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

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Introduction


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. ACID-PRUF is a programme of research aiming 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. ACID-PRUF 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.

A series of cross-linked tasks have been planned to directly address the specified programme requirements. The laboratory tasks have been targeted at those properties and processes at the core of the uncertainties in aerosol and cloud impacts on climate. Coupled with model sensitivity analyses, cloud resolving model simulations and large-scale model improvements, the programme will directly lead to improved quantification of, and reduction in, the uncertainties in estimates of radiative forcing and climate feedbacks relating to aerosol and cloud processes. The consortium draws on the extensive existing UK capacity built in part through the NERC APPRAISE Research Programme. The system under investigation is fundamentally complex and tightly coupled across the boundaries of disciplines conventionally regarded as distinct. ACID-PRUF brings together partners of appropriate expertise from within the UK community but from widely different disciplines and experience along with International partners of acknowledged expertise.

 

Objectives

 

With the stated overarching aim of the Aerosols and Clouds Programme being to reduce the uncertainty in estimates of radiative forcing and climate feedbacks relating to aerosol and cloud processes, the programme has at its core the primary interest in quantification of the following:

i) Indirect effects of aerosols on the radiative forcing due to their interaction with clouds and the underlying cloud/aerosol processes
ii) Ice particle processes in deep frontal, convective clouds and cirrus clouds.

Specific objectives to address this goal are to quantify in the laboratory the following processes and properties

1) Accommodation coefficient of water vapour on liquid droplets,
2) Impacts of semi-volatile components on aerosol water uptake,
3) Impacts of aerosol phase on water uptake,
4) Propensity for mineral particles to act as heterogeneous ice nuclei,
5) Impacts of aerosol phase on their IN behaviour,
6) Kinetic limitations to water uptake by ice crystals,
7) Impacts of ice crystal roughness,
8) Impacts of ice multiplication processes on ice crystal number,

Those processes that can be reasonably included in cloud parcel, cloud resolving and larger scale models will be directly constrained by the laboratory measurements and used to predict the impacts of the process quantification improvements on cloud properties and radiative forcing under a range of conditions. Sensitivity of climate important parameters to the improvement in the laboratory quantification of the investigated parameter will be evaluated systematically using a model emulator. Specific objectives that draw the laboratory studies through to impact evaluation are therefore:

9) Prediction of the impacts of the laboratory investigated parameters on cloud properties (e.g. cloud droplet number) at cloud-resolving and global scales,
10) Prediction of the impacts of the laboratory investigated parameters on radiative forcing at cloud resolving and global scales,
11) Evaluation of the sensitivity of climate-relevant parameters to the process quantification improvements at all scales,
12) Quantification of the reduction in uncertainties in estimates of radiative forcing and climate feedbacks relating to the investigated aerosol and cloud processes.