CR7.1

West Antarctica (WA), especially the Ross Ice Shelf (RIS), has experienced more frequent surface melting during austral summer over the past three decades. Surface melting will jeopardize the stability of ice shelves and cause potential ice loss in the future. We investigate four major melt cases over the RIS via Polar WRF simulations driven by ERA5 reanalysis data and MODIS observed albedo. Direct warm air advection, recurring foehn effect, and cloud/upper warm air introduced radiative warming are the three major regional causes of surface melting over WA. Warm marine air can warm the ice surface directly. With significant moisture transport occurring over more than 40% of the time during the melting period, the impact from net radiation can be amplified via the formation of low-level liquid water clouds. Consequently, extensive downward longwave radiation favors the melting expansion over the middle and coastal RIS. Also, for 3 of 4 melt cases, more than 50% of the melting period experiences foehn warming, which can cause a 2 – 4 ºC increase in surface temperature. Isentropic drawdown is usually the dominant foehn mechanism and contributes a 14 ºC temperature increase, especially when strong low-level blocking occurs on the upwind side. Foehn clearance and decreasing surface albedo respectively increase the downward shortwave radiation and decrease the upward shortwave radiation, significantly contributing to surface melting in areas like western Marie Byrd Land. Moreover, frequent foehn cases can enhance the turbulent mixing on the leeside and benefit sensible heat transfer when Froude number is around 1. With better understanding of the regional factors for the surface melting, the prediction of the future stability of West Antarctic Ice Shelves will be improved.

How to cite: Zou, X., Bromwich, D., Montenegro, A., Wang, S.-H., and Bai, L.: Major Surface Melting over the Ross Ice Shelf, Antarctica, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6628, https://doi.org/10.5194/egusphere-egu21-6628, 2021.

Global atmospheric reanalyses are frequently used to drive ocean-ice models but few data are available to assess the quality of these products in the Antarctic sea ice zone. We utilise measurements from three drifting buoys that were deployed on sea ice in the southern Weddell Sea in the austral summer of 2016 to validate the representation of near-surface atmospheric conditions in the ERA-Interim and ERA5 reanalyses produced by the European Centre for Medium Range Weather Forecasts (ECMWF). The buoys carried sensors to measure atmospheric pressure, air temperature and humidity, wind speed and direction, and downwelling shortwave and longwave radiation. One buoy remained in coastal fast ice for most of 2016 while the other two drifted northward through the austral winter and exited the pack ice during the following austral summer. Comparison of buoy measurements with reanalysis data indicates that both reanalyses represent the surface pressure field in this region accurately. Reanalysis temperatures are, however, biased warm by around 2 °C in both products, with the largest biases seen at the lowest temperatures. We suggest that this bias is a result of the simplified representation of sea ice in the reanalyses, in particular the lack of an insulating snow layer on top of the ice. We use a simple surface energy balance model to investigate the impact of the reanalysis biases on sea ice thermodynamics.

How to cite: King, J., Marshall, G., Colwell, S., Allen-Sader, C., and Phillips, T.: Validation of atmospheric reanalyses over the Weddell Sea, Antarctica, using observations from drifting buoys, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10127, https://doi.org/10.5194/egusphere-egu21-10127, 2021.

We systematically investigate the dynamical and thermodynamic processes that lead to 77 large-scale melt events affecting high-elevation regions of the Greenland Ice Sheet (GrIS) in June-August (JJA) 1979-2017. For that purpose, we compute 8 day kinematic backward trajectories from the lowermost ~500 m above the GrIS. The key synoptic feature accompanying the melt events is an upper-tropospheric ridge over Southeast Greenland associated with a surface high-pressure system. This circulation pattern is favorable to induce rapid poleward transport (up to 40° latitude) of warm (~15 K warmer than climatological air masses arriving on the GrIS) and moist air masses from the lower troposphere to the western GrIS and subsequently to distribute them in the anticyclonic flow over north and east Greenland. During transport to the GrIS, the melt event air masses cool by ~15 K due to ascent and radiation, which keeps them just above the critical threshold to induce melting.

The thermodynamic analyses reveal that the final warm anomaly of the air masses is primarily owed to anomalous horizontal transport from a climatologically warm region of origin. However, before being transported to the GrIS, i.e., in their region of origin, these air masses were not anomalously warm. Latent heating from condensation of water vapor, occurring as the airstreams are forced to ascend orographically or dynamically, is of secondary importance. These characteristics were particularly pronounced during the most extensive melt event in early July 2012. In this event, importantly, the warm anomaly was not preserved from anomalously warm source regions such as North America experiencing a record heat wave. Considering the impact of moisture on the surface energy balance, we find that radiative effects are closely linked to the air mass trajectories and enhance melt over the entire GrIS accumulation zone due to (i) enhanced downward longwave radiation related to poleward moisture transport and a shift in the cloud phase from ice to liquid primarily west of the ice divide and (ii) increased shortwave radiation in clear-sky regions east of the ice divide.

The temporal evolution, positioning, and intensity of synoptic scale weather systems deserve further attention as they are responsible for strong and partly opposing atmospheric forcing of the GrIS surface mass balance. Also, the mechanisms identified here are in contrast to melt events in the low-elevation high Arctic and to midlatitude heat waves, where the upper-tropospheric ridge is essential to induce adiabatic warming by large-scale subsidence. Given the ongoing increase in the frequency and the melt extent of large-scale melt events, the understanding of upper-tropospheric ridges over the North Atlantic, i.e., also Greenland blocking, and its representation in climate models is crucial in determining future GrIS accumulation zone melt and thus global sea level rise. 

How to cite: Hermann, M., Papritz, L., and Wernli, H.: Lagrangian analysis of the dynamical and thermodynamic drivers of large-scale Greenland melt events during 1979-2017, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-365, https://doi.org/10.5194/egusphere-egu21-365, 2021.

The Greenland Ice Sheet (GrIS) has been losing mass at an accelerated rate in the last few decades, of which, approximately 50% is related to surface melting and runoff (Imbie Team, 2020). Since the mid 2010’s, the highest melt anomalies were found in the northeast of Greenland, where the North East Greenland Ice Stream drains 8 - 12 % of the GrIS. Unsurprisingly, the vast majority of melting occurs in the summer months, however the increasing trend in air temperatures is larger in winter. Similarly, very warm winter periods have been observed recently in the north of Greenland and Arctic Ocean. Due to our previous focus on summer melting, our understanding of glacial hydrology and surface mass balance in winter is still poor.

Here, we present the frequency and amount of surface melting and precipitation, as simulated by the Modèle Atmosphérique Régional (MAR) at 15 km spatial resolution (from 1980 to 2018) and the COupled Snowpack and Ice surface energy and mass balance model in PYthon (COSIPY) at 1 km spatial resolution (from 2014 to 2018). Observations from two automatic weather stations are also used to analyse the meteorological setting. We find that both periods of winter melt and extreme precipitation are related to the presence of atmospheric rivers along the east coast of Greenland and in the Atlantic Ocean (specifically in the Greenland Sea and Fram Strait). On average, the detection of atmospheric rivers in the vicinity of the northeast of Greenland leads to a daily warming of +8°C and can raise temperatures to the melting point for a short period of time. We also present the changes in precipitation type (rainfall vs snowfall), from 1980 to 2018 from both MAR and the ERA5 reanalysis product, which are related to atmospheric rivers and passing storms.

How to cite: Turton, J., Mattingly, K., and Mölg, T.: The influence of atmospheric rivers on winter melt and accumulation in the northeast of Greenland., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6137, https://doi.org/10.5194/egusphere-egu21-6137, 2021.

Atmospheric Rivers (ARs), narrow filaments of concentrated water vapor transport, have direct impacts on the surface mass balance (SMB) of the western Greenland Ice Sheet through increased summer melting in the ablation area and increased snowfall in higher altitudes. Here, we show that an additional effect of ARs on SMB comes from the development of föhn winds, whereby the air is adiabatically warmed as it descends. As ARs pass over the ice sheet and deposit precipitation in northwest Greenland, the air subsequently flows down the leeward slope and the warm, dry conditions contribute to increased melting in the northeast, and more specifically on the Nioghalvfjerdsfjorden (or 79N) Glacier.

We identify föhn conditions using an automated detection algorithm applied to MAR and RACMO2 regional climate model output. These data are paired with an AR detection algorithm and self-organizing map (SOM) classification applied to MERRA-2 and ERA5 reanalyses, in order to investigate connections between regional circulation patterns, ARs, föhn winds, and ice sheet SMB. We find that föhn conditions and associated surface melt are increased for periods of 1–3 days after anomalous southerly and southwesterly water vapor transport by ARs through Baffin Bay and the Nares Strait. Approximately 70% of the ARs which make landfall in the northwest sector of Greenland lead to the development of föhn winds on the northeast coast. The frequency of AR-induced föhn conditions in the northeast has increased in the last 40 years, in line with an increase in the strongest ARs in the northwest. We also find that anomalous northerly moisture transport from the Lincoln Sea generates enhanced melt in the lowest (0–500m) elevations of northeast Greenland, while below-average surface melt occurs during all other identified moisture transport regimes.

How to cite: Mattingly, K., Turton, J., Wille, J., Fettweis, X., and Noël, B.: The contribution of föhn winds to northeast Greenland summer melt and their relationship with atmospheric rivers, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8002, https://doi.org/10.5194/egusphere-egu21-8002, 2021.

The development of the HARMONIE model system has led to huge advances in numerical weather prediction, including over Greenland where a numerical weather prediction (NWP) model is used to forecast daily surface mass budget over the Greenland ice sheet as presented on polarportal.dk. The new high resolution Copernicus Arctic Reanalysis further developed the possibilities in HARMONIE with full 3DVar data assimilation and extended use of quality-controlled local observations. Here, we discuss the development and current status of the climate version of the HARMONIE Climate model (HCLIM). The HCLIM system has opened up the possibility for flexible use of the model at a range of spatial scales using different physical schemes including HARMONIE-AROME, ALADIN and ALARO for different spatial and temporal resolutions and assimilating observations, including satellite data on sea ice concentration from ESA CCI+, to improve hindcasts. However, the range of possibilities means that documenting the effects of different physics and parameterisation schemes is important before widespread application. 

Here, we focus on HCLIM performance over the Greenland ice sheet, using observations to verify the different plausible set-ups and investigate biases in climate model outputs that affect the surface mass budget (SMB) of the Greenland ice sheet. 

The recently funded Horizon 2020 project PolarRES will use the HCLIM model for very high resolution regional downscaling, together with other regional climate models in both Arctic and Antarctic regions, and our analysis thus helps to optimise the use of HCLIM in the polar regions for different modelling purposes.

How to cite: Mottram, R., Landgren, O., Anker Pedersen, R., Pagh Nielsen, K., Bøssing Christensen, O., Olesen, M., Boberg, F., Hansen, N., Amstrup, B., and Yang, X.: Physics, Resolution and Data Assimilation: Making sense of Greenland climate and ice sheet Surface Mass Balance with HARMONIE Climate, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16100, https://doi.org/10.5194/egusphere-egu21-16100, 2021.

The surface radiation budget is an essential component of the total energy exchange between the atmosphere and the Earth’s surface. Measurements of radiative fluxes near/on ice surfaces are sparse in the polar regions, including on the Greenland Ice Sheet (GrIS), and the effects of cloud on radiative fluxes are still poorly studied. In this work, we assess the impacts of cloud on radiative fluxes using two metrics: the longwave-equivalent cloudiness, derived from long-wave radiation measurements, and the cloud transmittance factor, obtained from short-wave radiation. The metrics are applied to radiation data from two automatic weather stations located over the bare ground near the ice front of Helheim (HG) and Jakobshavn Isbræ (JI) on the GrIS. Comparisons of meteorological parameters, surface radiation fluxes, and cloud metrics show significant differences between the two sites. The cloud transmittance factor is higher at HG than at JI, and the incoming short-wave radiation in the summer at HG is 50.0 W m−2 larger than at JI. Cloud metrics derived at the two sites reveal   a high dependency on the wind direction. The total cloud radiative effect (CREnet) generally increases during melt season at the two stations due to long-wave CRE enhancement by cloud fraction.  CREnet decreases from May to June and increases afterward, due to the strengthened short-wave CRE. The annually averaged CREnet were 3.0 ± 7.4 W m-2 and 1.9 ± 15.1 W m−2 at JI and HG.  CREnet estimated from AWS indicates that clouds cool the JI and HG during melt season at different rates.

How to cite: Djoumna, G., Mernild, S. H., and Holland, D.: Cloud effects on surface radiation balance at Helheim and Jakobshavn Glaciers (Greenland) using ground-based observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8103, https://doi.org/10.5194/egusphere-egu21-8103, 2021.

Single extreme weather events such as intense storms or blocks can have a major impact on polar surface temperatures, the formation and melting rates of sea-ice, and, thus, on minimum and maximum sea-ice extent within a particular year. Anomalous weather conditions on the time scale of an entire season, for example resulting from an unusual sequence of storms, can affect the polar energy budget and sea-ice coverage even more. Here, we introduce the concept of an extreme season in a distinct region using an EOF analysis in the phase space spanned by anomalies of a set of surface parameters (surface temperature, precipitation, surface solar and thermal radiation and surface heat fluxes). To focus on dynamical instead of climate change aspects, we define anomalies as departures of the seasonal mean from a transient climatology. The goal of this work is to study the dynamical processes leading to such anomalous seasons in the polar regions, which have not yet been analysed. Specifically, we focus here on a detailed analysis of Arctic extreme seasons and their underlying atmospheric dynamics in the ERA5 reanalysis data set.

We find that in regions covered predominantly by sea ice, extreme seasons are mostly determined by anomalies of atmospheric dynamical features such as cyclones and blocking. In contrast, in regions including large areas of open water the formation of extreme seasons can also be partially due to preconditioning during previous seasons, leading to strong anomalies in the sea ice concentration and/or sea surface temperatures at the beginning of the extreme season.

Two particular extreme season case studies in the Kara-Barents Seas are discussed in more detail. In this region, the winter of 2011/12 shows the largest positive departure of surface temperature from the background warming trend together with a negative anomaly in the sea ice concentration. An analysis of the synoptic situation shows that the strongly reduced frequency of cold air outbreaks compared to climatology combined with several blocking events and the frequent occurrence of cyclones transporting warm air into the region favored the continuous anomalies of both parameters. In contrast, the winter of 2016/17, which shows a positive precipitation anomaly and negative anomaly in the surface energy balance, was favored by a strong surface preconditioning. An extremely warm summer and autumn in 2016 caused strongly reduced sea ice concentrations and increased sea surface temperatures in the Kara-Barents Seas at the beginning of the winter, favoring increased air-sea fluxes and precipitation during the following months.

Our results reveal a high degree of variability of the processes involved in the formation of extreme seasons in the Arctic. Quantifying and understanding these processes will also be important when considering climate change effects in polar regions and the ability of climate models in reproducing extreme seasons in the Arctic and Antarctica.

How to cite: Hartmuth, K., Papritz, L., Boettcher, M., and Wernli, H.: Dynamics and drivers of extreme seasons in the Arctic region, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1521, https://doi.org/10.5194/egusphere-egu21-1521, 2021.

The effects of Arctic cyclones on sea ice are the subject of many papers, however aside from individual case studies, few address the heterogeneity in the spatial pattern of the sea ice response.

We composite atmospheric conditions from ERA5 reanalysis and satellite sea ice concentrations on Arctic cyclones using a storm-centered approach to reveal the typical atmosphere and sea ice responses at different bearings and distances relative to an Arctic cyclone.

Asymmetry in the pattern of the sea ice concentration response to cyclones is revealed, with increased growth/reduced melt to the west of cyclones and decreased growth/increased melt to the east.

In part, this is explained by heterogeneity in the spatial patterns of atmospheric temperature and cloud fraction associated with cyclones, which result in heterogeneity in patterns of the surface energy fluxes.

Using the CICE sea ice model forced with prescribed atmospheric reanalysis from the Japan Meteorological Agency, we reveal the relative importance of the dynamic and thermodynamic forcing of cyclones on sea ice, as well as the spatial patterns of each. The dynamic and thermodynamic responses of sea ice concentration to cyclones are comparable in magnitude, however dynamic processes dominate the response of sea ice thickness.

These results highlight and explain important details often missed when answering “do cyclones cause an increase or decrease sea ice?”, as it appears the answer is both.

How to cite: Clancy, R., Bitz, C., Blanchard-Wrigglesworth, E., and McGraw, M.: Drivers of the Spatial Pattern of Arctic Sea Ice Response to Arctic Cyclones, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8130, https://doi.org/10.5194/egusphere-egu21-8130, 2021.

Polar lows are intense mesoscale cyclones that develop in polar marine air masses. Motivated by the large variety of their proposed intensification mechanisms, cloud structure, and ambient sub-synoptic environment, we use self-organising maps to classify polar lows.

We identify five different polar-low configurations which are characterised by the vertical wind shear vector, the change of the horizontal-wind vector with height, relative to the propagation direction. Four categories feature a strong shear with different orientations of the shear vector, whereas the fifth category contains conditions with weak shear. This confirms the relevance of a previously identified categorisation into forward and reverse-shear polar lows. We expand the categorisation with right and left-shear polar lows that propagate towards colder and warmer environments, respectively.

For the strong-shear categories, the shear vector organises the moist-baroclinic dynamics of the systems. This is apparent in the low-pressure anomaly tilting with height against the shear vector, and the main updrafts occurring along the warm front located in the forward-left direction relative to the shear vector. These main updrafts contribute to the intensification through latent-heat release and are typically associated with comma-shaped clouds.

Polar low situations with a weak shear, that often feature spirali-form clouds, occur mainly at decaying stages of the development. We thus find no evidence for hurricane-like intensification of polar lows and propose instead that spirali-form clouds are associated with a warm seclusion process.

How to cite: Stoll, P., Spengler, T., and Graversen, R. G.: Polar Lows - Moist Baroclinic Cyclones Developing in Four Different Vertical Wind Shear Environments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7737, https://doi.org/10.5194/egusphere-egu21-7737, 2021.

Improved understanding of evaporation and condensation processes is critical to improve the representation of the water cycle in atmospheric models. Thereby, in-situ measurements along the entire moisture transport pathway, covering evaporation, mixing between different air masses in the atmospheric boundary layer and the free troposphere, and resulting precipitation are highly valuable to obtain new insight. In particular, coherent measurements of the stable isotope composition in atmospheric vapour can provide additional constraints on phase change processes of water vapour from source to sink, enabling direct comparison within isotope-enabled models.

Here we present stable isotope measurements from the Iceland Greenland Seas Project field campaign that took place in February-March 2018. This unique dataset includes simultaneous measurements from a land-station in Husavik, Iceland, a ship and an air plane in the subpolar region. Alternation between cold-air outbreaks and mid-latitude airmasses characterized the measurement period. Here we focus on the stable water isotope composition in water vapour obtained from 10 research flights, covering a large geographic range (64 °N to 72 °N). Careful data treatment was applied to ensure the quality of isotope measurements in the predominant cold, dry conditions with large gradients in isotope composition and humidity.

From an intercomparison flight over the Husavik station, we find good agreement between ground and airborne measurements. Out of 7 flights dedicated to the study of atmosphere-ocean-ice interactions, with both low-levels legs and vertical sections in predominant Cold Air Outbreak (CAO) conditions, we focus on the marginal ice zone and regions covered by shallow cumulus clouds. For open water flights, we find the horizontal and vertical distribution of δ¹⁸O in the marine boundary layer to covary with cloud cover. Thereby, downdrafts bring dry and ¹⁸O-depleted air from the free troposphere towards the surface, corresponding to openings in cloud cover. For flights passing over sea ice edge, both δ¹⁸O and specific humidity show a clear east-west gradient, with increasing values towards the open sea reflecting ocean moisture availability. Additionally, open leads in the sea ice also have a visible impact on isotope values. Lastly, relatively low d-excess values are observed over the sea-ice, which could either be caused by local processes or advection.

How to cite: Touzeau, A., Steen-Larsen, H.-C., Renfrew, I., Jónsson, Þ., Elvidge, A., Lachlan-Cope, T., Weng, Y., Sveinbjörnsdóttìr, Á., Midtgarden Golid, H., Duscha, C., and Sodemann, H.: Airborne water vapor isotope measurements over the Iceland Sea in winter conditions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7357, https://doi.org/10.5194/egusphere-egu21-7357, 2021.