Global Aerosol Synthesis and Science Project (GASSP) to reduce the uncertainty in aerosol radiative forcing

Lead Research Organisation: University of Leeds
Department Name: School of Earth and Environment


The motivation for this project is that aerosols have persistently been assessed by the IPCC as the largest uncertainty in the radiative forcing of climate over the industrial period. This means that our ability to understand temperature changes over the industrial period is hampered by very poorly constrained aerosol processes in models. The main uncertainty is due to the effect that aerosols have on clouds - the so-called aerosol indirect effect by which anthropogenic aerosols make clouds more reflective. In the IPCC assessment, the range of predictions of the aerosol indirect forcing lies between -0.4 to -1.8 Wm-2, a far larger range than associated with CO2 forcing (1.6-1.9 Wm-2). Thus, to improve our understanding of climate change, we need to reduce the uncertainty in the aerosol indirect effect.

The controlling factor in the indirect effect is the concentration in the atmosphere of "cloud condensation nuclei" (CCN). CCN are a subset of the aerosol particles in the atmosphere, typically larger than 50 nm diameter and sufficiently water soluble to form cloud drops. Only recently, global models have been developed that are able to explicitly simulate CCN concentrations. This opens up the possibility of reducing model uncertainty by exploiting extensive measurements of CCN that have been made over many years.

We propose to undertake the first ever comprehensive synthesis of global CCN and related aerosol observations within the UK aerosol-chemistry-climate model. The overall aim is to reduce uncertainty in the indirect effect by constraining modern aerosol as much as possible based on present observing systems and models. We will reduce the uncertainty by producing a global model of CCN with well defined uncertainties that are constrained by worldwide observations. We will then use the "calibrated" aerosol model to quantify the indirect radiative forcing and its uncertainty. We will also use the new and better model to understand the sources of CCN in different environments, and thereby the factors that will drive future changes in the concentration. As a spin-off of the project we will also be able to use the model and data to identify the regions or environments in which new measurements would have the greatest impact on reducing the uncertainty further.

An important new aspect of the project will be the use of new uncertainty information about the global model. In most similar studies it has been possible to run the model only a few times. However, in reality the model has a wide uncertainty range due to the very large number of uncertain processes in the model. In this project we will use new information that tells us how the model behaves under all possible assumptions of uncertainty. From this collection of model runs we will be able to identify the best possible model in all parts of the world. This procedure is known as "calibration", and it has not been attempted before for a complex global model. With this approach we can be sure the model is as close to observations of CCN as can presently be achieved.

Planned Impact

GASSP is very well placed to have a direct impact on national and international climate and weather prediction centres, and therefore climate policy and the IPCC. The key to our engagement is that we are proposing to synthesise a vast number of existing measurements to constrain and improve aerosol forcing in the UK's climate model HadGEM. The unique compilation of harmonised observational data, synthesised to suitable formats / statistics, will then be available for wider community use and directly contribute to key international activities, such as the International Aerosol Intercomparison Project (AeroCom, Our engagement activities are a direct extension to our well-established collaborations.

Specifically in this project, our user engagement will include:

European Centre for Medium-Range Weather Forecasts (ECMWF): Our impact on the ECMWF will be through the EU Global Monitoring for Environment and Security MACC-II project ( MACC combines state-of-the-art atmospheric modelling with Earth observation data to provide information services covering European Air Quality, Global Atmospheric Composition, Climate, and UV and Solar Energy. As part of MACC-I Leeds incorporated the GLOMAP aerosol model (the same as in HadGEM) in the ECMWF forecast model to enable future aerosol microphysics data assimilation and operational forecasts of aerosols and air quality. MACC makes a direct connection to the public and environmental agencies. The GASSP project will have an impact on the ECMWF by providing a comprehensive microphysics dataset to evaluate their model. Three months of the project has been set aside for this work at the end of the project (see Pathways to Impact).

Met Office (formal project partner, see letter of support): Engagement with the Met Office will continue through our ongoing NCAS-funded collaboration (UKCA). The data synthesis will provide a level of model evaluation superior to that of any other model, and will lead to improvement of the UK's climate modelling capability. Met Office staff will be invited to the GASSP data workshop in month 26. The workshop will produce a community publication on the observational needs for reducing the uncertainties in aerosol radiative forcing. This paper will have a long-term impact on Met Office model development.

The international Aerosol Model Intercomparison Project (AeroCom) (formal project partner, see Kinne letter of support): AeroCom brings together the majority of global models in the world and its results have provided significant input to the IPCC concerning aerosol forcing of climate. In GASSP we will produce extensive harmonised datasets and new evaluation methodologies that can be taken up immediately by AeroCom. In our Pathways to Impact activity we are proposing to hold a session at an AeroCom workshop to discuss how our new knowledge on model uncertainty can be exploited by AeroCom. Through this activity, GASSP can have an impact on model evaluation and future development internationally.


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Description We have developed methodologies for quantifying uncertainty in global aerosol models using large perturbed parameter ensembles and model emulation.
We have quantified uncertainty in cloud condensation nuclei and aerosol-cloud radiative forcing in a global aerosol model.
We have compiled the world's largest database of aerosol measurements which can be used to constrain uncertainty in global models.
Exploitation Route The GASSP aerosol database can be considered as national capability. We plan to develop its use in model evaluation through NCAS.
Sectors Environment

Description Our results and datasets have been used by the Met Office in designing their climate projections and evaluating UKESM prior to submission to CMIP6. This has fed into IPCC assessments and acknowledged in the UKCP report.
First Year Of Impact 2018
Sector Environment
Title GASSP 
Description GASSP is a database of global aerosol microphysics measurements. We have collected and harmonised thousands of existing measurements from existing databases and from individual investigators. The measurements include aerosol particle concentrations, cloud condensation nuclei, PM2.5, black carbon etc. To our knowledge, this is the largest ever synthesis effort of such data, and it will become a huge resource for international aerosol modelling teams and climate modellers. The database is currently on JASMIN at CEDA and is private, but we will work with investigators to make the data public by the end of the GASSP project. 
Type Of Material Database/Collection of data 
Year Produced 2015 
Provided To Others? Yes  
Impact The database has so far enabled preliminary constraint of global aerosol model uncertainty using the GLOMAP model. Further work is underway with the Met Office to reduce uncertainty in climate model simulations of aerosol effects on climate in their ensemble prediction system. 
Title GASSP aerosol measurement database 
Description The GASSP aerosol measurement database contains aerosol measurements that have been processed to a standardized NetCDF format so that they can be easily compared with models. The database includes measurements from 86 field campaigns and long-term measurements from over 350 ground-based monitoring stations spanning 1990 to 2015. It includes 20 ship campaigns, 16 ground station campaigns, 29 aircraft campaigns and 21 campaigns involving multiple measurement platforms. We obtained data from fifteen repositories and, for 42 campaigns, through direct contact with investigators. The measurements include over 9500 flight hours (over 13,000 instrument hours) and 22,000 ship 162 hours (over 33,000 instrument hours). The data will be made available on CEDA this year. 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes  
Impact A subset of the GASSP database has been used by the Met Office to tune the UKESM Earth System Model. 
Title Statistical model emulators 
Description We have developed code to produce emulators of model output. 
Type Of Material Computer model/algorithm 
Year Produced 2015 
Provided To Others? Yes  
Impact The emulators have been applied by other groups to tackle problems outside the original focus of our work. Examples of other research facilitated: 1) NOAA ESRL analysis of cloud physics models; 2) Colorado State University analysis of meteorology of land-sea breezes; 3) University of Leeds analysis of volcanic eruption effects on climate. 
Description AEROCOM 
Organisation Norwegian Meteorological Institute
Country Norway 
Sector Public 
PI Contribution Contribution to multi-model assessments of global aerosol. Contribution of GLOMAP and UKCA model output. Leadership of intercomparison activities.
Collaborator Contribution Joint intercomparison activities
Impact Publication on intercomparison of global aerosol microphysics models by Mann et al. (Leeds) Publication on intercomparison of global organic aerosol (Tsigaridis et al.)
Start Year 2008
Description Met Office 
Organisation Meteorological Office UK
Country United Kingdom 
Sector Academic/University 
PI Contribution Development and improvement of a global aerosol model used in the UK Earth system model
Collaborator Contribution Modelling expertise
Impact None
Start Year 2008