Combining air-mass-following balloon observations and large-eddy simulations to build process-level understanding of arctic amplification

Known mechanisms causing Arctic amplification of global warming are lapse-rate and albedo feedbacks, as well as increased latent heat release due to enhanced moisture transport into the Arctic [1]. This classical understanding is based on the assumption of quasi-steady mean temperature and humidity states, described as a balance between advective transport from lower latitudes, surface fluxes, radiative exchange at the upper atmosphere, and related feedbacks [2]. While this integral, steady-state box-model framework has been instrumental in developing our current understanding, its inherent limitations hinder further progress in explaining Arctic amplification. For example, the box model cannot distinguish whether changes in heat or moisture transport are of fluid-dynamical or thermodynamical origin, nor can it describe how air masses entering the Arctic are transformed as they cool, mix, and exchange energy and moisture with surfaces and clouds. Yet Arctic air-mass transformations must be understood at the fundamental process level, as they are key to explaining, for instance, the necessary thermal conditions underlying the lapse-rate feedback.

Here we adopt an air-mass-centred framework that enables a physically consistent link between large-scale Arctic amplification of temperature and precipitation changes and the small-scale turbulent and microphysical processes governing the Arctic atmospheric boundary layer and its clouds. We combine air-mass-following balloon observations with large-eddy simulations (LES) to investigate the Lagrangian evolution of boundary-layer clouds during Arctic air-mass transformation events. Four such events have been tracked using CMET balloons [3] launched from Ny-Ålesund, Svalbard, providing unprecedented in situ measurements of thermodynamic quantities at controlled heights along air-mass trajectories. An additional campaign is planned for March 2026, during which up to twelve balloons will be launched from Station Nord, Greenland. The observational data are integrated into the LES code DALES [4] via time-dependent forcings and boundary conditions, yielding spatio-temporally resolved information on local thermo-fluid-dynamical processes and mixed-phase cloud microphysics along the air-mass pathways.

At the conference, we will present our collected field data and first LES results for one representative case. The focus of the current contribution is on establishing a robust Lagrangian LES framework, including domain-size sensitivity, grid-convergence behaviour, and basic physical plausibility checks against observations.

In perspective, this approach will allow us to compute vertical fluxes of energy and moisture during different transformation events and to analyse how the mean and final states of air masses, as well as their energy and moisture budgets, respond to varying climate conditions. Ultimately, we expect that this hybrid field–model study will enable us to test the following hypotheses: (i) liquid water path controls cloud persistence through cloud-top radiative cooling; and (ii) radiative cooling at cloud top (or in clear sky) drives air-mass transformation during both cloudy and clear states.

[1] M. Previdi, K.L. Smith, L.M. Polvani. Environ. Res. Lett., 2021, doi:10.1088/1748-9326/ac1c29.
[2] M. Cai. Geo. Res. Lett., 2005, doi:10.1029/2005gl024481.
[3] P. B. Voss. AIAA Balloon Systems Conference, 2009, doi:10.2514/6.2009-2810.
[4] T. Heus et al. Geosci. Model Dev., 2010, doi:10.5194/gmd-3-415-2010.