propagation
12
11
10
9
8
7
6
5
4
3
2
1
DC
STRONG UNIDIRECTIONAL WIND SHEAR
180°
8
6
1
10
12 km
4
2
270°
10
20
30
40 m/s
HODOGRAPH
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The Multicell Storm
(driven here by a DC)
Km
Propagation
12
11
10
9
8
7
6
5
4
3
2
1
DC
30 à 50 km
STRONG UNIDIRECTIONAL WIND SHEAR
180°
8
6
1
10
12 km
4
2
•
•
•
270°
10
20
30
HODOGRAPH
40 m/s
•
•
•
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
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Multicell Storm: Storm Motion
Mean wind
Cells motion
Discrete
Propagation
Adapted from Marwitz 1972
•
•
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
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0
The Supercell Storm
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The Supercell Storm
3D CONCEPTUAL MODEL
10 km
5 km
10 Km
1 km
5 km
DC
Adapted from Klemp, 1987
and Browning, 1964
DRY and
1 km
COLD AIR
0
WET and
WARM AIR
100 Km
Storm motion
STRONG and CURVED WIND SHEAR
180°
2
4
6
8
10 12
14 km
•
1
270°
20
30
40 m/s
•
A unique and giant cell, with a steady
structure in the storm-moving framework
Propagation = cell motion
Rotating vertical motions
HODOGRAPH
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The Supercell Storm
RADAR REFLECTIVITY PATTERNS
Km
15
14
13
D
C
12
11
50
10
60 km
9
40
8
7
30
6
5
4
B
A
3
20
2
50
1
40
30
20 dBz
20 km
20 km
VERTICAL CROSS SECTION (CD)
HORIZONTAL CROSS SECTION (AB)
SIGNATURES
180°
2
4
6
8
10 12
14 km
1
270°
20
30
Storm motion
HODOGRAPH
40 m/s
• Motion differs from the mean wind, to
the left or (more often) to the right
• Severe weather (hail, tornadoes)
• Hook-like structure at low level
• Weak echo regions or overhangs present
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The Supercell Storm
CONCEPTUAL SCHEME AT SURFACE
Storm motion
SUBSIDENT
MOTION
UPWARD MOTION
ROTATING ECHOS
TORNADOES
DC
Adapted from Lemon & Doswell, 1979 (quoted
by Rotunno & Klemp, 1982)
STRONG and CURVED WIND SHEAR
180°
2
4
6
8
10 12
14 km
1
270°
20
30
Storm motion
HODOGRAPH
40 m/s
. Motion differs from the mean wind, to the
left or (more often) to the right
. Severe weather (hail, tornadoes)
. Hook-like structure at low level
. Weak echo regions or overhangs present
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WIND SHEAR:
A CRUCIAL PARAMETER
WEAK SHEAR
180°
SINGLE-CELL
10 km
• No severe weather (or short-lived and
6
2
8
4
10
local)
• 3 to 10km, 30 to 50 minutes
270°
20 m/s
Storm motion
• Propagation at the mean wind of the
environment
HODOGRAPH
STRONG UNIDIRECTIONAL WIND SHEAR
180°
8
6
1
12 km
10
• Possibility of flooding and severe
weather (short-lived and local)
•The most frequent
•Cell Motion may differ from the
mean wind
4
2
270°
10
20
30
MULTICELL
40 m/s
HODOGRAPH
STRONG and CURVED WIND SHEAR
180°
2
4
6
8
10 12
14 km
1
270°
20
30
Storm motion
HODOGRAPH
40 m/s
SUPERCELL
• 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
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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.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.
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