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.

Third Indirect Effect Observed 8/8/2007 22:55 UTC Ship Ship

Third Indirect Effect Not Observed 8/23/2007 22:15 UTC

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) 08/07/2007 22:15 UTC

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)

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: 0.64 μm

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.

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.

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.