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Nocturnal Stratocumulus Clouds in the West African Monsoon
Based on “Nocturnal Stratiform Cloudiness during the West
African Monsoon” by Schrage, Augustyn and Fink
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On nonprecipitating nights…
• About ¼ of the nights were completely clear
• About ¼ of the nights were completely overcast at low levels
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Cloudy Clear
4 July 2002 5 July 2002
10 July 2002 8 July 2002
17 July 2002 11 July 2002
30 July 2002 20 July 2002
1 August 2002 27 July 2002
2 August 2002 3 August 2002
6 August 2002 10 August 2002
7 August 2002 28 August 2002
22 August 2002 3 September 2002
27 August 2002 8 September 2002
30 September 2002 17 September 2002
23 September 2002
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Composite Soundings
• Cloudy nights are:– More moist– Less stable (see the inversion on the clear nights)
Cloudy Nights Clear Nights
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925mb Flow
• Stronger monsoon flow on the cloudy nights; speed convergence over the Soudanian zone.
Cloudy Nights Clear Nights
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700mb Flow
• Cloudy nights have a weaker AEJ• Cloudy nights are in the trough of an
AEW
Cloudy Nights Clear Nights
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200mb Flow
• Strong TEJ on cloudy nights
Cloudy Nights Clear Nights
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Specific Humidity at 925mb
• Cloudy nights are more humid than clear nights… (notice that the greatest values are over the Soudanian Zone)
Cloudy Nights Clear Nights
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Moisture Advection at 925mb
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 5 10 15
latitude (°N)
DRY AIR ADVECTION from 12°N to the equator!!! (Know why!)
Solid=cloudy, Dotted=clear
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Moisture Divergence at 925mb
Soudanian Zone is a region of moisture CONVERGENCE on cloudy nights, but DIVERGENCE on clear nights!
-10
-8
-6
-4
-2
0
2
4
6
8
10
0 5 10 15
latitude (°N) Solid=cloudy, Dotted=clear
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Moisture Divergence…
• …is, mathematically, just q*DIV
• q is positive.• Therefore, an
area of moisture CONVERGENCE must be an area of CONVERGENCE
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• So why is there low-level convergence on cloudy nights in West Africa????
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A clue!
• On clear nights, there was a strong inversion at the surface.
• The boundary layer doesn’t experience friction from the surface!
Clear Nights
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A Clue!
• On cloudy nights, the boundary remains unable or statically neutral.
• Boundary layer air experiences friction from the surface.
Cloudy Nights
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• A painful, mathematical examination of the processes contributing to friction at 925mb showed that the main difference between the clear and the cloudy nights was the contribution of FRICTION!
D
y
CBA
v
F
x
v
v
u
v
uf
yvy
v
1
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• So why is there more FRICTION on cloudy nights and less friction on clear nights?
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On Clear Nights…
• The surface cools by longwave radiation.
• An inversion forms.
Cold!
Warmer!
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On Clear Nights…
• Winds in the boundary layer “decouple” from the surface and don’t experience friction.
Cold!
Warmer!
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On Cloudy Nights…
• Cloud cover prevents longwave radiation from escaping.
• The surface stays warm and turbulence occurs throughout the night.
Warmer!
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On Cloudy Nights…
• Monsoon flow experiences this turbulence as friction and slows down (remember the speed convergence on cloudy nights?)
• Results in MOISTURE CONVERGENCE!
Warmer!
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On Cloudy Nights…
• Moisture convergence and turbulence produce clouds, and the clouds keep the surface warm!
Warmer!
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New in 2009!
• From the AMMA (2006) experiment:– Ceilometer– Microwave retrievals of temperature and
moisture profiles– Many more radiosonde stations
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New in 2009!
• The ceilometer teaches us that:– These clouds are much more common than
we thought—they just tend to form somewhat after 0000 UTC.
– These clouds are extremely low—typically about 100m AGL!
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May 23, 2006
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June 27, 2006
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July 14, 2006