Aerosol-Cloud Interactions - A Directed Programme to Reduce Uncertainty in Forcing through a Targeted Laboratory and Modelling Programme

Lead Research Organisation: University of Hertfordshire
Department Name: Science and Technology RI

Abstract

Aerosol particles act as sites for cloud droplet and ice particle formation. Cloud properties can be perturbed through the addition of aerosol particles into the atmosphere from anthropogenic and natural processes. This addition influences cloud microphysical properties, and subsequently affects cloud dynamics and thermodynamics, and the way the cloud interacts with radiation. The Earth's radiation budget is very greatly affected by clouds, and human-induced changes to the particle loading affecting them, known as indirect effects, are large and highly uncertain. A large part of this uncertainty is the result of poor knowledge of the fundamental aerosol and cloud properties and processes, leading to their poor representation in models. A programme of research is proposed here to i) directly investigate these processes in the laboratory, ii) evaluate the sensitivity of climate relevant parameters to the studied processes, iii) interpret the laboratory studies with detailed model investigations and iv) to incorporate and test new descriptions of the studied processes in cloud-scale and, where possible, global scale models. The programme will thereby reduce the uncertainty in estimates of radiative forcing and climate feedbacks relating to aerosol and cloud processes. The studies are split into those affecting warm clouds (those containing only liquid droplets) and those affecting clouds containing ice particles. The programme brings together an interdisciplinary team of researchers with expertise in 'warm' and 'cold' cloud and aerosol processes combining laboratory and multiscale modelling activities to deliver the improved predictive capability. The 'warm' laboratory work focuses on two major aspects i) the rate at which water is taken up by growing aerosol particles as they become cloud droplets (or 'activate) and ii) the ability of aerosol particles of various compositions to act as seeds for cloud droplets. These studies use a number of techniques including single particle optical levitation and investigations in a large photochemical chamber coupled to a large number of chemical and physical probes of ensembles of particles formed in simulated atmospheric chemical processes. The 'cold' work uses a similar coupling of a large, well-instrumented cloud chamber experiments and single particle levitation studies. The chambers used in both aspects will be coupled to investigate the impacts of aerosol transformation conditions on warm and cold cloud formation, using the instrumental payload from both chambers. A range of detailed models will be used to explicitly describe the processes by which aerosol particles interact with increasing relative humidity and reducing temperature to form cloud droplet and ice crystals and to their properties. The processes and properties will be represented in dynamical frameworks to predict the interactions between aerosols and clouds and their radiative effects at cloud resolving scales and radiative forcing of some of the investigated properties on global radiative forcing and feedbacks. The sensitivity of climate relevant parameters to the fundamental parameters investigated in the laboratory programme and their improved quantification will be evaluated using a simplified model 'emulator'.

Publications

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Baumgardner D (2017) Cloud Ice Properties: In Situ Measurement Challenges in Meteorological Monographs

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Cotton R (2012) The effective density of small ice particles obtained from in situ aircraft observations of mid-latitude cirrus in Quarterly Journal of the Royal Meteorological Society

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Dandini P (2019) Halo ratio from ground-based all-sky imaging in Atmospheric Measurement Techniques

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Dandini P (2019) Halo ratio from ground based all-sky imaging in Atmospheric Measurement and Techniques

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Hesse E (2015) Modelling light scattering by absorbing smooth and slightly rough facetted particles in Journal of Quantitative Spectroscopy and Radiative Transfer

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Hesse E (2018) Discussion of a physical optics method and its application to absorbing smooth and slightly rough hexagonal prisms in Journal of Quantitative Spectroscopy and Radiative Transfer

 
Description We have identified some of the cloud processes that lead to the emergence of highly irregular shapes of atmospheric ice particles, which have recently been shown to dominate cold clouds. We have started quantifying the impact of the irregular shapes, and the processes that lead to them, on the radiative properties of clouds and hence on climate.
Exploitation Route The results can be used to improve weather and climate models by representing more accurately the properties of cold cloud particles.
Sectors Environment

 
Title Characterizing polar atmospheric ice using near-field lidar 
Description Ice crystals in a lidar beam have been observed to produce two-dimensional reflection patterns on snow surfaces. The patterns can to used to determine properties such as ice particle shape, size, roughness, alignment and altitude. 
Type Of Material Improvements to research infrastructure 
Year Produced 2017 
Provided To Others? Yes  
Impact Data obtained at Summit Station in Greenland showed the presence of hexagonal and scalene plates, columns and rounded plate. A variety of sizes was present. Two distinct, apparently non-overlapping subpopulations of ice crystals were identified: fast precipitating snow particles producing pronounced speckle, with sizes from 0.7 to 4.0 mm; and pristine ice particles consisting of smaller plates between 125 and 530 µm in size, many of which showing signs of sublimation, and some columns up to 560 µm in length. many crystals showed evidence of rough or irregular surfaces. 
 
Title Sizing small particles using laser speckle. 
Description The technique is for sizing small particles by taking advantage of laser speckle. It does not depend on instrument calibration. It can be used for cloud or aerosol particles, but was also shown to work for living cells such as spores or pollen grains. 
Type Of Material Improvements to research infrastructure 
Year Produced 2012 
Provided To Others? Yes  
Impact The technique was used in the subsequent "Co-ordinated Airborne Studies in the Tropics" project (CAST, NERC NE/J006157/1). 
 
Title All-sky camera 
Description A commercial all-sky camera was enhanced by the addition of a sun occulting disc and software for deriving approximate scattering phase functions from images. The phase functions can be used to obtain the halo ratio associated with cirrus clouds, which gives valuable information about the microphysical and radiative.properties of the clouds. 
Type Of Technology Systems, Materials & Instrumental Engineering 
Year Produced 2017 
Impact None yet. 
URL https://www.atmos-meas-tech.net/12/1295/