Kinematic Structure of the WAFR Monsoon ATS 553. 600mb NCEP Climatology Zonal Winds.
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Transcript of Kinematic Structure of the WAFR Monsoon ATS 553. 600mb NCEP Climatology Zonal Winds.
Kinematic Structure of the WAFR Monsoon
ATS 553
600mb NCEP Climatology Zonal Winds
Cross section of zonal winds along 0°E
Atlantic
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Sahara
Cross section of zonal winds along 0°E
Atlantic
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Sahara
Westerlies at 200mb:
• A weak subtropical jet?
Cross section of zonal winds along 0°E
Atlantic
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Sahara
Easterlies at 200mb:
• Near the coastline
• TROPICAL EASTERLY JET (TEJ)
TEJ=Equatorward Flank of Upper Level High Pressure Systems
(Above Heat Lows)
Streamlines at 200mb; colors are U winds at 200mb
HH
Heat Lows
1000mb
850mb
700mb
500mb
Cross section of zonal winds along 0°E
Atlantic
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Easterlies at 600mb:
• Centered near 12°N
• AFRICAN EASTERLY JET (AEJ)
Cross section of zonal winds along 0°E
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Low level westerlies:
• Extend to about 20°N
• THE MONSOON WESTERLIES
Cross section of zonal winds along 0°E
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Low level easterlies:
• Centered near 27°N
• HARMATTAN
Why is there an “African Easterly Jet”?
• Any time there is a change of wind speed with respect to height, you should be thinking “Thermal Wind Relationship”.
• Thermal Wind Relationship: U increases with height when T decreases to the north.
Why is there an “African Easterly Jet”?
• In West Africa, we have the opposite situation:– Temperature increases to the north– Therefore, U decreases with height.
– A negative U is a wind from the east—the AEJ!
Thermal Wind Relationship
Atlantic
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Thermal Wind Relationship
Atlantic
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500mb
400mb
500mb
400mb
Thermal Wind Relationship
Atlantic
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500mb
400mb
500mb
400mb
500mb
400mb
500mb
400mb
Thermal Wind Relationship
Atlantic
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Sahara
500mb
400mb
500mb
400mb
500mb
400mb
500mb
400mb
500mb
400mb
500mb
400mb Pressure
Gradient
Force
Meridional temperature gradient at the surface is key to the existence of the AEJ!
PGF
Coriolis
AEJ
• At this point, the students should have read the paper by Cook (1999) to explain the feedback between the temperature gradient and the AEJ.
Define 3 Regions
• Sahara
• Sahel
• ITCZ
Determine the heat budget for each of these three regions to determine why there is a temperature gradient in West Africa.
Heat Budget
SW↓:
Maybe the Sahara is hotter than the Sahel and the ITCZ simply because:•the sun angles are higher•the days are longer•the sky is more clear
SW↓ is greatest in the Sahara.
Heat Budget
SW↑:
However, the Sahara has a much higher albedo than the Sahel or the ITCZ region.
SW↑ is greatest in the Sahara.
Heat Budget
Net SW:
Still, the net shortwave radiation at the surface is greatest in the Sahara.
Heat Budget
LW↓:
Most LW↓ comes from:•cloud cover•greenhouse gases like water vapor
Therefore, the ITCZ region has the most LW↓, whereas the Sahara gains relatively little.
Heat Budget
LW↑:
Longwave radiation emitted by the surface is determined primarily by the temperature of the surface.
Therefore, the Saharan region loses the most LW↑, whereas the ITCZ region loses relatively little.
Heat Budget
Net LW:
The ITCZ has very little loss of heat by longwave radiation.
The Sahara loses a great deal of heat by longwave radiation.
Heat Budget
Net Radiative Heating:
The ITCZ region actually gains more heat by radiative processes than the Sahara does!
So, why is the Sahara warmer? (It seems like the coastal region should be warmer.)
Heat Budget
Sensible Heat flux:
•In the Sahara, the ground is much hotter than the air during the day, so SHF is very great.•In the ITCZ region, the air temperature and the ground temperature are approximately equal, so SHF is nearly zero.
Heat Budget
Latent Heat flux:
•In the Sahara, the ground is completely dry—there is basically no cooling by evaporation.•In the ITCZ region, soil moisture is high, so there is a great deal of evaporative cooling at the surface.
Heat Budget
• About equal amounts of heat are transferred from the ground to the atmosphere in the Sahara and in the ITCZ regions.– Sahara: Sensible Heat Flux– ITCZ: Latent Heat Flux
• But the difference between these two methods is very important…
Sensible Heat Flux in the Sahara
• Heat transfers from the surface into the atmosphere by conduction, warming only the air very near the surface.
• All of the heat becomes trapped in a very shallow layer.
• This layer becomes very hot.
Atlantic
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Latent Heat Flux in the ITCZ
• Heat transfers from the surface into the atmosphere by evaporation.
• This latent heat is released to the atmosphere at great heights.
Atlantic
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An Important Feedback
Atmospheric Temperature
Gradients Near the Surface
African Easterly Jet (AEJ)
African Easterly Waves (AEW)
Convection in the ITCZ
Region (Squalls,
MCSs, etc.)
Soil Moisture in
the ITCZ Region
Based on this feedback…
• …we should be able to make certain predictions about the AEJ.
Rain Rate (mm/d)
Over the course of the monsoon season, the precipitation moves progressively farther to the north, moistening the soil as it goes.
Therefore, we expect that the AEJ should north between June and August.
Zonal Wind at 700mb along 2.5°E (m/s)
And it does! In fact, notice that the peak winds are directly above the strongest gradient in the precipitation pattern!