Project to improve transport modelling of atmospheric composition tracers

CATRINE project logo

CATRINE, an EU-funded project coordinated by ECMWF, will improve the numerical aspects of the transport of atmospheric tracers, with an emphasis on long-lived greenhouse gases. It will support an operational anthropogenic greenhouse gas emissions Monitoring and Verification Support Capacity (CO2MVS).

CATRINE is a three-year project starting in January 2024. Its results will be just in time for CO2MVS, which will come into operation in 2026 as part of the EU’s Copernicus Atmosphere Monitoring Service (CAMS) implemented by ECMWF.

The aim is to improve the methods used to represent the transport of atmospheric tracers through the wind. CATRINE will focus on improving mass conservation in ECMWF’s Integrated Forecasting System (IFS) and on identifying other systematic errors in the various atmospheric transport and mixing processes.

“It’s a difficult problem that isn’t often addressed, so this is a very rare opportunity,” says ECMWF scientist Anna Agustí-Panareda, co-coordinator of CATRINE.

CATRINE’s role in CO2MVS

The goal of CO2MVS, to monitor the emissions of greenhouse gases caused by humans, is not very well supported by direct observations of the emissions. That is why accurate information from different components of the Earth system is required.

Sources of CO2 graphic

Carbon dioxide moves into and out of the atmosphere in multiple ways, only some of which are human-caused. Credit: CAMS/ECMWF

That includes observations from the surface and space, knowledge of the chemistry in the atmosphere, and crucially an understanding of the transport of greenhouse gases and other tracers in the atmosphere.

The two main long-lived greenhouse gases to which humans contribute are carbon dioxide (CO2) and methane (CH4). However, to know about the sources of human-caused emissions, it is possible to track other tracers that are co-emitted.

CATRINE is going to look at the atmospheric transport of tracers. “Knowledge of transport allows us to study how tracers move and mix, and to estimate the surface fluxes of the tracers by tracing back the signals in the atmospheric CO2 and CH4 to the emissions and surface fluxes from the land and the oceans,” Anna says.

The approach of estimating emissions and natural surface fluxes from atmospheric observations is known as atmospheric inversion modelling, and it is at the core of the CO2MVS. For this approach to work well, the tracer transport model needs to be accurate.

“Currently we don’t know how well the IFS and other tracer transport models used for atmospheric inversions perform in terms of the transport of long-lived tracers such as CO2 and CH4. We want to establish whether there are any systematic errors, quantify those, and better understand their origin.”

Main areas of work

The first task is to assess the quality of the transport of tracers in the IFS and other tracer transport models used operationally in CAMS and other European operational centres. The next step will be to improve it as required.

In the IFS, part of the problem lies with the advection scheme, in other words the numerical method used to solve the partial differential equations which model the transport of momentum, heat and mass in an atmospheric model. “The semi-Lagrangian scheme of the IFS is accurate and very efficient in forecasting the weather,” says ECMWF scientist Michail Diamantakis, co-coordinator of CATRINE. “But it has one weakness: it does not accurately conserve the mass of transported air constituents locally or globally.”

One question, addressed by some consortium partners in CATRINE, will thus be whether the transport of tracers can be adjusted to reduce local mass conservation problems and other transport errors. This is particularly hard in the case of CO2 and CH4 because of local point-source emissions, e.g. from power stations, industrial facilities or leaks.

“It all depends on the signal to error ratio,” Anna says. “The CO2 signal, for example, is very small compared to the water vapour signal, which means that our requirements for accuracy are much higher.”

Another area of work, pursued by other consortium partners, concerns the simulation of plumes that are much smaller than what the IFS can represent. These plumes are highly variable in size and magnitude depending on the atmospheric conditions, and they can emanate from different heights depending also on the source type.

The goal is to use very high-resolution local models to simulate the transport, mixing and chemistry in these small-scale plumes as accurately as possible, and to try to find ways to simplify these processes to include them in a large-scale model.

Emissions from multiple sources

In CO2MVS, it will have to be possible to trace very small plumes near power stations and cities back to a very small-scale or point source.

Once the model has been improved, it will have to be evaluated. “That will not be easy because it is difficult to separate emissions from transport in observations,” Anna says. “One way of doing this is based on the knowledge that different tracers have different emission errors, but they all have similar transport errors.”

Observations will be used to detect where systematic errors are particularly big and what the sources of uncertainty are. One of the challenges to be addressed by some consortium members is to come up with metrics to quantify the transport errors and identify the specific processes that need to be improved in the various tracer transport models used in CATRINE.

Benefits for numerical weather prediction

The work carried out in CATRINE is also expected to have some benefits for numerical weather prediction.

“A better tracer transport scheme will also improve the transport of moisture, which is very important for weather prediction,” says Michail. “This is true especially as the grid spacing becomes smaller towards the km-scale.”

In addition, numerical weather prediction increasingly relies on considering the Earth system as a whole. For example, the water fluxes from vegetation can have an influence on the formation of clouds above.

“We are trying to have a representation that is oriented more towards the Earth system, and to understand how important the coupling of different processes is,” says Anna. That could be important for forecasts at longer timescales and at very high resolutions.

Participants in CATRINE

CATRINE is coordinated by ECMWF and includes seven other research institutes: the French Alternative Energies and Atomic Energy Commission (CEA); Météo-France; Wageningen University in the Netherlands; the Karlsruhe Institute of Technology in Germany; Helsinki University in Finland; the University of Reims in France; and the University of Freiburg in Germany.

An outreach activity to other groups working on similar problems is also envisaged. “It will be good to have a community exchange on what works and what doesn’t work, so we’d like to have an intercomparison project,” says Anna. “There will first be an internal intercomparison, and then we’ll extend it to the international community.”

Funded by the EU

The CATRINE project (grant agreement No. 101135000) is funded by the European Union. Views and opinions expressed are, however, those of the authors only and do not necessarily reflect those of the European Union or the Commission. Neither the European Union nor the granting authority can be held responsible for them.