Figure: Schematic Diagram of processes governing cloud evolution of stratiform mixed-phase clouds perturbed by ship exhaust

Whilst many of these feedback processes and mechanisms have been explored in pure liquid clouds, little is know about different mechanisms that may occur in mixed-phase clouds, i.e. in clouds that contain a mixture of ice and liquid. These type of clouds are particularly common in the Arctic and over the Southern ocean. Due to their specific properties they are a significant contributor to the Earth's  energy balance and are therefore relevant for climate.

 

For this reason I and my co-authors from ETH Zurich (Ulrike Lohmann) and   Stockholm University (Annica Ekman) have conducted a study investigating feedback mechanisms triggered within Arctic stratiform mixed-phase clouds seeded by ship emissions. The key results of the study published in XX can be summarised as such:

1) Increases in cloud top radiative cooling lead to increased immersion freezing rates near cloud top.

2) Feedback mechanisims involving the ice phase reduce, if not suppress, changes to the cloud liquid water path triggered by ship exhaust.
 

​3) Changes in cloud condensation nuclei concentrations of 100 cm-3 were sufficient to shift the cloud state beyond its background variability.

In general my research is driven by the desire to further our process understanding of different phenomena or XX found within the Earth's atmosphere. In pursuing this goal I perform both high-resolution, short time scale studies to further understand local phenomena and small-scale processes that may have large-scale (temporal and spatial) impacts, as well as low-resolution climate simulations assessing more general climate impacts stemming from human activity.

Below you may find a brief overview of my research, which has focused on feedback mechanisms triggered in polluted marine boundary clouds, and possible climate impacts due to a large-scale deployment of wind power.

Many studies have shown that seeding clean marine low-lying clouds, such as coastal fog, with hygroscopic particles, or aerosols, 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 Fig X 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.

Figure: MODIS visible image of Bay of Biscay (Europe) showing ship tracks against clear sky (courtesy: MODIS, NASA).

Research Overview

At the kilometer 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 (see animation).

However when degrading the horizontal resolution further to levels approaching currently used GCM resolutions (O(100km)), huge biases became apparent in the simulated cloud perturbation and its radiative feedback.

These results therefore support the need for subgrid-scale variability of aerosol microphysics in GCMs, as is currently incorporated for clouds, but not for aerosols.