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Ships may alter cloud-radiative properties even when not detected from space

Figure: LHS: Cloud albedo of a shallow cloud scene where a ship track forms around the emission source. The ship is moving continuously through the domain from right to left. RHS: The same is shown for a different cloud scene (deeper boundary layer) where the same ships is passing through the domain. Although no ship track is simulated, the induced change in cloud reflectiveness averaged over the domain is as large.

The potential for ships to impact the amount of sunlight reflected by clouds over the oceans has long posed a challenging question to the community. It sits at the heart of a more general issue of how strongly aerosol emitted by human kind impact the climate by altering clouds. The response of low-level marine clouds to anthropogenic emissions constitutes the largest source of uncertainty in this problem.

That ship emissions can alter the cloud state and increase cloud albedo (measure of the cloud's reflectiveness) has long been known since the discovery of anomalous linear structures, so-called ship tracks forming in the wake of ocean-going container ships. We also know that the cloud albedo may not always be increased, but through interactions between the increased particular pollution, the cloud and the environment may actually be decreased instead. Yet, all these analyses rely on the structure of a ship track to be detected in the first place.

Yet ship tracks are extremely rare. It seems that merely 0.002% of ships generate a ship track. Furthermore, these tracks are only seen relatively close to the ground (within 800 m from the surface). When no track is simulated, it is extremely difficult to disentangle changes in cloud reflectivity caused by aerosol perturbations or other environmental factors.

In our recent study we show in high-resolution simulations, that in deep (up to 1600m above the surface) precipitating slightly convective boundary layers, ship exhaust may generate an equally strong change in cloud albedo as is found when a ship track is simulated. Furthermore we demonstrate that the change of reflectiveness within the cloud is largely caused by the smaller and more numerous cloud droplets (the so-called Twomey effect). At the same time the change in overall reflectiveness is attributed to the change in cloud area. Clouds cover 15% more of the dark ocean surface in simulations with the ship emissions.

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