The environment and characteristics of convective events over Niamey, Niger:
AMMA SOP2 observations and climatological context
Matthew A. Janiga1, Chris D. Thorncroft1, and Earle Williams2
University at Albany1, MIT Lincoln Laboratory2
Introduction
The genesis of convection that passed over Niamey
during AMMA SOP2 was analyzed to understand
whether terrain, gust fronts, or African easterly
waves (AEWs) played a role in the development.
These findings were placed within the context of the
mean AEW structures, monsoon, and diurnal cycle
from 2004-2008.
The relationship between adiabatic forcing for
ascent and moisture transport with convection
impacting Niamey was examined. Two events where
each process dominated were examined.
Monsoon Context
ERA Int. (0-10E) 12 UTC Pos. Area (J/kg) 2004-2008
Gust Front
J
F
M
A
M
J
J
A
S
O
N
D
ERA Int. (0-10E) Daily Avg. 600-900 Shear (m/s) 2004-2008
• In a composite AEW adiabatic forcing for ascent and
rainfall were found ahead of the mid-level vortex
(MLV). Moisture advection was found southeast of the
low-level vortex (LLV).
*
• Propagating convection dominates up to 10 degrees
downstream of the Aїr Mountains. Niamey has one of
the strongest nocturnal convection peaks in West
Africa.
• Moisture advection associated with the LLV or
adiabatic forcing associated with the MLV played a
role in the formation of 22 of the 30 SOP2 convective
events.
AEW Context
16
1000
14
750
12
500
10
250
8
0
6
4
3
6
9
12
15
18
21
2
The genesis hour (UTC, colors) of convective events
impacting Niamey during JAS 2006 overlaid on
elevation (m, grayscale).
25/18 600 PV, 850 Q-Vector Conv.
9
7
5
J
F
M
A
M
J
J
A
S
O
N
D
3
GPCP (0-10E) Daily Avg. Precip. (mm/day) 1997-2007
1
The hour (UTC) of the genesis and passage time of
the above events.
• The formation of most Niamey SOP2 convective
events occurred in distinct clusters over the Jos
Plateau and Aїr Mountains.
• The formation (passage) of these convective events
occurred in the late afternoon (early morning), except
in the case of gust front developments.
Jos Plateau
Cumulus Field
18
25/18 900 Q, Rel. Vor.
1250
25/14
25/16
1500
The Role of AEWs
1250
1000
J
F
M
A
M
J
J
A
S
O
N
25/23
D
26/02
750
500
250
The seasonal cycle of positive area, shear, and
rainfall from 0-10°E.
Diurnal Cycle Context
Track Density (10-2 passages day-1) of MLVs JAS 04-08
Relative Latitude
• Niamey has a brief climatological window of
favorable instability and shear (mid-July to midSeptember) for the development of organized
convection.
1500
AїrMountains
Mountains
Aїr
0
Summary
July 26 Event
SOP2 Convection
0
August 3 Event
Aїr Mountains
4
1
2
3
18
3/00 900 Q, Rel. Vor.
16
14
Niamey
Jos Plateau
12
Relative Longitude
10
The relative latitude and longitude of AEW mid-level
(MLVs, O) and low-level vortices (LLVs, X) to
convection genesis. Red LLVs (MLVs) are associated
with enhanced moisture transport (850 hPa q-vector
convergence).
8
6
4
2
• 16 events associated with LLV moisture transport
LLV
MLV
• 11 with MLV adiabatic forcing for ascent
The hour (UTC) of maximum <233 K pixel frequency
in 10.8 μm IR imagery during JAS 2004-2008 with
elevation greater than 500 m overlaid in blue.
1
Aїr Mountains
Convection
2
3/00 600 PV, 850 Q-Vector Conv.
• 10 with gust fronts from existing convection
9
• 4 with extratropical PV streamers interacting with
the baroclinic zone
7
5
Aїr Mountains
Convection
Low-level
Flow
Low-level
Vortex
(LLV)
N
3
1
Baroclinic
3
Aїr Mountains
Convection
Zone
AEJ
4
Local
Convection
Mid-level
PV Strip
Diabatic PV
Maxima
3/00
3/03
1500
Mid-level
Vortex
(MLV)
1250
1000
To determine the composite structure of AEWs
passing through the Niamey region during JAS
2004-2008, fields from the ERA Interim Reanalysis
and CMORPH were first 1-20 day band-pass filtered
to highlight the perturbation AEW structures.
The fraction of time pixels are cooler than 233 K in 10.8
μm IR imagery during JAS 2004-2008 at each 30 min
interval (UTC).
PDF of poster available at http://www.atmos.albany.edu/student/janiga/cv.htm
The AEW circulation tilts southeast with height
below the AEJ. Adiabatic forcing is found ahead of
the MLV associated with the low to mid-level shear.
Enhancement of the low-level monsoon flow and
moisture is found east of the LLV. Terrain and
mesoscale boundaries also aid in the development
of convection.
750
3/06
3/14
500
250
0
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