AS2.13

The polar-night stable boundary layer, observed during MOSAiC, gives rise to extreme stabilities not found in typical mid-latitude SBLs. In a steady or near-steady synoptic environment, the boundary layer even attains a quasi-steady state for several hours, i.e., boundary-layer processes and profiles are equilibrated.

Such lab-like conditions of extreme stability remain a major challenge for turbulence modelling across scales. Previously, we have demonstrated that such situations can be qualitatively modelled by carefully-designed large-eddy simulations, using wintertime observations from Dome C, Antarctica (Van der Linden et al., 2020: J. Atmos. Sci., 77, 3343–3360). However, a quantitative comparison remained elusive due to a lack of turbulent flux observations at Dome C.

The high-frequency observations of wind, temperature, radiation and turbulent fluxes during the MOSAiC-campaign do allow us to quantitatively disentangle different mixing processes facilitating a one-to-one comparison between observation and simulation. Here, we will show the results of our simulations and discuss the specific challenges of performing large-eddy simulations of such harsh, but fascinating conditions.

Our results show that atmospheric radiation, which is usually neglected in large-eddy simulations of very stable cases, is a key thermal process in the evolution of the boundary layer due to the large thermal inversion near the surface. Radiative exchanges result in a deeper boundary-layer, which is in line with the observed boundary-layer height, as compared to the simulations without radiative exchanges. Although a larger boundary-layer depth is obtained, discrepancies between the observed more exponentially-shaped temperature profile and the simulated temperature profile still persist.

This is suspected to be caused by a wrong magnitude of the turbulent mixing near the surface, related to the interplay of roughness lengths, Monin–Obukhov similarity theory and the extreme surface-layer stratification. Initial tests, however, remain inconclusive on this complex interplay.

How to cite: van der Linden, S. and Ansorge, C.: The coldest days of MOSAiC – an LES study, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9646, https://doi.org/10.5194/egusphere-egu22-9646, 2022.

Our physical understanding of boundary-layer processes, particularly under the long-lived stable conditions of the polar night, is limited. In these weak-wind stable boundary layers, the exchange of energy, momentum, and matter, can be dominated by submeso-scale motions on scales larger than those of classical turbulence, which have been found to violate assumptions of local similarity concepts.

We investigated (1) whether the magnitude of non-local flux contributions, possibly arising from submeso-scale motions in concert with terrain, is systematically connected to certain boundary-layer states and flow directions, and (2) to what extent non-local forcings impact  turbulent heat fluxes as well as the local surface energy balance including advection.

Data were collected during the NYTEFOX (NY Alesund TurbulencE Fiber-Optic eXperiment) field campaign 2020 at the scientific AWIPEV Arctic station in Ny-Ålesund, Svalbard at the end of polar night. Non-local influences were detected and quantified by comparing the measured sensible heat flux from sonic anemometry to the flux modeled from local first-order closure using temperature profile observations from fiber-optic distributed sensing. The spatial structure of the time-variant flow and of the horizontal advection were computed from a unique set of observations from a large horizontal fiber-optic distributed sensing array spanning hundreds of meters.

First results indicate an influence of cloud cover with stronger non-local flux contributions during clear skies causing an increased radiative cooling of the surface. Additionally, such contributions were generated and/or guided by the heterogeneous terrain in the source area of the incoming flow. Steep mountain slopes caused very cold katabatic currents and, hence, promoted vertical decoupling. This resulted in stronger non-local impact on the local fluxes, causing large disagreement between modeled and measured sensible heat fluxes. The conditions featuring large non-local flux contributions were also associated with large magnitudes of horizontal advection of sensible heat.

Advection, as well as conditions that promote strong surface-based inversions, appear to cause an increased violation of the assumption of local similarity in the Arctic weak-wind stable boundary layer.

How to cite: Huss, J.-M. and Thomas, C.: Local surface heterogeneity and terrain drive disagreement between measured and modeled fluxes in the cloud-free Arctic stable boundary layer, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6165, https://doi.org/10.5194/egusphere-egu22-6165, 2022.

Stable water isotopes are a tracer of hydrological processes and a paleoclimate proxy from ice core records. The interpretation of the latter relies on fractionation processes throughout the hydrological cycle, from the evaporation over the ocean, during each precipitation event, and during post-deposition processes, in particular due to the exchanges between the snow and the moisture in the atmosphere. Thanks to new developments in infrared spectroscopy, it is now possible to monitor not only the snow isotopic composition but also the vapour continuously, and thus document exchanges between the snow and the vapour. On the East Antarctic Plateau, records of water vapour isotopic composition in Kohnen and Dome C during summer have revealed significant diurnal variability which can be used to address the exchange between surface snow and atmospheric water vapour as well as the stability of the atmospheric boundary layer. 

In this study, we present the first vapour monitoring on a transect across East Antarctica for a period of 3 months from November 2019 to February 2020 during the EAIIST traverse, covering more than 3600 km. In parallel, we also monitored the vapour isotopic composition at two stations: Dumont D’Urville (DDU), the starting point, and Dome C, half way through. Efforts on the calibration on each monitoring station, as well as cross-calibration of the different instruments offer a unique opportunity to compare both the spatial and temporal (diurnal variability or at the scale of several days) gradients of humidity, temperature and water vapour isotopic composition in East Antarctica during the summer season. 

With the use of the Modele Atmospherique Régional (MAR), we compare the variability measured in water vapour isotopic composition, temperature and humidity with the different systems (fixed or mobile location). Although further comparisons with the surface snow isotopic composition are required to quantify the impact of the snow-atmosphere exchanges on the local surface mass balance, these three simultaneous measurements of the vapour isotopic composition show the potential of using water stables isotopes to evaluate hydrological processes in East Antarctica.  

How to cite: Casado, M., Leroy-Dos Santos, C., Fourré, E., Favier, V., Agosta, C., Arnaud, L., Prié, F., Akers, P. D., Janssen, L., Kittel, C., Savarino, J., and Landais, A.: Water vapor isotopic signature along the EAIIST traverse, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13362, https://doi.org/10.5194/egusphere-egu22-13362, 2022.

Understanding how and when aerosols are transported to the Arctic is crucial to evaluating the contribution of remote aerosol emissions on Arctic Amplification. Climate models show large discrepancies in long-range aerosol transport, significantly impacting estimates of local aerosol-driven forcing at the Arctic. Long-range aerosol transport is intimately linked to moisture transport. Aerosol transport is generally high during periods of low precipitation (i.e. low wet scavenging) and strong temperature inversions (with low vertical mixing). But studies have shown the importance of intense moisture intrusion events for moisture transport, with almost 30% of total annual moisture being transported during these intervals. These events are associated with warm, cloudy moist air transport that leads to strong downwelling longwave radiation and warm surface temperatures at the Arctic. Notably, the blocking patterns established during these events also give rise to favourable conditions for long-range aerosol transport from the mid-latitudes. Here, we use two reanalysis datasets – Copernicus Atmospheric Monitoring Service (CAMS) and Modern-Era Retrospective analysis for Research and Application (MERRA-2) – to investigate moisture and aerosol transport into the Arctic for a 20-year period. We present a comparison of the relative importance of intrusion events to the total annual moisture transport into to the Arctic during intrusion events for the two different datasets and whether aerosols correlate with these moisture intrusions. This comparison can advance our understanding of aerosol transport to the Arctic and improve the representation of seasonal cycle of aerosols in climate models.  Finally, we investigate whether changes in aerosol transport during these events could have led to significant changes in local forcing at the Arctic.

How to cite: Krishnan, S., Sand, M., Stjern, C., Ekman, A., and Böö, S.: Aerosol transport to the Arctic during moisture intrusion events, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9253, https://doi.org/10.5194/egusphere-egu22-9253, 2022.

The aerosol budget of the Arctic plays a key role in determining the behaviour of clouds, which are important for the surface energy balance and thus for the region’s climate. A key question is the extent to which cloud condensation nuclei in the high Arctic summertime boundary layer are controlled by local emission and formation processes or by transport from outside. Each of these sources is likely to respond differently to future changes in ice cover. Here we use a global model and observations from ship and aircraft field campaigns to understand the source of high-Arctic aerosol in late summer. We find that particles formed remotely are the dominant source of boundary layer Aitken mode particles during the sea ice melt period up to the end of August. Such a remote particle source, mostly entrained from the free troposphere, explains the remarkably stable nucleation mode concentrations of around 100 cm-3. This source from outside the high Arctic declines as photochemical rates decrease towards the end of summer, and is largely replaced by local new particle formation driven by iodic acid associated with freeze up. Such a local source is consistent with strong fluctuations in nucleation mode concentrations that occur in September. Our results suggest a high Arctic aerosol regime shift in late summer, and only after this shift do cloud condensation nuclei become sensitive to local aerosol processes.

How to cite: Price, R., Baccarini, A., Schmale, J., Zieger, P., Brooks, I., Field, P., and Carslaw, K.: Remote new particle formation dominates the nucleation and Aitken aerosol modes in the central Arctic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13169, https://doi.org/10.5194/egusphere-egu22-13169, 2022.

Marine organic aerosol is a major contributor to cloud condensation nuclei and ice nucleating particles over pristine open-ocean and coastal regions and thus has an important impact on radiation, precipitation, and atmospheric dynamics. In the Arctic, the summer-time loss of sea ice together with the rapid ice retreat are key factors for potentially increased marine aerosol emissions. In our planned studies with the aerosol-climate model ICON-HAM, we want to investigate the influence of primary marine organic aerosol on the Arctic climate and its rapid warming. Currently, the model development focuses on the implementation of a detailed, species-resolved ocean emission scheme. Here, we present the first results of an offline version following Burrows et al. (2014). The new emission scheme has been applied in ICON-HAM. This allows for including the marine organic aerosol’s life cycle and interactions with mixed–phase Arctic clouds, focusing on potential ice-active species.

How to cite: Leon, A., Heinold, B., and van Pinxteren, M.: Modelling marine organic aerosol and its impact on clouds in the Arctic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8559, https://doi.org/10.5194/egusphere-egu22-8559, 2022.

Aerosols represent a fundamental component of the atmosphere, and their behaviour in the Arctic surface layer determines deposition on snow or ice surfaces. Deposition processes lead to a decrease in the snow albedo enhancing its melting, which has major impacts on climate change in polar regions, particularly in the Arctic. Many aerosols properties have been investigated in the Arctic region, with regards to chemical composition (Quinn et al, 2009; Köllner et al, 2021), total number and mass concentrations (Croft et al, 2016), optical properties (Ferrero et al, 2019), their ability to act as cloud condensation nuclei (Bulatovic et al., 2021), their number and size distribution (Lupi et al., 2016). Relatively few cases exist of aerosol deposition measurements on snow or iced surfaces, especially by eddy‐correlation (EC) method. The first example was reported by Duann et al. (1988), who analysed deposition of particles in two size ranges (0.15–0.30 and 0.5–1.0 mm) using the EC in a snow covered field in central Pennsylvania. Successively, the deposition velocity of particles larger than 10 nm in diameter over an iced surface in the Arctic (Nilsson and Rannik, 2001) and Antarctic (Contini et al, 2010) was measured using the EC method. The aim of the present work is to analyse the deposition velocity of atmospheric particles on snow surfaces at Ny-Ålesund (Svalbard Islands) in relation to local micrometeorological conditions. This work reports an analysis of the concentration, size distribution, and size segregated deposition velocity of atmospheric particles. Measurements were performed using the eddy‐correlation method at the research laboratory of Gruvebadet from March to August 2021. The measurement system was based on a condensation particle counter (CPC) able to measure particles down to 5 nm in diameter with a 50% efficiency and an Optical Particle Counter (OPC) for evaluating particle size fluxes in the accumulation mode (0.25 < dp < 0.58 μm) and coarse mode (0.65 < dp < 3 μm). The average number concentration was 595 cm⁻³, 25 cm⁻³ and 0.7 cm⁻³ for ultrafine, accumulation and coarse particles mode. Higher concentrations were observed at low wind velocities. Results gave an average deposition velocity of 3.66 mm/s for ultrafine particles. Deposition velocity was 18.89 mm/s and 52.83 mm/s for accumulation and coarse particles, respectively. Deposition increased with friction velocity. We present an overview of the results discussed in terms of average concentration, deposition velocity, and the relationship between deposition, friction velocity, and atmospheric stability.

Bulatovic et al., Atmospheric Chemistry and Physics, 21, 3871-3897, 2021
Contini et al., Journal of Geophysical Research, 115, 2010
Croft et al., Atmospheric Chemistry and Physics, 16, 3665-3682, 2016
Duann, B., Journal of Applied Meteorology, 27, 642-652, 1988
Ferrero et al., Science of the Total Environment 686, 452-467, 2019
Köllner et al., Atmospheric Chemistry and Physics, 21, 6509-6539, 2021
Lupi et al., Rendiconti Lincei – Scienze Fisiche e Naturali, ISSN 2037-4631, 27, 2016
Nilsson, E.D., and U. Rannik, Journal of Geophysical Research 106(D23), 32125-32137, 2001
Quinn et al., Atmos. Chem. Phys., 9, 8883-8888, 2009

How to cite: Donateo, A., Pappaccogli, G., Mazzola, M., Decesari, S., and Famulari, D.: Characterization of size-segregated turbulent fluxes and deposition velocity by eddy correlation method in an Arctic site, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1968, https://doi.org/10.5194/egusphere-egu22-1968, 2022.

The British Antarctic Survey (BAS) operates one of the most remote, advanced, and scientifically important research stations on the Antarctic continent – Halley. Located on the floating Brunt ice shelf, the station has provided meteorological and atmospheric observations since it was established in 1956. However, in the face of glaciological uncertainty, Halley Research Station had to close for the first time in its history during winter 2017. To overcome the subsequent data loss from the unmanned research station, engineering and science teams at BAS began automating the station.

In 2018-19, the Halley automation project began with scientific equipment adapted and the installation of an innovative micro-turbine electrical generator. Science experiments ran uninterrupted throughout the nine-month winter period, with the station preserving core science data streams such as Meteorology and Ozone Monitoring, Tropospheric Chemistry and Climate, and Space Weather and Upper Atmospheric Observations. The system proved its ability to withstand the Antarctic environment during the 2019 winter; unaffected by ambient temperatures as low as -55˚C and winds gusting up to 70 knots.

Work is ongoing to automate and reinstate the long-term atmospheric monitoring experiments at Halley. In December 2021, a new automated CO₂ and CH₄ analyser was installed in Halley’s Clean Air Sector (CAS) laboratory which will run continuously over the coming Antarctic winter. Halley’s coastal location provides an ideal platform to explore air-sea CO₂ exchange in the Southern Ocean region. The Southern Ocean is a globally important carbon sink, estimated to account for ~75% of global ocean CO₂ uptake but a sparsity of observations in the region has contributed to uncertainty around the inter-annual and seasonal nature of the Southern Ocean sink.

CO₂ mixing ratios have been measured at Halley at high temporal resolution since 2013. Before the installation of the new autonomous system at Halley, measurements were relocated to the German coastal Antarctic research station, Neumayer, at the end of 2017. Both the Halley and Neumayer records show short-term variability in CO₂ mixing ratios during the summer, with up to ~0.5 ppb decreases in CO₂ over the course of a day, about 1/6 of the average annual growth rate. Trajectory analysis suggests that these decreases in mixing ratio correspond to periods where the air sampled has spent time over the Southern Ocean, suggesting CO₂ uptake has occurred. This work will explore the possible drivers for the short-term variability in CO₂ mixing ratios. An overview of the automation work carried out so far at Halley and plans for future seasons will also be presented.

How to cite: Squires, F., Barningham, T., Jones, A., Rose, M., France, J., Weller, R., and Ort, L.: Short-term variability in atmospheric carbon dioxide as observed from coastal Antarctica and an introduction to the Halley Autonomous Long-term Observational Science (HALOS) Platform, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6621, https://doi.org/10.5194/egusphere-egu22-6621, 2022.