Real Case Simulations of Ship Tracks in the Bay of Biscay: from the Kilometer Scale towards the GCM Scale

Abstract:

Ship tracks are clear manifestations of aerosol-cloud interactions (ACI), which occur within confined linear structures and exclusively within strongly constrained environmental conditions. Furthermore a wide range of processes including the large-scale forcing, radiation, cloud dynamics and cloud microphysics are essential in the formation of such tracks. We therefore argue that they provide an ideal platform for studying and evaluating existing parameterisations and their interaction within the marine stratocumulus regime in real-case simulations.

We are using the COSMO model at resolutions between 1 and 50 km to investigate an episode of observed ship tracks over the Bay of Biscay. We have previously shown that the COSMO model with extensions from Zubler et al. (2011) including two-moment aerosol (Vignati et al 2004) and cloud microphysical parameterisations (Seifert and Beheng, 2006) is able to capture the essence of ship tracks in warm-phase stratocumulus in a shallow (cloud top below 700m) boundary layer at a 2-km resolution (Possner et al, 2015). The boundary layer structure of these simulations closely matched observed soundings obtained along the coast of the Bay of Biscay (Possner et al, 2014). The cloud brightening of the track (in terms of optical thickness τ) as observed by MODIS was found to be in good agreement with the simulations. Furthermore, the cloud microphysical response (in terms of cloud droplet number concentration and effective radius) was found to be in range of observed responses in various field campaigns.

Building upon these initial results, we further explore the resolution dependence of the ACI in stratocumulus as we approach GCM resolution from the kilometer scale. Decreasing the spatio-temporal resolution from 2 km (20s) down to 50 km (180s) was found to lead to significant changes in the cloud radiative effect of the tracks of almost 300%, which can be tied to changes in vertical mixing and loss of spatial variability. Implications of this effect for low-resolution ship-track studies will also be discussed.