Occurrence of lower cloud albedo in ship tracks

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Abstract

The concept of geoengineering by marine cloud brightening is based on seeding marine stratocumulus clouds with sub-micrometer sea-salt particles to enhance the cloud droplet number concentration and cloud albedo, thereby producing a climate cooling effect. The efficacy of this as a strategy for global cooling rests on the extent to which aerosol-perturbed marine clouds will respond with increased albedo. Ship tracks, quasi-linear cloud features prevalent in oceanic regions impacted by ship exhaust, are a well-known manifestation of the effect of aerosol injection on marine clouds. We present here an analysis of the albedo responses in ship tracks, based on in situ aircraft measurements and three years of satellite observations of 589 individual ship tracks. It is found that the sign (increase or decrease) and magnitude of the albedo response in ship tracks depends on the mesoscale cloud structure, the free tropospheric humidity, and cloud top height. In a closed cell structure (cloud cells ringed by a perimeter of clear air), nearly 30% of ship tracks exhibited a decreased albedo. Detailed cloud responses must be accounted for in global studies of the potential efficacy of sea-spray geoengineering as a means to counteract global warming. © Author(s) 2012.

Figures

  • Table 1. Instrumentation Payload on CIRPAS Twin Otter.
  • Fig. 1. Spiral soundings of clean and ship exhaust perturbed areas in E-PEACE research flight 20 and 24 (4 and 10 August 2011, respectively). Flight path is colored according to aerosol number concentration (particle diameter > 120 nm).
  • Table 2. Aerosol/cloud properties measured during E-PEACE Research Flights 18, 19, 20, and 24. For the cloud structure, closed/open means closed or open cloud cellular structure. Cloud layer is defined with cloud droplet number concentration > 10 cm−3 and liquid water content > 0.01 g m−3. Mean Na, Nd, re (cloud drop effective radius), and k (droplet spectral shape parameter) are geometric mean values. BL average w′w′ is the mean vertical velocity variance in the boundary layer. Standard deviation is in parenthesis.
  • Fig. 2. Cloud microphysical parameters measured along the flight tracks. Each symbol represents data over a 1 s increment. Cloud droplet number concentration [cm−3] is colored on a logarithmic scale; droplet effective radius (re) is given by the size of symbols varying between ∼ 4 and 19 µm. Clean and perturbed cloud data are presented by crosses and open circles, respectively.
  • Fig. 3. GOES satellite images. Satellite images during (a) RF20 (4 August 2011) and (b) RF24 (10 August 2011) off coast of Monterey, CA, exemplifying open and closed cell cloud structures, respectively. Flight path is colored according to aerosol number concentration (particle diameter > 10 nm).
  • Table 3. Cloud LWP, optical properties, and environmental conditions measured during E-PEACE Research Flights 18, 19, 20, and 24. Standard deviation is in parenthesis.
  • Fig. 4. Magnitude of cloud susceptibility in four E-PEACE cases. Twomey effect (red circle), dispersion effect (green circle), cloud thickness effect (blue circle), and total cloud albedo susceptibility based on Eq. (3) (black circle) and Eq. (1) (black cross) for RF18, RF24, RF19, and RF20 (order from low to high cloud albedo susceptibility).
  • Fig. 5. Frequency distribution of different parameters for 589 individual ship tracks from June 2006–December 2009 A-Train observations. The parameters include: (a) dew point depression, (b) cloud top height, (c) effective radius, and (d) optical depth. Albedo enhancement (brightening) and decrease (dimming) cases are shown by red and blue lines, respectively. Means and (standard deviations) are given at the top of each panel. The cloud top height, effective radius, and optical depth are averaged over the unpolluted cloudy sections of each ship track.

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CITATION STYLE

APA

Chen, Y. C., Christensen, M. W., Xue, L., Sorooshian, A., Stephens, G. L., Rasmussen, R. M., & Seinfeld, J. H. (2012). Occurrence of lower cloud albedo in ship tracks. Atmospheric Chemistry and Physics, 12(17), 8223–8235. https://doi.org/10.5194/acp-12-8223-2012

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