Impact of ship emissions on the microphysical, optical and radiative properties of marine stratus: a case study

Abstract

Abstract. Modifications of existing clouds by the exhaust of ships are well-known but inadequately quantified impacts, which could contribute to climate change. The perturbation of a cloud layer by ship-generated aerosol changes the cloud reflectivity and is identified by long curves in satellite images, known as ship tracks. As ship tracks indicate a pollution of a very clean marine environment and also affect the radiation budget below and above the cloud, it is important to investigate their radiative and climatic effects. Satellitedata from MODIS on Terra are used to examine a scene from 10 February 2003 where ship tracks were detected close to the North American West-Coast. The cloud optical and microphysical properties are derived using a semi-analytical retrieval technique combined with a look-up-table approach. An algorithm is presented to distinguish ship-track-pixels from the unperturbed cloud pixels in the scene and from this the optical properties of the former are compared to those of the latter. Within the ship tracks a significant change in the droplet number concentration, the effective radius and the optical thickness are found compared to the unaffected cloud. The resulting cloud properties are used to calculate the radiation budget below and above the cloud. Assuming a mean solar zenith angle of 63 • for the selected scene, the mean solar surface radiation below the ship track is decreased by 43.2 Wm −2 and the mean reflectance at top of atmosphere (TOA) is increased by 40.8 Wm −2 . For the entire analyzed scene the ship emission decreases the solar radiation at the surface by 2.1 Wm −2 and increases the backscattered solar radiation at TOA by 2.0 Wm −2 , whereas no significant effect on thermal radiation was detected.

Global ship track distribution and radiative forcing from 1 year of AATSR data

“…[19] While previous studies have focused on local impacts of ship tracks [e.g., Durkee et al, 2000;Platnick et al, 2000;Schreier et al, 2006], this study presents the first global distribution of ship track coverage and the resulting change on the Earth's radiation budget. However, there are still some limitations for this analysis.…”

“…The high variability of ship tracks results in a high variation of their radiative behavior. Therefore, in the third step the cloud optical properties like effective radius and cloud optical thickness are calculated for each individual pixel using a combination of the SACURA method (Semi-Analytical Cloud Retrieval Algorithm [Kokhanovsky et al, 2003]) and a look-up-tableapproach calculated with the libRadtran package [Mayer and Kylling, 2005] as described by Schreier et al [2006]. Using these cloud optical parameters, the backscattered radiative flux at TOA is calculated for the possible solar zenith angles, depending on the latitude and season, within the ship tracks and in the surrounding region.…”

“…The satellite data analysis presented here is restricted to the visually detectable impact of ship emissions on clouds, identified by elongated structures in satellite images, known as ship tracks. Ship tracks have been shown to change the radiation budget on a local scale [Schreier et al, 2006]. Thus, there is considerable interest to estimate their radiative impact on global scale.…”

Update on emissions and environmental impacts from the international fleet of ships: the contribution from major ship types and ports

“…Aerosols can also act as condensation nuclei, modify cloud properties and precipitation rates and through that have indirect climate effects (Ramanathan et al, 2001). Measurements have revealed both direct and indirect effects of aerosols from ship emissions Durkee et al, 2000;Ferek et al, 2000;Schreier et al, 2006;Schreier et al, 2007;Devasthale et al, 2006) Several studies have made estimates on the ship emissions impact on climate for one or more components (Capaldo et al, 1999;Endresen et al, 2003Endresen et al, , 2008Eyring et al, 2007Eyring et al, , 2008Lee et al, 2007;Lauer et al, 2007;Dalsøren et al, 2007;Fuglestvedt et al, 2008;Berntsen and Fugelstvedt, 2008). Fuglestvedt et al (2008), Berntsen and Fuglestvedt (2008), Endresen et al (2008) and Eyring et al (2008) give measures or reviews of the overall impact.…”

Multi-model simulations of the impact of international shipping on Atmospheric Chemistry and Climate in 2000 and 2030

“…The direct effect results from enhanced scattering of solar radiation (Haywood and Shine, 1995). The indirect effect of particles from shipping results from changes in the microphysical, optical and radiative properties of low marine clouds (Scorer, 1987;Capaldo et al, 1999, Durkee et al, 2000Schreier et al, 2006).…”