The Dynamics and Microphysics of Atmospheric Ship Tracks at the Mesoscale

Ship tracks are clear manifestations of aerosol-cloud interactions (ACI) in marine warmphase boundary layer clouds. Since their discovery in satellite retrievals, field measurements have confirmed that increases in internally mixed aerosol particle concentrations within the exhaust plume can indeed induce locally increased cloud droplet number concentrations and increased cloud albedo under suitable environmental conditions. However, the variety and extent of the occurring ACI, their feedbacks and representation in microphysical and boundary layer parameterisations, remain highly uncertain. Indeed, low clouds and ACIs within low clouds in general remain amongst the largest uncertainties in climate projections. In order to evaluate the parameterised effects in ship tracks qualitatively and quantitatively against observations, real case simulations at the meso-gamma scale are explored. At a 2km horizontal resolution, the track structure (∼ 12 km in width) should be captured, while boundary layer and ACI processes occur predominantly on the sub-grid scale. The real case approach allows for the direct evaluation of the simulated features against observations. In particular, the simulations are focused on a case of ship tracks detected within MODIS retrievals obtained at a 1-km horizontal resolution in the Bay of Biscay on January 26 2003. In the first part of this PhD thesis, the simulated environmental conditions of the case study are evaluated against boundary layer soundings and satellite imagery. It is shown that for subsidence rates of −0.5ms−1 to −1.5ms−1, inversions of 10 – 12K are simulated, provided the prescribed minimal eddy diffusivity threshold implemented in the model is lowered. Reducing the eddy diffusivity limiter from 1m2 s−1 to less than 0.4m2 s−1 increases the realism of the simulated vertical mixing and cloud distribution in the boundary layer considerably, whilst the sensitivity of these entities to the horizontal resolution was found to be less pronounced. Introducing mobile ship emissions to the simulations induces local increases in cloud droplet number concentration and decreases in effective radii which are comparable to observations. In simulations with varied emission specifications, which are based on marine ship exhaust measurements, the spatial structure and magnitude of the cloud response are evaluated. All simulations display increments of cloud optical thickness between 3 to 15, which are comparable to the MODIS observations on this day. However, only simulations of up-scaled emission fluxes displayed in-plume aerosol-size distribution perturbations comparable with field observations and ship tracks of realistic spatial extent. As well as microphysical impacts of ship exhausts, feedbacks of the macrophysical state are also simulated. In particular, two-fold increases in the liquid water path are found within 25% of the ship track regions, which is attributed in most cases to a local increase in liquid water content by drizzle suppression, and a subsequent lowering of the cloud base by vertical mixing. In the final chapter, the potential resolution dependence of these findings is addressed. As the boundary layer and microphysical parameterisations of the employed model are also used in many global climate models, such an analysis can provide valuable insights into the limitations of global radiative impact assessments of ship emissions. Our simulations indicate significant variations of the exerted shortwave cloud radiative effect for varying degrees of emission dilution. It was found to be stronger by −2.3Wm−2 when emissions were diluted from 2 km down to 50 km. Therefore our results indicate that the cloud radiative forcing due to ship emissions is overestimated in global climate models due to their coarse resolution. Overall, it is demonstrated in this thesis that real case regional-scale simulations of ship tracks provide a valuable platform for the evaluation of parameterisations involved in boundary layer ACIs. We believe that this approach can contribute to the general assessment of individual physical parameterisations, and their effects.