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Solving The Entrainment Puzzle

Periodic Reporting for period 1 - STEP (Solving The Entrainment Puzzle)

Reporting period: 2019-05-01 to 2021-04-30

This Marie Skłodowska Curie Action (MSCA) is named "Solving The Entrainment Puzzle" (STEP) and is aimed exactly at doing that.
The problem:
As a cloud moves through environmental air, cloudy and cloud free air mix at the cloud edges in the entrainment process. Entrainment is a key cloud process central to understanding cloud morphology and microphysical processes such as precipitation formation. Entrainment changes cloud particle properties such as number concentrations and sizes, which also modifies the radiative properties of the cloud, and is responsible for the depletion of cloud water content, along with precipitation. It has thus important implications for cloud lifetime. After decades of research, no reliable formulation exists that allows to describe and understand entrainment in terms of cloud- and environmental physical quantities ("the entrainment puzzle"). Differences in representation of entrainment in climate models are a main cause for spread in climate sensitivity estimates. The realistic treatment of entrainment in climate models will help to reduce a large part of the uncertainty in predictions of climate sensitivity. This will in turn help to inform climate adaptation and mitigation strategies. STEP is thus addressing one of the Europe 2020 strategy main targets "Climate Change and Energy"

Objectives of this project have been to 1) identify the main underlying physical parameters governing entrainment using laboratory measurements in a unique turbulent wind tunnel (the Turbulent Leipzig Aerosol and Cloud Interaction Simulator, short LACIS-T) and understand in what way these parameters influence entrainment; 2) use Computational Fluid Dynamics (CFD) simulations that accompany the laboratory measurements in order to close the scale gap between the mm-scale (CFD) and 10 m-scale, where Large Eddy Simulations (LES, which can be embedded in weather forecast models) can perform. This will be used to formulate a new entrainment calculation scheme; 3) to verify the new entrainment calculation using comparison of in situ cloud observations and LES case studies of those cloud cases; 4) to foster the development of the individual researcher - which is me.
The work in this project was performed in five work packages, three of which comprised the scientific work, one contained the training activities and one work package dealt with communication and outreach.

The scientific work packages were focusing on laboratory work using the turbulent wind tunnel LACIS-T (WP 1), and on computer simulations using models at different scales - one simulating the conditions that occurred during the measurements in the wind tunnel (WP 2) and one for simulating cases of observed clouds from a field experiment (WP 3).

LACIS-T uses two air streams which can be controlled and conditioned separately concerning temperature, humidity and flow speed (for more information on the wind tunnel see The two air streams are then turbulently mixed at the inlet of the measurement section. In the measurement setup for STEP one stream was kept at constant conditions whereas one parameter in the second stream was varied to test the influence of that parameter on entrainment and its impact on droplet size distributions. Water droplets were injected into the measurement section by a droplet generator.
The Computational Fluid Dynamics model was adapted to properly simulate the behaviour of the water droplets and simulations were run for all measurement conditions. These simulations provided the full 3 dimensional fields, where measurements were only able to provide point measurements. Large Eddy Simulations were setup to simulate two observed cloud cases from the ACORES campaign, which took place in July 2017 above the Azores.

Most interesting results were found when varying the flow speed, while the change in temperature and humidity did not lead to major differences in the observed size distributions. The results raise questions in regard to further turbulent cloud microphysical processes beyond the scope of STEP, which the fellow is planning to address in future projects.

The results have been presented at three major international conferences: A poster at the International Conference on Clouds and Precipitation in 2021, and oral presentations at the American Geophysical Union fall meeting 2021, and the 16th American Meteorological Society Conference on Cloud Physics, 2022, which led to further fruitful discussions. As the Covid-19 pandemic impeded in-person events, most public engagement activities were happening online, mainly through activities on twitter. For the researcher's training, the Fellow attended six training workshops and conferences on various topics from HPC computing, didactic, gender issues to science diplomacy, plus the seminars and workshops of the Leibniz mentoring.
The computing intensive work packages (WP2 and WP3) secured computing time on High Performance Computing clusters to pursue the simulations: 1000 node hours on Mistral and 700 node hours on Levante at DKRZ, Hamburg, Germany, and 470,000 hours CPU-time and 1200
GPU hours on taurus at ZIH Dresden, Germany.
STEP significantly contributed to establish the turbulent wind tunnel LACIS-T, which started operation in 2017, as a world leading facility for turbulent process studies on clouds, by giving it visibility e.g. at international high ranking conferences. Presentations there have led to fruitful discussions and interest in collaborations, and it is expected that the results will be published in peer-reviewed journals in the near future.
Furthermore, some outcomes of the action have given rise to the planning of future projects involving the impact of turbulence on processes as collision and coalescence (i.e. the formation of new cloud droplets from two colliding drops).

The results of this MSCA are expected to provide the basis for model parameterisation improvement which will lead to climate model improvement. By translating the results into physically sound parameterisations of the entrainment process in weather forecast and climate models, uncertainties will be reduced and this will help inform policies for climate change adaptation and mitigation. Hence, the project has wider, indirect societal and socio-economic implications.

The MSCA allowed the fellow to greatly enhance her career prospects, not only be training received on new research tools (operation of turbulent wind tunnel, usage of computational fluid dynamics model) to broaden her research portfolio, but also through specific training courses. Most notably in the latter regard was the Leibniz Mentoring which provided a wealth of seminars on various career development topics.