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Summit Aerosol Cloud Experiment (SACE): Examining the Role of Aerosol in Cloud Over the Top of the Greenland Ice Sheet

Dr Ryan Neely (SEE), Prof Ken Carslaw (SEE), Dr Dan Grosvenor (NCAS)

Contact email: r.neely@leeds.ac.uk

Overview and scientific need

Clouds are fundamentally important to Earth’s climate since they play a central role in the radiation budget — and hence surface temperatures —and the hydrological cycle. Further, clouds are not passive tracers within the atmosphere. Rather clouds are agents of change that actively influence atmospheric circulation on all scales (Stevens and Bony, 2013; Bony et al. 2015). Aerosols play a critical role in determining cloud formation and their resulting radiative properties by acting as cloud condensation nuclei (CCN), which provide seeding sites for available water vapour to condense onto, and ice nucleating particles (INP), which modulate the formation of ice in clouds.

Yet, aerosol-cloud interactions, especially in regards to clouds that are comprised of both ice and supercooled liquid water (i.e. mixed-phase clouds), remain one of the greatest sources of uncertainty in weather and climate models. This is despite the fact that decades of laboratory experiments, observations, modelling and analysis have contributed to the state-of-the-art knowledge (Morrison et al. 2012).

Though found globally, mixed-phase clouds are more commonly found in the in the polar regions where they typically cover large areas throughout the year. Generally, polar mixed-phased clouds occur as single or multiple layers of supercooled liquid water from which ice crystals form and precipitate (Morrison et al. 2012; Lawson and Gettelman, 2014). This produces the characteristic structure of liquid near the cloud top and ice within and below the liquid layer as seen in Figure 1 from observations made at Summit, Greenland. Such clouds occur frequently during all seasons in these regions, where they often persist for many days at a time.

Figure 1. Polarised lidar and Doppler cloud radar observations of the characteristic structure of Arctic mixed-phase clouds. Observations at Summit, Greenland on September 23, 2016, show that supercooled liquid water persists for more than 24 hours despite a continual loss of mass due to ice precipitation. Lidar depolarization ratio (a) indicates the presence of differing phases with low values indicating liquid. Lidar photon counts (b) are dominated by the much smaller, yet more numerous, droplets found in liquid layers at ~1.5km while radar reflectivity (c) is dominated by the relatively large ice crystals that form in and fall from the supercooled liquid cloud layers. The strong lidar signal lower down is due a thick layer of blowing snow at the surface. Cloud radar mean Doppler velocity (d) depicts the local vertical circulation. 

The persistence of mixed-phase clouds in the polar regions is due to feedbacks between a complex web of local processes and feedbacks (Morrison et al. 2012). These include the formation and growth of ice and cloud droplets, radiative cooling, turbulence, entrainment and surface fluxes of heat and moisture. As well as the existence of the persistent mixed-phase cloud state there is another distinct atmospheric state in the polar regions, which is characterized by radiatively clear conditions. The occurrence of either state seems to be related, in part, to aerosol interactions and to large-scale environmental conditions (which may be linked). Large-scale changes in regional climate will drive changes in aerosol populations which have the potential to significantly alter the local processes important for mixed-phase clouds and, thus, surface mass and energy budgets. Evidence already suggests that large-scale changes in circulation are decreasing cloud cover over the Greenland Ice Sheet (GrIS), which is, in turn, accelerating mass loss of the GrIS (Hofer et al. 2017). Changes in atmospheric dynamics are thought to drive this change but the corresponding role of changes in aerosol is unexplored.

A problem with understanding mixed-phase cloud-aerosol interactions is the inherent complexity of the system and our limited ability to accurately observe all the necessary variables to describe this multi-dimensional space. The problem is entangled further due to the complex roles played by changes in atmospheric state (i.e. temperature and humidity) driven by boundary layer, mesoscale, and synoptic scale dynamics. To reduce the complexity, this project will examine aerosol-cloud interactions in the natural laboratory created by the central GrIS. In particular, observations from the on-going Integrated Characterization of Energy, Clouds, Atmospheric State and Precipitation at Summit (ICECAPS) project at the U.S. NSF-funded observatory located at Summit, Greenland will provide the core set of measurements for the analyses to be conducted.

Figure 2.  Sunset at Summit, Greenland in August, 2017.

For examining the role of aerosols, Greenland is unique because, once away from the coastal edge, there are no local sources of aerosol besides blowing snow. Yet, due to its relative proximity to both large sources of natural and anthropogenic emissions, plumes of aerosol are transported over the GrIS episodically where they can interact with clouds (Figure 3). In addition, due to its unique geography (i.e. its high latitude, high altitude and ice sheet), the atmosphere above the GrIS is an ideal environment for examining the role of aerosol in mixed-phase clouds. Though an ideal region to study aerosol-cloud interactions, very little is known about how aerosols and clouds interact over the GrIS.

Figure 3.  Schematic depicting the possible sources and pathways of aerosol over the GrIS.

Understanding the role of aerosol-cloud interactions over the GrIS is an important problem but has yet to be attempted. Clouds over the GrIS play a fundamental role in the regional and, in turn, global climate, by acting as both sources, via precipitation, and as potential sinks, via modulation of the surface energy budget, in the mass budget of the cryosphere.

Figure 4.  Longwave CRE observed by the CERES satellite (A) and as represented in the HadGEM-ES climate model.

In the last two decades, the rate of decline in the GrIS has accelerated. Several recent studies have examined the role of clouds in this decline and have found that both the existence of clouds in certain conditions as well as the overall decrease in cloud cover play significant roles. In particular, low-level liquid clouds are thought to have enhanced surface melting of the GrIS by one-third (van den Broeke et al. 2009; Van Tricht et al. 2016). The recent decline of the GrIS has also been punctuated by several extreme melt events (i.e. where a majority of the GrIS has been observed to be melting). This includes the event in July 2012 when the GrIS sustained the first melt event over its entire surface since 1889. Bennartz et al. (2013) directly linked this and other recent extreme melt events to the warming effect that low-level mixed phase clouds have on the surface of the GrIS. This warming role of clouds is set within a regional context of synoptic scale changes in the atmospheric circulation which are decreasing cloud cover over the GrIS (Hofer et al. 2017).

Finally, the results of the observational analyses to be conducted are needed to drive the next generation of physical parameterisations in climate models. Current climate models used in CMIP5 (Taylor et al. 2012) show a very large spread in their estimates of the cloud radiative effects (CRE) over the GrIS (Boeke et al., 2016). Figure 4 shows observations of the longwave cloud radiative effect (i.e. the warming of the surface due to clouds) averaged over June, July and August versus the results of the HadGEM-ES simulations for CMIP5. The large discrepancy has a significant impact on the surface energy balance of the GrIS which, in turn, drives surface melting of the icesheet. In addition, the CMIP5 simulations exhibit diverse combinations of cloud occurrence, thicknesses and phase that govern CRE; many models have CREs close to those observed, but they are the result of compensating biases (Boeke et al., 2016). Crucially, this misrepresentation of the current cloud state indicates that projections of future cloud changes and the impact over Greenland are erroneous.

Scientific Objectives

  • Objective 1. Using ground-based remote sensing and in situ observations from Summit, Greenland in conjunction with satellite observations, determine to what extent different properties of aerosol produce differences in the microphysical and macro-physical properties of clouds over the GrIS.
  • Objective 2. Quantify the influence CCN and INP have on the surface energy balance of the GrIS through modulation of cloud microphysical properties.
  • Objective 3. Evaluate regional and global model performance against the observations to identify the model parameterizations that contribute to biases in cloud occurrence, cloud thickness and phase partitioning that contribute to the LW summertime CRE bias over the GrIS; develop, test and deliver new parameterizations to improve model fidelity. This synthesizing objective will be accomplished via modelling through an integrated analysis of the cloud and aerosol observations, and process evaluations in Obj. 1 and 2.

As part of accomplishing these objectives, there is an opportunity to travel Greenland to get hands-on field experience and make personal observations in this unique environment.

Financial support

In addition to the stipend and research money that comes with a NERC studentship, this work will be supported by NCAS and the NSF-funded ICECAPS project. This includes the potential for doing field work at Summit, Greenland.

Potential for high impact outcome

The role of CCN and INP in the formation of ice in cloud remains a major limitation in our quantitative understanding of clouds in the climate system. Through the highly multidisciplinary teams working on ice within ICAS, NCAS and the U.S.-based ICECAPS team (with whom we have strong connections) we are very well positioned to produce more high impact papers. In this project, we foresee publications on: i) state-of-the-art cloud property retrievals using multiple remote sensing instruments, ii) the first detailed analysis of the role of aerosol in clouds over Summit, and the top of the GrIS, iii) the role aerosol plays in modulating the surface energy budget over Summit and the GrIS, iv) the regional role of aerosol over the wider GrIS during the past, present and future.

Training

You will work directly under the supervision of Dr Ryan R. Neely III, Prof. Ken Carslaw and Dr Dan Grosvenor within ICAS. You will also become an active member of each PIs research group and, thus, benefit from working within an active and multidisciplinary group of scientists within SEE, ICAS and NCAS.

This project will equip you with the necessary expertise to become a leader in the next generation of atmospheric scientists, ready to carry out your own programme of innovative scientific research. These skills will be developed by a mixture of hands on experience, attending external training courses, national and international conferences and taking part in the Leeds – York NERC Doctoral Training Partnership programme. This includes access to a broad spectrum of training workshops put on by the Faculty that consist of a range of extensive training workshops that will help you manage your degree and prepare for your viva (http://www.emeskillstraining.leeds.ac.uk/).

Specifically, this project will provide highly transferrable scientific training in a range of growth areas/skills including: i) the use of remote sensing instrumentation; ii) making measurements in the field; iii) numerical modelling; iv) communication of these results to the broader scientific community at international meetings and v) working in a team with a broad range of expertise both within and outside of Leeds.

Requirements

A good first degree (1 or good 2i), or a good Masters degree in a physical or mathematical discipline, such as mathematics, physics, geophysics, engineering or meteorology is required. Experience in programming (e.g. Python, Matlab, IDL, R…) and fieldwork is an advantage.

Contact Information

Contact is strongly encourage before application so that we may discuss your interests and project specifics.  Help with the application process may also be provided. Enquires should be made by contacting Dr Ryan Neely, Lecturer of Observational Atmospheric Science (R.Neely@leeds.ac.uk).

References

Bennartz, R., Shupe, M. D., Turner, D. D., Walden, V. P., Steffen, K., Cox, C. J., Kulie, M. S., Miller, N. B. and Pettersen, C.: July 2012 Greenland melt extent enhanced by low-level liquid clouds, Nature, 496(7443), 83–86, doi:10.1038/nature12002, 2013.

Bony, S., Stevens, B., Frierson, D. M. W., Jakob, C., Kageyama, M., Pincus, R., Shepherd, T. G., Sherwood, S. C., Siebesma, A. P., Sobel, A. H., Watanabe, M. and Webb, M. J.: Clouds, circulation and climate sensitivity, Nature Geoscience, 8(4), 261–268, doi:10.1038/ngeo2398, 2015.

van den Broeke, M. R., Enderlin, E. M. and Howat, I. M.: On the recent contribution of the Greenland ice sheet to sea level change, The Cryosphere, 10(5), 1933–1946, doi:10.5194/tc-10-1933-2016, 2016.

Hofer, S., Tedstone, A. J., Fettweis, X. and Bamber, J. L.: Decreasing cloud cover drives the recent mass loss on the Greenland Ice Sheet, Science Advances, 3(6), e1700584, doi:10.1126/sciadv.1700584, 2017.

Lawson, R. P. and Gettelman, A.: Impact of Antarctic mixed-phase clouds on climate, Proceedings of the National Academy of Sciences, 111(51), 18156–18161, doi:10.1073/pnas.1418197111, 2014.

Morrison, H., de Boer, G., Feingold, G., Harrington, J., Shupe, M. D. and Sulia, K.: Resilience of persistent Arctic mixed-phase clouds, Nature Geoscience, 5(1), 11–17, doi:10.1038/ngeo1332, 2012.

Stevens, B. and Bony, S.: What Are Climate Models Missing? Science, 340(6136), 1053–1054, doi:10.1126/science.1237554, 2013.

Shupe, M. D., Turner, D. D., Walden, V. P., Bennartz, R., Cadeddu, M. P., Castellani, B. B., Cox, C. J., Hudak, D. R., Kulie, M. S., Miller, N. B., Neely, R. R., III, Neff, W. D. and Rowe, P. M.: High and Dry: New Observations of Tropospheric and Cloud Properties above the Greenland Ice Sheet, Bulletin of the American Meteorological Society, 94(2), 169–186, doi:10.1175/BAMS-D-11-00249.1, 2013.

Van Tricht, K., Lhermitte, S. and Lenaerts, J.: Clouds enhance Greenland ice sheet meltwater runoff, Nat Comms, 7, 10266, doi:10.1038/ncomms10266, 2016.

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