Rantanen, M., Karpechko, A. Y., Lipponen, A., Nordling, K., Hyvärinen, O., Ruosteenoja, K., Vihma, T., and Laaksonen, A.: The Arctic has warmed nearly four times faster than the globe since 1979, Commun. Earth Environ., 3, 168, https://doi.org/10.1038/s43247-022-00498-3, 2022.

Rinke, A., Knudsen, E., Mewes, D., Dorn, W., Handorf, D., Dethloff, K., and Moore, J. C.: Arctic summer sea-ice melt and related atmospheric conditions in coupled regional climate model simulations and observations, J. Geophys. Res., 124, 6027–6039, https://doi.org/10.1029/2018JD030207, 2019.

Ripple, W. J., Wolf, C., Lenton, T. M., Gregg, J. W., Natali, S. M., Duffy, P. B., Rockström, J., and Schellnhuber, H. J.: Many risky feedback loops amplify the need for climate action, One Earth, 6, 86–91, https://doi.org/10.1016/j.oneear.2023.01.004, 2023.

Roberts, T. J., Dütsch, M., Hole, L. R., and Voss, P. B.: Controlled meteorological (CMET) free balloon profiling of the Arctic atmospheric boundary layer around Spitsbergen compared to ERA-Interim and Arctic System Reanalyses, Atmos. Chem. Phys., 16, 12383–12396, https://doi.org/10.5194/acp-16-12383-2016, 2016.

Sato, Y., Miura, H., Yashiro, H., Goto, D., Takemura, T., Tomita, H., and Nakajima, T.: Unrealistically pristine air in the Arctic produced by current global scale models, Sci. Rep., 6, 26561, https://doi.org/10.1038/srep26561, 2016.

Schäfer, M., Wolf, K., Ehrlich, A., Hallbauer, C., Jäkel, E., Jansen, F., Luebke, A. E., Müller, J., Thoböll, J., Röschen-thaler, T., Stevens, B., and Wendisch, M.: VELOX – a new thermal infrared imager for airborne remote sensing of cloud and surface properties, Atmos. Meas. Tech., 15, 1491–1509, https://doi.org/10.5194/amt-15-1491-2022, 2022.

Schirmacher, I., Kollias, P., Lamer, K., Mech, M., Pfitzenmaier, L., Wendisch, M., and Crewell, S.: Assessing Arctic low-level clouds and precipitation from above – a radar perspective, Atmos. Meas. Tech., 16, 4081–4100, https://doi.org/10.5194/amt-16-4081-2023, 2023.

Schirmacher, I., Schnitt, S., Klingebiel, M., Maherndl, N., Kir-bus, B., Ehrlich, A., Mech, M., and Crewell, S.: Clouds and precipitation in the initial phase of marine cold air outbreaks as observed by airborne remote sensing, EGUSphere [preprint], https://doi.org/10.5194/egusphere-2024-850, 2024.

Schmale, J., Zieger, P., and Ekman, A. M. L.: Aerosols in current and future Arctic climate, Nat. Clim. Change, 11, 95–105, https://doi.org/10.1038/s41558-020-00969-5, 2021.

Screen, J. A.: An ice-free Arctic: What could it mean for European weather?, Weather, 76, 327–328, https://doi.org/10.1002/wea.4069, 2021.

Serreze, M. C. and Meier, W. N.: The Arctic's sea ice cover: trends, variability, predictability, and comparisons to the Antarctic, Ann. NY Acad. Sci., 1436, 36–53, https://doi.org/10.1111/nyas.13856, 2019.

Shapiro, M. A. and Keyser, D.: Fronts, Jet Streams and the Tropopause, American Meteorological Society, Boston, MA, 167–191, ISBN 978-1-944970-33-8, https://doi.org/10.1007/978-1-944970-33-8_10, 1990.

Shupe, M. D., Daniel, J. S., de Boer, G., Eloranta, E. W., Kollias, P., Long, C. N., Luke, E. P., Turner, D. D., and Verlinde, J.: A focus on mixed-phase clouds: The status of ground-based observational methods, B. Am. Meteorol. Soc., 89, 1549–1562, https://doi.org/10.1175/2008BAMS2378.1, 2008.

Shupe, M. D., Rex, M., Blomquist, B., Persson, P. O. G., Schmale, J., Uttal, T., Althausen, D., Angot, H., Archer, S., Bariteau, L., Beck, I., Bilberry, J., Bussi, S., Buck, C., Boyer, M., Brasseur, Z., Brooks, I. M., Calmer, R., Cassano, J., Castro, V., Chu, D., Costa, D., Cox, C. J., Creamean, J., Crewell, S., Dahlke, S., Damm, E., de Boer, G., Deckelmann, H., Dethloff, K., Dütsch, M., Ebell, K., Ehrlich, A., Ellis, J., Engelmann, R., Fong, A. A., Frey, M. M., Gallagher, M. R., Ganzeveld, L., Gradinger, R., Graeser, J., Greenamyer, V., Griesche, H., Griffiths, S., Hamilton, J., Heinemann, G., Helmig, D., Herber, A., Heuzé, C., Hofer, J., Houchens, T., Howard, D., Inoue, J., Jacobi, H.-W., Jaiser, R., Jokinen, T., Jourdan, O., Jozef, G., King, W., Kirchgaessner, A., Klingebiel, M., Krassovski, M., Krumpen, T., Lampert, A., Landing, W., Laurila, T., Lawrence, D., Lonardi, M., Loose, B., Lüpkes, C., Maahn, M., Macke, A., Maslowski, W., Marsay, C., Maturilli, M., Mech, M., Morris, S., Moser, M., Nicolaus, M., Ortega, P., Osborn, J., Pätzold, F., Perovich, D. K., Petäjä, T., Pilz, C., Pirazzini, R., Posman, K., Powers, H., Pratt, K. A., Preußer, A., Quéléver, L., Radenz, M., Rabe, B., Rinke, A., Sachs, T., Schulz, A., Siebert, H., Silva, T., Solomon, A., Sommerfeld, A., Sreen, G., Stephens, M., Stohl, A., Svensson, G., Uin, J., Viegas, J., Voigt, C., von der Gathen, P., Wehner, B., Welker, J. M., Wendisch, M., Werner, M., Xie, Z., and Yue, F.: Overview of the MOSAiC expedition: Atmosphere, Elem. Sci. Anth., 10, 00060, https://doi.org/10.1525/elementa.2021.00060, 2022.

Smith, A. J. P., Smith, D. M., Cohen, J., and Jones, M. W.: Arctic warming amplifies climate change and its impacts, in: Critical Issues in Climate Change Science, edited by: Liss, P., Le Quéré, C., and Forster, P., Zenodo, https://doi.org/10.5281/zenodo.5596791, 2021

Smith, D. M., Screen, J. A., Deser, C., Cohen, J., Fyfe, J. C., García-Serrano, J., Jung, T., Kattsov, V., Matei, D., Msadek, R., Peings, Y., Sigmond, M., Ukita, J., Yoon, J.-H., and Zhang, X.: The Polar Amplification Model Intercomparison Project (PAMIP) contribution to CMIP6: investigating the causes and consequences of polar amplification, Geosci. Model Dev., 12, 1139–1164, https://doi.org/10.5194/gmd-12-1139-2019, 2019.

Smith, D. M., Eade, R., Andrews, M. B., Ayres, H., Clark, A., Chripko, S., Deser, C., Dunstone, N. J., García-Serrano, J., Gastineau, G., Graff, L. S., Hardiman, S. C., He, B., Hermanson, L., Jung, T., Knight, J., Levine, X., Magnusdottir, G., Manzini, E., Matei, D., Mori, M., Msadek, R., Ortega, P., Peings, Y., Scaife, A. A., Screen, J. A., Seabrook, M., Semmler, T., Sigmond, M., Streffing, J., Sun, L., and Walsh, A.: Robust but weak winter atmospheric circulation response to future Arctic sea ice loss, Nat. Commun., 13, 727, https://doi.org/10.1038/s41467-022-28283-y, 2022.

Sobel, A. H., Nilsson, J., and Polvani, L. M.: The Weak Temperature Gradient Approximation and Balanced Tropical Moisture Waves, J. Atmos. Sci., 58, 3650–3665, https://doi.org/10.1175/1520-0469(2001)058<3650:TWTGAA>2.0.CO;2, 2001.

Sprenger, M. and Wernli, H.: The LAGRANTO Lagrangian analysis tool – version 2.0, Geosci. Model Dev., 8, 2569–2586, https://doi.org/10.5194/gmd-8-2569-2015, 2015.

Stapf, J., Ehrlich, A., Jäkel, E., Lüpkes, C., and Wendisch, M.: Reassessment of shortwave surface cloud radiative forcing in the Arctic: consideration of surface-albedo-cloud interactions, Atmos. Chem. Phys., 20, 9895–9914, https://doi.org/10.5194/acp-20-9895-2020, 2020.
<figure><img src="image_47.png" /></figure>

We also look at the chemical composition of aerosol particles and cloud residuals and their variation with season. For this purpose, we compare observations collected during ACLOUD (late spring/early summer) (Wendisch et al., 2019) with our data available from HALO-(AC)³ (late winter/early spring). The single particle mass spectrometer ALABAMA (Brands et al., 2011; Clemen et al., 2020) was used to measure the chemical composition and size of single particles in the size range of 0.23–3 µm (50% cutoff diameter with respect to detection efficiency). The ALABAMA was connected to the CVI inlet allowing for sampling of CDR and ambient aerosol particles, dependent on the counter-flow settings. Simultaneous measurements of trace gases such as CO, CO₂, and O₃ were used to identify different air mass origins, e.g. to distinguish polluted from non-polluted air masses. The results (Fig. 16) show a large abundance of particulate amines in ambient air and a dominance of amines in CDR during late spring/early summer (ACLOUD), emphasizing the importance of marine biogenic sources for summertime Arctic cloud processes. In contrast, amine-containing particles were rarely observed during late winter/early spring (HALO-(AC)³).

Furthermore, the abundance of elemental carbon (EC) containing particles was higher during HALO-(AC)³, accompanied by higher CO mixing ratios than during ACLOUD, indicating the presence of anthropogenic pollution typical for the 'Arctic Haze' season in spring. These differences between the data obtained during ACLOUD and HALO-(AC)³ reflect the general observation that ACLOUD was dominated by WAIs, while CAOs predominated during the HALO-(AC)³ measurement flights of P6. The composition of CDR is similar to that of the particles in ambient air during HALO-(AC)³, but with a higher contribution of fresh and aged sea salt particles to the CDR, while in summer the amine-containing particles dominate the CDR. In future work, a detailed analysis of different air mass situations (WAIs versus CAOs) combined with air mass history analysis (e.g., air mass trajectories) will be included to investigate the sources of the identified particle types.

<figure><img src="image_48.png" /><figcaption>**Figure 16.** Number fraction of different particle types for ambient particles and cloud droplet residuals (CDR) during the aircraft missions ACLOUD and HALO-(AC)³ analyzed by the ALABAMA. amm.sulf.: ammonium sulfate, DCA: dicarboxylic acids, van.: Vanadium, EC: elemental carbon. Particle types are assigned by the dominating peaks in the mass spectra.</figcaption></figure>
Stevens, B. and Kluft, L.: A colorful look at climate sensitivity, Atmos. Chem. Phys., 23, 14673–14689, https://doi.org/10.5194/acp-23-14673-2023, 2023.

Stevens, B., Ament, F., Bony, S., Crewell, S., Ewald, F., Gross, S., Hansen, A., Hirsch, L., Jacob, M., Kölling, T., Konow, H., Mayer, B., Wendisch, M., Wirth, M., Wolf, K., Bakan, S., Bauer-Pfundstein, B., Brueck, M., Delanoë, J., Ehrlich, A., Farrell, D., Forde, M., Gödde, F., Grob, H., Hagen, M., Jäkel, E., Jansen, F., Klepp, C., Klingebiel, M., Mech, M., Peters, G., Rapp, M., Wing, A. A., and Zinner, T.: A High-Altitude Long-Range Aircraft Configured as a Cloud Observatory: The NARVAL Expeditions, B. Am. Meteorol. Soc., 100, 1061–1077, https://doi.org/10.1175/BAMS-D-18-0198.1, 2019.

Stroeve, J., Holland, M. M., Meier, W., Scambos, T., and Serreze, M.: Arctic sea ice decline: Faster than forecast, Geophys. Res. Lett., 34, L09501, https://doi.org/10.1029/2007GL029703, 2007.

Svensson, G., Murto, S., Shupe, M. D., Pithan, F., Magnusson, L., Day, J. J., Doyle, J. D., Renfrew, I. A., Spengler, T., and Vihma, T.: Warm air intrusions reaching the MOSAiC expedition in April 2020 – The YOPP targeted observing period (TOP), Elem. Sci. Anth., 11, 00016, https://doi.org/10.1525/elementa.2023.00016, 2023.

Tan, I. and Storelvmo, T.: Evidence of Strong Contributions From Mixed-Phase Clouds to Arctic Climate Change, Geophys. Res. Lett., 46, 2894–2902, https://doi.org/10.1029/2018GL081871, 2019.

Taylor, P. C., Boeke, R. C., Boisvert, L. N., Feldl, N., Henry, M., Huang, Y., Langen, P. L., Liu, W., Pithan, F., Sejas, S. A., and Tan, I.: Process drivers, inter-model spread, and the path forward: A review of amplified Arctic warming, Front. Earth Sci., 9, 758361, https://doi.org/10.3389/feart.2021.758361, 2022.

Tjernström, M. and Graversen, R. G.: The vertical structure of the lower Arctic troposphere analysed from observations and the ERA-40 reanalysis, Q. J. R. Meteorol. Soc., 135, 431–443, https://doi.org/10.1002/qj.380, 2009.

Tjernström, M., Zagar, M., Svensson, G., Cassano, J. J., Pfeifer, S., Rinke, A., Wyser, K., Dethloff, K., Jones, C., Semmler, T., and Shaw, M.: Modelling the Arctic Boundary Layer: An Evaluation of Six Arcmip Regional-Scale Models using Data from the Sheba Project, Bound.-Lay. Meteorol., 117, 337–381, https://doi.org/10.1007/s10546-004-7954-z, 2005.

Tjernström, M., Shupe, M. D., Brooks, I. M., Achtert, P., Prytherch, J., and Sedlar, J.: Arctic Summer Airmass Transformation, Surface Inversions, and the Surface Energy Budget, J. Climate, 32, 769–789, https://doi.org/10.1175/JCLI-D-18-0216.1, 2019.

Urbanek, B., Groß, S., Schäfler, A., and Wirth, M.: Determining stages of cirrus evolution: a cloud classification scheme, Atmos. Meas. Tech., 10, 1653–1664, https://doi.org/10.5194/amt-10-1653-2017, 2017.

Uttal, T., Curry, J. A., McPhee, M. G., Perovich, D. K., Moritz, R. E., Maslanik, J. A., Guest, P. S., Stern, H., Moore, J. A., Turenne, R., Heiberg, A., Serreze, M. C., Wylie, D. P., Persson, O. G., Paulson, C. A., Halle, C., Morison, J. H., Wheeler, P. A., Makshtas, A., Welch, H., Shupe, M. D., Intrieri, J. M., Stamnes, K., Lindsey, R. W., Pinkel, R., Pegau, W. S., Stanton, T. P., and Grenfeld, T. C.: Surface heat budget of the arctic ocean, B. Am. Meteorol. Soc., 83, 255–276, https://doi.org/10.1175/1520-0477(2002)083<0255:SHBOTA>2.3.CO;2, 2002.

Vihma, T., Hartmann, J., and Lüpkes, C.: A case study of an on-ice air flow over the Arctic marginal sea ice zone, Bound.-Lay. Meteorol., 107, 189–217, https://doi.org/10.1023/A:1021599601948, 2003.

Volkmer, L., Kölling, T., Zinner, T., and Mayer, B.: Consideration of the cloud motion for aircraft-based stereographically derived cloud geometry and cloud top heights, Atmos. Meas. Tech. Discuss. [preprint], https://doi.org/10.5194/amt-2024-19, in review, 2024.

Walbröl, A., Michaelis, J., Becker, S., Dorff, H., Ebell, K., Gorodetskaya, I., Heinold, B., Kirbus, B., Lauer, M., Maherndl, N., Maturilli, M., Mayer, J., Müller, H., Neggers, R. A. J., Paulus, F. M., Röttenbacher, J., Rückert, J. E., Schirmacher, I., Slätberg, N., Ehrlich, A., Wendisch, M., and Crewell, S.: Contrasting extremely warm and long-lasting cold air anomalies in the North Atlantic sector of the Arctic during the HALO-(AC)³ campaign, Atmos. Chem. Phys., 24, 8007–8029, https://doi.org/10.5194/acp-24-8007-2024, 2024.

Weber, A., Kölling, T., Pörtge, V., Baumgartner, A., Rammeloo, C., Zinner, T., and Mayer, B.: Polarization upgrade of specMACS: calibration and characterization of the 2D RGB polarization-resolving cameras, Atmos. Meas. Tech., 17, 1419–1439, https://doi.org/10.5194/amt-17-1419-2024, 2024.

Wendisch, M. and Br enguier, J.-L.: Airborne Measurements for Environmental Research – Methods and Instruments, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, Weinheim, Germany, ISBN 978-3-527-40996-9, 2013.

Wendisch, M., Macke, A., Ehrlich, A., Lüpkes, C., Mech, M., Chechin, D., Barrientos, C., Bozem, H., Brückner, M., Clemen, H.-C., Crewell, S., Donth, T., Dupuy, R., Ebell, K., Egerer, U., Engelmann, R., Engler, C., Eppers, O., Gehrmann, M., Gong, X., Gottschalk, M., Gourbeyre, C., Griesche, H., Hartmann, J., Hartmann, M., Herber, A., Herrmann, H., Heygster, G., Hoor, P., Jafarerajehlou, S., Jäkel, E., Järvinen, E., Jourdan, O., Kästner, U., Kecorius, S., Knudsen, E. M., Köllner, F., Kretzschmar, J., Lelli, L., Leroy, D., Maturilli, M., Mei, L., Mertes, S., Mioche, G., Neuber, R., Nicolaus, M., Nomokonova, T., Notholt, J., Palm, M., van Pinxteren, M., Quaas, J., Richter, P., Ruiz-Donoso, E., Schäfer, M., Schmieder, K., Schnaiter, M., Schneider, J., Schwarzenböck, A., Seifert, P., Shupe, M. D., Siebert, H., Spreen, G., Stapf, J., Stratmann, F., Vogl, T., Welti, A., Wex, H., Wiedensohler, A., Zanatta, M., Zeppenfeld, S., Dethloff, K., and Heinold, B.: The Arctic Cloud Puzzle: Using ACLOUD-/PASCAL Multi-Platform Observations to Unravel the Role of Clouds and Aerosol Particles in Arctic Amplification, B. Am. Meteorol. Soc., 100, 841–871, https://doi.org/10.1175/BAMS-D-18-0072.1, 2019.

Wendisch, M., Handorf, D., Tegen, I., Neggers, R. A. J., and Spreen, G.: Glimpsing the ins and outs of the Arctic atmospheric cauldron, EOS, https://doi.org/10.1029/2021EO155959, 2021.

Wendisch, M., Brückner, M., Crewell, S., Ehrlich, A., Notholt, J., Lüpkes, C., Macke, A., Burrows, J. P., Rinke, A., Quaas, J., Maturilli, M., Schemann, V., Shupe, M. D., Akansu, E. F., Barrientos-Velasco, C., Bärfuss, K., Blechschmidt, A.-M., Block, K., Bougoudis, I., Bozem, H., Böckmann, C., Bracher, A., Bresson, H., Bretschneider, L., Buschmann, M., Chechin, D. G., Chylik, J., Dahlke, S., Deneke, H., Dethloff, K., Donth, T., Dorn, W., Dupuy, R., Ebell, K., Egerer, U., Engelmann, R., Eppers, O., Gerdes, R., Gierens, R., Gorodetskaya, I. V., Gottschalk,
<figure><img src="image_49.png" /></figure>

## 4.4 Proof of concept to measure mesoscale divergence and subsidence in the Arctic

ABL cloud transformations at high latitudes play a key role in the Arctic and are partially controlled by large-scale dynamics such as divergence and subsidence. During several research flights (RFs) of HALO-(AC)³, we have successfully applied a measurement technique using the data from multiple dropsonde releases in circular flight patterns to estimate mesoscale properties including divergence, associated subsidence, pressure gradients, and advective tendencies (Paulus et al., 2024). As illustrated in Fig. 17a, HALO RF10 and RF11 probed a weak CAO in a quasi-Lagrangian way, sampling it at multiple points during two days along its southbound trajectory from the central Arctic into the Fram Strait. The four selected locations were determined before take-off, using trajectory estimates based on forecast data. Mesoscale flight patterns surrounding these locations were then incorporated in the HALO flight plan, in principle allowing the calculation of mesoscale gradients across the area (Bony and Stevens, 2019).

The scientific objective of this effort is twofold. Firstly, we aim to test the hypothesis that this novel sampling technique (the sondes) can reliably yield mesoscale profiles of (thermo-)dynamic circulation properties also at high latitudes, in particular in cold, transforming low level air masses. Various studies have reported encouraging results with this method in marine subtropical areas (George et al., 2021; Bony and Stevens, 2019) and the mid-latitudes (Li et al., 2022). We show that the method works equally well at high latitudes, given the absence of large-scale weak temperature gradients (Charney, 1963; Sobel et al., 2001) and the (partially) associated high transience in synoptic weather. The second objective is to achieve a dataset of circulation properties sampled by HALO at mesoscales that is suitable for driving scientific process models of clouds in CAOs along the trajectory. While CAO cases for Large-Eddy Simulations (LES) and Single-Column Models (SCM) have been generated before (de Roode et al., 2019), typically the observational data to realistically constrain the simulation in the upstream areas was lacking. Previous LES studies in the Arctic have shown a strong dependence of Arctic mixed layers on larger-scale forcings, in particular subsidence (Neggers et al., 2019; Dimitrelos et al., 2023), prioritizing the need for such observational data. The HALO-(AC)³ campaign supplies this crucial data for the first time, a unique opportunity for exclusively forcing scientific process model experiments for this weather regime with mesoscale data sampled along the trajectory. This has not been achieved before and would represent a step forward in anchoring Arctic process model studies in reality.

Figures 17b-e show the vertical profiles of subsidence as derived from dropsonde data from four “mesoscale circle” flight segments that were flown at selected locations along the two-day trajectory. The northernmost circle was sampled by HALO RF10, while the southern three circles were flown a day later by RF11. The third HALO circle on the trajectory (RF11 C02) located just south of the sea ice edge was also probed in collocation by the P5 and P6 aircraft, providing additional in-situ measurements of clouds, turbulence, radiation, and aerosol properties. We obtain robust profiles of mesoscale subsidence at all four circle sites. For reference the measured profiles are cross-compared with ERA5 reanalysis data, reporting less-than-optimal agreement in general. At circle RF11 C02 the subsidence profile sampled by HALO is reproduced by independent dropsonde data from the P5 aircraft, conducted an hour later in the same area and covering only the lowest few kilometers
M., Griesche, H., Gryanik, V. M., Handorf, D., Harm-Altstädter, B., Hartmann, J., Hartmann, M., Heinold, B., Herber, A., Herrmann, H., Heygster, G., Höschel, I., Hofmann, Z., Hälemann, J., Hünerbein, A., Jafariserajehlou, S., Jäkel, E., Jacobi, C., Janout, M., Jansen, F., Jourdan, O., Jurányi, Z., Kalesse-Los, H., Kanzow, T., Käthner, R., Kliesch, L. L., Klingebiel, M., Knudsen, E. M., Kovács, T., Körtke, W., Krampe, D., Kretzschmar, J., Kreyling, D., Kulla, B., Kunkel, D., Lampert, A., Lauer, M., Lelli, L., von Lerber, A., Linke, O., Löhnert, U., Lonardi, M., Losa, S. N., Losch, M., Maahn, M., Mech, M., Mei, L., Mertes, S., Metzner, E., Mewes, D., Michaelis, J., Mioche, G., Moser, M., Nakoudi, K., Neggers, R., Neuber, R., Nomokonova, T., Oelker, J., Papakonstantinou-Presvelou, I., Pätzold, F., Pefanis, V., Pohl, C., van Pinxteren, M., Radovan, A., Rhein, M., Rex, M., Richter, A., Risse, N., Ritter, C., Rostosky, P., Rozanov, V. V., Ruiz-Donoso, E., Saavedra-Garfias, P., Salzmann, M., Schacht, J., Schäfer, M., Schneider, J., Schnierstein, N., Seifert, P., Seo, S., Siebert, H., Soppa, M. A., Sreen, G., Stachlewska, I. S., Stapf, J., Stratmann, F., Tegen, I., Viceto, C., Voigt, C., Vountas, M., Walbröl, A., Walter, M., Wehner, B., Wex, H., Willmes, S., Zanatta, M., and Zeppenfeld, S.: Atmospheric and Surface Processes, and Feedback Mechanisms Determining Arctic Amplification: A Review of First Results and Prospects of the (AC)³ Project, B. Am. Meteorol. Soc., 104, E208–E242, https://doi.org/10.1175/bams-d-21-0218.1, 2023.

Wesche, C., Steinhage, D., and Nixdorf, U.: Polar aircraft Polar5 and Polar6 operated by the Alfred Wegener Institute, J. Large-Scale Res. Facilities, 2, https://doi.org/10.17815/jlsrf-2-153, 2016.

Wirth, M., Fix, A., Mahnke, P., Schwarzer, H., Schrandt, F., and Ehret, G.: The airborne multi-wavelength water vapor differential absorption lidar WALES: system design and performance, Appl. Phys. B, 96, 201–213, https://doi.org/10.1007/s00340-009-3365-7, 2009.

Woods, C. and Caballero, R.: The Role of Moist Intrusions in Winter Arctic Warming and Sea Ice Decline, J. Climate, 29, 4473–4485, https://doi.org/10.1175/JCLI-D-15-0773.1, 2016.

You, C., Tjernström, M., and Devasthale, A.: Warm-air advection over melting sea-ice: A Lagrangian case study, Bound.-Lay. Meteorol., 179, 99–116, https://doi.org/10.1007/s10546-020-00590-1, 2021a.

You, C., Tjernström, M., and Devasthale, A.: Eulerian and Lagrangian views of warm and moist air intrusions into summer Arctic, Atmos. Res., 256, 105586, https://doi.org/10.1016/j.atmosres.2021.105586, 2021b.

You, C., Tjernström, M., and Devasthale, A.: Warm and moist air intrusions into the winter Arctic: a Lagrangian view on the near-surface energy budgets, Atmos. Chem. Phys., 22, 8037–8057, https://doi.org/10.5194/acp-22-8037-2022, 2022.

Yuval, J. and Kaspi, Y.: Eddy Activity Response to Global Warming-Like Temperature Changes, J. Climate, 33, 1381–1404, https://doi.org/10.1175/JCLI-D-19-0190.1, 2020.
<figure><img src="image_50.png" /></figure>

below the P5 aircraft. This agreement, in particular, suggests that the HALO profiles of subsidence are realistic, and provides proof of principle for the applicability of the mesoscale dropsonde technique also at high latitudes.

The encouraging results obtained for this CAO case are fully described by Paulus et al. (2024). They motivate taking the next step and configure LES and SCM experiments for this case that are purely based on HALO measurements. Apart from divergence and associated subsidence, the data also yields profiles of horizontal pressure gradients (or geostrophic wind) and advective tendencies of temperature, humidity, and momentum. The in-situ and independent P5 and P6 data can well be used to evaluate the process model simulations, yielding a complete package for realistic model studies of this CAO. This research is currently in progress.

Cold Air Outbreak (CAO): 29 and 30 March 2022

<figure><img src="image_51.png" /><figcaption>**Figure 17.** Overview of mesoscale flight patterns flown by HALO (dark blue) and P5 (green) sampling a low level air mass on 29 and 30 March 2022 at four locations along its southbound trajectory (light blue). Dropsonde launch locations are marked by crosses. b)-e) Profiles of pressure velocity Ω calculated from dropsonde data (colored) and ERA5 reanalysis data (grey dashed).</figcaption></figure>

5 Summary

520

In this paper, we illustrate the application of a quasi-Lagrangian aircraft measurement approach to observe air mass transfor-
mations in the Arctic. The data were collected during the HALO–(AC)³ aircraft campaign that took place between Scandinavia
<figure><img src="image_52.png" /></figure>

and the North Pole in March and April 2022. We have employed three research aircraft with distinct objectives: The low-flying (mainly within and below clouds) Polar 6 (P6) was equipped with in-situ instrumentation to measure aerosol, cloud, and precipitation properties, whereas the higher-flying (mostly above clouds) Polar 5 (P5) aircraft conducted remote sensing observations of water vapor, aerosol particle, cloud, and precipitation characteristics. The third research aircraft, the High Altitude and Long Range Research Aircraft (HALO) was flying in about 10 km altitude. Whenever possible, HALO was flying closely collocated with P5 and P6. The in-situ and remote sensing payloads of the three aircraft were complemented by numerous dropsonde launches from P5 and HALO.

The focus of the observations was on air mass transformations during moist and warm air intrusions (WAIs) and marine cold air outbreaks (CAOs). The processes during these transformations were observed in a quasi-Lagrangian manner, probing the same air mass parcel twice on its way into or out of the Arctic, either on the same flight (e.g., at its beginning and end) or on two flights during consecutive days. To plan such flights, dedicated air mass parcel trajectory simulations along the flight paths were employed. A substantial number of matches of the same air mass parcels was obtained enabling statistical analysis of the temporal tendencies of potential temperature and moisture during WAIs and CAOs. For the CAO cases, a strong surface-driven diabatic heating between 1–3 K h⁻¹ and a near-surface moistening between 0.05–0.3 g kg⁻¹ h⁻¹ were observed within the lowest about 0.5 km. For the WAI observations, a weak diabatic cooling of up to 0.4 K h⁻¹ and a moisture loss of up to 0.1 g kg⁻¹ h⁻¹ were obtained from the ground to about 5 km altitude.

Furthermore, we followed the evolution of cloud properties (cloud top height and cover, liquid water path, precipitation rate, thermodynamic phase, and radar reflectivity) during CAOs. We have shown that for a strong CAO case, cloud tops are higher, and more liquid water-topped clouds exist. The liquid water path and mean radar reflectivity increase compared to a weaker CAO. In addition, we see how the cloud parameters evolved with distance over the open sea, with the ABL deepening and CTH rising. We observed that for the stronger CAO case, the characteristic features such as the formation of cloud streets and the onset of precipitation occur closer to the sea ice edge.

Furthermore, the moisture budget of a WAIs was quantified for a strong WAI. ICON-based and observational estimates of the moisture budget components agree reasonably well for the moisture tendency due to mass convergence and surface evaporation, while there are remarkable discrepancies regarding the local tendency of water vapor in ICON compared to the HALO data.

The vertical distribution of the thermodynamic phase in low-level Arctic clouds was quantified in a statistical manner using in-situ cloud measurements carried out over the open ocean and sea ice. The clouds over sea ice are dominated by the ice phase. In the upper part of the ABL, liquid clouds are frequently detected. Over sea ice, only a small fraction of the observed clouds is attributed to mixed-phase clouds. In contrast, over the open ocean, the cloud phase distribution is more variable in height, as mixed-phase clouds and pure liquid clouds are detected over the entire altitude range. Some reasons for the longevity of mixed-phase clouds were discussed by using a three-dimensional characterization of the thermodynamic phase partitioning.

Typical aerosol characteristics were quantified, and the chemical composition of the aerosol particles was studied. Values of the median total aerosol particle number concentration of 303 cm⁻³, and a median CCN number concentration at 0.1 % supersaturation of 155 cm⁻³ were derived. No altitude trend in particle numbers is evident up to roughly 1000 m above sea
<figure><img src="image_53.png" /></figure>

level, but a decreasing trend was sometimes observed higher up. With regard to the chemical composition of the aerosol particles, a large abundance of particulate amines in ambient air and a dominance of amines in cloud droplet residuals (CDR) during late spring/early summer is shown, emphasizing the importance of marine biogenic sources for summertime Arctic cloud processes. In contrast, amine-containing particles were rarely observed during late winter/early spring.

It was shown that circular flight patterns with sufficiently frequent dropsonde releases provide appropriate data to estimate mesoscale gradients, which can be used to derive subsidence and advective tendencies in the Arctic. These data are highly valuable, for example, to make available data to constrain the initial conditions for large eddy simulations avoiding the use of numerical models with a coarser resolution.

Delivering data for evaluating and testing numerical models and reanalysis is a major objective of the HALO-(AC)³ campaign. This objective will further be pursued using the wealth of observations from the campaign.

Analysis of the results of the HALO-(AC)³ aircraft campaign is ongoing, and resulting papers will be published in a Special Issue of Atmospheric Chemistry and Physics (ACP) on "HALO-(AC)³ – an airborne campaign to study air mass transformations during warm-air intrusions and cold-air outbreaks", see: https://acp.copernicus.org/articles/special issue1272.html.

*Data availability.* All data collected during the HALO-(AC)³ aircraft campaign are being published by Ehrlich et al. (2024).

*Author contributions.* All authors provided text and figures and contributed to the editing of the article, and to the analysis and discussion of the results.

*Competing interests.* One of the co-authors (Birgit Wehner) is a member of the editorial board of ACP.

*Acknowledgements.* We gratefully acknowledge the funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – Project Number 268020496 – TRR 172, within the framework of the Transregional Collaborative Research Center “ArctiC Amplification: Climate Relevant Atmospheric and SurfaCe Processes, and Feedback Mechanisms (AC)³”. The authors are grateful to AWI for providing and operating the two Polar 5 and Polar 6 aircraft. We thank the crews and the technicians of the three research aircraft for excellent technical and logistical support. The generous funding of the flight hours for the Polar 5 and Polar 6 aircraft by AWI, and for HALO by DFG, Max-Planck-Institut für Meteorologie (MPIM), and Deutsches Zentrum für Luft- und Raumfahrt (DLR) is greatly appreciated. We are further grateful for funding of project grant number 316646266 by DFG within the framework of Priority Program SPP 1294 to promote research with HALO. Oliver Eppers, Philipp Joppe, and Johanna Mayer acknowledge funding by the DFG – TRR 301 – Project-ID 428312742. Hans-Christian Clemen acknowledges funding by the DFG, project No. 442647984 (SCHN 1138/8-1). Timo Vihma acknowledges funding from the European Commission Horizon 2020 project Polar Regions in the Earth System project (PolarRES, 101003590). Gunilla Svensson and Michail
<figure><img src="image_54.png" /></figure>

<figure><img src="image_55.png" /></figure>

Karalis acknowledge funding from the Swedish Research Council (VR, project 2020-04064). Manuel Moser is grateful for funding by DFG SPP 2115 PROM (VO 1504/5-1), Johannes Lucke appreciates funding within the EU project SENS4ICE (grant agreement number 824253), and Elena De La Torre Castro acknowledges support by DFG SPP HALO 1294 (VO 1504/7-1). Harald Sodemann acknowledges funding from the European Research Council (Consolidator Grant Nr. 773245). This publication was supported by the Open Access Publishing Fund of Leipzig University.

590
<figure><img src="image_56.png" /></figure>

<figure><img src="image_57.png" /></figure>

Appendix A: A hypothesis on humidity fields in WAIs

We investigate the typical structure of the atmospheric moisture field, how it typically evolves during a WAI, and what processes drive this development. Relative humidity varies strongly and determines the structure of the moisture field in the cross-flow perspective going from 100% in the core of the WAI to around 50% within no more than 100 km distance from the core. Temperature determines the structure of the moisture field in an along-flow direction, as relative humidity is at saturation throughout the core of the intrusion. In the ABL and close to the sea ice edge, temperatures change by 3-4 K over a distance of about 150 km. Clearly an atmospheric river is channeled in the Fram Strait between sea ice (20°W) and Svalbard (0°). In future work we will investigate the hypothesis that a secondary circulation acts to spread out moisture from the initial river-like intrusion in the cross-flow direction (Fig. 18). This hypothesized circulation consists of uplift in the core of the intrusions, divergence in the upper troposphere above the core, and convergence and subsidence of drier air on the flanks of the intrusion.

<figure><img src="image_58.png" /><figcaption>**Figure 18.** Zonal vertical cross-section of relative humidity with respect to liquid water during a moist and warm air intrusion (WAI) on the basis of ERA5 data. Arrows indicate hypothesized circulation.</figcaption></figure>
<figure><img src="image_59.png" /></figure>

## Appendix B: The particular mode structure of the Arctic radiant energy budget

The radiant energy budget (REB) in the Arctic depends on different surface, cloud, and thermodynamic properties (Sedlar et al., 2011; Stapf et al., 2021). At the surface, Wendisch et al. (2023b) identified four modes (regimes) of the REB, depending on the surface type (sea ice or open ocean) and the atmospheric state (cloudy or cloud-free conditions). Models often struggle to correctly represent this mode structure due to limitations in the treatment of sub-grid processes including clouds and the sea ice albedo (Kretzschmar et al., 2020; Solomon et al., 2023). Consequently, detailed observation-model comparisons are helpful to identify potential misrepresentations of properties affecting the REB.

During HALO-(AC)³, the coordinated combination of measurements from three aircraft operating in different altitude regimes allowed to extend the statistical analysis of the Arctic REB to higher altitudes. Beside REB measurements performed by P6 close to the surface, the REB above the ABL clouds and close to the tropopause was observed by P5 and HALO, respectively (Fig. 19). The common mode structure (cloudy and cloud-free, over sea ice and open ocean) is most prominently illustrated in the near-surface REB (modes indicated by 3, 4, and 5, Fig. 19c). Modes 4 and 5 result from the observations in cloudy and cloud-free conditions above P6 when flying over the open ocean. Mode 3 corresponds to cloud-free situations over sea ice (only few clouds were observed over sea ice). For P5 (mid-level regime, Fig. 19b), clouds were not present above the aircraft, leaving a combined mode for weaker emitting cases above sea ice or low clouds (mode 1) and a mode for cloud-free observations above the open ocean (mode 2). The REB in high altitudes shows a continuous transition between modes 1 and 2. In general, the thermal-infrared net irradiance becomes more negative with increasing altitude. Furthermore, the surface and top of atmosphere (TOA) REBs obtained from ERA5 along the flight tracks are compared to the P6 and HALO measurements (not shown). The near-surface thermal-infrared REB is less negative than in the P6 observations. The TOA REB in ERA5 is slightly shifted towards lower solar net irradiances and is more narrow than the distribution of the HALO observations due to the lower horizontal ERA5 resolution. Such comparisons will help to constrain the REB in numerical models which aim to reproduce HALO-(AC)³ observations.
<figure><img src="image_60.png" /></figure>

<figure><img src="image_61.png" /><figcaption>**Figure 19.** Two-dimensional probability density function of the solar (normalized by the solar downward irradiance) and thermal-infrared net irradiances observed in different altitudes: (a) HALO, (b) P5, and (c) P6 flight altitudes. Mode numbers: 1 – above clouds or sea ice, 2 – above open ocean, 3 – cloud-free over sea ice, 4 – cloud-free over open ocean, 5 – cloudy over open ocean.</figcaption></figure>
<figure><img src="image_62.png" /></figure>

## Appendix C: An extraordinary CAO case covered during HALO-(AC)³

HALO-(AC)³ has provided an opportunity to expand the limited data base of airborne CAO studies that are known from the literature (Brümmer, 1997, 1999; Chechin et al., 2013; Dahlke et al., 2022; Geerts et al., 2022; Schirmacher et al., 2024). Here we integrate our data obtained during two CAO events with measurements following approximately a North-South trajectory (29 March and 9 April 2022) with literature data. The 9 April case was extraordinary for two reasons. Firstly, it was characterized by cloud-free conditions along the entire 180 km North-South flight track across the MIZ to the open ocean, which is very rare and unusual. Only at the southernmost position, convective clouds appeared. There and over sea ice, vertical profile measurements were performed with P6 (RF12). Secondly, the southernmost position was influenced by the front of a polar low, which was a remnant of a polar low over Fram Strait on the preceding day. Figure 20 shows profiles of mean and turbulent thermodynamic quantities measured over open sea during the two HALO-(AC)³ flights in comparison to previous measurements. For 29 March 2022, the results fit well to the earlier measurements except some peaks in the momentum fluxes, which could be traced back to orographic influences of Svalbard mountains at the northern coast. However, on 9 April (the cloudless day), unusually high temperatures and untypically low wind speeds were observed leading to extraordinarily weak turbulent convection over open sea when compared to previous CAO events.

<figure><img src="image_63.png" /><figcaption>**Figure 20.** Comparison of atmospheric boundary layer (ABL) profiles, with *z* representing altitude, of mean and turbulent quantities (normalized by the ABL height *z*ᵢ) obtained by airborne measurements during CAOs on 29 March (orange) and 9 April (red) 2022 during HALO-(AC)³. Data from previous aircraft campaigns (ARTIST, Hartmann et al. (1999), and AFLUX, Mech et al. (2022)) are indicated by black lines with different symbols. TKE is the turbulent kinetic energy, and *w*'/2 represents the vertical velocity variance.</figcaption></figure>
<figure><img src="image_64.png" /></figure>

References

Ali, S. M. and Pithan, F.: Following moist intrusions into the Arctic using SHEBA observations in a Lagrangian perspective, Q. J. R. Meteorol. Soc., 146, 3522–3533, https://doi.org/10.1002/qj.3859, 2020.

640 Aubry, C., Delanoë, J., Groß, S., Ewald, F., Tridon, F., Jourdan, O., and Mioche, G.: Lidar-radar synergistic method to retrieve ice, supercooled and mixed-phase clouds properties, Atmos. Meas. Technol. Discuss., https://doi.org/10.5194/amt-2023-252, 2024.

Bauer, R., Grooß, J.-U., Ungermann, J., Bär, M., Geldenhuys, M., and Hoffmann, L.: The Mission Support System (MSS v7.0.4) and its use in planning for the SouthTRAC aircraft campaign, Geosci. Model Dev., 15, 8983–8997, https://doi.org/10.5194/gmd-15-8983-2022, 2022.

645 Becker, S., Ehrlich, A., Schäfer, M., and Wendisch, M.: Airborne observations of the surface cloud radiative effect during different seasons over sea ice and open ocean in the Fram Strait, Atmos. Chem. Phys., 23, 7015–7031, https://doi.org/10.5194/acp-23-7015-2023, 2023.

Beer, C., Zimov, N., Olofsson, J., Porada, P., and Zimov, S.: Protection of Permafrost Soils from Thawing by Increasing Herbivore Density, Sci. Rep., 10, 4170, https://doi.org/10.1038/s41598-020-60938-y, 2020.

650 Blackport, R. and Screen, J. A.: Insignificant effect of Arctic amplification on the amplitude of midlatitude atmospheric waves, Sci. Adv., 6, 2880, https://doi.org/10.1126/sciadv.aay2880, 2020.

Block, K., Schneider, F., Mülmenstädt, J., Salzmann, M., and Quaas, J.: Climate models disagree on the sign of total radiative feedback in the Arctic, Tellus A: Dyn. Meteorol. Oceanogr., 72:1, 1–14, https://doi.org/10.1080/16000870.2019.1696139, 2020.

Boettcher, M., Schäfler, A., Sprenger, M., Sodemann, H., Kaufmann, S., Voigt, C., Schlager, H., Summa, D., Di Girolamo, P., Nerini, D., Germann, U., and Wernli, H.: Lagrangian matches between observations from aircraft, lidar and radar in a warm conveyor belt crossing orography, Atmos. Chem. Phys., 21, 5477–5498, https://doi.org/10.5194/acp-21-5477-2021, 2021.

Bony, S. and Stevens, B.: Measuring Area-Averaged Vertical Motions with Dropsondes, J. Atmos. Sci., 76, 767–783, https://doi.org/10.1175/JAS-D-18-0141.1, 2019.

Bossioli, E., Sotiuropoulou, G., Methymaki, G., and Tombrou, M.: Modeling extreme warm-air advection in the Arctic during summer: The effect of mid-latitude pollution inflow on cloud properties, J. Geophys. Res., 126, https://doi.org/10.1029/2020JD033291, 2021.

660 Brands, M., Kamphus, M., Böttger, T., Schneider, J., Drewnick, F., Roth, A., Curtius, J., Voigt, C., Borbon, A., Beekmann, M., Bourdon, A., Perrin, T., and Borrmann, S.: Characterization of a Newly Developed Aircraft-Based Laser Ablation Aerosol Mass Spectrometer (ALABAMA) and First Field Deployment in Urban Pollution Plumes over Paris During MEGAPOLI 2009, Aerosol Sci. Technol., 45, 46–64, https://doi.org/10.1080/02786826.2010.517813, 2011.

Brümmer, B.: Boundary Layer Mass, Water, and Heat Budgets in Wintertime Cold-Air Outbreaks from the Arctic Sea Ice, Mon. Wea. Rev., 125, 1824–1837, https://doi.org/10.1175/1520-0493(1997)125<1824:BLMWAH>2.0.CO;2, 1997.

Brümmer, B.: Roll and Cell Convection in Wintertime Arctic Cold-Air Outbreaks, J. Atmos. Sci., 56, 2613–2636, https://doi.org/10.1175/1520-0469(1999)056<2613:RACCIW>2.0.CO;2, 1999.

Brümmer, B. and Thiemann, S.: The Atmospheric Boundary Layer In An Arctic Wintertime On-Ice Air Flow, Boundary-Layer Meteorol., 104, 53–72, https://doi.org/10.1023/A:1015547922104, 2002.

670 Businger, S., Chiswell, S. R., Ulmer, W. C., and Johnson, R.: Balloons as a Lagrangian measurement platform for atmospheric research, J. Geophys. Res., 101, 4363–4376, https://doi.org/10.1029/95JD00559, 1996.

Businger, S., Johnson, R., and Talbot, R.: Scientific Insights from Four Generations of Lagrangian Smart Balloons in Atmospheric Research, Bull. Amer. Meteorol. Soc., 87, 1539–1554, http://www.jstor.org/stable/26217226, 2006.
<figure><img src="image_65.png" /></figure>

Charney, J. G.: A note on large-scale motions in the tropics, J. Atmos. Sci., 20, 607–609, https://doi.org/10.1175/1520-0469(1963)020<0607:ANOLSM>2.0.CO;2, 1963.

675

Chechin, D. G., Lüpkes, C., Repina, I. A., and Gryanik, V. M.: Idealized dry quasi 2-D mesoscale simulations of cold-air outbreaks over the marginal sea ice zone with fine and coarse resolution, J. Geophys. Res. Atmos., 118, 8787–8813, https://doi.org/10.1002/jgrd.50679, 2013.

Chylek, P., Folland, C., Klett, J. D., Wang, M., Hengartner, N., Lesins, G., and Dubey, M. K.: Annual mean Arctic amplification 1970–2020: Observed and simulated by CMIP6 climate models, Geophys. Res. Lett., 49, e2022GL099371, https://doi.org/10.1029/2022GL099371, 2022.

680

Clemen, H.-C., Schneider, J., Klimach, T., Helleis, F., Köllner, F., Hünig, A., Rubach, F., Mertes, S., Wex, H., Stratmann, F., Welti, A., Kohl, R., Frank, F., and Borrmann, S.: Optimizing the detection, ablation, and ion extraction efficiency of a single-particle laser ablation mass spectrometer for application in environments with low aerosol particle concentrations, Atmos. Meas. Technol., 13, 5923–5953, https://doi.org/10.5194/amt-13-5923-2020, 2020.

685

Cohen, J., Screen, J. A., Furtado, J. C., Barlow, M., Whittleston, D., Coumou, D., Francis, J. A., Dethloff, K., Entekhabi, D., Overland, J. E., and Jones, J.: Recent Arctic amplification and extreme mid-latitude weather, Nat. Geosci., 7, 627–637, https://doi.org/10.1038/ngeo2234, 2014.

Cohen, J., Zhang, X., Francis, J., Jung, T., Kwok, R., Overland, J., Ballinger, T., Bhatt, U. S., Chen, H. W., Coumou, D., Feldstein, S., Handorf, D., Henderson, G., Ionita, M., Kretschmer, M., Laliberte, F., Lee, S., Linderholm, H. W., Maslowski, W., Peings, Y., Pfeiffer, K., Rigor, I., Semmler, T., Stroeve, J., Taylor, P. C., Vavrus, S., Vihma, T., Wang, S., Wendisch, M., Wu, Y., and Yoon, J.: Divergent consensuses on Arctic amplification influence on midlatitude severe winter weather, Nat. Clim. Change, 10, 20–29, https://doi.org/10.1038/s41558-019-0662-y, 2020.

690

Cohen, J., Agel, L., Barlow, M., and Entekhabi, D.: No detectable trend in mid-latitude cold extremes during the recent period of Arctic amplification, Commun. Earth Environ., 4, https://doi.org/10.1038/s43247-023-01008-9, 2023.

Dada, L., Angot, H., Beck, I., Baccarini, A., Quéléver, L. L. J., Boyer, M., Laurila, T., Brasseur, Z., Jozef, G., de Boer, G., Shupe, M. D., Henning, S., Bucci, S., Dütsch, M., Stohl, A., Petäjä, T., Daellenbach, K. R., Jokinen, T., and Schmale, J.: A central Arctic extreme aerosol event triggered by a warm air-mass intrusion, Nat. Commun., 13, https://doi.org/10.1038/s41467-022-32872-2, 2022.

Dahlke, S., Solbès, A., and Maturilli, M.: Cold Air Outbreaks in Fram Strait: Climatology, Trends, and Observations During an Extreme Season in 2020, J. Geophys. Res. Atmos., 127, e2021JD035741, https://doi.org/10.1029/2021JD035741, 2022.

695

De La Torre Castro, E., Jurkat-Witschas, T., Afchine, A., Grewe, V., Hahn, V., Kirschler, S., Krämer, M., Lucke, J., Spelten, N., Wernli, H., Zöger, M., and Voigt, C.: Differences in microphysical properties of cirrus at high and mid-latitudes, Atmos. Chem. Phys., 23, 1367–1389, https://doi.org/10.5194/acp-23-1367-2023, 2023.

de Roode, S. R., Frederikse, T., Siebesma, A. P., Ackerman, A. S., Chylik, J., Field, P. R., Fricke, J., Gryschka, M., Hill, A., Honnert, R., Krueger, S. K., Lac, C., Lesage, A. T., and Tomassini, L.: Turbulent Transport in the Gray Zone: A Large Eddy Model Intercomparison Study of the CONSTRAIN Cold Air Outbreak Case, J. Adv. Model. Earth Syst., https://doi.org/10.1029/2018MS001443, 2019.

700

Dekoutsidis, G., Groß, S., Wirth, M., Krämer, M., and Rolf, C.: Characteristics of supersaturation in midlatitude cirrus clouds and their adjacent cloud-free air, Atmos. Chem. Phys., 23, 3103–3117, https://doi.org/10.5194/acp-23-3103-2023, 2023.

Dimitrelos, A., Ekman, A. M., Caballero, R., and Savre, J.: A sensitivity study of Arctic air-mass transformation using large eddy simulation, J. Geophys. Res., 125, e2019JD031738, https://doi.org/10.1029/2019JD031738, 2020.

710
<figure><img src="image_66.png" /></figure>

Dimitrelos, A., Caballero, R., and Ekman, A. M. L.: Controls on Surface Warming by Winter Arctic Moist Intrusions in Idealized Large-Eddy Simulations, J. Climate, 36, 1287–1300, https://doi.org/10.1175/JCLI-D-22-0174.1, 2023.

Druckenmiller, M. L., Thoman, R. L., and Moon, T. A.: Arctic Report Card 2022: Executive Summary, NOAA Arctic Report Card 2021, 22–01, https://doi.org/10.25923/yjx6-r184, 2022.

Ehrlich, A., Bierwirth, E., Wendisch, M., Gayet, J.-F., Mioche, G., Lampert, A., and Heintzenberg, J.: Cloud phase identification of Arctic boundary-layer clouds from airborne spectral reflection measurements: Test of three approaches, Atmos. Chem. Phys., 8, 7493–7505, https://doi.org/10.5194/acp-8-7493-2008, 2008.

Ehrlich, A., Wendisch, M., Lüpkes, C., Buschmann, M., Bozem, H., Chechin, D., Clemen, H.-C., Dupuy, R., Eppers, O., Hartmann, J., Herber, A., Jäkel, E., Järvinen, E., Jourdan, O., Kästner, U., Kliesch, L.-L., Köllner, F., Mech, M., Mertes, S., Neuber, R., Ruiz-Donoso, E., Schnaiter, M., Schneider, J., Stapf, J., and Zanatta, M.: A comprehensive in situ and remote sensing data set from the Arctic CLoud Observations Using airborne measurements during polar Day (ACLOUD) campaign, Earth Syst. Sci. Data, 11, 1853–1881, https://doi.org/10.5194/essd-11-1853-2019, 2019.

Ehrlich, A., Luebke, A. E., Klingebiel, M., Dorff, H., Walbröl, A., Becker, S., Schäfer, M., Rosenberg, S., Müller, H., Kirbus, B., Maahn, M., Maherndl, N., Ewald, F., Schäfler, A., Pörtge, V., Weber, A., and Wendisch, M.: A comprehensive in situ and remote sensing data set collected during the HALO-(AC)³ airborne campaign, for Submission to Earth Syst. Sci. Data, 2024.

Ewald, F., Kölling, T., Baumgartner, A., Zinner, T., and Mayer, B.: Design and characterization of specMACS, a multipurpose hyperspectral cloud and sky imager, Atmos. Meas. Technol., 9, 2015–2042, https://doi.org/10.5194/amt-9-2015-2016, 2016.

Forster, P. M., Smith, C. J., Walsh, T., Lamb, W. F., Lamboll, R., Hauser, M., Ribes, A., Rosen, D., Gillett, N., Palmer, M. D., Rogelj, J., von Schuckmann, K., Seneviratne, S. I., Trewin, B., Zhang, X., Allen, M., Andrew, R., Birt, A., Borger, A., Boyer, T., Broersma, J. A., Cheng, L., Dentener, F., Friedlingstein, P., Gutiérrez, J. M., Gütschow, J., Hall, B., Ishii, M., Jenkins, S., Lan, X., Lee, J.-Y., Morice, C., Kadow, C., Kennedy, J., Killick, R., Minx, J. C., Naik, V., Peters, G. P., Pirani, A., Pongratz, J., Schleussner, C.-F., Szopa, S., Thorne, P., Rohde, R., Rojas Corradi, M., Schumacher, D., Vose, R., Zickfeld, K., Masson-Delmotte, V., and Zhai, P.: Indicators of Global Climate Change 2022: annual update of large-scale indicators of the state of the climate system and human influence, Earth Syst. Sci. Data, 15, 2295–2327, https://doi.org/10.5194/essd-15-2295-2023, 2023.

Francis, J. A. and Vavrus, S. J.: Evidence for a wavier jet stream in response to rapid Arctic warming, Environ. Res. Lett., 10, 014005, https://doi.org/10.1088/1748-9326/10/1/014005, 2015.

Geerts, B., Giangrande, S. E., McFarquhar, G. M., Xue, L., Abel, S. J., Comstock, J. M., Crewell, S., DeMott, P. J., Ebell, K., Field, P., Hill, T. C. J., Hunzinger, A., Jensen, M. P., Johnson, K. L., Juliano, T. W., Kollias, P., Kosovic, B., Lackner, C., Luke, E., Lüpkes, C., Matthews, A. A., Neggers, R., Ovchinnikov, M., Powers, H., Shupe, M. D., Spengler, T., Swanson, B. E., Tjernström, M., Theisen, A. K., Wales, N. A., Wang, Y., Wendisch, M., and Wu, P.: The COMBLE Campaign: A Study of Marine Boundary Layer Clouds in Arctic Cold-Air Outbreaks, Bull. Amer. Meteorol. Soc., 103, E1371–E1389, https://doi.org/10.1175/BAMS-D-21-0044.1, 2022.

George, G., Stevens, B., Bony, S., Pincus, R., Fairall, C., Schulz, H., Kölling, T., Kalen, Q. T., Klingebiel, M., Konow, H., Lundry, A., Prange, M., and Radtke, J.: JOANNE: Joint dropsonde Observations of the Atmosphere in tropical North atlaNtic meso-scale Environments, Earth Syst. Sci. Data, 13, 5253–5272, https://doi.org/10.5194/essd-13-5253-2021, 2021.

Gong, X., Zhang, J., Croft, B., Yang, X., Frey, M. M., Bergner, N., Chang, R. Y.-W., Creamean, J. M., Kuang, C., Martin, R. V., Ranjithkumar, A., Sedlacek, A. J., Uin, J., Willmes, S., Zawadowicz, M. A., Pierce, J. R., Shupe, M. D., Schmale, J., and Wang, J.: Arctic warming by abundant fine sea salt aerosols from blowing snow, Nat. Geosci., 16, 768–774, https://doi.org/10.1038/s41561-023-01254-8, 2023.
<figure><img src="image_67.png" /></figure>

Graham, R., Cohen, L., Ritzhaupt, N., Segger, B., Graversen, R., Rinke, A., Walden, V. P., Granskog, M. A., and Hudson, S. R.: Evaluation of six atmospheric reanalyses over Arctic sea ice from winter to early summer, J. Clim., 32, 4121–4143, https://doi.org/10.1175/JCLI-D-18-0643.1, 2019a.

750

Graham, R. M., Hudson, S. R., and Maturilli, M.: Improved performance of ERA5 in Arctic gateway relative to four global atmospheric reanalyses, Geophys. Res. Lett., 46, 6138–6147, https://doi.org/10.1029/2019GL082781, 2019b.

Graversen, R. G., Mauritsen, T., Tjernström, M., Källén, E., and Svensson, G.: Vertical structure of recent Arctic warming, Nature, 451, 53–56, https://doi.org/10.1038/nature06502, 2008.

755

Groß, S., Wirth, M., Schäfler, A., Fix, A., Kaufmann, S., and Voigt, C.: Potential of airborne lidar measurements for cirrus cloud studies, Atmos. Meas. Technol., 7, 2745–2755, https://doi.org/10.5194/amt-7-2745-2014, 2014.

Gryanik, V. M. and Lüpkes, C.: A Package Momentum and Heat Transfer Coefficients for the Stable Surface Layer Extended by New Coefficients over Sea Ice, Boundary-Layer Meteorol., 187, 41–72, https://doi.org/10.1007/s10546-022-00730-9, 2023.

Hartmann, J., Kottmeier, C., and Raasch, S.: Roll vortices and boundary-layer development during a cold air outbreak, Bound.-Lay. Meteorol., 84, 45–65, https://doi.org/10.1023/A:1000392931768, 1997.

760

Hartmann, J., Albers, F., Argentini, S., Bochert, A., Bonafé, U., Cohrs, W., Conidi, A., Freese, D., Georgiadis, T., Ippoliti, A., Kaleschke, L., Lüpkes, C., Maixner, U., Mastrantonio, G., Ravegnani, F., Reuter, A., Trivellone, G., and Viola, A.: Arctic Radiation and Turbulence Interaction Study (ARTIST), https://doi.org/10.2312/BzP_0305_1999, 1999.

Hersbach, H., Bell, B., Berrisford, P., Hirahara, S., Horányi, A., Muñoz-Sabater, J., Nicolas, J., Peubey, C., Radu, R., Schepers, D., Simmons, A., Soci, C., Abdalla, S., Abellan, X., Balsamo, G., Bechtold, P., Biavati, G., Bidlot, J., Bonavita, M., De Chiara, G., Dahlgren, P., Dee, D., Diamantakis, M., Dragani, R., Flemming, J., Forbes, R., Fuentes, M., Geer, A., Haimberger, L., Healy, S., Hogan, R. J., Hólm, E., Janisková, M., Keeley, S., Laloyaux, P., Lopez, P., Lupu, C., Radnoti, G., de Rosnay, P., Rozum, I., Vamborg, F., Villaume, S., and Thépaut, J.-N.: The ERA5 global reanalysis, Q. J. R. Meteorol. Soc., 146, 1999–2049, https://doi.org/10.1002/qj.3803, 2020.

765

Holland, M. M. and Landrum, L.: The emergence and transient nature of Arctic amplification in coupled climate models, Front. Earth Sci., 9, https://doi.org/10.3389/feart.2021.719024, 2021.

770

Holtslag, B., Svensson, G., Baas, P., Basu, S., Beare, R., Beljaars, A., Bosveld, F., Cuxart, J., Lindvall, J., Steeneveld, G.-J., Tjernström, M., and Van de Wiel, B.: Stable Atmospheric Boundary Layers and Diurnal Cycles: Challenges for Weather and Climate Models, Bull. Amer. Meteorol. Soc., 94, 1691–1706, https://doi.org/10.1175/BAMS-D-11-00187.1, 2013.

Huang, Y., Dong, X., Bailey, D. A., Holland, M. M., Xi, B., DuVivier, A. K., Kay, J. E., Landrum, L. L., and Deng, Y.: Thicker clouds and accelerated Arctic sea ice decline: The atmosphere-sea ice interactions in spring, Geophys. Res. Lett., 46, 6980–6989, https://doi.org/10.1029/2019GL082791, 2019.

775

Jeffries, M. O., Overland, J. E. E., and Perovich, D. K.: The Arctic shifts to a new normal, Phys. Today, 66, 35–40, https://doi.org/10.1063/PT.3.2147, 2013.

Johansson, A. M., King, J. A., Doulgeris, A. P., Gerland, S., Singha, S., Sreen, G., and Busche, T.: Combined observations of Arctic sea ice with near-coincident colocated X-band, C-band, and L-band SAR satellite remote sensing and helicopter-borne measurements, J. Geophys. Res., 122, 669–691, https://doi.org/10.1002/2016jc012273, 2017.

780

Johnson, R., Businger, S., and Baerman, A.: Lagrangian air mass tracking with smart balloons during ACE-2, Tellus B: Chem. Phys. Meteorol., 52, 321–334, https://doi.org/10.3402/tellusb.v52i2.16114, 2000.

Jung, T., Gordon, N. D., Bauer, P., Bromwich, D. H., Chevallier, M., Day, J. J., Dawson, J., Doblas-Reyes, F., Fairall, C., Goessling, H. F., Holland, M., Inoue, J., Iversen, T., Klebe, S., Lemke, P., Losch, M., Makshtas, A., Mills, B., Nurmi, P., Perovich, D., Reid, P., Renfrew,

785
<figure><img src="image_68.png" /></figure>

I. A., Smith, G., Svensson, G., Tolstykh, M., and Yang, Q.: Advancing Polar Prediction Capabilities on Daily to Seasonal Time Scales, Bull. Am. Meteorol. Soc., 97, 1631–1647, https://doi.org/10.1175/BAMS-D-14-00246.1, 2016.

Kirbus, B., Chylik, J., Ehrlich, A., Becker, S., Schäfer, M., Neggers, R., and Wendisch, M.: Analysis of an Arctic cold air outbreak during autumn and related air mass transformations forced by surface changes and advection in higher altitudes, Elem. Sci. Anth., 11, https://doi.org/10.1525/elementa.2023.00079, 2023a.

790

Kirbus, B., Schirmacher, I., Klingebiel, M., Schäfer, M., Ehrlich, A., Slättberg, N., Lucke, J., Moser, M., Müller, H., and Wendisch, M.: Thermodynamic and cloud evolution in a cold air outbreak during HALO-(AC)³: Quasi-Lagrangian observations compared to the ERA5 and CARRA reanalyses, EGUSphere [preprint], https://doi.org/10.5194/egusphere-2023-2989, 2023b.

Kirschler, S., Voigt, C., Anderson, B. E., Chen, G., Crosbie, E. C., Ferrare, R. A., Hahn, V., Hair, J. W., Kaufmann, S., Moore, R. H., Painemal, D., Robinson, C. E., Sanchez, K. J., Scarino, A. J., Shingler, T. J., Shook, M. A., Thornhill, K. L., Winstead, E. L., Ziemba, L. D., and Sorooshian, A.: Overview and statistical analysis of boundary layer clouds and precipitation over the western North Atlantic Ocean, Atmos. Chem. Phys., 23, 10731–10750, https://doi.org/10.5194/acp-23-10731-2023, 2023.

795

Kölling, T., Zinner, T., and Mayer, B.: Aircraft-based stereographic reconstruction of 3-D cloud geometry, Atmos. Meas. Technol., 12, 1155–1166, https://doi.org/10.5194/amt-12-1155-2019, 2019.

Konow, H., Ewald, F., George, G., Jacob, M., Klingebiel, M., Kölling, T., Luebke, A. E., Mieslinger, T., Pörtge, V., Radtke, J., Schäfer, M., Schulz, H., Vogel, R., Wirth, M., Bony, S., Crewell, S., Ehrlich, A., Forster, L., Giez, A., Gödde, F., Groß, S., Gutleben, M., Hagen, M., Hirsch, L., Jansen, F., Lang, T., Mayer, B., Mech, M., Prange, M., Schnitt, S., Vial, J., Walbröl, A., Wendisch, M., Wolf, K., Zinner, T., Zöger, M., Ament, F., and Stevens, B.: EUREC⁴A's HALO, Earth Syst. Sci. Data, 13, 5545–5563, https://doi.org/10.5194/essd-13-5545-2021, 2021.

800

Kretzschmar, J., Stapf, J., Klocke, D., Wendisch, M., and Quaas, J.: Employing airborne radiation and cloud microphysics observations to improve cloud representation in ICON at kilometer-scale resolution in the Arctic, Atmos. Chem. Phys., 20, 13145–13165, https://doi.org/10.5194/acp-20-13145-2020, 2020.

805

Leaitch, W. R., Aliabadi, A., Korolev, A. A., Burkart, J., Willis, M. D., Abbatt, J. P. D., Bozem, H., Hoor, P., Köllner, F., Schneider, J., Herber, A., Konrad, C., and Brauner, R.: Effects of 20–100nm particles on liquid clouds in the clean summertime Arctic, Atmos. Chem. Phys., 16, 11107–11124, https://doi.org/10.5194/acp-16-11107-2016, 2016.

810

Li, X.-Y., Wang, H., Chen, J., Endo, S., George, G., Cairns, B., Chellappan, S., Zeng, X., Kirschler, S., Voigt, C., Sorooshian, A., Crosbie, E., Chen, G., Ferrare, R. A., Gustafson, W. I., Hair, J. W., Kleb, M. M., Liu, H., Moore, R., Painemal, D., Robinson, C., Scarino, A. J., Shook, M., Shingler, T. J., Thornhill, K. L., Tornow, F., Xiao, H., Ziemba, L. D., and Zuidema, P.: Large-Eddy Simulations of Marine Boundary Layer Clouds Associated with Cold-Air Outbreaks during the ACTIVATE Campaign. Part I: Case Setup and Sensitivities to Large-Scale Forcings, J. Atmos. Sci., 79, 73–100, https://doi.org/10.1175/JAS-D-21-0123.1, 2022.

815

Linke, O., Quaas, J., Baumer, F., Becker, S., Chylik, J., Dahlke, S., Ehrlich, A., Handorf, D., Jacobi, C., Kalesse-Los, H., Lelli, L., Mehrdad, S., Neggers, R. A., Riebold, J., Garfias, P. S., Schnierstein, N., Shupe, M. D., Smith, C., Sreen, G., Verneuil, B., Vinjamuri, K. S., Vountas, M., and Wendisch, M.: Constraints on simulated past Arctic amplification and lapse-rate feedback from observations, Atmos. Chem. Phys., 23, 9963–9992, https://doi.org/10.5194/acp-23-9963-2023, 2023.

Lohmann, U. and Neubauer, D.: The importance of mixed-phase and ice clouds for climate sensitivity in the global aerosol–climate model ECHAM6-HAM2, Atmos. Chem. Phys., 18, 8807–8828, https://doi.org/10.5194/acp-18-8807-2018, 2018.

820
<figure><img src="image_69.png" /></figure>

Lonardi, M., Akansu, E. F., Ehrlich, A., Mazzola, M., Pilz, C., Shupe, M. D., Siebert, H., and Wendisch, M.: Tethered balloon-borne observations of thermal-infrared irradiance and cooling rate profiles in the Arctic atmospheric boundary layer, Atmos. Chem. Phys., 24, 1961–1978, https://doi.org/10.5194/acp-24-1961-2024, 2024.

Lüpkes, C., Vihma, T., Birnbaum, G., Dierer, S., Garbrecht, T., Gryanik, V. M., Gryschka, M., Hartmann, J., Heinemann, G., Kaleschke, L., Raasch, S., Savijürvi, H., Schlünzen, K. H., and Wacker, U.: Mesoscale modelling of the Arctic atmospheric boundary layer and its interaction with sea ice, vol. 43, chap. ARCTIC Climate Change - The ACSYS Decade and Beyond (edited by Peter Lemke and Hans-Werner Jacobi), Springer, Atmospheric and Oceanographic Sciences Library, https://doi.org/10.1007/978-94-007-2027-5, 2012.

Maherndl, N., Moser, M., Lucke, J., Mech, M., Risse, N., Schirmacher, I., and Maahn, M.: Quantifying riming from airborne data during the HALO-(AC)³ campaign, Atmos. Meas. Technol., 17, 1475–1495, https://doi.org/10.5194/amt-17-1475-2024, 2024.

Manabe, S. and Wetherald, R. T.: The effects of doubling the CO₂ concentration on the climate of a general circulation model, J. Atmos. Sci., 32, 3–15, https://doi.org/10.1175/1520-0469(1975)032<0003:TEODTC>2.0.CO;2, 1975.

Masson-Delmotte, V., Zhai, P., Pirani, A., Connors, S. L., Péan, C., Berger, S., Caud, N., Chen, Y., Goldfarb, L., Gomis, M. I., Huang, M., Leitzell, K., Lonnoy, E., Matthews, J. B. R., Maycock, T. K., Waterfield, T., Yelekçi, O., Yu, R., and Zhou, B.: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, UK, https://doi.org/10.1017/9781009157896, 2021.

Mateling, M. E., Pettersen, C., Kulie, M. S., Ecuyer, L., and Tristan, S.: Marine Cold-Air Outbreak Snowfall in the North Atlantic: A CloudSat Perspective, J. Geophys. Res., 128, e2022JD038053, https://doi.org/10.1029/2022JD038053, 2023.

McCoy, D. T., Hartmann, D. L., Zelinka, M. D., Ceppi, P., and Grosvenor, D. P.: Mixed-phase cloud physics and Southern Ocean cloud feedback in climate models, J. Geophys. Res., 120, 9539–9554, https://doi.org/10.1002/2015JD023603, 2015.

Mech, M., Orlandi, E., Crewell, S., Ament, F., Hirsch, L., Hagen, M., Peters, G., and Stevens, B.: HAMP – the microwave package on the High Altitude and LOng range research aircraft (HALO), Atmos. Meas. Technol., 7, 4539–4553, https://doi.org/10.5194/amt-7-4539-2014, 2014.

Mech, M., Kliesch, L.-L., Anhäuser, A., Rose, T., Kollias, P., and Crewell, S.: Microwave Radar/radiometer for Arctic Clouds (MiRAC): first insights from the ACLOUD campaign, Atmos. Meas. Technol., 12, 5019–5037, https://doi.org/10.5194/amt-12-5019-2019, 2019.

Mech, M., Ehrlich, A., Herber, A., Lüpkes, C., Wendisch, M., Crewell, S., Becker, S., Boose, Y., Chechin, D., Dupuy, R., Gourbeyre, C., Hartmann, J., Jäkel, E., Jourdan, O., Kliesch, L.-L., Kulla, B. S., Mioche, G., Moser, M., Risse, N., Schäfer, M., Klingebiel, M., Stapf, J., and Voigt, C.: MOSAiC-ACA and AFLUX: Arctic airborne campaigns characterizing the exit area of MOSAiC, Sci. Data, 9, https://doi.org/10.1038/s41597-022-01900-7, 2022.

Moser, M., Voigt, C., Jurkat-Witschas, T., Hahn, V., Mioche, G., Jourdan, O., Dupuy, R., Gourbeyre, C., Schwarzenboeck, A., Lucke, J., Boose, Y., Mech, M., Borrmann, S., Ehrlich, A., Herber, A., Lüpkes, C., and Wendisch, M.: Microphysical and thermodynamic phase analyses of Arctic low-level clouds measured above the sea ice and the open ocean in spring and summer, Atmos. Chem. Phys., 23, 7257–7280, https://doi.org/10.5194/acp-23-7257-2023, 2023.

Murray-Watson, R. J., Gryspeerdt, E., and Goren, T.: Investigating the development of clouds within marine cold-air outbreaks, Atmos. Chem. Phys., 23, 9365–9383, https://doi.org/10.5194/acp-23-9365-2023, 2023.

Nash, D., Waliser, D., Guan, B., Ye, H., and Ralph, F. M.: The role of atmospheric rivers in extratropical and polar hydroclimate, J. Geophys. Res.: Atmos., 123, 6804–6821, https://doi.org/10.1029/2017JD028130, 2018.

Neggers, R. A. J., Chylík, J., Egerer, U., Griesche, H., Schemann, V., Seifert, P., Siebert, H., and Macke, A.: Local and remote controls on Arctic mixed-layer evolution, J. Adv. Model. Earth Syst., 11, 2214–2237, https://doi.org/10.1029/2019MS001671, 2019.
<figure><img src="image_70.png" /></figure>

860 Nygård, T., Papritz, L., Naakka, T., and Vihma, T.: Cold wintertime air masses over Europe: where do they come from and how do they form?, Weath. Clim. Dyn., 4, 943–961, https://doi.org/10.5194/wcd-4-943-2023, 2023.

Ogren, J. A., Heintzenberg, J., and Charlson, R. J.: In-situ sampling of clouds with a droplet to aerosol converter, Geophys. Res. Lett., 12, 121–124, https://doi.org/10.1029/GL012i003p00121, 1985.

865 Olonscheck, D., Mauritsen, T., and Notz, D.: Arctic sea-ice variability is primarily driven by atmospheric temperature fluctuations, Nat. Geosci., 12, 430–434, https://doi.org/10.1038/s41561-019-0363-1, 2019.

Overland, J. E., Wood, K. R., and Wang, M.: Warm Arctic-cold conditions: Impacts of the newly open Arctic Sea, Polar Res., 30, 15,787, https://doi.org/10.3402/polar.v30i0.15787, 2011.

Paulus, F. M., Karalis, M., George, G., Svensson, G., Wendisch, M., and Neggers, R. A. J.: Airborne measurements of mesoscale divergence at high latitudes during HALO-(AC)³, Submitted to J. Atmos. Sci., 2024.

870 Pefanis, V., Losa, S. N., Losch, M., Janout, M. A., and Bracher, A.: Amplified Arctic Surface Warming and Sea Ice Loss Due to Phytoplankton and Colored Dissolved Material, Geophys. Res. Lett., 47, e2020GL088795, https://doi.org/10.1029/2020GL088795, 2020.

Pithan, F., Medeiros, B., and Mauritsen, T.: Mixed-phase clouds cause climate model biases in Arctic wintertime temperature inversions, Clim. Dyn., 43, 289–303, https://doi.org/10.1007/s00382-013-1964-9, 2014.

875 Pithan, F., Ackerman, A., Angevine, W. M., Hartung, K., Ickes, L., Kelley, M., Medeiros, B., Sandu, I., Steeneveld, G.-J., Sterk, H. A. M., Svensson, G., Vaillancourt, P. A., and Zadra, A.: Select strengths and biases of models in representing the Arctic winter boundary layer over sea ice: the Larcform 1 single column model intercomparison, J. Adv. Model. Earth Syst., 8, 1345–1357, https://doi.org/10.1002/2016MS000630, 2016.

Pithan, F., Svensson, G., Caballero, R., Chechin, D., Cronin, T., Ekman, A., Neggers, R. A. J., Shupe, M., Solomon, A., Tjernström, M., and Wendisch, M.: Role of air-mass transformations in exchange between the Arctic and mid-latitudes, Nat. Geosci., 11, 805–812, https://doi.org/10.1038/s41561-018-0234-1, 2018.

880 Pithan, F., Athanase, M., Dahlke, S., Sánchez-Benítez, A., Shupe, M. D., Sledd, A., Streffing, J., Svensson, G., and Jung, T.: Nudging allows direct evaluation of coupled climate models with in situ observations: a case study from the MOSAiC expedition, Geosci. Model Dev., 16, 1857–1873, https://doi.org/10.5194/gmd-16-1857-2023, 2023.

Polkova, I., Afargan-Gerstman, H., Domeisen, D. I. V., King, M. P., Ruggieri, P., Athanasiadis, P., Dobrynin, M., Aarnes, O., Kretschmer, M., and Baehr, J.: Predictors and prediction skill for marine cold-air outbreaks over the Barents Sea, Q. J. R. Meteorol. Soc., 147, 2638–2656, https://doi.org/10.1002/qj.4038, 2021.

885 Previdi, M., Smith, K. L., and Polvani, L. M.: Arctic amplification of climate change: a review of underlying mechanisms, Environ. Res. Lett., 16, 093003, https://doi.org/10.1088/1748-9326/ac1c29, 2021.

Rantanen, M., Karpechko, A. Y., Lipponen, A., Nordling, K., Hyvärinen, O., Ruosteenoja, K., Vihma, T., and Laaksonen, A.: The Arctic has warmed nearly four times faster than the globe since 1979, Commun. Earth Environ., 3, 168, https://doi.org/10.1038/s43247-022-00498-3, 2022.

890 Rinke, A., Knudsen, E., Mewes, D., Dorn, W., Handorf, D., Dethloff, K., and Moore, J. C.: Arctic summer sea-ice melt and related atmospheric conditions in coupled regional climate model simulations and observations, J. Geophys. Res., 124, 6027–6039, https://doi.org/10.1029/2018JD030207, 2019.

Ripple, W. J., Wolf, C., Lenton, T. M., Gregg, J. W., Natali, S. M., Duffy, P. B., Rockström, J., and Schellnhuber, H. J.: Many risky feedback loops amplify the need for climate action, One Earth, 6, 86–91, https://doi.org/10.1016/j.oneear.2023.01.004, 2023.
<figure><img src="image_71.png" /></figure>

Roberts, T. J., Dütsch, M., Hole, L. R., and Voss, P. B.: Controlled meteorological (CMET) free balloon profiling of the Arctic atmospheric boundary layer around Spitsbergen compared to ERA-Interim and Arctic System Reanalyses, Atmos. Chem. Phys., 16, 12383–12396, https://doi.org/10.5194/acp-16-12383-2016, 2016.

900 Sato, Y., Miura, H., Yashiro, H., Goto, D., Takemura, T., Tomita, H., and Nakajima, T.: Unrealistically pristine air in the Arctic produced by current global scale models, Sci. Rep., 6, 26561, https://doi.org/10.1038/srep26561, 2016.

Schäfer, M., Wolf, K., Ehrlich, A., Hallbauer, C., Jäkel, E., Jansen, F., Luebke, A. E., Müller, J., Thoböll, J., Röschenthaler, T., Stevens, B., and Wendisch, M.: VELOX – a new thermal infrared imager for airborne remote sensing of cloud and surface properties, Atmos. Meas. Technol., 15, 1491–1509, https://doi.org/10.5194/amt-15-1491-2022, 2022.

905 Schirmacher, I., Kollias, P., Lamer, K., Mech, M., Pfitzenmaier, L., Wendisch, M., and Crewell, S.: Assessing Arctic low-level clouds and precipitation from above – a radar perspective, EGUSphere, pp. 1–30, https://doi.org/10.5194/egusphere-2023-636, [preprint], 2023.

Schirmacher, I., Schnitt, S., Klingebiel, M., Maherndl, N., Kirbus, B., and Crewell, S.: Clouds and precipitation in the initial phase of marine cold air outbreaks as observed by airborne remote sensing, To be submitted to Special Issue of Atmos. Chem. Phys., 2024.

910 Schmale, J., Zieger, P., and Ekman, A. M. L.: Aerosols in current and future Arctic climate, Nat. Clim. Change, 11, 95–105, https://doi.org/10.1038/s41558-020-00969-5, 2021.

Screen, J. A.: An ice-free Arctic: What could it mean for European weather?, Weather, 76, 327–328, https://doi.org/10.1002/wea.4069, 2021.

Sedlar, J., Tjernström, M., Mauritsen, T., Shupe, M. D., Brooks, I. M., Persson, P. O. G., Birch, C. E., Leck, C., Sirevaag, A., and Nicolaus, M.: A transitioning Arctic surface energy budget: the impacts of solar zenith angle, surface albedo and cloud radiative forcing, Clim. Dyn., 37, 1643–1660, https://doi.org/10.1007/s00382-010-0937-5, 2011.

915 Serreze, M. C. and Barry, R. C.: Processes and impacts of Arctic amplification: A research synthesis, Glob. Planet. Change, 77, 85–96, https://doi.org/10.1016/j.gloplacha.2011.03.004, 2011.

Serreze, M. C. and Francis, J. A.: The Arctic Amplification debate, Clim. Change, 76, 241–264, https://doi.org/10.1007/s10584-005-9017-y, 2006.

920 Serreze, M. C. and Meier, W. N.: The Arctic's sea ice cover: trends, variability, predictability, and comparisons to the Antarctic, Annals of the New York Academy of Sciences, 1436, 36–53, https://doi.org/10.1111/nyas.13856, 2019.

Serreze, M. C., Barrett, A. P., Stroeve, J. C., Kindig, D. N., and Holland, M. M.: The emergence of surface-based Arctic amplification, The Cryosphere, 3, 11–19, https://doi.org/10.5194/tc-3-11-2009, 2009.

Shapiro, M. A. and Keyser, D.: Fronts, Jet Streams and the Tropopause, pp. 167–191, American Meteorological Society, Boston, MA, https://doi.org/10.1007/978-1-944970-33-8_10, 1990.

925 Shupe, M. D., Rex, M., Blomquist, B., Persson, P. O. G., Schmale, J., Uttal, T., Althausen, D., Angot, H., Archer, S., Bariteau, L., Beck, I., Bilberry, J., Bussi, S., Buck, C., Boyer, M., Brasseur, Z., Brooks, I. M., Calmer, R., Cassano, J., Castro, V., Chu, D., Costa, D., Cox, C. J., Creamean, J., Crewell, S., Dahlke, S., Damm, E., de Boer, G., Deckelmann, H., Dethloff, K., Dütsch, M., Ebell, K., Ehrlich, A., Ellis, J., Engelmann, R., Fong, A. A., Frey, M. M., Gallagher, M. R., Ganzeveld, L., Gradinger, R., Graeser, J., Greenamyer, V., Griesche, H., Griffiths, S., Hamilton, J., Heinemann, G., Helmig, D., Herber, A., Heuzé, C., Hofer, J., Houchens, T., Howard, D., Inoue, J., Jacobi, H.-W., Jaiser, R., Jokinen, T., Jourdan, O., Jozef, G., King, W., Kirchgaessner, A., Klingebiel, M., Krassovski, M., Krumpen, T., Lampert, A., Landing, W., Laurila, T., Lawrence, D., Lonardi, M., Loose, B., Lüpkes, C., Maahn, M., Macke, A., Maslowski, W., Marsay, C., Maturilli, M., Mech, M., Morris, S., Moser, M., Nicolaus, M., Ortega, P., Osborn, J., Pätzold, F., Perovich, D. K., Petäjä, T., Pilz, C., Pirazzini, R., Posman, K., Powers, H., Pratt, K. A., Preußer, A., Quéléver, L., Radenz, M., Rabe, B., Rinke, A., Sachs, T., Schulz, A., Siebert, H., Silva, T., Solomon, A., Sommerfeld, A., Sreen, G., Stephens, M., Stohl, A., Svensson, G., Uin, J., Viegas, J., Voigt, C., von der Gathen, P.,
<figure><img src="image_72.png" /></figure>

935 Wehner, B., Welker, J. M., Wendisch, M., Werner, M., Xie, Z., and Yue, F.: Overview of the MOSAiC expedition: Atmosphere, Elem. Sci. Anth., 10, https://doi.org/10.1525/elementa.2021.00060, 2022.

Smith, A. J. P., Smith, D. M., Cohen, J., and Jones, M. W.: Arctic warming amplifies climate change and its impacts, Zenodo, https://doi.org/10.5281/zenodo.5596791, 2021.

940 Smith, D. M., Screen, J. A., Deser, C., Cohen, J., Fyfe, J. C., García-Serrano, J., Jung, T., Kattsov, V., Matei, D., Msadek, R., Peings, Y., Sigmond, M., Ukita, J., Yoon, J.-H., and Zhang, X.: The Polar Amplification Model Intercomparison Project (PAMIP) contribution to CMIP6: investigating the causes and consequences of polar amplification, Geosci. Model Dev., 12, 1139–1164, https://doi.org/10.5194/gmd-12-1139-2019, 2019.

945 Smith, D. M., Eade, R., Andrews, M. B., Ayres, H., Clark, A., Chripko, S., Deser, C., Dunstone, N. J., García-Serrano, J., Gastineau, G., Graff, L. S., Hardiman, S. C., He, B., Hermanson, L., Jung, T., Knight, J., Levine, X., Magnusdottir, G., Manzini, E., Matei, D., Mori, M., Msadek, R., Ortega, P., Peings, Y., Scaife, A. A., Screen, J. A., Seabrook, M., Semmler, T., Sigmond, M., Streffing, J., Sun, L., and Walsh, A.: Robust but weak winter atmospheric circulation response to future Arctic sea ice loss, Nat. Commun., 13, 727, https://doi.org/10.1038/s41467-022-28283-y, 2022.

Sobel, A. H., Nilsson, J., and Polvani, L. M.: The Weak Temperature Gradient Approximation and Balanced Tropical Moisture Waves, J. Atmos. Sci., 58, 3650–3665, https://doi.org/10.1175/1520-0469(2001)058<3650:TWTGAA>2.0.CO;2, 2001.

950 Solomon, A., Shupe, M. D., Svensson, G., Barton, N. P., Batrak, Y., Bazile, E., Day, J. J., Doyle, J. D., Frank, H. P., Keeley, S., Remes, T., and Tolstykh, M.: The winter central Arctic surface energy budget: A model evaluation using observations from the MOSAiC campaign, Elem. Sci. Anth., 11, 00104, https://doi.org/10.1525/elementa.2022.00104, 2023.

Sprenger, M. and Wernli, H.: The LAGRANTO Lagrangian analysis tool - version 2.0, Geosci. Model Dev., 8, 2569–2586, https://doi.org/10.5194/gmd-8-2569-2015, 2015.

955 Stapf, J., Ehrlich, A., Jäkel, E., Lüpkes, C., and Wendisch, M.: Reassessment of shortwave surface cloud radiative forcing in the Arctic: consideration of surface-albedo-cloud interactions, Atmos. Chem. Phys., 20, 9895–9914, https://doi.org/10.5194/acp-20-9895-2020, 2020.

Stapf, J., Ehrlich, A., and Wendisch, M.: Influence of Thermodynamic State Changes on Surface Cloud Radiative Forcing in the Arctic: A Comparison of Two Approaches Using Data From AFLUX and SHEBA, J. Geophys. Res. Atmos., 126, e2020JD033589, https://doi.org/10.1029/2020JD033589, 2021.

960 Stevens, B. and Kluft, L.: A colorful look at climate sensitivity, Atmos. Chem. Phys., 23, 14673–14689, https://doi.org/10.5194/acp-23-14673-2023, 2023.

Stevens, B., Ament, F., Bony, S., Crewell, S., Ewald, F., Gross, S., Hansen, A., Hirsch, L., Jacob, M., Kölling, T., Konow, H., Mayer, B., Wendisch, M., Wirth, M., Wolf, K., Bakan, S., Bauer-Pfundstein, B., Brueck, M., Delanoë, J., Ehrlich, A., Farrell, D., Forde, M., Gödde, F., Grob, H., Hagen, M., Jäkel, E., Jansen, F., Klepp, C., Klingebiel, M., Mech, M., Peters, G., Rapp, M., Wing, A. A., and Zinner, T.: A high-altitude long-range aircraft configured as a cloud observatory—the NARVAL expeditions, Bull. Am. Meteorol. Soc., 100, 1061–1077, https://doi.org/10.1175/BAMS-D-18-0198.1, 2019.

Stroeve, J., Holland, M. M., Meier, W., Scambos, T., and Serreze, M.: Arctic sea ice decline: Faster than forecast, Geophys. Res. Lett., 34, https://doi.org/10.1029/2007GL029703, 2007.

Svensson, G., Murto, S., Shupe, M. D., Pithan, F., Magnusson, L., Day, J. J., Doyle, J. D., Renfrew, I. A., Spengler, T., and Vihma, T.: Warm air intrusions reaching the MOSAiC expedition in April 2020—The YOPP targeted observing period (TOP), Elem. Sci. Anth., 11, 00106, https://doi.org/10.1525/elementa.2023.00016, 2023.
<figure><img src="image_73.png" /></figure>

Tan, I. and Storelvmo, T.: Evidence of Strong Contributions From Mixed-Phase Clouds to Arctic Climate Change, Geophys. Res. Lett., 46, 2894–2902, https://doi.org/10.1029/2018GL081871, 2019.

Taylor, P. C., Boeke, R. C., Boisvert, L. N., Feldl, N., Henry, M., Huang, Y., Langen, P. L., Liu, W., Pithan, F., Sejas, S. A., and Tan, I.: Process drivers, inter-model spread, and the path forward: A review of amplified Arctic warming, Front. Earth Sci., 9, 758–761, https://doi.org/10.3389/feart.2021.758361, 2022.

Tjernström, M. and Graversen, R. G.: The vertical structure of the lower Arctic troposphere analysed from observations and the ERA-40 reanalysis, Q. J. R. Meteorol. Soc., 135, 431–443, https://doi.org/10.1002/qj.380, 2009.

Tjernström, M., Zagar, M., Svensson, G., Cassano, J. J., Pfeifer, S., Rinke, A., Wyser, K., Dethloff, K., Jones, C., Semmler, T., and Shaw, M.: Modelling the Arctic Boundary Layer: An Evaluation of Six Arcmip Regional-Scale Models using Data from the Sheba Project, Bound.-Layer Meteorol., 117, 337–381, https://doi.org/10.1007/s10546-004-7954-z, 2005.

Tjernström, M., Shupe, M. D., Brooks, I. M., Achtert, P., Prytherch, J., and Sedlar, J.: Arctic Summer Airmass Transformation, Surface Inversions, and the Surface Energy Budget, J. Clim., 29, 769–789, https://doi.org/10.1175/JCLI-D-18-0216.1, 2019.

Urbanek, B., Groß, S., Schäfler, A., and Wirth, M.: Determining stages of cirrus evolution: a cloud classification scheme, Atmos. Meas. Technol., 10, 1653–1664, https://doi.org/10.5194/amt-10-1653-2017, 2017.

Uttal, T., Curry, J. A., McPhee, M. G., Perovich, D. K., Moritz, R. E., Maslanik, J. A., Guest, P. S., Stern, H., Moore, J. A., Turenne, R., Heiberg, A., Serreze, M. C., Wylie, D. P., Persson, O. G., Paulson, C. A., Halle, C., Morison, J. H., Wheeler, P. A., Makshtas, A., Welch, H., Shupe, M. D., Intrieri, J. M., Stamnes, K., Lindsey, R. W., Pinkel, R., Pegau, W. S., Stanton, T. P., and Grenfeld, T. C.: Surface heat budget of the arctic ocean, Bull. Am. Meteorol. Soc., 83, 255–276, https://doi.org/10.1175/1520-0477(2002)083<0255:SHBOTA>2.3.CO;2, 2002.

Vihma, T., Hartmann, J., and Lüpkes, C.: A case study of an on-ice air flow over the Arctic marginal sea ice zone, Bound.-Layer Meteorol., 107, 189–217, https://doi.org/10.1023/A:1021599601948, 2003.

Volkmer, L., Kölling, T., Zinner, T., and Mayer, B.: Consideration of the cloud motion for aircraft-based stereographically derived cloud geometry and cloud top heights, Atmos. Meas. Technol. Discuss., 2024, 1–13, https://doi.org/10.5194/amt-2024-19, 2024.

von Lerber, A., Mech, M., Rinke, A., Zhang, D., Lauer, M., Radovan, A., Gorodetskaya, I., and Crewell, S.: Evaluating seasonal and regional distribution of snowfall in regional climate model simulations in the Arctic, Atmos. Chem. Phys., 22, 7287–7317, https://doi.org/10.5194/acp-22-7287-2022, 2022.

Walbröl, A., Michaelis, J., Becker, S., Dorff, H., Gorodetskaya, I., Kirbus, B., Lauer, M., Maherndl, N., Maturilli, M., Mayer, J., Müller, H., Rückert, J., Schirmacher, I., Slättberg, N., Röttenbacher, J., Ehrlich, A., Wendisch, M., and Crewell, S.: Environmental conditions during the HALO-(AC)³ campaign in the North Atlantic sector of the Arctic, EGUSphere, https://doi.org/10.5194/egusphere-2023-668, [preprint], 2023.

Weber, A., Kölling, T., Pörtge, V., Baumgartner, A., Rammeloo, C., Zinner, T., and Mayer, B.: Polarization upgrade of specMACS: calibration and characterization of the 2D RGB polarization-resolving cameras, Atmos. Meas. Technol., 17, 1419–1439, https://doi.org/10.5194/amt-17-1419-2024, 2024.

Wendisch, M. and Brenguier, J.-L.: Airborne Measurements for Environmental Research – Methods and Instruments, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, Weinheim, Germany, ISBN: 978-3-527-40996-9, 2013.

Wendisch, M., Macke, A., Ehrlich, A., Lüpkes, C., Mech, M., Chechin, D., Barrientos, C., Bozem, H., Brückner, M., Clemen, H.-C., Crewell, S., Donth, T., Dupuy, R., Ebell, K., Egerer, U., Engelmann, R., Engler, C., Eppers, O., Gehrmann, M., Gong, X., Gottschalk, M., Gourbeyre, C., Griesche, H., Hartmann, J., Hartmann, M., Herber, A., Herrmann, H., Heygster, G., Hoor, P., Jafariserajehlou, S., Jäkel,
<figure><img src="image_74.png" /></figure>

1010 E., Järvinen, E., Jourdan, O., Kästner, U., Kecorius, S., Knudsen, E. M., Köllner, F., Kretzschmar, J., Lelli, L., Leroy, D., Maturilli, M., Mei, L., Mertes, S., Mioche, G., Neuber, R., Nicolaus, M., Nomokonova, T., Notholt, J., Palm, M., van Pinxteren, M., Quaas, J., Richter, P., Ruiz-Donoso, E., Schäfer, M., Schmieder, K., Schnaiter, M., Schneider, J., Schwarzenböck, A., Seifert, P., Shupe, M. D., Siebert, H., Sreen, G., Stapf, J., Stratmann, F., Vogl, T., Welti, A., Wex, H., Wiedensohler, A., Zanatta, M., Zeppenfeld, S., Dethloff, K., and Heinold, B.: The Arctic Cloud Puzzle: Using ACLOUD/PASCAL Multi-Platform Observations to Unravel the Role of Clouds and Aerosol Particles in Arctic Amplification, Bull. Am. Meteorol. Soc., 100 (5), 841–871, https://doi.org/10.1175/BAMS-D-18-0072.1, 2019.

1015 Wendisch, M., Handorf, D., Tegen, I., Neggers, R. A. J., and Sreen, G.: Glimpsing the ins and outs of the Arctic atmospheric cauldron, EOS, 102, https://doi.org/10.1029/2021EO155959, 2021.

1020 Wendisch, M., Brückner, M., Crewell, S., Ehrlich, A., Notholt, J., Lüpkes, C., Macke, A., Burrows, J. P., Rinke, A., Quaas, J., Maturilli, M., Schemann, V., Shupe, M. D., Akansu, E. F., Barrientos-Velasco, C., Bärfuss, K., Blechschmidt, A.-M., Block, K., Bougoudis, I., Bozem, H., Böckmann, C., Bracher, A., Bresson, H., Bretschneider, L., Buschmann, M., Chechin, D. G., Chylik, J., Dahlke, S., Deneke, H., Dethloff, K., Donth, T., Dorn, W., Dupuy, R., Ebell, K., Egerer, U., Engelmann, R., Eppers, O., Gerdes, R., Gierens, R., Gorodetskaya, I. V., Gottschalk, M., Griesche, H., Gryanik, V. M., Handorf, D., Harm-Altstädter, B., Hartmann, J., Hartmann, M., Heinold, B., Herber, A., Herrmann, H., Heygster, G., Höschel, I., Hofmann, Z., Hälemann, J., Hünerbein, A., Jafarerajehlou, S., Jäkel, E., Jacobi, C., Janout, M., Jansen, F., Jourdan, O., Jurányi, Z., Kalesse-Los, H., Kanzow, T., Käthner, R., Kliesch, L. L., Klingebiel, M., Knudsen, E. M., Kovács, T., Körtke, W., Krampe, D., Kretzschmar, J., Kreyling, D., Kulla, B., Kunkel, D., Lampert, A., Lauer, M., Lelli, L., von Lerber, A., Linke, O., Löhnert, U., Lonardi, M., Losa, S. N., Losch, M., Maahn, M., Mech, M., Mei, L., Mertes, S., Metzner, E., Mewes, D., Michaelis, J., Mioche, G., Moser, M., Nakoudi, K., Neggers, R., Neuber, R., Nomokonova, T., Oelker, J., Papakonstantinou-Presvelou, I., Pätzold, F., Pefanis, V., Pohl, C., van Pinxteren, M., Radovan, A., Rhein, M., Rex, M., Richter, A., Risse, N., Ritter, C., Rostosky, P., Rozanov, V. V., Ruiz-Donoso, E., Saavedra-Garfias, P., Salzmann, M., Schacht, J., Schäfer, M., Schneider, J., Schnierstein, N., Seifert, P., Seo, S., Siebert, H., Soppa, M. A., Sreen, G., Stachlewska, I. S., Stapf, J., Stratmann, F., Tegen, I., Viceto, C., Voigt, C., Vountas, M., Walbröl, A., Walter, M., Wehner, B., Wex, H., Willmes, S., Zanatta, M., and Zeppenfeld, S.: Atmospheric and Surface Processes, and Feedback Mechanisms Determining Arctic Amplification: A Review of First Results and Prospects of the (AC)³ Project, Bull. Am. Meteorol. Soc., 104, E208–E242, https://doi.org/10.1175/bams-d-21-0218.1, 2023a.

1030 Wendisch, M., Stapf, J., Becker, S., Ehrlich, A., Jäkel, E., Klingebiel, M., Lüpkes, C., Schäfer, M., and Shupe, M. D.: Effects of variable ice-ocean surface properties and air mass transformation on the Arctic radiative energy budget, Atmos. Chem. Phys., 23, 9647–9667, https://doi.org/10.5194/acp-23-9647-2023, 2023b.

1035 Wesche, C., Steinhage, D., and Nixdorf, U.: Polar aircraft Polar5 and Polar6 operated by the Alfred Wegener Institute, J. Large-Scale Res. Facilities, 2, https://doi.org/10.17815/jlsrf-2-153, 2016.

1040 Wirth, M., Fix, A., Mahnke, P., Schwarzer, H., Schrandt, F., and Ehret, G.: The airborne multi-wavelength water vapor differential absorption lidar WALES: system design and performance, Appl. Phys. B, 96, 201–213, https://doi.org/10.1007/s00340-009-3365-7, 2009.

1045 You, C., Tjernström, M., and Devasthale, A.: Warm-air advection over melting sea-ice: A Lagrangian case study, Bound.-Layer Meteorol., 179, 99–116, https://doi.org/10.1007/s10546-020-00590-1, 2021a.

1050 You, C., Tjernström, M., and Devasthale, A.: Eulerian and Lagrangian views of warm and moist air intrusions into summer Arctic, Atmos. Res., 256, 105–116, https://doi.org/10.1016/j.atmosres.2021.105586, 2021b.

1055 Yuval, J. and Kaspi, Y.: Eddy Activity Response to Global Warming-Like Temperature Changes, J. Climate, 33, 1381–1404, https://doi.org/10.1175/JCLI-D-19-0190.1, 2020.