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 Mid-East 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

March 6, 2000

DUST

DUST

Areas covered by TRMM overpass

March 1, 2000 March 6, 2000

Aerosol Map

Blue: smoke

Orange: sulfates

Green: Desert Dust

PR pass from March 1

clear

dusty

Only precip

1 2

3 4

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 DistributionsCCN 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

Dust

Levy et al. (2007)

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 1957-1980 (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