Sebastian Westermann

• Etzelmüller, Bernd; Patton, Henry; Schomacker, Anders; Czekirda, Justyna; Girod, Luc Maurice Ramuntcho; Hubbard, Alun Lloyd; Lilleøren, Karianne Staalesen & Westermann, Sebastian (2020). Icelandic permafrost dynamics since the Last Glacial Maximum – model results and geomorphological implications. Quaternary Science Reviews.  ISSN 0277-3791.  233:106236 . doi: 10.1016/j.quascirev.2020.106236 Full text in Research Archive. Show summary

  Iceland’s periglacial realm is one of the most dynamic on the planet, with active geomorphologicalprocesses and high weathering rates of young bedrock resulting in high sediment yields and ongoingmass movement. Permafrost is discontinuous in Iceland’s highlands and mountains over c. 800 m a.s.l,and sporadic in palsa mires in the central highlands. During the late Pleistocene and Holocene, Iceland’speriglacial environment varied considerably in time and space, dominated by glacialfluctuations andperiglacial processes. To evaluate the dynamics of permafrost in Iceland since the last deglaciation, weuse the output of a coupled climate/ice sheet model to force a transient permafrost model (CryoGRID 2)from the Last Glacial Maximum (LGM) through to the present. Wefind that permafrost was widespreadacross the deglaciated areas of western, northern and eastern Iceland after the LGM, and that up to 20% ofIceland’s terrestrial area was underlain by permafrost throughout the late Pleistocene. This influencedgeomorphological processes and landform generation: the early collapse of the marine-based ice sheettogether with the aggradation of permafrost in these zones initiated the formation of abundant and nowrelict rock glaciers across coastal margins. Permafrost degraded rapidly after the Younger Dryas, with amarked impact on slope stability. Permafrost that formed during the Little Ice Age is again thawingrapidly, and an escalation in slope failure and mass-movement might be currently underway. Our studydemonstrates that large regions of Iceland have been underlain by permafrost for millennia, facilitatinglandform development and influencing the stability of steep slopes

• Aalstad, Kristoffer; Westermann, Sebastian & Bertino, Laurent (2020). Evaluating satellite retrieved fractional snow-covered area at a high-Arctic site using terrestrial photography. Remote Sensing of Environment.  ISSN 0034-4257.  239 . doi: 10.1016/j.rse.2019.111618 Full text in Research Archive. Show summary

  The seasonal snow-cover is one of the most rapidly varying natural surface features on Earth. It strongly modulates the terrestrial water, energy, and carbon balance. Fractional snow-covered area (fSCA) is an essential snow variable that can be retrieved from multispectral satellite imagery. In this study, we evaluate fSCA retrievals from multiple sensors that are currently in polar orbit: the operational land imager (OLI) on-board Landsat 8, the multispectral instrument (MSI) on-board the Sentinel-2 satellites, and the moderate resolution imaging spectroradiometer (MODIS) on-board Terra and Aqua. We consider several retrieval algorithms that fall into three classes: thresholding of the normalized difference snow index (NDSI), regression on the NDSI, and spectral unmixing. We conduct the evaluation at a high-Arctic site in Svalbard, Norway, by comparing satellite retrieved fSCA to coincident high-resolution snow-cover maps obtained from a terrestrial automatic camera system. For the lower resolution MODIS retrievals, the regression-based retrievals outperformed the unmixing-based retrievals for all metrics but the bias. For the higher resolution sensors (OLI and MSI), retrievals based on NDSI thresholding overestimated the fSCA due to the mixed pixel problem whereas spectral unmixing retrievals provided the most reliable estimates across the board. We therefore encourage the operationalization of spectral unmixing retrievals of fSCA from both OLI and MSI.

• Angelopoulos, Michael; Overduin, Pier Paul; Westermann, Sebastian; Tronicke, Jens; Strauss, Jens; Schirrmeister, Lutz; Biskaborn, Boris K.; Liebner, Susanne; Maksimov, Georgii; Grigoriev, Mikhail N & Grosse, Guido (2020). Thermokarst Lake to Lagoon Transitions in Eastern Siberia: Do Submerged Taliks Refreeze?. Journal of Geophysical Research (JGR): Earth Surface.  ISSN 2169-9003.  125(10) . doi: 10.1029/2019JF005424
• Cai, Lei; Lee, Hanna; Aas, Kjetil Schanke & Westermann, Sebastian (2020). Projecting circum-Arctic excess-ground-ice melt with a sub-grid representation in the Community Land Model. The Cryosphere.  ISSN 1994-0416.  14(12), s 4611- 4626 . doi: 10.5194/tc-14-4611-2020 Full text in Research Archive. Show summary

  To address the long-standing underrepresentation of the influences of highly variable ground ice content on the trajectory of permafrost conditions simulated in Earth system models under a warming climate, we implement a sub-grid representation of excess ground ice within permafrost soils using the latest version of the Community Land Model (CLM5). Based on the original CLM5 tiling hierarchy, we duplicate the natural vegetated land unit by building extra tiles for up to three cryostratigraphies with different amounts of excess ice for each grid cell. For the same total amount of excess ice, introducing sub-grid variability in excess-ice contents leads to different excess-ice melting rates at the grid level. In addition, there are impacts on permafrost thermal properties and local hydrology with sub-grid representation. We evaluate this new development with single-point simulations at the Lena River delta, Siberia, where three sub-regions with distinctively different excess-ice conditions are observed. A triple-land-unit case accounting for this spatial variability conforms well to previous model studies for the Lena River delta and displays markedly different dynamics of future excess-ice thaw compared to a single-land-unit case initialized with average excess-ice contents. For global simulations, we prescribed a tiling scheme combined with our sub-grid representation to the global permafrost region using presently available circum-Arctic ground ice data. The sub-grid-scale excess ice produces significant melting of excess ice under a warming climate and enhances the representation of sub-grid variability of surface subsidence on a global scale. Our model development makes it possible to portray more details on the permafrost degradation trajectory depending on the sub-grid soil thermal regime and excess-ice melting, which also shows a strong indication that accounting for excess ice is a prerequisite of a reasonable projection of permafrost thaw. The modeled permafrost degradation with sub-grid excess ice follows the pathway that continuous permafrost transforms into discontinuous permafrost before it disappears, including surface subsidence and talik formation, which are highly permafrost-relevant landscape changes excluded from most land models. Our development of sub-grid representation of excess ice demonstrates a way forward to improve the realism of excess-ice melt in global land models, but further developments require substantially improved global observational datasets on both the horizontal and vertical distributions of excess ground ice.

• Chadburn, Sarah E.; Aalto, Tuula; Aurela, Mika; Baldocchi, Dennis; Biasi, Christina; Boike, Julia; Burke, Eleanor J.; Comyn-platt, Edward; Dolman, A Johannes; Duran-Rojas, Carolina; Fan, Yuanchao; Friborg, Thomas; Gao, Yao; Gedney, Nicola; Göckede, Mathias; Hayman, Garry D.; Holl, David; Hugelius, Gustaf; Kutzbach, Lars; Lee, Hanna; Lohila, Annalea; Parmentier, Frans-Jan W.; Sachs, Torsten; Shurpali, Narasinha J. & Westermann, Sebastian (2020). Modeled Microbial Dynamics Explain the Apparent Temperature Sensitivity of Wetland Methane Emissions. Global Biogeochemical Cycles.  ISSN 0886-6236.  34(11) . doi: 10.1029/2020GB006678 Full text in Research Archive.
• Chaudhary, Nitin; Westermann, Sebastian; Lamba, Shubhangi; Shurpali, Narasinha J.; Sannel, A. Britta K.; Schurgers, Guy; Miller, Paul A. & Smith, Benjamin (2020). Modelling past and future peatland carbon dynamics across the pan‐Arctic. Global Change Biology.  ISSN 1354-1013.  26(7), s 4119- 4133 . doi: 10.1111/gcb.15099
• Nitzbon, Jan; Westermann, Sebastian; Langer, Moritz; Martin, Leo Celestin Paul; Strauss, Jens; Laboor, Sebastian & Boike, Julia (2020). Fast response of cold ice-rich permafrost in northeast Siberia to a warming climate. Nature Communications.  ISSN 2041-1723.  11 . doi: 10.1038/s41467-020-15725-8
• Obu, Jaroslav; Westermann, Sebastian; Vieira, Gonçalo; Abramov, Andrey; Balks, Megan Ruby; Bartsch, Annett; Hrbáček, Filip; Kääb, Andreas & Ramos, Miguel (2020). Pan-Antarctic map of near-surface permafrost temperatures at 1 km2 scale. The Cryosphere.  ISSN 1994-0416.  14(2), s 497- 519 . doi: 10.5194/tc-14-497-2020 Full text in Research Archive. Show summary

  Permafrost is present within almost all of the Antarctic's ice-free areas, but little is known about spatial variations in permafrost temperatures except for a few areas with established ground temperature measurements. We modelled a temperature at the top of the permafrost (TTOP) for all the ice-free areas of the Antarctic mainland and Antarctic islands at 1 km2 resolution during 2000–2017. The model was driven by remotely sensed land surface temperatures and downscaled ERA-Interim climate reanalysis data, and subgrid permafrost variability was simulated by variable snow cover. The results were validated against in situ-measured ground temperatures from 40 permafrost boreholes, and the resulting root-mean-square error was 1.9 ∘C. The lowest near-surface permafrost temperature of −36 ∘C was modelled at Mount Markham in the Queen Elizabeth Range in the Transantarctic Mountains. This is the lowest permafrost temperature on Earth, according to global-scale modelling results. The temperatures were most commonly modelled between −23 and −18 ∘C for mountainous areas rising above the Antarctic Ice Sheet and between −14 and −8 ∘C for coastal areas. The model performance was good where snow conditions were modelled realistically, but errors of up to 4 ∘C occurred at sites with strong wind-driven redistribution of snow.

• Pastorello, Gilberto; Trotta, Carlo; Canfora, Eleonora; Chu, Housen; Christianson, Danielle; Cheah, You-Wei; Poindexter, Cristina; Chen, Jiquan; Elbashandy, Abdelrahman; Humphrey, Marty; Isaac, Peter; Polidori, Diego; Ribeca, Alessio; van Ingen, Catharine; Zhang, Leiming; Amiro, Brian; Ammann, Christof; Arain, M. Altaf; Ardö, Jonas; Arkebauer, Timothy; Arndt, Stefan K.; Arriga, Nicola; Aubinet, Marc; Aurela, Mika; Baldocchi, Dennis D.; Barr, Alan; Beamesderfer, Eric; Marchesini, Luca Belelli; Bergeron, Onil; Beringer, Jason; Bernhofer, Christian; Berveiller, Daniel; Billesbach, Dave; Black, Thomas Andrew; Blanken, Peter D.; Bohrer, Gil; Boike, Julia; Bolstad, Paul V.; Bonal, Damien; Bonnefond, Jean-Marc; Bowling, David R.; Bracho, Rosvel; Brodeur, Jason; Brümmer, Christian; Buchmann, Nina; Burban, Benoit; Burns, Sean P.; Buysse, Pauline; Cale, Peter; Cavagna, Mauro; Cellier, Pierre; Chen, Shiping; Chini, Isaac; Christensen, Torben R.; Cleverly, James; Collalti, Alessio; Consalvo, Claudia; Cook, Bruce D.; Cook, David; Coursolle, Carole; Cremonese, Edoardo; Curtis, Peter S.; D’Andrea, Ettore; da Rocha, Humberto; Dai, Xiaoqing; Davis, Kenneth J.; De Cinti, Bruno; de Grandcourt, Agnes; De Ligne, Anne; De Oliveira, Raimundo C.; Delpierre, Nicolas; Desai, Ankur R.; Di Bella, Carlos Marcelo; di Tommasi, Paul; Dolman, Han; Domingo, Francisco; Dong, Gang; Dore, Sabina; Duce, Pierpaolo; Dufrene, Eric; Dunn, Allison; Dušek, Jiří; Eamus, Derek; Eichelmann, Uwe; ElKhidir, Hatim Abdalla M.; Eugster, Werner; Ewenz, Cacilia M.; Ewers, Brent; Famulari, Daniela; Fares, Silvano; Feigenwinter, Iris; Feitz, Andrew; Fensholt, Rasmus; Filippa, Gianluca; Fischer, Marc; Frank, John; Galvagno, Marta; Gharun, Mana; Gianelle, Damiano; Gielen, Bert; Gioli, Beniamino; Gitelson, Anatoly; Goded, Ignacio; Goeckede, Mathias; Goldstein, Allen H.; Gough, Christopher M.; Goulden, Michael L.; Graf, Alexander; Griebel, Anne; Gruening, Carsten; Grünwald, Thomas; Hammerle, Albin; Han, Shijie; Han, Xingguo; Hansen, Birger Ulf; Hanson, Chad; Hatakka, Juha; He, Yongtao; Hehn, Markus; Heinesch, Bernard; Hinko-Najera, Nina; Hörtnagl, Nina; Hutley, Lindsay; Ibrom, Andreas; Ikawa, Hiroki; Jackowicz-Korczynski, Marcin; Janous, Dalibor; Jans, Wilma; Jassal, Rachhpal; Jiang, Shicheng; Kato, Tomomichi; Khomik, Myroslava; Klatt, Janina; Knohl, Alexander; Knox, Sarah; Kobayashi, Hideki; Koerber, Georgia; Kolle, Olaf; Kosugi, Yoshiko; Kotani, Ayumi; Kowalski, Andrew; Kruijt, Bart; Kurbatova, Julia; Kutsch, Werner Leo; Kwon, Hyojung; Launiainen, Samuli; Laurila, Tuomas; Law, Bev; Leuning, Ray; Li, Yingnian; Liddell, Michael; Limousin, Jean-Marc; Lion, Marryanna; Liska, Adam J.; Lohila, Annalea; López-Ballesteros, Ana; López-Blanco, Efrén; Loubet, Benjamin; Loustau, Denis; Lucas-Moffat, Antje; Lüers, Johannes; Ma, Siyan; Macfarlane, Craig; Magliulo, Vincenzo; Maier, Regine; Mammarella, Ivan; Manca, Giovanni; Marcolla, Barbara; Margolis, Hank A.; Marras, Serena; Massman, William; Mastepanov, Mikhail; Matamala, Roser; Matthes, Jaclyn Hatala; Mazzenga, Francesco; McCaughey, Harry; McHugh, Ian; McMillan, Andrew M. S.; Merbold, Lutz; Meyer, Wayne; Meyers, Tilden; Miller, Scott D.; Minerbi, Stefano; Moderow, Uta; Monson, Russell K.; Montagnani, Leonardo; Moore, Caitlin E.; Moors, Eddy; Moreaux, Virginie; Moureaux, Christine; Munger, J. William; Nakai, Taro; Neirynck, Johan; Nesic, Zoran; Nicolini, Giacomo; Noormets, Asko; Northwood, Matthew; Nosetto, Marcelo; Nouvellon, Yann; Novick, Kimberly; Oechel, Walter C.; Olesen, Jørgen Eivind; Ourcival, Jean-Marc; Papuga, Shirley A.; Parmentier, Frans-Jan W.; Paul-Limoges, Eugenie; Pavelka, Marian; Peichl, Matthias; Pendall, Elise; Phillips, Richard P.; Pilegaard, Kim; Pirk, Norbert; Posse, Gabriela; Powell, Thomas; Prasse, Heiko; Prober, Suzanne M.; Rambal, Serge; Rannik, Üllar; Raz-Yaseef, Naama; Reed, David; de Dios, Victor Resco; Restrepo-Coupe, Natalia; Reverter, Borja R.; Roland, Marilyn; Sabbatini, Simone; Sachs, Torsten; Saleska, Scott R.; Sánchez-Cañete, Enrique P.; Sanchez-Mejia, Zulia M.; Schmid, Hans Peter; Schmidt, Marius; Schneider, Karl; Schrader, Frederik; Schroder, Ivan; Scott, Russell L.; Sedlák, Pavel; Serrano-Ortíz, Penélope; Shao, Changliang; Shi, Peilin; Shironya, Ivan; Siebicke, Lukas; Šigut, Ladislav; Silberstein, Richard; Sirca, Costantino; Spano, Donatella; Steinbrecher, Rainer; Stevens, Robert M.; Sturtevant, Cove; Suyker, Andy; Tagesson, Torbern; Takanashi, Satoru; Tang, Yanhong; Tapper, Nigel; Thom, Jonathan; Tiedemann, Frank; Tomassucci, Michele; Tuovinen, Juha-Pekka; Urbanski, Shawn P.; Valentini, Riccardo; Van Der Molen, Michiel K.; van Gorsel, Eva; van Huissteden, Ko; Varlagin, Andrej; Verfaillie, Joseph; Vesala, Timo; Vincke, Caroline; Vitale, Domenico; Vygodskaya, Natalia; Walker, Jeffrey P.; Walter-Shea, Elizabeth; Wang, Huimin; Weber, Robin; Westermann, Sebastian; Wille, Christian; Wofsy, Steven C.; Wohlfahrt, Georg; Wolf, Sebastian; Woodgate, William; Li, Yuelin; Zampedri, Roberto; Zhang, Junhui; Zhou, Guoyi; Zona, Donatella; Agarwal, Deb; Biraud, Sébastien; Torn, Margaret S. & Papale, Dario (2020). The FLUXNET2015 dataset and the ONEFlux processing pipeline for eddy covariance data. Scientific Data.  ISSN 2052-4463.  7 . doi: 10.1038/s41597-020-0534-3 Full text in Research Archive.
• Sayedi, Sayedeh Sara; Abbott, Benjamin W.; Thornton, Brett F.; Frederick, Jennifer M.; Vonk, Jorien E.; Overduin, Paul; Schädel, Christina; Schuur, Edward A.G.; Bourbonnais, Annie; Demidov, Nikita; Gavrilov, Anatoly; He, Shengping; Hugelius, Gustaf; Jakobsson, Martin; Jones, Miriam C.; Joung, DongJoo; Kraev, Gleb; Macdonald, Robie W.; McGuire, A. David; Mu, Cuicui; O'Regan, Matt; Schreiner, Kathryn M.; Stranne, Christian; Pizhankova, Elena; Vasiliev, Alexander; Westermann, Sebastian; Zarnetske, Jay P.; Zhang, Tingjun; Ghandehari, Mehran; Baeumler, Sarah; Brown, Brian C. & Frei, Rebecca J. (2020). Subsea permafrost carbon stocks and climate change sensitivity estimated by expert assessment. Environmental Research Letters.  ISSN 1748-9326.  15(12) . doi: 10.1088/1748-9326/abcc29 Full text in Research Archive.
• Aas, Kjetil Schanke; Martin, Leo Celestin Paul; Nitzbon, Jan; Langer, Moritz; Boike, Julia; Lee, Hanna; Berntsen, Terje Koren & Westermann, Sebastian (2019). Thaw processes in ice-rich permafrost landscapes represented with laterally coupled tiles in a land surface model. The Cryosphere.  ISSN 1994-0416.  13(2), s 591- 609 . doi: 10.5194/tc-13-591-2019 Full text in Research Archive.
• Angelopoulos, Michael; Westermann, Sebastian; Overduin, Paul; Faguet, Alexey; Olenchenko, Vladimir; Grosse, Guido & Grigoriev, Mikhail N (2019). Heat and Salt Flow in Subsea Permafrost Modeled with CryoGRID2. Journal of Geophysical Research (JGR): Earth Surface.  ISSN 2169-9003.  124, s 920- 937 . doi: 10.1029/2018JF004823 Full text in Research Archive.
• Czekirda, Justyna; Westermann, Sebastian; Etzelmüller, Bernd & Jóhannesson, Tómas (2019). Transient Modelling of Permafrost Distribution in Iceland. Frontiers in Earth Science.  ISSN 2296-6463.  7(130) . doi: 10.3389/feart.2019.00130 Full text in Research Archive.
• Fiddes, Joel; Aalstad, Kristoffer & Westermann, Sebastian (2019). Hyper-resolution ensemble-based snow reanalysis in mountain regions using clustering. Hydrology and Earth System Sciences.  ISSN 1027-5606.  23(11), s 4717- 4736 . doi: 10.5194/hess-23-4717-2019 Full text in Research Archive. Show summary

  Spatial variability in high-relief landscapes is immense, and grid-based models cannot be run at spatial resolutions to explicitly represent important physical processes. This hampers the assessment of the current and future evolution of important issues such as water availability or mass movement hazards. Here, we present a new processing chain that couples an efficient sub-grid method with a downscaling tool and a data assimilation method with the purpose of improving numerical simulation of surface processes at multiple spatial and temporal scales in ungauged basins. The novelty of the approach is that while we add 1–2 orders of magnitude of computational cost due to ensemble simulations, we save 4–5 orders of magnitude over explicitly simulating a high-resolution grid. This approach makes data assimilation at large spatio-temporal scales feasible. In addition, this approach utilizes only freely available global datasets and is therefore able to run globally. We demonstrate marked improvements in estimating snow height and snow water equivalent at various scales using this approach that assimilates retrievals from a MODIS snow cover product. We propose that this as a suitable method for a wide variety of operational and research applications where surface models need to be run at large scales with sparse to non-existent ground observations and with the flexibility to assimilate diverse variables retrieved by Earth observation missions.

• Karjalainen, Olli; Aalto, Juha; Luoto, Miska; Westermann, Sebastian; Romanovsky, Vladimir E; Nelson, Frederick E.; Etzelmüller, Bernd & Hjort, Jan (2019). Data Descriptor: Circumpolar permafrost maps and geohazard indices for near-future infrastructure risk assessments. Scientific Data.  ISSN 2052-4463.  6 . doi: 10.1038/sdata.2019.37 Full text in Research Archive.
• Magnin, Florence; Etzelmüller, Bernd; Westermann, Sebastian; Isaksen, Ketil; Hilger, Paula & Hermanns, Reginald (2019). Permafrost distribution in steep rock slopes in Norway: measurements, statistical modelling and implications for geomorphological processes. Earth Surface Dynamics.  ISSN 2196-6311.  7(4), s 1019- 1040 . doi: 10.5194/esurf-7-1019-2019 Full text in Research Archive.
• Martin, Leo Celestin Paul; Nitzbon, Jan; Aas, Kjetil Schanke; Etzelmüller, Bernd; Kristiansen, Håvard & Westermann, Sebastian (2019). Stability Conditions of Peat Plateaus and Palsas in Northern Norway. Journal of Geophysical Research (JGR): Earth Surface.  ISSN 2169-9003.  124(3), s 705- 719 . doi: 10.1029/2018JF004945 Full text in Research Archive.
• Myhra, Kristin Sæterdal; Westermann, Sebastian & Etzelmüller, Bernd (2019). Modelling conductive heat flow between steep rock walls and talus slopes–thermal processes and geomorphological implications. Frontiers in Earth Science.  ISSN 2296-6463.  7, s 1- 14 . doi: 10.3389/feart.2019.00192 Full text in Research Archive.
• Nitzbon, Jan; Langer, Moritz; Westermann, Sebastian; Martin, Leo Celestin Paul; Aas, Kjetil Schanke & Boike, Julia (2019). Pathways of ice-wedge degradation in polygonal tundra under different hydrological conditions. The Cryosphere.  ISSN 1994-0416.  13(4), s 1089- 1123 . doi: 10.5194/tc-13-1089-2019 Full text in Research Archive.
• Obu, Jaroslav; Westermann, Sebastian; Bartsch, Annett; Berdnikov, Nikolai M.; Christiansen, Hanne H; Dashtseren, Avirmed; Delaloye, Reynald; Elberling, Bo; Etzelmüller, Bernd; Kholodov, Alexander; Khomutov, Artem V.; Kääb, Andreas; Leibman, Marina O.; Lewkowicz, Antoni G.; Panda, Santosh K.; Romanovsky, Vladimir E.; Way, Robert G.; Westergaard-Nielsen, Andreas; Wu, Tonghua; Yamkhin, Jambaljav & Zou, Defu (2019). Northern Hemisphere permafrost map based on TTOP modelling for 2000–2016 at 1 km2 scale. Earth-Science Reviews.  ISSN 0012-8252.  193, s 299- 316 . doi: 10.1016/j.earscirev.2019.04.023 Full text in Research Archive.
• Overduin, PP; Schneider von Deimling, T; Miesner, F; Grigoriev, Mikhail N; Ruppel, Carolyn; Vasiliev, Alexander; Lantuit, H; Juhls, Bennet & Westermann, Sebastian (2019). Submarine Permafrost Map in the Arctic Modeled Using 1-D Transient Heat Flux (SuPerMAP). Journal of Geophysical Research (JGR): Oceans.  ISSN 2169-9275.  124(6), s 3490- 3507 . doi: 10.1029/2018JC014675
• Zwieback, Simon; Westermann, Sebastian; Langer, Moritz; Boike, Julia; Marsh, Philip & Berg, Aaron (2019). Improving Permafrost Modeling by Assimilating Remotely Sensed Soil Moisture. Water Resources Research.  ISSN 0043-1397.  55(3), s 1814- 1832 . doi: 10.1029/2018WR023247
• Aalstad, Kristoffer; Westermann, Sebastian; Schuler, Thomas; Boike, Julia & Bertino, Laurent (2018). Ensemble-based assimilation of fractional snow-covered area satellite retrievals to estimate the snow distribution at Arctic sites. The Cryosphere.  ISSN 1994-0416.  12(1), s 247- 270 . doi: 10.5194/tc-12-247-2018 Full text in Research Archive. Show summary

  With its high albedo, low thermal conductivity and large water storing capacity, snow strongly modulates the surface energy and water balance, which makes it a critical factor in mid- to high-latitude and mountain en- vironments. However, estimating the snow water equiva- lent (SWE) is challenging in remote-sensing applications al- ready at medium spatial resolutions of 1km. We present an ensemble-based data assimilation framework that estimates the peak subgrid SWE distribution (SSD) at the 1km scale by assimilating fractional snow-covered area (fSCA) satel- lite retrievals in a simple snow model forced by downscaled reanalysis data. The basic idea is to relate the timing of the snow cover depletion (accessible from satellite products) to the peak SSD. Peak subgrid SWE is assumed to be log- normally distributed, which can be translated to a modeled time series of fSCA through the snow model. Assimilation of satellite-derived fSCA facilitates the estimation of the peak SSD, while taking into account uncertainties in both the model and the assimilated data sets. As an extension to previ- ous studies, our method makes use of the novel (to snow data assimilation) ensemble smoother with multiple data assimi- lation (ES-MDA) scheme combined with analytical Gaussian anamorphosis to assimilate time series of Moderate Reso- lution Imaging Spectroradiometer (MODIS) and Sentinel-2 fSCA retrievals. The scheme is applied to Arctic sites near Ny-Ålesund (79◦ N, Svalbard, Norway) where field measure- ments of fSCA and SWE distributions are available. The method is able to successfully recover accurate estimates of peak SSD on most of the occasions considered. Through the ES-MDA assimilation, the root-mean-square error (RMSE) for the fSCA, peak mean SWE and peak subgrid coefficient of variation is improved by around 75, 60 and 20%, re- spectively, when compared to the prior, yielding RMSEs of 0.01, 0.09m water equivalent (w.e.) and 0.13, respectively. The ES-MDA either outperforms or at least nearly matches the performance of other ensemble-based batch smoother schemes with regards to various evaluation metrics. Given the modularity of the method, it could prove valuable for a range of satellite-era hydrometeorological reanalyses.

• Boike, Julia; Juszak, Inge; Lange, Stephan; Chadburn, Sarah; Burke, Eleanor; Paul Overduin, Pier; Roth, Kurt; Ippisch, Olaf; Bornemann, Niko; Stern, Lielle; Gouttevin, Isabelle; Hauber, Ernst & Westermann, Sebastian (2018). A 20-year record (1998-2017) of permafrost, active layer and meteorological conditions at a high Arctic permafrost research site (Bayelva, Spitsbergen). Earth System Science Data.  ISSN 1866-3508.  10(1), s 355- 390 . doi: 10.5194/essd-10-355-2018 Full text in Research Archive.
• Hjort, Jan; Karjalainen, Olli; Aalto, Juha; Westermann, Sebastian; Romanovsky, Vladimir E; Nelson, Frederick E.; Etzelmüller, Bernd & Luoto, Miska (2018). Degrading permafrost puts Arctic infrastructure at risk by mid-century. Nature Communications.  ISSN 2041-1723.  9 . doi: 10.1038/s41467-018-07557-4 Full text in Research Archive.
• Kjellman, Sofia E.; Axelsson, Pia E.; Etzelmüller, Bernd; Westermann, Sebastian & Sannel, A. Britta K. (2018). Holocene development of subarctic permafrost peatlands in Finnmark, northern Norway. The Holocene.  ISSN 0959-6836.  28(12), s 1855- 1869 . doi: 10.1177/0959683618798126 Full text in Research Archive. Show summary

  Subarctic permafrost peatlands are important soil organic carbon pools, and improved knowledge about peat properties and peatland sensitivity to past climate change is essential when predicting future response to a warmer climate and associated feedback mechanisms. In this study, Holocene peatland development and permafrost dynamics of four subarctic peat plateaus in Finnmark, northern Norway have been investigated through detailed analyses of plant macrofossils and geochemical properties. Peatland inception occurred around 9800 cal. yr BP and 9200 cal. yr BP at the two continental sites Suossjavri and Iskoras. Younger basal peat ages were found at the two coastal locations Lakselv and Karlebotn, at least partly caused by the time lag between deglaciation and emergence of land by isostatic uplift. Here, peatland development started around 6150 cal. yr BP and 5150 cal. yr BP, respectively. All four peatlands developed as wet fens throughout most of the Holocene. Permafrost aggradation, causing frost heave and a shift in the vegetation assemblage from wet fen to dry bog species, probably did not occur until during the last millennium, ca. 950 cal. yr BP in Karlebotn and ca. 800 cal. yr BP in Iskoras, and before ca. 150 cal. yr BP in Lakselv and ca. 100 cal. yr BP in Suossjavri. In Karlebotn, there are indications of a possible earlier permafrost phase around 2200 cal. yr BP due to climatic cooling at the late Subboreal to early Subatlantic transition. The mean long-term Holocene carbon accumulation rate at all four sites was 12.3 +/- 4.1 gC m-2 yr-1 (+/- SD) and the mean soil organic carbon storage was 97 +/- 46 kgC m-2.

• Strozzi, Tazio; Antonova, Sofia; Günther, Frank; Mätzler, Eva; Vieira, Gonçalo; Wegmüller, Urs; Westermann, Sebastian & Bartsch, Annett (2018). Sentinel-1 SAR interferometry for surface deformation monitoring in low-land permafrost areas. Remote Sensing.  ISSN 2072-4292.  10(9) . doi: 10.3390/rs10091360 Full text in Research Archive.
• Westergaard-Nielsen, Andreas; Karami, Mojtaba; Hansen, Birger Ulf; Westermann, Sebastian & Elberling, Bo (2018). Contrasting temperature trends across the ice-free part of Greenland. Scientific Reports.  ISSN 2045-2322.  8 . doi: 10.1038/s41598-018-19992-w Full text in Research Archive.
• Aas, Kjetil Schanke; Gisnås, Kjersti; Westermann, Sebastian & Berntsen, Terje Koren (2017). A Tiling Approach to Represent Subgrid Snow Variability in Coupled Land Surface–Atmosphere Models. Journal of Hydrometeorology.  ISSN 1525-755X.  18(1), s 49- 63 . doi: 10.1175/JHM-D-16-0026.1 Full text in Research Archive.
• Borge, Amund Frogner; Westermann, Sebastian; Solheim, Ingvild & Etzelmüller, Bernd (2017). Strong degradation of palsas and peat plateaus in northern Norway during the last 60 years. The Cryosphere.  ISSN 1994-0416.  11(1), s 1- 16 . doi: 10.5194/tc-11-1-2017 Full text in Research Archive. Show summary

  Solheim, Ingvild og Etzelmuller, Bernd er ikke (P)-personer. Fikset ms 26/9

• Chadburn, Sarah E.; Burke, Eleanor J.; Cox, Peter M.; Friedlingstein, Pierre; Hugelius, Gustaf & Westermann, Sebastian (2017). An observation-based constraint on permafrost loss as a function of global warming. Nature Climate Change.  ISSN 1758-678X.  7(5), s 340- 344 . doi: 10.1038/nclimate3262
• Chadburn, Sarah E.; Krinner, Gerhard; Porada, Philipp; Bartsch, Annett; Beer, Christian; Belelli Marchesini, Luca; Boike, Julia; Ekici, Altug; Elberling, Bo; Friborg, Thomas; Hugelius, Gustaf; Johansson, Margareta; Kuhry, Peter; Kutzbach, Lars; Langer, Moritz; Lund, Magnus; Parmentier, Frans-Jan Willem; Peng, Shushi; van Huissteden, Ko; Wang, Tao; Westermann, Sebastian; Zhu, Dan & Burke, Eleanor J. (2017). Carbon stocks and fluxes in the high latitudes: Using site-level data to evaluate Earth system models. Biogeosciences.  ISSN 1726-4170.  14(22), s 5143- 5169 . doi: 10.5194/bg-14-5143-2017 Full text in Research Archive.
• Gisnås, Kjersti; Etzelmüller, Bernd; Lussana, Cristian; Hjort, Jan; Sannel, Britta; Isaksen, Ketil; Westermann, Sebastian; Kuhry, Peter; Christiansen, Hanne H; Frampton, Andrew & Åkermann, Jonas (2017). Permafrost Map for Norway, Sweden and Finland. Permafrost and Periglacial Processes.  ISSN 1045-6740.  28(2), s 359- 378 . doi: 10.1002/ppp.1922
• Kepski, Daniel; Luks, Bartek; Migala, K.; Wawrzyniak, Tomasz; Westermann, Sebastian & Wojtun, B. (2017). Terrestrial Remote Sensing of Snowmelt in a Diverse High-Arctic Tundra Environment Using Time-Lapse Imagery. Remote Sensing.  ISSN 2072-4292.  9(7) . doi: 10.3390/rs9070733 Full text in Research Archive.
• Magnin, Florence; Westermann, Sebastian; Pogliotti, Paolo; Ravanel, Ludovico; Deline, Phillip & Malet, Eric (2017). Snow control on active layer thickness in steep alpine rock walls (Aiguille du Midi, 3842 m a.s.l., Mont Blanc massif). CATENA.  ISSN 0341-8162.  149(Part 2), s 648- 662 . doi: 10.1016/j.catena.2016.06.006
• Myhra, Kristin Sæterdal; Westermann, Sebastian & Etzelmuller, Bernd (2017). Modelled Distribution and Temporal Evolution of Permafrost in Steep Rock Walls Along a Latitudinal Transect in Norway by CryoGrid 2D. Permafrost and Periglacial Processes.  ISSN 1045-6740.  28(1), s 172- 182 . doi: 10.1002/ppp.1884
• Trofaier, Anna Maria; Westermann, Sebastian & Bartsch, Annett (2017). Progress in space-borne studies of permafrost for climate science: towards a multi-ECV approach. Remote Sensing of Environment.  ISSN 0034-4257.  203, s 55- 70 . doi: 10.1016/j.rse.2017.05.021 Full text in Research Archive.
• Westermann, Sebastian; Peter, Maria; Langer, Moritz; Schwamborn, Georg; Schirrmeister, Lutz; Etzelmüller, Bernd & Boike, Julia (2017). Transient modeling of the ground thermal conditions using satellite data in the Lena River delta, Siberia. The Cryosphere.  ISSN 1994-0416.  11(3), s 1441- 1463 . doi: 10.5194/tc-11-1441-2017 Full text in Research Archive.
• Antonova, Sonya; Duguay, Claude R.; Kääb, Andreas; Heim, Birgit; Langer, Moritz; Westermann, Sebastian & Boike, Julia (2016). Monitoring ice phenology and bedfast ice in lakes of the Lena River Delta using TerraSAR-X backscatter and coherence time series. Remote Sensing.  ISSN 2072-4292.  8(11) . doi: 10.3390/rs8110903
• Bartsch, Annett; Grosse, Guido; Kääb, Andreas; Westermann, Sebastian; Strozzi, Tazio; Wiesmann, Andreas; Duguay, Claude; Seifert, Frank Martin; Obu, Jaroslav & Goler, Robert (2016). GlobPermafrost - How space-based earth observation supports understanding of permafrost. ESA SP.  ISSN 0379-6566.  SP-740
• Gisnås, Kjersti; Westermann, Sebastian; Schuler, Thomas; Melvold, Kjetil & Etzelmüller, Bernd (2016). Small-scale variation of snow in a regional permafrost model. The Cryosphere.  ISSN 1994-0416.  10(3), s 1201- 1215 . doi: 10.5194/tc-10-1201-2016
• Langer, Moritz; Westermann, Sebastian; Boike, Julia; Kirillin, Sergej; Grosse, Guido; Peng, Shushi & Krinner, Gerhard (2016). Rapid degradation of permafrost underneath waterbodies in tundra landscapes - Toward a representation of thermokarst in land surface models. Journal of Geophysical Research (JGR): Earth Surface.  ISSN 2169-9003.  121(12), s 2446- 2470 . doi: 10.1002/2016JF003956
• Steiger, Céline; Etzelmüller, Bernd; Westermann, Sebastian & Myhra, Kristin Sæterdal (2016). Modelling the permafrost distribution in steep rock walls in Norway. Norsk Geologisk Tidsskrift.  ISSN 0029-196X.  96(4), s 1- 13 . doi: 10.17850/njg96-4-04
• Wawrzyniak, Tomasz; Osuch, Marzena; Napiorkowski, Jaroslaw & Westermann, Sebastian (2016). Modelling of the thermal regime of permafrost during 1990-2014 in Hornsund, Svalbard. Polish Polar Research.  ISSN 0138-0338.  37(2), s 219- 242 . doi: 10.1515/popore-2016-0013
• Westermann, Sebastian; Langer, Moritz; Boike, Julia; Heikenfeld, Max; Peter, Maria; Etzelmuller, Bernd & Krinner, Gerhard (2016). Simulating the thermal regime and thaw processes of ice-rich permafrost ground with the land-surface model CryoGrid 3. Geoscientific Model Development.  ISSN 1991-959X.  9(2), s 523- 546 . doi: 10.5194/gmd-9-523-2016
• Aas, Kjetil Schanke; Berntsen, Terje Koren; Boike, Julia; Etzelmuller, Bernd; Kristjansson, Jon Egill; Maturilli, Marion; Schuler, Thomas; Stordal, Frode & Westermann, Sebastian (2015). A comparison between simulated and observed surface energy balance at the Svalbard Archipelago. Journal of Applied Meteorology and Climatology.  ISSN 1558-8424.  54(5), s 1102- 1119 . doi: 10.1175/JAMC-D-14-0080.1
• Gisnås, Kjersti; Westermann, Sebastian; Schuler, Thomas; Melvold, Kjetil & Etzelmuller, Bernd (2015). Small-scale variation of snow in a regional permafrost model. The Cryosphere Discussions.  ISSN 1994-0432.  9, s 6661- 6696 . doi: 10.5194/tcd-9-6661-2015
• Langer, M; Westermann, Sebastian; Walter Anthony, K.; Wischnewski, K. & Boike, J. (2015). Frozen ponds: Production and storage of methane during the Arctic winter in a lowland tundra landscape in northern Siberia, Lena River delta. Biogeosciences.  ISSN 1726-4170.  12(4), s 977- 990 . doi: 10.5194/bg-12-977-2015
• Westermann, Sebastian; Elberling, B.; Højlund Pedersen, S.; Stendel, M.; Hansen, B. U. & Liston, G.E. (2015). Future permafrost conditions along environmental gradients in Zackenberg, Greenland. The Cryosphere.  ISSN 1994-0416.  9(2), s 719- 735 . doi: 10.5194/tc-9-719-2015
• Westermann, Sebastian; Østby, Torbjørn; Gisnås, Kjersti; Schuler, Thomas & Etzelmuller, Bernd (2015). A ground temperature map of the North Atlantic permafrost region based on remote sensing and reanalysis data. The Cryosphere.  ISSN 1994-0416.  9(3), s 1303- 1319 . doi: 10.5194/tc-9-1303-2015
• Gisnås, Kjersti; Westermann, Sebastian; Schuler, Thomas; Litherland, Tobias; Isaksen, Ketil; Boike, Julia & Etzelmuller, Bernd (2014). A statistical approach to represent small-scale variability of permafrost temperatures due to snow cover. The Cryosphere Discussions.  ISSN 1994-0432.  8, s 509- 536 . doi: 10.5194/tcd-8-509-2014
• Gisnås, Kjersti; Westermann, Sebastian; Schuler, Thomas; Litherland, Tobias; Isaksen, Ketil; Boike, Julia & Etzelmuller, Bernd (2014). A statistical approach to represent small-scale variability of permafrost temperatures due to snow cover. The Cryosphere.  ISSN 1994-0416.  8, s 2063- 2074 . doi: 10.5194/tc-8-2063-2014
• Jore, Solveig; Vanwambeke, Sophie O; Viljugrein, Hildegunn; Isaksen, Ketil; Kristoffersen, Anja Bråthen; Woldehiwet, Zerai; Johansen, Bernt; Brun, Edgar; Brun-Hansen, Hege C; Westermann, Sebastian; Larsen, Inger Lise; Ytrehus, Bjørnar & Hofshagen, Merete (2014). Climate and environmental change drives Ixodes ricinus geographical expansion at the northern range margin. Parasites & Vectors.  ISSN 1756-3305.  7(1) . doi: 10.1186/1756-3305-7-11 Full text in Research Archive.
• Eberhardt, A.; Scholz, L.; Westermann, Sebastian; Sachs, T; Langer, M; Wöllenstein, J. & Palzer, S. (2014). Low cost, mobile sensor system for measurement of carbon dioxide in permafrost areas. Procedia Engineering.  ISSN 1877-7058.  87, s 1318- 1321 . doi: 10.1016/j.proeng.2014.11.690
• Hipp, Tobias Florian; Etzelmuller, Bernd & Westermann, Sebastian (2014). Permafrost in Alpine Rock Faces from Jotunheimen and Hurrungane, Southern Norway. Permafrost and Periglacial Processes.  ISSN 1045-6740.  25(1), s 1- 13 . doi: 10.1002/ppp.1799
• Langer, Moritz; Westermann, Sebastian; Walter-Anthony, Katey; Wischniewski, Karoline & Boike, Julia (2014). Frozen ponds – production and storage of methane during the Arctic winter. Biogeosciences Discussions.  ISSN 1810-6277.  11, s 11061- 11094 . doi: 10.5194/bgd-11-11061-2014
• Lüers, Johannes; Westermann, Sebastian; Piel, Konstanze & Boike, Julia (2014). Annual CO2 budget and seasonal CO2 exchange signals at a High Arctic permafrost site on Spitsbergen, Svalbard archipelago. Biogeosciences Discussions.  ISSN 1810-6277.  11, s 1535-1559 . doi: 10.5194/bgd-11-1535-2014
• Lüers, Johannes; Westermann, Sebastian; Piel, Konstanze & Boike, Julia (2014). Annual CO<sub>2</sub> budget and seasonal CO<sub>2</sub> exchange signals at a high Arctic permafrost site on Spitsbergen, Svalbard archipelago. Biogeosciences.  ISSN 1726-4170.  11(22), s 6307- 6322 . doi: 10.5194/bg-11-6307-2014

View all works in Cristin

• Aalstad, Kristoffer & Westermann, Sebastian (2020). ACS_Bayelva_class: 302 high-resolution snow cover maps covering the 2012-2017 snowmelt seasons in the Bayelva catchment (Svalbard, Norway). Show summary

  The ACS_Bayelva_class dataset contains 302 high-resolution binary snow cover images that were obtained by classifying orthrorectified photographs of a 1.77 km^2 area of interest in the Bayelva catchment. This catchment is close to Ny-Ålesund, the northernmost permanent civilian settlement in the world and a major hub for polar research, in the Norwegian high-Arctic Svalbard archipelago. The imagery has a (roughly) daily temporal resolution and a ground sampling distance (pixel spacing) of 0.5 m. The dataset spans 6 snowmelt seasons, covering the months May-August for the period 2012-2017. The orthophotos were obtained by processing oblique time-lapse photographs taken by a terrestiral automatic camera system (ACS) mounted at 562 m a.s.l. near the summit of Scheteligfjellet (719 m a.s.l.) a few kilometers west of Ny-Ålesund. The orthophotos were manually classified into binary snow cover images (0=no snow, 1=snow) by iteratively selecting a (visually) optimal threshold on the intensity in the blue band for each image. More details are provided in the study of Aalstad et al. (2020) [a copy is available in this repository] where this dataset was created. The ACS was maintained by scientists from the group of Sebastian Westermann at the Section for Physical Geography and Hydrology in the Department of Geosciences at the University of Oslo, Oslo, Norway.

• Aalstad, Kristoffer; Westermann, Sebastian; Fiddes, Joel; Karsten, Logan; McCreight, James & Bertino, Laurent (2020). Testing ensemble-based snow reanalysis in the Lakes basin using the ASO.
• Aalstad, Kristoffer; Westermann, Sebastian; Fiddes, Joel; McCreight, James & Bertino, Laurent (2020). Retrieving, validating, and assimilating fractional snow-covered area from emerging optical satellites for snow reanalysis. Show summary

  Accurately estimating the snow water equivalent (SWE) that is stored in the worlds mountains remains a challenging and important unsolved problem. The SWE reconstruction approach, where the remotely sensed seasonal depletion of fractional snow-covered area (fSCA) is used with a snow model to build up the snowpack in reverse, has been used for decades to help tackle this problem retrospectively. Despite some success, this deterministic approach ignores uncertainties in the snow model, the meteorological forcing, and the remotely sensed fSCA. A trade-off has also existed between the desired temporal and spatial resolution of the satellite-retrieved fSCA depletion. Recently, ensemble-based data assimilation techniques that can account for the uncertainties inherent in the reconstruction exercise have allowed for probabilistic snow reanalyses. In addition, new higher resolution optical satellite constellations such as Sentinel-2 and the PlanetScope cubesats have been launched into polar orbit, potentially eliminating the aforementioned trade-off. We combine these two developments, namely ensemble-based data assimilation and the emerging remotely sensed data streams, to see if snow reanalyses can be improved at the hillslope (100 m) scale in complex terrain. As a first step, we develop accurate high-resolution binary snow-cover maps using a terrestrial automatic camera system installed on a mountaintop near Ny-Ålesund (Svalbard, Norway). These maps are used to validate fSCA retrieved from various satellite sensors (MODIS, Sentinel-2 MSI, and Landsat 8 OLI) using algorithms ranging from simple thresholding of the normalized difference snow index to spectral unmixing. Through the validation, we demonstrate that the spectral unmixing technique can obtain unbiased fSCA retrievals at the hillslope scale. Next, we move to the Mammoth Lakes basin in the Californian Sierra Nevada, USA, where we have access to independent validation data retrieved from several Airborne Snow Observatory (ASO) and Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) flights. Using these airborne retrievals as a reference, we show that fSCA can be retrieved at the hillslope scale with reasonable accuracy at an unprecedented near daily revisit period using a combination of the Landsat, Sentinel-2 MSI, RapidEye, and PlanetScope satellite constellations. In a series of data assimilation experiments we show how the combination of these constellations can lead to significant improvements in hillslope scale snow reanalyses as gauged by various evaluation metrics. Furthermore, it is suggested that an iterative ensemble smoother data assimilation scheme can provide more robust SWE estimates than other smoothers that have previously been proposed for snow reanalysis. We briefly conclude with thoughts as to the current impediments to conducting a global hillslope scale snow reanalysis and propose avenues for further research, such as how snow reanalyses can help in the prediction exercise.

• Aalstad, Kristoffer; Westermann, Sebastian; Pirk, Norbert; Fiddes, Joel; Karsten, Logan; McCreight, James; Alonso-Gonzalez, Esteban & Bertino, Laurent (2020). Constraining land surface models with emerging observations.
• Althuizen, Inge; Christiansen, Casper Tai; Michelsen, Anders; Pirk, Norbert; Risk, David; Westermann, Sebastian & Lee, Hanna (2020). Annual ecosystem carbon budgets across an abrupt permafrost thaw gradient in Northern Norway.
• Althuizen, Inge; Christiansen, Casper Tai; Michelsen, Anders; Pirk, Norbert; Risk, David; Westermann, Sebastian & Lee, Hanna (2020). Annual ecosystem carbon budgets across an abrupt permafrost thaw gradient in Northern Norway.
• Bryn, Anders; Dalen, Thea Grobstok; Finne, Eirik Aasmo; Heiberg, Hanne; Nilsen, Irene Brox; Parmentier, Frans-Jan W.; Snekkenes, Christine; Stordal, Frode; Aas, Kjetil Schanke; Althuizen, Inge; Berntsen, Terje Koren; Bjerke, Jarle W.; Bright, Ryan M.; Dyrrdal, Anita Verpe; Geange, Sonya Rita; Pirk, Norbert; Puschmann, Oskar; Tang, Hui; Torma, Michal; Vollsnes, Ane Victoria; Westermann, Sebastian & Yilmaz, Yeliz (2020). Natur i endring.
• Christiansen, Casper Tai; Michelsen, Anders; Pirk, Norbert; Risk, David; Westermann, Sebastian & Lee, Hanna (2020). Annual ecosystem carbon budgets across an abrupt permafrost thaw gradient in Northern Norway.
• Christiansen, Casper Tai; Michelsen, Anders; Westermann, Sebastian; Pirk, Norbert; Risk, David & Lee, Hanna (2020). Annual ecosystem carbon budgets across an abrupt permafrost thaw gradient in Northern Norway.
• Aalstad, Kristoffer; Westermann, Sebastian & Bertino, Laurent (2019). Ensemble-based retrospective analysis of the seasonal snowpack. Series of dissertations submitted to the Faculty of Mathematics and Natural Sciences, University of Oslo.. 2186. Show summary

  This thesis presents a satellite-based modeling framework that can estimate how much snow was stored in the terrain. These estimates can help guide climate analysis and prediction. Accurate quantification of Earth’s snow mass is a long-standing problem to which a solution is direly needed with ongoing climate change. Snow plays an essential role in the climate system and snowmelt is a vital source of freshwater for a quarter of the world’s population. The framework combines satellite imagery and historic weather data to remotely estimate snow mass by leveraging enhanced ensemble-based data assimilation algorithms. The result is a retrospective analysis (reanalysis) of the snow mass that can be obtained for any location on Earth. So far, this framework has been successfully implemented in three different environments: Svalbard, the Californian Sierra Nevada, and the Swiss Alps. In the future, snow reanalyses could be used to train algorithms to predict snow mass in near real time. They may also help validate and subsequently improve climate models. Ultimately this would allow us to make even more informed future projections of the possible fate of the environment that sustains us.

• Aalstad, Kristoffer; Westermann, Sebastian; Fiddes, Joel & Bertino, Laurent (2019). Retrieving the depletion of snow-covered area from multiple optical satellite sensors with applications for SWE reanalysis.
• Bilt, Willem van der; Bakke, Jostein; Smedsrud, Lars H.; Sund, Monica; Schuler, Thomas; Westermann, Sebastian; Wong, Wai Kwok; Sandven, Stein; Simpson, Matthew James Ross; Skogen, Morten D.; Pavlova, O.; Ravndal, Ole; Risebrobakken, Bjørg; Saloranta, Tuomo; Mezghani, Abdelkader; Nilsen, Jan Even Øie; Nilsen, Irene Brox; Kierulf, Halfdan; Kohler, Jack; Melvold, Kjetil; Gjelten, Herdis Motrøen; Gundersen, Jeanette; Jaedicke, Christian; Dobler, Andreas; Engeset, Rune; Frauenfelder, Regula; Gerland, Sebastian; Christiansen, Hanne H; Børsheim, Knut Yngve; Breivik, Øyvind; Breili, Kristian; Borstad, Christopher Paul; Benestad, Rasmus; Isaksen, Ketil; Adakudlu, Muralidhar; Førland, Eirik; Hisdal, Hege; Lutz, Julia; Mayer, Stephanie; Hanssen-Bauer, Inger; Sandø, Anne Britt; Andersen, Jess; Nilsen, Frank; Sorteberg, Asgeir; Li, Hong; Bogen, Jim & Beldring, Stein (2019). Climate in Svalbard 2100. Norwegian Centre for Climate Services Reports. 1/2019. Full text in Research Archive.
• Cai, Lei; Lee, Hanna; Westermann, Sebastian & Aas, Kjetil Schanke (2019). Development of the subgrid excess ground ice framework in the Community Land Model.
• Etzelmüller, Bernd; Patton, Henry; Schomacker, Anders; Hubbard, Alun Lloyd; Czekirda, Justyna & Westermann, Sebastian (2019). Permafrost dynamics in Iceland between 18 ka BP and today – model results and geomorphological implications. Show summary

  Periglacial processes and the dynamics of permafrost is a decisive factor for slope stability locally, and for understanding landscape development over longer time scales. Iceland has a highly dynamic landscape because of young bedrock and associated high geomorphological process rates, leading to large material production and frequent gravitational processes in the present periglacial realm. At present, permafrost in Iceland is widespread in mountain settings over c. 800 m a.s.l. and sporadically in palsa mires in the central Highlands. However, during the late Pleistocene and Holocene, the periglacial environment in Iceland must have varied strongly in time and space, with subsequent imprint in the landscape. To evaluate the dynamics of permafrost in Iceland since the onset of the last deglaciation, we used the forcing and output of a 3D, time-integrated ice sheet model to run a transient permafrost model (CryoGRID 2) between the onset of the last deglaciation (c. 18 ka BP) until today. The permafrost model was forced by either modeled sub-glacial temperatures if ice-covered, or air temperatures if the area was deglaciated. The results give insights into the possible age of permafrost in Iceland, distinguish areas with wide-spread paleo-permafrost and let us determine the persistence of permafrost in the different areas. The presentation discusses these results in the light of periglacial processes, landforms and landscape development

• Aalstad, Kristoffer; Westermann, Sebastian; Bertino, Laurent; Schuler, Thomas; Boike, Julia & Karsten, Logan (2018). Towards high-resolution Bayesian snow reconstruction in permafrost regions.
• Aalstad, Kristoffer; Westermann, Sebastian; Karsten, Logan; Fiddes, Joel & Bertino, Laurent (2018). Snow history matching in mountainous terrain.
• Aalstad, Kristoffer; Westermann, Sebastian; Karsten, Logan; Gutmann, Ethan; McCreight, James; Fiddes, Joel & Bertino, Laurent (2018). Ensemble-based reanalysis of the seasonal montane snowpack: Lessons from the ASO.
• Christiansen, Casper Tai; Westermann, Sebastian; Risk, David & Lee, Hanna (2018). Advancing permafrost carbon climate feedback – improvements and evaluations of the Norwegian Earth System Model with observations.
• Lee, Hanna; Ekici, Altug; Robson, Benjamin Aubrey; Fan, Yuanchao; Westermann, Sebastian & Langer, Moritz (2018). Vulnerability of permafrost thaw and the emerging risks for the Arctic infrastructure.
• Aalstad, Kristoffer; Westermann, Sebastian; Schuler, Thomas; Boike, Julia & Bertino, Laurent (2017). Ensemble-based subgrid snow data assimilation.
• Aalstad, Kristoffer; Westermann, Sebastian; Schuler, Thomas; Boike, Julia & Bertino, Laurent (2017). Towards High-Resolution SWE Mapping in Permafrost Regions.
• Aalstad, Kristoffer; Westermann, Sebastian; Schuler, Thomas; Boike, Julia & Bertino, Laurent (2017). Towards High-Resolution SWE Mapping in Permafrost Regions.
• Aas, Kjetil Schanke; Westermann, Sebastian; Martin, Leo Celestin Paul & Berntsen, Terje Koren (2017). Degrading Palsa Mires in Northern Norway Simulated with a Regional Climate Model with a Subgrid Snow Scheme.
• Lee, Hanna; Christiansen, Casper Tai & Westermann, Sebastian (2017). Advancing permafrost carbon climate feedback – improvements and evaluations of the Norwegian Earth System Model with observations (FEEDBACK).
• Aalstad, Kristoffer; Bertino, Laurent & Westermann, Sebastian (2016). An ensemble-based subgrid snow data assimilation framework.
• Aalstad, Kristoffer; Westermann, Sebastian & Bertino, Laurent (2016). An ensemble-based snow data assimilation framework.
• Aalstad, Kristoffer; Westermann, Sebastian & Bertino, Laurent (2016). An ensemble-based subgrid snow data assimilation framework.
• Aalstad, Kristoffer; Westermann, Sebastian & Bertino, Laurent (2016). An ensemble-based subgrid snow data assimilation framework applied to the southern Swiss alps.
• Aalstad, Kristoffer; Westermann, Sebastian; Boike, Julia; Bertino, Laurent & Aas, Kjetil Schanke (2016). An ensemble-based snow data assimilation framework with applications to permafrost modeling.
• Hilger, Paula; Hermanns, Reginald; Etzelmüller, Bernd; Westermann, Sebastian; Krautblatter, Michael; Gosse, John C.; Magnin, Florence & Jacobs, Benjamin (2016). Evaluation of rock-slope failures in steep permafrost slopes in Norway - first insights in the geological history of the Mannen unstable rock slope in Møre og Romsdal.
• Myhra, Kristin Sæterdal; Etzelmuller, Bernd & Westermann, Sebastian (2016). The Influence of Steep Rock Walls on the Thermal Regime of Talus Slopes.
• Nuth, Christopher; Kääb, Andreas; Westermann, Sebastian & Winsvold, Solveig Havstad (2016). Climate Change from Global Monitoring of Glaciers and Permafrost.
• Obu, Jaroslav; Westermann, Sebastian; Kääb, Andreas & Bartsch, Annett (2016). Mapping the Thermal State of Permafrost Through Modelling and Remote Sensing in ESA GlobPermafrost Project.
• Schuler, Thomas; Aalstad, Kristoffer; Aas, Kjetil Schanke; Burkhart, John; Dunse, Thorben; Filhol, Simon; Hulth, John; Østby, Torbjørn Ims & Westermann, Sebastian (2016). Towards real-time snow products for Svalbard.
• Westermann, Sebastian; Langer, Moritz; Østby, Torbjørn Ims; Peter, Maria; Boike, Julia; Gisnås, Kjersti; Aalstad, Kristoffer; Schuler, Thomas; Etzelmüller, Bernd; Jaroslav, Obu; Georg, Schwamborn & Lutz, Schirrmeister (2016). Mapping the thermal state of permafrost through modeling and remote sensing.
• Gisnås, Kjersti; Westermann, Sebastian; Etzelmuller, Bernd; Schuler, Thomas; Isaksen, Ketil; Litherland, Tobias & Boike, Julia (2014). A probabilistic approach to represent small-scale variability of permafrost temperatures due to snow cover.
• Aas, Kjetil Schanke; Berntsen, Terje Koren; Boike, Julia; Etzelmuller, Bernd; Gisnås, Kjersti & Westermann, Sebastian (2014). Simulating the surface energy balance on Svalbard with the Weather Research and Forecasting model.
• Antonova, Sonya; Langer, Moritz; Kääb, Andreas; Heim, Birgit; Boike, Julia; Bartsch, Annett & Westermann, Sebastian (2014). Spatial and temporal variability of the interferometric coherence based on TerraSAR-X imagery of the central Lena River Delta.
• Gisnås, Kjersti; Westermann, Sebastian; Schuler, Thomas & Etzelmuller, Bernd (2014). Towards Inclusion of Sub-grid Variability of Snow in Distributed Permafrost Models.
• Myhra, Kristin Sæterdal; Etzelmuller, Bernd & Westermann, Sebastian (2014). Sensitivity of Rock Slope Stability in Relation to Thermal Regime - a Study From Norway.
• Myhra, Kristin Sæterdal; Etzelmuller, Bernd & Westermann, Sebastian (2014). The Sensitivity of Rock Wall Temperatures Along a Latitudinal Transect in Norway.
• Peter, Maria; Langer, Moritz; Boike, Julia; Westermann, Sebastian & Etzelmuller, Bernd (2014). Transient permafrost modeling of the Lena River Delta, Siberia.

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