What Can We Learn from Ship Tracks about the Importance of Aerosol-cloud Interactions in Shallow Clouds? (Invited Presentation)

The effective radiative forcing due to aerosol-cloud interactions (ERFaci) remains one of the largest uncertainties since pre-industrial times (IPCC, 2013). While progress has been made over the last 2 decades to improve both climate models and satellite data retrievals and analysis, from which ERFaci can be deduced, new sources of uncertainty have emerged at the same time.

Ship tracks are sometimes visible in satellite images because the aerosol in the exhaust plume increases the optical depth of the perturbed clouds over the neighboring clean clouds. However, not every ship track is visible and the global cloud radiative effect of ship exhaust has been estimated between being negligible up to -0.6 W/m2.

In this talk I will show simulations of ship tracks in the Bay of Biscay using the regional model COSMO with horizontal resolutions between 1 and 50 km. If ship emission mass fluxes are scaled up (ten-fold scaling) in order to match observed near-exhaust in-plume size distributions, COSMO can nicely capture ship tracks. The increase in cloud optical depth in the ship track was found to agree well with MODIS observations. The increase in cloud optical depth in ship tracks, however, depends on the horizontal resolution of the underlying model and is overestimated in coarser resolutions.