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The Interaction between Convection and African
Easterly Waves a Model Case Study
Juliane Schwendike and Sarah Jones
http//www.sat.dundee.ac.uk/abin/geobrowse/MSG/200
6/
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13/09/2006, 00 UTC
10/09/2006, 00 UTC
Tropical Depression
10/09/2006, 12 UTC
13/09/2006, 12 UTC
11/09/2006, 00 UTC
14/09/2006, 00 UTC
Tropical Storm
17/09/2006, 00 UTC
11/09/2006, 12 UTC
Hurricane Helene
Meteosat images http//www.sat.dundee.ac.uk/abin/
geobrowse/MSG/2006/9Channel 5 water vapour 5.35
- 7.15 µm.
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The AEW in the 700-hPa ECMWF Analysis

• Propagation speed 10 m s-1
• 9 Sep Initiation of convection ahead of the
  trough of the AEW
• 10-11Sep crossing from Land to water
• 12 Sep tropical depression
• 14 Sep Tropical Storm Helene
• 16 Sep Hurricane Helene

Tropical Depression
TS Helene
Convective system in satellite images
Developing tropical depression in satellite images
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Model simulations with COSMO 4.0
orography m

• Forecast model of the Deutscher Wetterdienst
  (DWD) COSMO 4.0
• Horizontal resolution 28 km and 2.8 km
• 50 levels
• No parameterisation of convection with 2.8km
  resolution
• 72-h forecasts
• Initiated with ECMWF analyses

Model regions
100
200
400
800
1200
1000
1400
1800
1600
2000
2200
2400
2600
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The AEW in the 700-hPa ECMWF Analysis and in
COSMO
COSMO, 28 km resolution72h Forecast initialised
at 9 September 2006, 12 UTC
ECMWF Analysis
Hovmoeller between 6N and 16N
COSMO Relative vorticity at 700 hPa
Hovmoeller between 6N and 16N
ECMWF Relative vorticity at 700 hPa
12UTC
12UTC
10Sep
10Sep
12UTC
12UTC
11Sep
11Sep
12UTC
12UTC
12Sep
12Sep
12UTC
12UTC
10E
35W
0
10E
35W
0
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Convection over West Africa
10/09/2006, 00 UTC
16N
12N
8N
18W
12W
6W
10/09/2006, 18 UTC
17N
16N
12N
8N
7N
18W
12W
6W
19W
1W
Image courtesy for the RTD Product of Météo
France.
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Convection over West Africa and the Atlantic
12h forecast initiated at 10 Sep 2006, 12
UTC Height (1000 hPa)
COSMO T12 Vert. integral of
humidity, cloud water and ice (kg/m2) wind
(m/s)
11/09/2006, 00 UTC
12N
8N
24W
18W
12W
21h forecast initiated at 11 Sep 2006, 00
UTC Height (1000 hPa)
COSMO T21 Vert. integral of
humidity, cloud water and ice (kg/m2) wind
(m/s)
11/09/2006, 12 UTC
10N
5N
28W
22W
16W
10W
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Becoming a Tropical Depression
12h forecast initiated at 12 Sep 2006, 00
UTC Height (1000 hPa)
COSMO T12 Vert. integral of
humidity, cloud water and ice (kg/m2) wind
(m/s)
12/09/2006, 12 UTC
17N
14N
10N
6N
5N
24W
12W
30W
18W
32W
11W
12h forecast initiated at 13 Sep 2006, 00
UTC Height (1000 hPa)
COSMO T12 Vert. integral of
humidity, cloud water and ice (kg/m2) wind
(m/s)
13/09/2006, 12 UTC
17N
16N
12N
8N
5N
32W
11W
16W
22W
28W
34W
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12h forecast initiated at 16 Sep 2006, 00 UTC
Height (700 hPa)
COSMO T12 Specific humidity (kg/kg)
and wind (m/s)
Tropical Storm Helene
22N
16/09/2006, 12 UTC
18N
14N
56W
50W
44W
Image courtesy of Jason Dunion, HRD CIMSS.
Saharan Air Layer (SAL)
TS Helene 16 Sep 2006, 12 UTC
Image courtesy to CIMSS.
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Convection over Land vs. Ocean
West Africa
rel. vorticity (contours) diabatic heating
(shaded)
zonal wind (contours) vertical velocity (shaded)
B
Atlantic
D
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Potential Temperature Budget Analysis Over Land
Average over box A
300 400
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Potential Temperature Budget Analysis Over Land
Average over box B
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Potential Temperature Budget Analysis Over Land
Average over box C
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T tendency for MCS over West Africa
Ascent
Descent
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Potential Vorticity Budget
Potential vorticity (PV)

PV budget
PV budget
very small
Diabatic PV changes
Frictional PV changes
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Potential Vorticity Budget
PV budget

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PV tendency for the whole boxes for MCS over West
Africa
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Potential Temperature Budget Analysis Over Water
Average over box D
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Potential Temperature Budget Analysis Over Water
Average over box E
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D
E
T tendency for convective system over the Ocean
T tendency for strong ascent region
E
D
T tendency for strong descent region
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PV tendency for convective system over the Ocean
Diabatic PV tendency for the whole boxes
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Mean PV at the beginning and end of the
convective systems life cycle
West Africa
Eastern Atlantic
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Summary

• Updraught, heating and rel. vorticity are tilted
  over West Africa and upright over the Atlantic
• Diabatic PV tendency at low-levels positive and
  negative at mid and upper levels
• Increased low-level PV could enhance low-level
  monsoon flow behind the system and strengthen the
  AEW trough and AEJ at 700 hPa

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Outlook Interaction with SAL
Modelling with COSMO-ART (Aerosol and Reactive
Trace gases).
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Convection over Land vs. Ocean
West Africa
rel. vorticity (contours) diabatic heating
(shaded)
zonal wind (contours) vertical velocity (shaded)
B
Atlantic
D
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PV tendency for the ascent regions for the MCS
over West Africa
A
B
C
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PV tendency for the descent regions for the MCS
over West Africa
A
C
B
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D
PV tendency for convective system over the Ocean
E
Diabatic PV tendency for strong ascent region
D
E
Diabatic PV tendency for strong descent region