Solving the entrainment puzzle

The "turbulent Leipzig Aerosol Cloud Interaction Simulator" (LACIS-T) is a multi-purpose instrument for investigating atmospherically relevant interactions in clouds. Tilo Arnhold, TROPOS

With this programme, the EU supports the career development of scientists. It is the first such MSCA fellowship to be awarded to an employee of TROPOS.

Clouds and the realistic consideration of their feedbacks remain one of the greatest challenges in predicting the future climate. Climate models are not capable to resolve relevant small-scale cloud processes, due to their coarse resolution.

Unresolved processes are represented in terms of statistical formulations – parameterisations – that are inherently uncertain; this is particularly true if the underlying processes are non-linear and if they are poorly understood.

One of the key small-scale processes that necessarily require parameterisation is the turbulent mixing of cloudy and cloud-free air, i.e. entrainment.

Since clouds are very dynamic structures and make an important contribution to the radiation balance of the earth, a better understanding of these mixing processes at the cloud edges is necessary.

A new project at TROPOS wants to better understand the mixing processes at the cloud edges. These processes change the cloud particle properties such as number concentrations and sizes, which also modifies the radiative properties of the cloud.

Mixing processes at the cloud edges have furthermore important implications for cloud lifetime. The mixing processes and their treatment in the numerical models have been found to be responsible for much of the large spread found in climate sensitivity estimates.

“No reliable formulation exists to date that allows understanding and describing entrainment in terms of cloud- and environmental physical quantities (“the entrainment puzzle”). The model of the European Centre for Medium-Range Weather Forecasts (ECMWF), for example, employs an entrainment rate that decreases with increasing relative humidity, whereas another widely used scheme (by Kain and Fritsch) does the opposite. This highlights that the entrainment process itself is not well understood. My project is therefore entitled 'Solving The Entrainment Puzzle'”, explains Dr. Wiebke Frey.

Dr Frey wants to employ the wind tunnel LACIS-T of the TROPOS cloud laboratory to characterise the mixing processes. The conditions for the experiments will be based on observations performed during the “Azores stratoCumulus measurements Of Radiation, turbulEnce and aeroSols” (ACORES) field campaign. An international research team led by TROPOS had investigated clouds around the archipelago of the Azores in the North Atlantic, in July 2017. Parts of the atmospheric measurements were performed by the helicopter borne measuring platform ACTOS.

The experiments in the wind tunnel and accompanying high-resolution simulations (direct numerical simulations with resolution in the millimetre range) will help to obtain a more exact physical description for the entrainment process. Wiebke Frey intends to integrate this into coarser simulations, so-called Large Eddy Simulations (LES with resolution in the 10-meter range) in the following step.

The field observations by ACTOS will be used to verify the new model entrainment calculation. The atmospheric model ICON-LEM, developed jointly by the German Weather Service and the Max Planck Institute for Meteorology, will be used. For the modelling part of the project, Dr Wiebke Frey will be collaborating with the renowned cloud modellers Prof. Johannes Quaas from the University of Leipzig and Prof. Bjorn Stevens from the Max Planck Institute for Meteorology in Hamburg.

“We are very pleased that Dr Frey has joined our group and is dedicating her activities to a difficult but exciting topic. With our unique wind tunnel LACIS-T it will be possible for the first time to investigate the physical parameters that control these mixing processes in clouds. Probably the difference of air velocity inside and outside the cloud plays an important role, which we can adjust very precisely in LACIS-T.

This accuracy has made the design of the system very complex. We are therefore very excited to see the results, which will be available in about two years' time,” stresses Dr. Frank Stratmann, head of the cloud working group at TROPOS.

Dr. Wiebke Frey studied meteorology at Leibniz University in Hanover and received her doctorate at Johannes Gutenberg University in Mainz on aircraft measurements of ice particles in the tropical troposppause region. As a postdoc she has worked at the Max Planck Institute for Chemistry in Mainz, the University of Melbourne in Australia and the University of Manchester in the UK. Her project in Leipzig is funded by the European Union under the Marie Skłodowska-Curie Actions (MSCA). The programme is part of the European Framework Programme for Research and Innovation (“Horizon 2020”).

It is part of the EU's “Excellence in Science” programme and has a budget of six billion euros over the lifetime of Horizon 2020 (2014-2020). The programme aims to strengthen research and technology in Europe and promote careers as innovative researchers. Researchers with a doctorate or at least four years of full-time postgraduate research experience can apply for an Individual Fellowship (IF).

The Marie Skłodowska Curie Actions promote mobility and transfer: researchers shall expand or deepen their competences in another country and share their knowledge with the host institution. Tilo Arnhold

Solving The Entrainment Puzzle (STEP) project @ Twitter:

Cloud Workgroup at TROPOS:…

Messkampagne „Azores stratoCumulus measurements Of Radiation, turbulEnce and aeroSols“ (ACORES) 2017:

Unique turbulent wind channel started operation (2017):…

Marie Skłodowska-Curie Actions (MSCA):….…

The Leibniz-Institute for Tropospheric Research (TROPOS) is member of the Leibniz Association, which connects 95 independent research institutions that range in focus from the natural, engineering and environmental sciences via economics, spatial and social sciences to the humanities. Leibniz Institutes address issues of social, economic and ecological relevance. They conduct knowledge-driven and applied basic research, maintain scientific infrastructure and provide research-based services.
The Leibniz Association identifies focus areas for knowledge transfer to policy-makers, academia, business and the public. Leibniz institutions collaborate intensively with universities – in the form of “Leibniz ScienceCampi” (thematic partnerships between university and non-university research institutes), for example – as well as with industry and other partners at home and abroad.
They are subject to an independent evaluation procedure that is unparalleled in its transparency. Due to the importance of the institutions for the country as a whole, they are funded jointly by the Federation and the Länder, employing some 20,000 individuals, including 10,000 researchers. The entire budget of all the institutes is approximately 1.9 billion Euros.

Dr. Wiebke Frey
Scientist, Cloud Workgroup
Leibniz Institute for Tropospheric Research (TROPOS)
Tel. +49 341 2717- 7386
Dr. Frank Stratmann
Group leader of the working group Clouds
Leibniz Institute for Tropospheric Research (TROPOS)
Tel. +49 341 2717- 7142
Tilo Arnhold
TROPOS Public Relation
Tel. +49-341-2717-7189…

Media Contact

Tilo Arnhold idw - Informationsdienst Wissenschaft

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