1. Evidence for a Third aerosol Indirect Effect from Ship Tracks Observed by Calipso
Matthew Christensen and Graeme Stephens
Department of Atmospheric Sciences
Colorado State University
GOAL: Use ship tracks, that are created by aerosol plumes, to assess the microphysical and dynamical changes to marine stratus.

2. Third Indirect Effect Observed
8/8/ :55 UTC
Ship
Ship

3. Third Indirect Effect Not Observed
8/23/ :15 UTC

4. Ship Track Identification
1. Find Ship tracks
2. Automated Pixel Identification
Region: Eastern Pacific
Annual: 6/15/06 – 1/1/09
432 Ship track Profiles 3σ
Collocate Calipso
to Modis pixels.
MODIS: 2.1 μm
4. Construct 20km segments around each cross-section
Droplet Radius
Ship pixels have smaller cloud droplets than the nearby unpolluted control pixels.
(1st Indirect Effect)
Region
100 km2
20km
Pixel Identification
Calipso
Orbit
Cloud Optical Properties
 MOD06 cloud retrievals
Cloud Albedo
BUGSrad (Stephens et al., 2001)
08/07/ :15 UTC

5. Ship Track Identification
5. Collocate CloudSat to Modis.
CloudSat Reflectivity & Precipitation Flags
(Haynes et al., 2006)
Radar reflectivity PDFS are constructed from MODIS retrievals of Re and Tau corresponding to the Contoured Frequency by Optical Depth climatology (Nakajima et al., 2009) derived from CloudSat & MODIS.
ECMWF-AUX
dew point
depression
Polluted
Unpolluted
Stability  Estimated Inversion Strength (Wood et al., 2006).
Moisture  Averaged dew point depression above the boundary layer to 700mb.
**Precipitation is reduced in polluted clouds**
(2nd Indirect Effect)

6. Cloud Type Identification
Stratocumulus cloud type was identified by visual inspection.
While there are many cloud types, only closed and open celled clouds were used in the analysis.
NIR: 3.7 μm
Open
Closed
3.7 μm
Visible: μm

7. Differences in Cloud Depth (Ship – Controls)
# of Tracks
164 Closed Celled
44 Open Celled
Moist
Dry
Height differences are most pronounced under low static stability, high moisture content above the boundary layer, and low cloud cover fraction.
Polluted clouds in open celled convection are ~15% deeper than the unpolluted clouds.
 Increased static stability squelches vertical cloud development.
 Entrainment of dry air above the boundary layer impedes vertical cloud
development as shown in LES models (Ackermann et al., 2004).
 Open celled convection supports vertical cloud development for polluted
clouds.

8. Differences in Cloud Properties (Ship – Controls)
Precipitation is reduced in polluted clouds and the response is more pronounced in open celled convection.
Smaller cloud droplets in polluted clouds inhibits collision coalescence efficiency and the
ability for the clouds to precipitate (Albrecht et al., 1989).
The deeper polluted clouds in open celled convection have enhanced liquid water paths.
In an unstable and relatively moist environment suppressed precipitation enables clouds to
grow deeper and accumulate more liquid water than nearby unpolluted clouds (Pincus and
Baker, 1994).
Polluted clouds have enhanced cloud albedos.
Enhanced albedo in open celled polluted clouds is likely due to both microphysical and
dynamical changes (increased cloud cover) caused by the aerosol.

9. Summary of Findings
Evidence for the 3rd indirect effect is clearly demonstrated by the use of
vertically resolved profiles of ship track cloud top heights from Calipso.
Open celled convective clouds exhibit large cloud top differences between
polluted and nearby unpolluted clouds, while closed celled do not.
The aerosol indirect effect strongly depends on the atmospheric stability
and moisture above the boundary layer.
Ship pixels exhibited a smaller raining fraction than control pixels as
expected due to the suppression of precipitation in polluted clouds (and
the response is different between open and closed celled).
Deeper polluted clouds have larger liquid water paths and cloud albedos
than their nearby unpolluted counterparts.
These results could be useful for validating large eddy simulations of
stratocumulus.