Desert Dust Suppressing
Precipitation: A possible
Feedback Loop
Paper by Daniel Rosenfeld et al.
Presented by Derek Ortt
February 19, 2007
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?
If it were, I would not be giving this
talk!!!
• Dust storm over MidEast on March 16,
1998
• Red = areas of dust
• Satellite, aircraft,
and laboratory
observations suggest
that droplets formed
with dust as CCN are
<14 micrometers,
which favors clouds,
but little precip
(Rosenfeld, 1994)
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
March 1, 2000
DUST
DUST
March 6, 2000
Areas covered by TRMM
overpass
Aerosol Map
Blue: smoke
Orange:
sulfates
Green: Desert
Dust
March 1, 2000
March 6, 2000
PR pass from March 1
3
clear
1
4
2
dusty
Only precip
Lines 1-4
correspond to
boxes 1-4 on
previous slide
Line 5 from
heavy dust
storm
1: Dust laden clouds, 2: Dust free clouds, 3: Smoke
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
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 (d=diameter)
• 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
Climatic Effects
Levy et al. (2007)
Dust
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…
Solid line: frequency of dust occurrence at Gao from 19571980 (left axis, Bars: Rainfall anamolies for Sahel as a whole
*Dust is represented as number of days with dust haze
Monthly Mean Rainfall
Hours of reduced visibility on left axis (<5km solid,
5<vis<10 open)
mm of rainfall on right axis
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