Ice-Multiplication Mechanisms: Their Role in the Linkage between Aerosols, Cold Clouds and Climate
Lead Supervisor: Vaughan Phillips (SEE, v.phillips(at)leeds.ac.uk; Tel.: 0113-343 6476); Co-supervisors: Steven Dobbie, Benjamin Murray, Alan Blyth.
Background
Impacts from aerosol pollution on cloud properties account for most of the uncertainty in prediction of climate change. Aerosols are submicron particles found everywhere in the troposphere at high concentrations (100s – 10000s per cm3). Globally, aerosol loadings have been greatly altered by pollution. Indeed, as much as about half of past global warming from greenhouse gas emissions since 1800 may have been offset by aerosol pollution altering cloud properties. This is because clouds control the heating of the Earth's surface by sunlight and infrared radiation, and are formed by the growth of aerosols to become cloud-particles. Total number concentrations of cloud-particles depend on aerosol loadings and govern clouds' production of precipitation, as well as their extent and reflection of sunlight to space.
Cold clouds are pivotal for impacts from aerosols on climate. Most of the volume of the troposphere has sub-zero temperatures, where clouds are glaciated. In cold clouds, initiation of ice is poorly understood. Initial ice particles are formed by aerosols but their numbers may be greatly increased subsequently by fragmentation. This “ice multiplication” has been observed by aircraft to dominate total number concentrations of cloud-particles, which in turn govern properties of cold clouds. Lab studies have revealed multiple mechanisms of ice multiplication by fragmentation of ice. Recent aircraft data show that fragmentation in real clouds can occur (Fig. 1):- (a) in collisions between ice particles; and (b) during freezing of drops, as seen in the lab. Yet scarcely any modeling studies have assessed the roles of both mechanisms in the atmosphere.
The aim of the project would be to extend knowledge about aerosols’ effects to glaciated clouds, by understanding how the multiplication of their ice particles controls the linkage between aerosols, cold clouds and climate.
Objectives
- Develop a theory for two mechanisms of ice multiplication, namely fragmentation in ice-ice collisions and during drop-freezing, by analysing data from published lab studies.
- Verify the theory by comparing simulations of selected clouds with aircraft observations.
- Quantify how these ice-multiplication mechanisms modify the impact from aerosol pollution on cold clouds and climate (e.g. clouds' ice concentrations, precipitation, radiative fluxes), by performing large-scale simulations of the European region.
Potential for high impact outcome
Initiation of ice by multiplication in cold clouds and its role in the linkage between aerosols and climate change is one of the greatest uncertainties in cloud physics and climate science. This topic will be addressed in the PhD project. You will develop a new theory and use state-of-the-art atmospheric models, with the potential for a major effect on climate science and clouds physics.
Ice is complex in its morphology and physics of initiation, and this complexity has ramifications for the climate system via the effects from cold-cloud properties. Much progress in modelling climate change can arise from considering key ice-microphysical processes in detail, as in this PhD.
- Figure 1. Fragmented ice crystals with fewer than the 6 original branches (left; published by Schwarzenboeck and colleagues in 2009) with natural fractures (white circles), and ice fragments with rounded portions due to fragmentation during raindrop-freezing (right; from Rangno in 2008). Images are from aircraft observations of clouds in the Arctic (left) and near Washington State (right).
Drs Vaughan Phillips, Steven Dobbie and Benjamin Murray, and Professor Alan Blyth, have a long history of high-impact publications about aerosols, clouds and climate. Dr Vaughan Phillips has published papers about mechanisms for precipitation production in glaciated clouds and initiation of ice, some of which were cited in the latest report in 2007 by the Intergovernmental Panel on Climate Change (IPCC). Professor Alan Blyth leads a field campaign to study glaciated clouds over England and ice multiplication in laboratory studies. Dr Steven Dobbie leads an international working group to study cirrus modeling. The PhD will benefit from this world-leading expertise.
Training
The expertise of a group of scientists in the Dynamics and Clouds Group will inform your training. This group has trained many successful PhD students. You will learn much about cloud physics and climate science from this Group, and about the techniques for development and validation of advanced models of the atmosphere. After obtaining your PhD degree, you will have a great advantage if you choose to continue working in research. This is because the PhD course will give you the capability to develop, or even create, new atmospheric models, and you will gain an understanding of the physics of cloud and radiation underpinning them. You will learn about how to process the observational aircraft data used for validating and initialising models. You will acquire many transferrable skills generally applicable, including skills not only in computing but also in how to present complex ideas to audiences by diverse media. You will be able to attend the NCAS Earth System Science summer school during the PhD and to attend international conferences such as the European Geophysical Union (EGU) held every year in countries such as Austria or France.
Further reading
To find out more about our exciting research projects, and about opportunities in research with our group at Leeds, we suggest that you visit the webpage of the Dynamics and Clouds group and Dr Vaughan Phillips's home page.