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Clouds and Arctic Climate Change

Pof Ken Carslaw (SEE), Prof Paul Field (Met Office/SEE), Prof Ian Brooks (SEE)

Contact email: k.s.carslaw@leeds.ac.uk

Project Summary

The climate is changing in the Arctic more rapidly than anywhere else on the planet. The decline of Arctic sea ice (currently 13% per decade) leads to regions of open water, which leads to more of the sun’s energy being absorbed and further acceleration of the warming. However, persistent low-level cloud cover in the Arctic, particularly in the summer and autumn, can have a dramatic effect on the surface temperature, but currently these clouds are not well understood and are therefore poorly handled in climate models. As a consequence, our ability to accurately predict future changes in Arctic climate is hampered. This PhD project aims to improve our understanding of clouds in the Arctic using sophisticated computer models as well as existing and new measurements.

Decline of Arctic sea ice

This PhD project is supervised by Prof Ken Carslaw, who leads an Aerosol modelling research group, Prof Ian Brooks, who has long experience in Arctic measurements, and Prof Paul Field, who leads the clouds and microphysics group at the Met Office. You will use and further develop an advanced high-resolution weather prediction model and observations from several previous and upcoming ship expeditions to the Arctic. Collaboration with the UK Met Office will enable the research to feed through to an improved understanding of Arctic weather and climate.

Objectives

The overall aim of this project is to understand what controls the behaviour of Arctic low-level clouds, how their behaviour may change in future, and what impact any changes will have on the climate of the Arctic. Your research will involve:

  1. Running and further developing the Met Office’s weather prediction model to simulate Arctic clouds and the way the clouds interact with the ocean, sea ice and sources of aerosol particles. Ultimately we need to be able to simulate the clouds in global climate models, but the first step will be to use much more detailed models running at very high resolution so we can capture the fine-scale variations. We have had some success modelling the clouds under some situations, but there are many other complicated environments that we have not explored yet.
  2. Analysing field measurements and satellite observations of clouds, aerosols and surface radiation to test and improve the model. 
  3. Simulating how Arctic clouds respond to reductions in sea ice cover and how the clouds themselves affect the sea ice. Loss of sea ice leads to a change in the meteorology of the surface, a greater supply of moisture to the clouds, and also allows more natural sources of aerosol particles to enter the clouds, such as from sea spray and marine biological processes. It’s not well understood how all these factors work together.
  4. Exploring the effect of new understanding and improved modelling capability on Arctic climate using global climate models.

Background

Clouds are one of the most problematic parts of climate models and one of the principal sources of uncertainty in understanding how rapidly the climate will warm in future (see Schneider et al.). Clouds are an even bigger problem in the Arctic, partly because they interact in complex ways with ocean water and sea ice, and partly because they have some unique features that makes simulating them in climate models very tricky. 

Arctic clouds can be extremely thin but still have a significant effect on the sea ice. Photo Ian Brooks from the ASCOS expedition.

Why are Arctic clouds so hard to capture in climate models? First, the clouds are often very tenuous and almost fog-like, which makes them particularly difficult to simulate in low-resolution global models. Second, they are also very different to clouds at lower latitudes because they can warm the surface rather than cool it (they are often too thin to block out much sunlight but they are thick enough to radiate heat back to the surface by the same process that prevents frost forming on cloudy nights). A third reason is that the summertime Arctic is often so pristine that there are barely enough aerosol particles to form cloud droplets on. This means the clouds can dissipate quickly (by drizzling). The clouds therefore become very sensitive to changes in air pollution or new sources of aerosols from the surface (like when sea ice breaks up). Another complication is that the clouds often contain a mixture of water droplets and ice crystals, but we know very little about the special particles that cause the ice to form.

In a future climate with less coverage of sea ice, Arctic clouds are likely to be very different. But how will the change in clouds affect the climate and what can we learn from recent changes? The clouds could change in such a way as to cause more cooling of the surface than they do now (offsetting some of the loss of ice) – see the article by Ridley et al. Or it is possible that large-scale changes in clouds could contribute to enhanced sea-ice melting (see Kay et al.). The supply of aerosol particles will also change (see Browse et al.), which will affect the number of cloud drops and ice crystals and influence rain formation.

The big challenge is that climate models struggle to reproduce even some quite basic features of the clouds (see Kay et al.), and they are not sophisticated enough to simulate the effects of changes in aerosols and ice formation. In this project the aim will therefore be to study Arctic clouds using much more detailed models.

Methodologies

The key modelling tool is the Met Office weather prediction model, which we have recently developed at the Met Office and Leeds for research purposes to simulate the details of aerosol and cloud microphysics (the number of cloud droplets and ice crystals and how they interact to form precipitation). See for example Grosvenor et al. The image below shows the simulation of a cyclone over the Southern Ocean, which demonstrates the detailed cloud features that can be simulated.

The student will also have access to cloud observations from Arctic cruises (involving co-supervisor Prof Brooks), including several weeks of remote sensing retrievals of cloud properties from high summer melt through early autumn freeze up, and open water to dense pack ice. The research will also make extensive use of satellite observations from sensors such as CERES, MODIS and CloudSat-CALIPSO.

Simulation of a cyclone over the Southern Ocean using the CASIM model to be used in this project. The figure shows reflected shortwave (solar) radiation. Data produced by Leeds PhD student Jesus Vergara-Temprado.

Requirements

Undergraduate training in any physical/chemical science, computing or mathematics. An interest in developing in-depth understanding of high performance computing and analysis of big climate and observation datasets.

Training

Students will receive training in running and visualizing global model results both at the Met Office and through the institute’s new Centre of Excellence for Modelling the Atmosphere and Climate (CEMAC: https://www.cemac.leeds.ac.uk/).

Co-supervision will involve regular meetings between all partners. In addition the successful PhD student will have access to a broad spectrum of training workshops put on by the Faculty that include an extensive range of supportive workshops in skills such as managing your degree to preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/).  There will also be opportunities to take part in field campaigns, international conferences, and training courses offered by other organisations. You will have the opportunity to make regular visits to the Met Office.

Research environment

The student will join the vibrant Atmospheric Composition and Cloud Physics research groups of 10 Academic Staff. There are about 50 PhD students across the Institute for Climate and Atmospheric Science (ICAS) covering climate, dynamics, impacts, with extensive programmes in observations, modelling and lab studies.  Wider interdisciplinary experience is guaranteed through our new cross-campus Priestley Centre (http://climate.leeds.ac.uk). Peer exchange and learning occurs through frequent institute and group seminars, discussion meetings and paper review groups.

The Met Office is a world class centre working across observations, climate processes, climate model development and assessment. This environment offers opportunities for the student to engage with a cross section of current weather and climate research (via regular seminar series and peer group meetings).

The collaboration between Leeds and the Met Office goes back over nearly 20 years and is now formalised as the Met Office Academic Partnership (MOAP: http://www.metoffice.gov.uk/research/partnership). There is therefore an active group of Met Office-facing scientists at Leeds.

Further reading

Cloudy with a chance of warming: https://phys.org/news/2015-12-cloudy-chance.html#nRlv

Grosvenor et al. https://www.atmos-chem-phys.net/17/5155/2017/

Ridley et al: The transformation of Arctic clouds with warming http://rdcu.be/wjg0

Browse et al. The complex response of Arctic aerosol to sea-ice retreat: https://www.atmos-chem-phys.net/14/7543/2014/

Kay et al. The contribution of cloud and radiation anomalies to the 2007 Arctic sea ice extent minimum http://rdcu.be/wjkg/

Lenaerts et al. Polar clouds and radiation in satellite observations, reanalyses, and climate models, http://onlinelibrary.wiley.com/doi/10.1002/2016GL072242/full

Schneider et al. Climate goals and computing the future of clouds, http://rdcu.be/wjkS

Related undergraduate subjects:

  • Applied mathematics
  • Atmospheric science
  • Chemical engineering
  • Chemistry
  • Civil engineering
  • Computer science
  • Computing
  • Earth system science
  • Engineering
  • Environmental science
  • Geophysical science
  • Geoscience
  • Mathematics
  • Meteorology
  • Natural sciences
  • Physical science
  • Physics
  • Remote sensing