PhD Studentship: Improving the representation of the interaction between aerosol particles and the cloud droplet size distribution in climate models.

Location: Streatham Campus, University of Exeter, EX4 4QJ

Project Description:

Motivation

Clouds are a defining feature of the climate system. The only tools at our disposal to predict the impact of clouds on the future climate are numerical models, known as general circulation models (GCMs). Reducing current uncertainty surrounding climate change projections is contingent on us improving the representation of cloud processes in GCMs, in particular, the complex interactions between clouds and aerosols - tiny particles suspended in the atmosphere.

One limitation in the representation of aerosol-cloud interactions in GCMs is the accurate treatment of the impact aerosols have on the shape of the cloud droplet size distribution. In the real world the cloud droplet spectral shape is highly variable and dependent on numerous atmospheric processes acting across a wide range of temporal and spatial scales. However, GCMs assume it can be approximated by constant distributions. Currently there is no firm physical basis for selecting a specific function to describe droplet size distributions in GCMs.

Primary Aim

In this college funded PhD studentship the overall aim is to improve the representation of cloud microphysical properties in GCMs. To reach this goal we will make use of a new novel modelling framework in which a cloud model has been embedded within a GCM that explicitly calculates the droplet spectral shape as a function of the prevailing aerosol environment.

Project Strategy

• Statistical analysis

The project will tailor existing methods to perform statistical analysis of GCM estimates of cloud microphysical properties. By employing a statistical technique termed clustering, previously applied to aerosol size-distributions, we will generate climatologies and explore the global variability in droplet size distributions. These results will be used to investigate current assumptions made concerning the droplet spectral shape in GCMs when calculating the radiative impact of clouds on the climate system.

• GCM Model development

The project will employ existing modelling tools and apply them to the GCM HadGem-UKCA used by the UK Met Office. We will investigate relationships between the shape of the simulated droplet size distribution and aerosol-cloud microphysical processes in the form of novel Monte Carlo sensitivity simulations with cloud models. In addition we will compare cloud properties simulated by the embedded cloud parcel model with the existing GCM cloud microphysics scheme. These results will guide the development and improvement of the representation of the cloud droplet spectral shape within HadGem-UKCA. The impact of these model developments on estimates of climate change will be assessed. The research will be conducted in close collaboration with the Met Office. The PhD project has the potential to produce key academic publications as well as real improvements in estimates of climate change. The project is interdisciplinary in nature, drawing on numerical modelling, applied statistics, as well as weather and climate science.

Objectives

The main objectives of the project are:
1. to develop and explore novel methods for statistical analysis of cloud microphysical properties from GCMs for comparison against observations.

2. to improve current understanding of the sensitivity of cloud properties to processes involved in aerosol-cloud interactions that control the shape of the cloud droplet size distribution.

3. to improve the representation of the cloud droplet size distribution in GCMs and subsequent estimates of precipitation, cloud radiative forcing and climate change.