Testing model parameterisations using ship track simulations in the Bay of Biscay

Figure. MODIS visible image taken on Jan. 27th 2003 over the Bay of Biscay (Europe) showing ship tracks against clear sky (Courtesy: MODIS, NASA).

Many studies have shown that emmitting hygroscopic particles (i.e. water affine aerosol) into clean marine low-lying clouds, such as coastal fog, can alter the cloud state and induce local cloud brightening effects.

In pure liquid clouds, and clouds containing a mixture of ice and liquid, it has been observed that highly concentrated plumes of ship exhaust may induce local cloud brightening.

These so-called ship tracks, shown in the above figure, are rare and are not likely to exert a climate relevant radiative forcing. However, they provide an ideal test bed to understand feedback mechanisms induced by variations in aerosol concentrations within these types of clouds.

Cloud droplet number burden (CDNB) and cloud optical thickness shown at mid-day on Jan. 26th 2003. Grey values indicate missing information. Note that CDNB can only be diagnosed at grid-scale saturation.

At the kilometre scale resolution, which is sufficiently small to resolve the ship track structure but insufficient to resolve any of the involved dynamic, thermodynamic or microphysical processes, we demonstrated that the parameterisations used to prescribe these unresolved processes within the model were indeed able to capture the essence of a ship track.