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 Areas covered by TRMM overpass

March 1, 2000March 6, 2000 Aerosol Map Blue: smoke Orange: sulfates Green: Desert Dust

PR pass from March 1 clear 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 1km 2 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) – (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 (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