Desert Dust Suppressing Precipitation: A possible Feedback Loop

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Desert Dust Suppressing Precipitation A possible
Feedback Loop

• Paper by Daniel Rosenfeld et al.
• Presented by Derek Ortt
• February 19, 2007

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Background

• Twomey et al. (1987) and Rosenfeld (2000) found
  that areosols from smoke and anthropogenic
  pollution cause high concentrations of small CCN
• Small CCN results in formation of fewer
  precipitation droplets
• Levin et al. (1996, 2000) found that desert dust
  leads to the formation of giant CCN, and enhances
  precipitation
• Giant CCN enhance collision and coalescence,
  allowing for the formation of more raindrops
• Is the Levin et al. hypothesis correct?

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If it were, I would not be giving this talk!!!

• Dust storm over Mid-East on March 16, 1998
• Red areas of dust
• Satellite, aircraft, and laboratory observations
  suggest that droplets formed with dust as CCN are
  lt14 micrometers, which favors clouds, but little
  precip (Rosenfeld, 1994)

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For all plots effective radius on x-axis,
temperature on y axis Long dash 15
percentile Solid 50 percentile Short dash 85
percentile Black and red lines correspond to
dusty boxes Vertical green line is 14 micrometer
precip threshold
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March 1, 2000
DUST
DUST
March 6, 2000
Areas covered by TRMM overpass
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Aerosol Map
Blue smoke Orange sulfates Green Desert Dust
March 1, 2000
March 6, 2000
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PR pass from March 1
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3
clear
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2
dusty
Only precip
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Lines 1-4 correspond to boxes 1-4 on previous
slide
Line 5 from heavy dust storm
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1 Dust laden clouds, 2 Dust free clouds, 3
Smoke
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Particle Size Distributions
CCN concentrations assume all vertically
integrated particles lie within a 1km2 column
Black Concentrations from sky radiometers
(Sde-Boker solid, Cape Verde dashed) Blue
Conversion of dust into CCN Red CCN concentration
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How dust acts as CCN

• 65 of particles contained sulfur from ground
• Sulfur accumulates on dust particle via following
  relation log(S) 2.13 log(d) 13.44
  (ddiameter)
• Calculate the equivalent NaCl CCN using drop size
  distribution and sulfur mass
• Transformation shown on previous slide shifts the
  dust particle distribution to smaller sizes of CCN

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Climatic Effects
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Levy et al. (2007)
Dust
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Dust effects on Climate

• Leads to cloud formation with little
  precipitation
• Increases atmospheric albedo, cooling the earth
• Increases greenhouse effect warming middle
  troposphere
• New temperature profile creates a more stable
  atmosphere, further suppressing precip
• Continous feedback cycle could lead to

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Solid line frequency of dust occurrence at Gao
from 1957-1980 (left axis, Bars Rainfall
anamolies for Sahel as a whole Dust is
represented as number of days with dust haze
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Monthly Mean Rainfall
Hours of reduced visibility on left axis (lt5km
solid, 5ltvislt10 open) mm of rainfall on right axis
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Conclusions

• Desert dust is a source of CCN
• Droplets typically do not grow to precipitation
  size when dust is CCN
• Dust creates a more stable atmosphere
• Feedback develops and could lead to a further
  expansion of the desert areas