CR7.2

Recent Greenland Ice Sheet (GrIS) surface mass loss has been attributed to the expansion of the bare ice area following the upward migration of the snowline along with persistent blocking systems. Given the temporal fluctuations and spatial heterogeneity of the ablation zone, the local impacts of atmospheric drivers on the GrIS surface energy and mass balance at different elevations and under various atmospheric circulation patterns remain poorly known.

Based on the 1959-2020 period, we present a new indicator of the North Atlantic influence over Greenland (NAG) as the combination of the North Atlantic Oscillation Index (NAO), the Greenland Blocking Index (GBI) and the vertically integrated water vapor over the GrIS. We explore the NAG monthly frequency and the inter-annual evolution along with large-scale spatial anomalies. With the support of a high-resolution regional climate model (RACMO2.3p2), we investigate the influence of spatio-temporal NAG fluctuations on atmospheric drivers, surface energy and mass balance fluxes, that triggered the expansion of the ablation zone to higher elevations. Finally, we assess NAG performance by comparing its results with NAO and GBI alone.

How to cite: Silva, T., Abermann, J., Noël, B., Shahi, S., van der Schot, J., and Schöner, W.: Atmospheric drivers of Greenland ice sheet surface energy and mass balance changes as a function of elevation and circulation patterns, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11895, https://doi.org/10.5194/egusphere-egu22-11895, 2022.

Cyclone events in the Arctic strongly affect both atmospheric variables, such as wind, air temperature and clouds, and surface variables, including sea ice concentration (SIC) and turbulent heat fluxes. However, despite the progress via recent statistical studies, the overall impact of cyclones on Arctic weather, sea ice, and feedback processes between them is not quantitatively well known.

In this study we built up on previous publications and present further details on cyclone impacts on Arctic sea ice in winter by covering a wider range of timescales than before and evaluating our results separately for three different marginal seas of the Arctic Ocean. Hereby we make use of the ERA5 reanalysis and a storm tracking algorithm to analyze the temporal evolution of SIC up to two weeks around the occurrence of each cyclone and compare it with a non-cyclone reference state.

The results show an initial decrease in SIC associated with the occurrence of a cyclone for the Barents and Kara Seas, which is balanced by an increase during the following days. On the contrary, in the Greenland Sea SIC remains lower after a cyclone event for the whole analyzed time period. For all the marginal seas considered, the impact of cyclones on sea ice is intensified, if SIC at a grid cell is low and if the intensity of a cyclone is high. Ongoing work consists of providing more details about the mechanisms responsible for the identified regional differences in cyclone influence on sea ice.

How to cite: Aue, L., Akperov, M., Uotila, P., Vihma, T., and Rinke, A.: Regional differences in cyclone impacts on Arctic sea ice concentration during winter, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8386, https://doi.org/10.5194/egusphere-egu22-8386, 2022.

Rapid changes in the sea ice cover are commonly attributed to periods of strong winds, which in turn are often associated with cyclones and their fronts. In addition to geographically redistributing sea ice, and thereby potentially increasing its export from the Arctic, cyclones also transport moist warm air masses into the Arctic which can lead to local sea ice melt while the cyclone’s cold sector might lead to freezing and sea ice formation. Furthermore, cold air outbreaks associated with the withdrawal of cold air masses over the open ocean usually lead to sea-ice formation. The relative contribution of these competing effects of weather events on the sea ice is so far poorly understood.

We climatologically assess these competing effects of cyclones on sea ice using detected cyclones, fronts, and cold-air outbreaks in the coupled ECMWF CERA-SAT reanalyses. We then decompose the climatological sea-ice increases and decreases during the different seasons into the components that occur in the vicinity or at larger distance from the different weather events. Preliminary results indicate that the amplitude of both positive and negative sea ice changes increases around cyclones, with an overall net effect of reducing sea-ice concentration during most seasons. Thus, the effect of the wind and warm intrusions within cyclones dominates over the effect of the cyclone’s cold sector. In contrast, cold-air outbreaks are associated with sea-ice growth at any time of the year, but exhibit a clear seasonality in their frequency of occurrence.

How to cite: Weyland, F., Spensberger, C., and Spengler, T.: Climatology of sea ice changes attributed to cyclones, fronts, and cold-air outbreaks, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8466, https://doi.org/10.5194/egusphere-egu22-8466, 2022.

This study investigates in a synthetic way to what extent saw-tooth flight patterns from long-range research aircrafts can close the moisture budget of arctic atmospheric rivers (ARs). Such ARs dominate the moisture transport into the Arctic. The analysis of the moisture budget in AR corridors is key to understand the spatiotemporal AR evolution, resulting air mass transformations along their pathway and precipitation efficiency of ARs. However, the determination of moisture budget components in arctic ARs is challenging due to sparse observations. Dedicated research flight campaigns require the quantification of divergence of integral water vapour transport (IVT) using dropsondes along AR cross sections and remote sensing capturing internal water vapour load and precipitation rate. However, limited number of dropsondes and curtain-restricted remote sensing may deteriorate the AR moisture budget. Uncertainties in airborne representation of AR moisture components have to be assessed. We consider seven arctic ARs from spring season of last decade. They cover pathways over the North Atlantic and Siberia and a broad range of AR conditions representative for the Arctic. To assess airborne budget closure capabilities, we include outputs from the new C3S Arctic Regional Reanalysis (CARRA) and simulations from an adapted ICON model configuration. Both have a horizontal resolution of around 2.5 km and deliver reasonable AR representation with high spatial variability in moisture budget components. By generating synthetic flights and mirroring airborne observations (e.g. dropsondes) in both gridded datasets, we identify major sources of error that arise in the airborne quantification of IVT variability. We determine the representativeness of total precipitation and hydrometeor content derived from diagonal legs for entire AR sectors. For all ARs, levels where specific humidity and wind speed contribute most to IVT are located below 1500 m. Along horizontal AR transects, maximum IVT values and highest lateral IVT variability are located around low-level jets. Frequent soundings near the low-level jet are fundamental to lower uncertainties in moisture flux convergence that dominate against other budget terms. In CARRA, having less than six soundings within the AR cross-section causes biases of total IVT by more than 10 %. Samples along diagonal flight legs through AR sectors can reproduce mean internal precipitation rate, whereas the statistical distribution of hydrometeor contents for the entire sector differs due to the complex cold-front composition near the AR. Evaporation shows minor budget contributions in arctic ARs. While moisture convergence uncertainties are highest close to the AR centre, uncertainty of precipitation rate increases in the AR outflow region. Moreover, we give first insights on very preliminary observations from the HALO-(AC)³ flight campaign in March and April, 2022.

How to cite: Dorff, H., Konow, H., Schemann, V., and Ament, F.: Moisture Budget Closure of Arctic Atmospheric Rivers from Saw-Tooth Flight Pattern – A Feasibility Study in High-Resolution Model Data, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13313, https://doi.org/10.5194/egusphere-egu22-13313, 2022.

Authors:

Michelle L. Maclennan, Jan T. M. Lenaerts, Christine A. Shields, Andrew O. Hoffman, Nander Wever, Megan Thompson-Munson, Erin C. Pettit, Theodore A. Scambos, and Jonathan D. Wille.

While Antarctic Ice Sheet (AIS) mass loss is dominated by accelerated ice discharge from the West Antarctic Ice Sheet (WAIS) due to ocean-induced basal melting, surface mass balance (SMB) processes return mass to the WAIS through snowfall. On Thwaites Glacier (TG) in West Antarctica, snowfall is the primary driver for SMB (125 ± 16 Gt snowfall per year), and extreme snowfall events contribute more than 60% of the total snowfall over TG ice shelf, and 30-50% of the total snowfall over grounded TG. Many of these extreme snowfall events are associated with the landfall of atmospheric rivers (ARs). ARs are long, narrow bands of warm and moist air that contribute intense precipitation and surface melting on the AIS, meaning they contribute both positively and negatively to the SMB. Here, we use an Antarctic-specific AR detection tool combined with MERRA-2 and ERA5 reanalyses to develop a climatology of AR events that made landfall over TG and the WAIS from 1980-2020, including their frequency and duration. We quantify the snowfall and surface melt attributed to AR events to determine their impacts on WAIS SMB. Using two case studies of AR events in December 1999 and February 2020, we illustrate the spatial patterns in snowfall and surface melt associated with AR landfall. We then compare the seasonal and spatial patterns in AR-attributed snowfall to the climatology of all snowfall over the WAIS. Finally, we highlight the interannual and decadal variability of West Antarctic AR events and their relationships to large-scale modes of atmospheric variability. Our results enable us to quantify the past impacts of ARs on WAIS SMB and characterize their interannual variability and trends, enabling a better assessment of future AR-driven changes in SMB.

How to cite: Maclennan, M. and Lenaerts, J.: Climatology of West Antarctic Atmospheric Rivers and their Impacts on Surface Mass Balance, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13315, https://doi.org/10.5194/egusphere-egu22-13315, 2022.

Snowfall in Antarctica is the main input to ice sheet mass balance, which is heavily influenced by the frequency and intensity of maritime moisture intrusions from lower latitudes. The most intense moisture incursions often occur as narrow corridors of enhanced vapor transport, called atmospheric rivers (ARs). However, the fate of ARs depends on the state of the coastal boundary layer. For instance, katabatic or foehn winds can lead to a subsaturated boundary layer, which can cause total snowfall sublimation. In this study, we use recent data collected during the Precipitation over Land And The Southern Ocean (PLATO) campaign to investigate how the synoptic evolution and the local orography influenced the sublimation of snowfall during an AR event (08 – 10 January 2019) at Davis, East Antarctica. The dataset includes scanning polarimetric and vertically pointing Doppler radar, radiosounding, and Raman lidar measurements. We also make use of simulations from the Weather Research and Forecasting (WRF) model. Our analysis revealed that orographic gravity waves (OGWs), generated by a north-easterly flow impinging on the ice ridge upstream of Davis, were responsible for snowfall sublimation through a foehn effect. Despite the strong meridional moisture advection associated with the AR during this event, almost no precipitation reached the ground at Davis. We found that the direction of the synoptic flow with respect to the orography determined the intensity of OGWs over Davis, which in turn directly influenced the snowfall microphysics. We hypothesize that turbulence induced by the OGWs likely enhanced the aggregation process, as identified thanks to dual-polarization and dual-frequency radar observations. This study suggests that despite the intense AR, the snowfall distribution was determined by local processes tied to the orography. It also stresses the importance of studying local effects when interpreting the impact of ARs on the Antarctic surface masse balance. Finally, the mechanisms found in this case study could contribute to the extremely dry climate of the Vestfold Hills, one of the main Antarctic oases.

How to cite: Gehring, J., Vignon, E., Billault--Roux, A.-C., Ferrone, A., Protat, A., Alexander, S. P., and Berne, A.: Orographic Flow Influence on Precipitation During an Atmospheric River Event at Davis, Antarctica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4757, https://doi.org/10.5194/egusphere-egu22-4757, 2022.

Atmospheric rivers (ARs) that reach the Antarctic Ice Sheet (AIS) transport anomalous moisture from lower latitudes and can impact the AIS via extreme precipitation and increased downward longwave radiation. ARs contribute significantly to the interannual variability of precipitation over the AIS and thus are likely to play a key role in understanding future changes in the surface mass balance of the AIS. While ARs impact the entire coastal AIS, coastal Dronning Maud Land (DML) is one of four East Antarctic maxima in AR frequency. Along with the high frequency of ARs, the variability of large-scale flow patterns associated with ARs around DML motivates further investigation of synoptic regimes favoring ARs in this region.

This study utilizes a self-organizing map (SOM) to identify synoptic-scale regimes associated with landfalling ARs in and near DML. The catalogue of ARs used in this research is output from a detection algorithm developed specifically for Antarctic ARs, and AR landfalls are identified at timesteps in which an AR overlaps with the AIS between 30°W and 30°E. To determine synoptic regimes conducive to AR landfall, sea level pressure anomalies between 60°W and 60°E from MERRA-2 at the time of AR landfalls are used to train a 16 node SOM. Analysis of precipitation attributable to each SOM node reveals three out of the 16 synoptic regimes are responsible for 28% of the AR precipitation despite representing only 24% of the AR timesteps. Subsequent analysis of this SOM will provide insight into the synoptic drivers and thermodynamic characteristics of the synoptic regimes conducive to the most impactful ARs in the region.

How to cite: Baiman, R., Winters, A., and Lenaerts, J.: Synoptic Drivers of Landfalling Atmospheric Rivers Near Dronning Maud Land, Antarctica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13314, https://doi.org/10.5194/egusphere-egu22-13314, 2022.

Measurements of precipitation in inland Antarctica are scarce, with estimates often derived by indirect means. This scarcity contrasts with the importance of snowfall, which constitutes, together with water vapor deposition, the main water mass input to the Antarctic ice sheet.

During the austral summer 2019-2020, a transect of three vertically-pointing K-band Doppler radars (MRR-PRO) was deployed across the Sør Rondane Mountains, directly south of Princess Elisabeth Antarctica (PEA). The instruments have been placed at different stages of the interaction between the typical flow of the precipitation systems and the orography. A vertically-pointing W-band Doppler cloud radar was also deployed at the base.

Using the data collected by these four radars, alongside information derived from the ERA5 reanalysis and a set of high-resolution WRF simulations covering the previous three years, we investigated the behavior of precipitation across the transect.

A significant difference in the proportion of virga and precipitation has been observed between the three locations. One of the three MRR-PRO was deployed in a valley, connecting the plateau to the lower plains, at the lowest elevation among the radars in the transect. At this location we observed the highest amount of virga. This behavior is consistent with the presence of a thick dry layer, whose height has been estimated to approximately 1.2 km above the level of PEA. Its existence was noticed in both the reanalysis and the simulations, and the reflectivity factor recorded by the cloud profiling radar decreases with height for most of the layer.

The other two MRR-PRO were deployed at higher altitudes, and both of them recorded a lower fraction of virga. We hypothetize that the higher elevation implies a shorter time spent by precipitating particles in the dry layer, limiting the sublimation of hydrometeors. However, despite being at a slightly lower elevation than the MRR-PRO on the plateau, the MRR-PRO installed amid the mountains recorded precipitation reaching the ground for a higher amount of time steps. This may be caused by the localized precipitation systems frequently observed near the top of the mountains south of PEA.

This study shows that complex terrain in the vicinity of PEA increases the variability in precipitation occurrence, depending on the relative position with respect to the incoming flow and to the dry katabatic layer. This variability questions the representativity of measurements collected at a few stations in the mountainous regions of Antarctica.

How to cite: Ferrone, A. and Berne, A.: Summer snowfall in the Sør Rondane Mountains, Antarctica: characterization using a transect of K-band Doppler profilers, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5553, https://doi.org/10.5194/egusphere-egu22-5553, 2022.

Surface winds in Antarctica are amongst the strongest and most persistent winds on Earth. They play a key role in defining the surface climate.
While new proxys are being developed in order to understand their past evolution, it is a crucial to understand the processes controlling their temporal variability. 

Here, we investigate the drivers of surface winds variability in East Antarctica at present-day. To do so, we separate the wind-speed temporal variations from daily outputs of the regional atmospheric model MAR at 35 km resolution into different terms of the dynamic equations.
 Our study focuses on a transect running through Adelie Land, where numerous meteorological measurements are being conducted.  
 
We identify the combination of terms that correlates best in winter to the wind speed in this region.
On the Antarctic plateau, wind speed is controlled by the balance between large-scale pressure gradient acceleration and turbulence.
At mid-slope, the katabatic term is the greatest but does not correlate with wind-speed. One of the reason that explains this result is that increasing positive katabatic forcing is counteracted by increasing turbulence (negative term, deceleration effect). Consequently, the combination of the turbulence and katabatic terms correlates slightly better to wind-speed intensity.

At the coast, wind-speed intensity mainly results from the katabatic and thermal wind terms. 

As a conclusion, the study of a smaller number of contribution terms in the budget equation will help evaluating the drivers of past and future evolution of wind speed in this region.

How to cite: Davrinche, C., Agosta, C., Amory, C., Kittel, C., and Orsi, A.: Drivers of surface winds variability in Antarctica, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9082, https://doi.org/10.5194/egusphere-egu22-9082, 2022.

Modelling the surface mass balance of Antarctica and snow and ice surfaces in general is challenging, yet it is important for making reliable projections of sea level rise. One of the terms with the largest uncertainties is sublimation (and vapor deposition) of drifting and blowing snow. Large-scale atmospheric models strongly simplify or completely neglect the underlying physical processes. In particular, they do not resolve the vertical profiles of particle concentration and sublimation in the saltation layer, corresponding roughly to the lowest 10 cm of the atmosphere. However, small-scale studies based on large-eddy simulations (LES) demonstrate that most of the sublimation of drifting and blowing snow can take place in the saltation layer, at least for shallow layers of drifting snow. As these events occur very frequently, current large-scale models may strongly underestimate snow sublimation. Even in deep blowing snow layers, the saltation layer may be relevant for the overall moisture exchange because strong vapor deposition may occur in an oversaturated layer with a high particle concentration close to the surface. The goals of this study are to (i) propose a parametrization for sublimation of drifting snow in the saltation layer and (ii) evaluate two parametrization options using LES simulations as a reference. The simulations reproduce four situations with different weather conditions measured at the Syowa and Davis Stations, Antarctica. We focus on a suitable parametrization of air temperature, humidity, and sublimation, not yet the representation of the drifting snow concentration. We implement our parametrization in a simple one-dimensional (1D) model that is inspired by the large-scale model CRYOWRF and can be compared to the LES simulations. The 1D model computes temperature and specific humidity at ten vertical levels between the surface and a height of 9 m, of which six levels are in the lowest 0.1 m. The first option uses a prognostic solver at all levels, accounting for turbulent transport and the exchange of moisture and heat between snow particles and the atmosphere. The second, simpler option, uses Monin-Obukhov bulk formulas to estimate the profiles below a height of 2.25 m. The concentrations of drifting and blowing snow are taken from the LES simulations and assumed to remain constant in time. The parametrization computes sublimation of drifting snow using the common formula of Thorpe and Mason (1966). On the contrary, the LES model applies a more accurate approach based on the transient mass and heat balance equations for Lagrangian particles. Only the lowest 9 m of the LES domain (38 x 19 x 18 m³) are used for comparison with the 1D model to limit undesirable effects of the Neumann upper boundary conditions. The prognostic parametrization option yields satisfactory results, while the bulk formulas can lead to a significant bias. We show how the 1D model performs in different weather conditions and discuss the benefits and remaining challenges of the parametrization.

How to cite: Sigmund, A., Sharma, V., Melo, D. B., Comola, F., Dujardin, J., Gerber, F., Huwald, H., and Lehning, M.: Parametrizing drifting snow sublimation in the saltation layer, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11853, https://doi.org/10.5194/egusphere-egu22-11853, 2022.

The mass balance of the Antarctic ice sheet is intricately linked to the state of the atmosphere and ocean surrounding the continent. As a direct result, improving projections of future sea level change relies on understanding change in the Antarctic atmosphere and Southern Ocean, as well as the processes that couple these systems. Here, we explore the influence of sea ice cover on the overlying atmosphere and subsequently the energy and mass budgets of the adjacent Antarctic ice sheet. We investigate these processes using simulations of the Community Earth System Model 2 (CESM2) developed as part of the Polar Amplification Model Intercomparison Project (PAMIP). Specifically, we explore an ensemble of atmosphere-only time slice experiments where the sea ice cover is altered. Results highlight atmospheric warming in all seasons in response to sea ice loss, but particularly pronounced warming at the surface and during non-summer seasons. Sea ice reductions further drive positive anomalies in atmospheric moisture and liquid-bearing clouds, resulting in both enhanced precipitation and downward longwave radiative fluxes over the ice sheet, particularly in West Antarctica. We furthermore explore the impact of sea ice loss on primary modes of atmospheric variability, including the Amundsen Sea low and Southern Annular Mode. These results highlight the potential impact and importance of proper simulation of the Southern Ocean sea ice cover for determining the surface mass balance of the adjacent Antarctic ice sheet. Given that the current generation of coupled climate models struggle with representing observed sea ice dynamics, our results indicate this may likely contribute to uncertainties in the simulation of recent and future Antarctic ice sheet mass balance.

How to cite: Trusel, L., Kromer, J., and Lenaerts, J.: Atmospheric response to reduced Antarctic sea ice drives ice sheet mass and energy flux anomalies, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13312, https://doi.org/10.5194/egusphere-egu22-13312, 2022.