Our research addresses physical climate processes in the context of anthropogenic perturbations to the earth system as the underlying cause of climate change and air pollution.

Owing to the complexity of clouds and aerosols in the climate system, spanning scales from nanometers to thousands of kilometres, none of the sub-disciplines of Theory, Modelling and Observations have led to a significant reduction of uncertainties in our understanding of their role in the climate system, despite significant research over decades. In the Climate Processes group we strive to bridge these gaps and combine modelling closely with observational constraints.

Modelling Observations
Figure: Aerosol mass-mixing ratios isosurfaces colour-coded by component as simulated with the aerosol-climate model ECHAM5-HAM. Contour levels individually adjusted. Figure: Illustration of the A-Train concept of multiple earth observation satellites providing co-located measurements from multiple instruments within minutes of consecutive overpass times.
Theory Machine Learning
Figure: We develop complex parameterisations of convection for use in climate models. The Convective Cloud Field Model represents the evolution of a spectrum of clouds through a Lotka-Volterra equation describing a competitive system of interacting species (as used in population dynamics). Figure: We increasingly use machine learning methods to analyse Earth Observations and to improve atmospheric models and our understanding of their uncertainty. The figure shows a mask (blue) of a Pocket of Open Cells in stratocumulus clouds detected using deep convolutional neural networks based on the ResUnet architecture.

Key areas of research include:

Aerosol physics and radiative effects

We develop models and utilise observations to gain insights into the global distribution and microphysical state of atmospheric aerosols and their radiative effects (through scattering and absorption).

Figure: Smog over Los Angeles: the fact that it appears bright to the observer illustrates the scattering of solar light by aerosol particles. Enhanced scattering of solar radiation back to space introduces a cooling effect on the climate system, partially masking greenhouse gas warming.

Selected Publications:

Cloud physics and feedbacks

Clouds play a key role in the climate system. Modelling clouds from the cloud to the global scale in combination with in-situ and satellite observations we seek to improve their representation in global climate models as basis for more reliable climate predictions. Convective clouds are becoming the focus of our research activities.

Figure: Convective clouds linearly organised on the meso-scale ("Squall Line") over Florida as seen from the Space Shuttle. The representation of convection in global climate models is a source of key uncertainty (from NASA).

Selected Publications:

Aerosol-Cloud interactions and radiative effects

Clouds play a key role in the climate system. Modelling clouds from the cloud to the global scale in combination with in-situ and satellite observations .

Figure: MODIS true colour satellite image of ship tracks over the north Atlantic: aerosol emitted by ships passing below a deck of stratocumulus clouds increase the number of cloud condensation nuclei leading to (under the assumption of constant liquid water) smaller cloud droplets with higher reflectance. This illustrates the importance of aerosol effects on clouds and the global radiation balance. (From NASA Earth Observatory)

Selected Publications:

The role of aerosols and clouds for climate change and air pollution

Our research converges around one central theme: to what extent affect anthropogenic perturbations to the global aerosol and cloud systems global climate change and air pollution?

Figure: Figure: Year 2000 mean anthropogenic direct aerosol radiative forcing [Wm-2], as simulated with the ECHAM5-HAM aerosol-climate model. Positive (red) perturbations indicate a warming effect, negative (blue) perturbations indicate a cooling effect of anthropogenic aerosols on the earth system. (From Stier et al., Atmos. Chem. Phys., 2007).

Selected Publications: