Sascha Willmes

• PhD
• Research and teaching associate at Trier University

The presence of sea-ice leads represents a key feature of the Arctic sea ice cover. Leads promote the flux of sensible and latent heat from the ocean to the cold winter atmosphere and are thereby crucial for air-sea-ice-ocean interactions. We here apply a binary segmentation procedure to identify leads from MODIS thermal infrared imagery on a daily...

The opacity of clouds is the main problem for optical and thermal space-borne sensors, like the Moderate-Resolution Imaging Spectroradiometer (MODIS). Especially during polar nighttime, the low thermal contrast between clouds and the underlying snow/ice results in deficiencies of the MODIS cloud mask and affected products. There are different appro...

Polynyas and leads are key elements of the wintertime Arctic sea-ice cover. They play a crucial role in surface heat loss, potential ice formation and consequently in the seasonal sea-ice budget. While polynyas are generally sufficiently large to be observed with passive microwave satellite sensors, the monitoring of narrow leads requires the use o...

Satellite observations of microwave brightness temperatures between 19 GHz and 85 GHz are the main data source for operational sea-ice monitoring. However, the sea ice microwave emissivity is subject to pronounced seasonal variations and shows significant hemispheric contrasts that mainly arise from differences in the rate and strength of snow meta...

The temporal and spatial variability of sea-ice radar signatures in the Southern Ocean during late winter, spring and early summer from QuikSCAT data is presented. We observe a circumpolar and broad band of sea-ice close to the marginal ice zone that is characterized by very high radar backscatter. This feature is explained through detailed in situ...

Sea-ice leads play a key role in the climate system by facilitating heat and moisture exchanges between the ocean and atmosphere, as well as by providing essential habitats for marine life. This study presents a new dataset on monthly sea-ice leads in the Southern Ocean and a first comprehensive analysis of spatial patterns, seasonal variability, a...

Sea-ice leads are narrow, linear fractures in sea ice, and are an important basis for understanding the mechanism of the atmosphere-sea ice-ocean system in the Southern Ocean. We use monthly sea-ice lead frequencies based on satellite thermal imagery with 1 km2 grid resolution to investigate potential causes for the observed spatial and temporal va...

Leads and fractures in sea ice play a crucial role in the heat and gas exchange between the ocean and atmosphere, impacting atmospheric, ecological, and oceanic processes. We estimated lead fractions from high-resolution divergence obtained from satellite synthetic aperture radar (SAR) data and evaluated them against existing lead products. We deri...

The Arctic warms nearly four times faster than the global average, and aerosols play an increasingly important role in Arctic climate change. In the Arctic, sea salt is a major aerosol component in terms of mass concentration during winter and spring. However, the mechanisms of sea salt aerosol production remain unclear. Sea salt aerosols are typic...

Leads and fractures in sea ice play a crucial role in the heat and gas exchange between the ocean and atmosphere, impacting atmospheric, ecological, and oceanic processes. Our aim was to estimate lead fractions from high-resolution divergence obtained from satellite synthetic-aperture radar (SAR) data and to evaluate it against existing lead produc...

We use a novel sea-ice lead climatology for the winters of 2002/03 to 2020/21 based on satellite observations with 1 km2 spatial resolution to identify predominant patterns in Arctic wintertime sea-ice leads. The causes for the observed spatial and temporal variabilities are investigated using ocean surface current velocities and eddy kinetic energ...

We use a novel sea-ice lead climatology based on satellite observations with 1 km2 spatial resolution to identify predominant patterns in Arctic wintertime sea-ice leads. The causes for the observed spatial and temporal variabilities are investigated using ocean surface current velocities and eddy kinetic energies from an ocean model (FESOM) and wi...

Mechanisms behind the phenomenon of Arctic amplification are widely discussed. To contribute to this debate, the ( AC ) ³ project has been established in 2016 ( http://www.ac3-tr.de/ ). It comprises modeling and data analysis efforts as well as observational elements. The project has assembled a wealth of ground-based, airborne, ship-borne, and sat...

Climate observations inform about the past and present state of the climate system. They underpin climate science, feed into policies for adaptation and mitigation, and increase awareness of the impacts of climate change. The Global Climate Observing System (GCOS), a body of the World Meteorological Organization (WMO) assesses the maturity of the r...

The ship-based experiment MOSAiC 2019/2020 was carried out during a full year in the Arctic and yielded an excellent data set to test the parameterizations of ocean/sea-ice/atmosphere interaction processes in regional climate models (RCMs). In the present paper, near-surface data during MOSAiC are used for the verification of the RCM COnsortium for...

The Arctic is warming faster than anywhere else on Earth, prompting glacial melt, permafrost thaw, and sea ice decline. These severe consequences induce feedbacks that contribute to amplified warming, affecting weather and climate globally. Aerosols and clouds play a critical role in regulating radiation reaching the Arctic surface. However, the ma...

Knowledge of the wintertime sea-ice production in Arctic polynyas is an important requirement for estimations of the dense water formation, which drives vertical mixing in the upper ocean. Satellite-based techniques incorporating relatively high resolution thermal-infrared data from MODIS in combination with atmospheric reanalysis data have proven...

Climate observations inform about the past and present state of the climate system. They underpin climate science, feed into policies for adaptation and mitigation, and increase awareness of the impacts of climate change. The Global Climate Observing System (GCOS), a body of the World Meteorological Organization (WMO) assesses the maturity of the r...

We combine satellite data products to provide a first and general overview of the physical sea ice conditions along the drift of the international Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition and a comparison with previous years (2005–2006 to 2018–2019). We find that the MOSAiC drift was around 20 % fas...

We combine satellite data products to provide a first and general overview of the sea-ice conditions along the MOSAiC drift and a comparison with previous years. We find that the MOSAiC drift was around 25 % faster than the climatological mean drift, as a consequence of large-scale low-pressure anomalies prevailing around the Barents-Kara-Laptev Se...

The parameterization of ocean/sea-ice/atmosphere interaction processes is a challenge for regional climate models (RCMs) of the Arctic, particularly for wintertime conditions, when small fractions of thin ice or open water cause strong modifications of the boundary layer. Thus, the treatment of sea ice and sub-grid flux parameterizations in RCMs is...

Landfast sea ice (fast ice) is an important component of the Antarctic nearshore marine environment, where it strongly modulates ice sheet–ocean–atmosphere interactions and biological and biogeochemical processes, forms a key habitat, and affects logistical operations. Given the wide-ranging importance of Antarctic fast ice and its sensitivity to c...

Plain Language Summary Clouds are a key factor in the energy budget of the Arctic atmosphere. Ice nucleating particles (INPs) can modify the radiation properties and lifetime of clouds by affecting the relative abundance of liquid and frozen droplets in a cloud. Despite this important ability, knowledge about the INP concentration above ground leve...

The presence of sea ice leads in the sea ice cover represents a key feature in polar regions by controlling the heat exchange between the relatively warm ocean and cold atmosphere due to increased fluxes of turbulent sensible and latent heat. Sea ice leads contribute to the sea ice production and are sources for the formation of dense water which a...

Landfast sea ice (fast ice) is an important component of the Antarctic nearshore marine environment, where it strongly modulates ice sheet-ocean-atmosphere interactions and biological and biogeochemical processes, forms a key habitat, and affects logistical operations. Given the wide-ranging importance of Antarctic fast ice and its sensitivity to c...

Sea ice is of substantial importance for the Southern Ocean, as it insulates the relatively warm ocean from the cold atmosphere. Due to mechanical stress induced by wind and ocean currents, sea ice leads occur, which are characterized by open water and thin ice causing an increase of energy and moisture fluxes between ocean and atmosphere. Furtherm...

Antarctic coastal polynyas serve as crucially-important sea-ice “factories” and are (in certain cases) of global significance as sites of Antarctic Bottom Water (AABW) formation e.g., the Cape Darnley Polynya (CDP) in East Antarctica. As such, understanding change and variability in their behaviour, and the factors responsible, is a high priority i...

Precise knowledge of wintertime sea ice production in Arctic polynyas is not only required to enhance our understanding of atmosphere‐sea ice‐ocean interactions but also to verify frequently utilized climate and ocean models. Here, a high‐resolution (2‐km) Moderate Resolution Imaging Spectroradiometer (MODIS) thermal infrared satellite data set fea...

It is well-known that katabatic winds can be detected as warm signatures in the surface temperature over the slopes of the Antarctic ice sheets. For appropriate synoptic forcing and/or topographic channeling, katabatic surges occur, which result in warm signatures also over adjacent ice shelves. Moderate Resolution Imaging Spectroradiometer (MODIS)...

A substantial part of the Asian coast line borders seasonally sea-ice covered waters. This chapter deals with the sea-ice cover along the northern shores facing the Arctic Ocean. Long-term year-round, sustained monitoring of this sea-ice cover is, on the one hand, crucial for shipping, off-shore activities, near-coastal transport, and marine safety...

High-resolution MODIS thermal infrared satellite data are used to infer spatial and temporal characteristics of 17 prominent coastal polynya regions over the entire Arctic basin. Thin-ice thickness (TIT) distributions ( ≤ 20cm) are calculated from MODIS ice-surface temperatures, combined with ECMWF ERA-Interim atmospheric reanalysis data in an ener...

The quantification of sea-ice production in the Laptev Sea polynyas is important for the Arctic sea-ice budget and the heat loss to the atmosphere. We estimated the ice production for the winter season 2007/2008 (November–April) based on simulations with the regional climate model COSMO-CLM at a horizontal resolution of 5 km and compared it to remo...

In the early 1980s, Germany started a new era of modern Antarctic research. The Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research (AWI) was founded and important research platforms such as the German permanent station in Antarctica, today called Neumayer III, and the research icebreaker Polarstern were installed. The research...

High-resolution MODIS thermal infrared satellite data are used to infer spatial and temporal characteristics of 16 prominent coastal polynya regions over the entire Arctic basin. Thin-ice thickness distributions (≤ 20 cm) are calculated from MODIS ice-surface temperatures, combined with ECMWF ERA-Interim atmospheric reanalysis data in an energy bal...

An improved understanding of the temporal variability and the spatial distribution of snowmelt on Antarctic sea ice is crucial to better quantify atmosphere-ice-ocean interactions, in particular sea-ice mass and energy budgets. It is therefore important to understand the mechanisms that drive snowmelt, both at different times of the year and in dif...

A tile-approach (TA) for the calculation of the energy balance over fractional sea ice was implemented into the standard version of the COSMO-CLM (CCLM) model. The tile-approach accounts for subgrid-scale energy exchange within polynyas and leads, which are neglected in the CCLM standard version. We perform six simulations for the area of the Lapte...

Sea ice leads play an essential role in ocean-ice-atmosphere exchange, in ocean circulation, geochemistry, and in ice dynamics. Their precise detection is crucial for altimetric estimations of sea ice thickness and volume. This study evaluates the performance of the SARAL/AltiKa (Satellite with ARgos and ALtiKa) altimeter to detect leads and to mon...

The North Water (NOW) Polynya is a regularly-forming area of open-water and thin-ice, located between northwestern Greenland and Ellesmere Island (Canada) at the northern tip of Baffin Bay. Due to its large spatial extent, it is of high importance for a variety of physical and biological processes, especially in wintertime. Here, we present a long-...

Based upon thermal-infrared satellite imagery in combination with ERA-Interim atmospheric reanalysis data, we derive long-term polynya characteristics such as polynya area, thin-ice thickness distribution, and ice-production rates for a 13-year investigation period (2002–2014) for the austral winter (1 April to 30 September) in the Antarctic southe...

Based upon high-resolution thermal-infrared Moderate-Resolution Imaging Spectroradiometer (MODIS) satellite imagery in combination with ERA-Interim atmospheric reanalysis data, we derived long-term polynya parameters such as polynya area, thin-ice thickness distribution and ice-production rates from daily cloud-cover corrected thin-ice thickness co...

Spatial and temporal characteristics of the Storfjorden polynya, which forms regularly in the proximity of the islands Spitsbergen, Barentsøya and Edgeøya in the Svalbard archipelago under the influence of strong northeasterly winds, have been investigated for the period of 2002/2003 to 2013/2014 using thermal infrared satellite imagery. Thin-ice t...

Sea ice leads play an essential role in ocean-ice-atmosphere exchange, in ocean circulation, geochemistry and in ice dynamics. Their precise detection is crucial for altimetric estimations of sea ice thickness and volume. This study evaluates the performance of the SARAL/AltiKa altimeter to detect leads and to monitor their spatio-temporal dynamics...

Basal melt of ice shelves may lead to an accumulation of disc-shaped ice platelets underneath nearby sea ice, to form a sub-ice platelet layer. Here we present the seasonal cycle of sea ice attached to the Ekström Ice Shelf, Antarctica, and the underlying platelet layer in 2012. Ice platelets emerged from the cavity and interacted with the fast-ice...

Up to now, snow cover on Antarctic sea ice and its impact on radar backscatter, particularly after the onset of freeze-thaw processes, are not well understood. Here, we present a combined analysis of in situ observations of snow properties from the landfast sea ice in Atka Bay (Antarctica) and high-resolution TerraSAR-X backscatter data, for the tr...

Based upon high-resolution thermal-infrared Moderate-Resolution Imaging Spectroradiometer (MODIS) satellite imagery in combination with ERA-Interim atmospheric reanalysis data, we derived long-term polynya parameters such as polynya area, thin-ice thickness distribution and ice-production rates from daily cloud-cover corrected thin-ice thickness co...

Up to now, snow cover on Antarctic sea ice and its impact on radar backscatter, particularly after the onset of freeze/thaw processes, are not well understood. Here we present a combined analysis of in situ observations of snow properties from the landfast sea ice in Atka Bay, Antarctica, and high-resolution TerraSAR-X backscatter data, for the tra...

The presence of sea-ice leads represents a key feature of the Arctic sea ice cover. Leads promote the flux of sensible and latent heat from the ocean to the cold winter atmosphere and are thereby crucial for air-sea-ice-ocean interactions. We here apply a binary segmentation procedure to identify leads from MODIS thermal infrared imagery on a daily...

The North Water (NOW) Polynya is a regularly-forming area of open-water and thin-ice, located between northwestern Greenland and Ellesmere Island (Canada) at the northern tip of Baffin Bay. Due to its large spatial extent, it is of high importance for a variety of physical and biological processes, especially in wintertime. Here, we present a long-...

Spatial and temporal characteristics of the Storfjorden polynya, which forms regularly in the proximity of the islands Spitsbergen, Barentsøya and Edgeøya in the Svalbard archipelago under the influence of strong north-easterly winds, have been investigated for the period 2002/2003 to 2013/2014 using thermal infrared satellite imagery. Thin-ice thi...

Satellite observations of microwave brightness temperatures between 19 GHz and 85 GHz are the main data sources for operational sea-ice monitoring and retrieval of ice concentrations. However, microwave brightness temperatures depend on the emissivity of snow and ice, which is subject to pronounced seasonal variations and shows significant hemisphe...

Storfjorden, which hosts a latent heat polynya, is a well known region of dense water formation. This Brine-enriched Shelf Water (BSW) displays substantial year to year variability in its properties, which is partly linked to interannual variations in ice production. Here we have developed a model based on high-resolution AMSR-E satellite sea-ice c...

Basal melt of ice shelves may lead to an accumulation of disc-shaped ice platelets underneath nearby sea ice, to form a sub-ice platelet layer. Here we present the seasonal cycle of sea ice attached to the Ekström Ice Shelf, Antarctica, and the underlying platelet layer in 2012. Ice platelets emerged from the cavity and interacted with the fast-ice...

Arctic flaw polynyas are considered to be highly productive areas for the formation of sea-ice throughout the winter season. Most estimates of sea-ice production are based on the surface energy balance equation and use global reanalyses as atmospheric forcing, which are too coarse to take into account the impact of polynyas on the atmosphere. Addit...

Considering the sea ice decline in the Arctic during the last decades, polynyas are of high research interest since these features are core areas of new ice formation. The determination of ice formation requires accurate retrieval of polynya area and thin-ice thickness (TIT) distribution within the polynya. We use an established energy balance mode...

The polynyas of the Laptev Sea are regions of particular interest due to the strong formation of Arctic sea ice. The determination of ice formation requires accurate retrieval of polynya area and thin-ice thickness distribution within the polynya. Based on ice-surface temperatures from MODIS satellite data and NCEP atmospheric reanalysis data daily...

This report described the sea-ice measurements on Atka Bay landfast sea ice in 2012.

Variability and trends in seasonal and interannual ice area export out of the Laptev Sea between 1992 and 2011 are investigated using satellite-based sea ice drift and concentration data. We found an average total winter (October to May) ice area transport across the northern and eastern Laptev Sea boundaries (NB and EB) of 3.48 × 105 km2. The aver...

Accurate knowledge of ice-production rates within the marginal ice zones of the Arctic Ocean requires monitoring of the thin-ice distribution within polynyas. The thickness of the ice layer controls the heat loss and hence the new-ice formation. An established thin-ice algorithm using high-resolution MODIS data allows deriving the ice-thickness dis...

A simple polynya flux model driven by standard atmospheric forcing is used to investigate the ice formation that took place during an exceptionally strong and consistent western New Siberian (WNS) polynya event in 2004 in the Laptev Sea. Whether formation rates are high enough to erode the stratification of the water column beneath is examined by a...

Previous versions of the Consortium for Small-scale Modelling (COSMO) numerical weather prediction model have used a constant sea-ice surface temperature, but observations show a high degree of variability on sub-daily timescales. To account for this, we have implemented a thermodynamic sea-ice module in COSMO and performed simulations at a resolut...

Sea-ice concentrations in the Laptev Sea simulated by the coupled North Atlantic-Arctic Ocean-Sea-Ice Model and Finite Element Sea-Ice Ocean Model are evaluated using sea-ice concentrations from Advanced Microwave Scanning Radiometer-Earth Observing System satellite data and a polynya classification method for winter 2007/08. While developed to sim...

Advanced Microwave Scanning Radiometer (AMSR-E) snow-depth data for Antarctic sea ice are compared with ship-based visual observations of snow depth, ice type and ridged-ice fraction, and with satellite C-band and Ku-band radar backscatter observations for two ship cruises into the Weddell Sea (ISPOL 2004–05, WWOS 2006) and one cruise into the Bell...

We test the ability of a two-dimensional flux model to simulate polynya events with narrow open-water zones by comparing model results to ice-thickness and ice-production estimates derived from thermal infrared Moderate Resolution Imaging Spectroradiometer (MODIS) observations in conjunction with an atmospheric dataset. Given a polynya boundary and...

Polynyas in the Laptev Sea are examined with respect to recurrence and interannual wintertime ice production.We use a polynya classification method based on passive microwave satellite data to derive daily polynya area from long-term sea-ice concentrations. This provides insight into the spatial and temporal variability of open-water and thin-ice r...

Combined salinity and δ18O data from summer 2007 reveal a significant change in brine production in the Laptev Sea relative to summer 1994. The distribution of river water and brine-enriched waters on the Laptev Sea shelf is derived based on mass balance calculations using salinity and δ18O data. While in 1994 maximal influence of brines is seen wi...

This paper examines the role of atmospheric forcing in modifying the pathways of riverine water on the Laptev Sea shelf, using summer-to-winter hydrographic surveys from 2007 to 2009. Over the two consecutive winter seasons of 2007–2008 and 2008–2009 in the area of the winter coastal polynya, our data clearly link winter surface salinity fields to...

Polynyas in the Laptev Sea are examined with respect to recurrence and inter-annual wintertime ice production. We use a polynya classification method based on passive microwave satellite data to derive daily polynya area from long-term sea-ice concentrations. This provides insight into the spatial and temporal variability of open water and thin ice...

The Laptev Sea polynyas play a key role for the shelf areas of the Siberian Arctic due to their impact on ice production. Changes in polynya dynamics result in modified fluxes of energy, momentum and matter in the atmosphere-ocean-sea ice system. An improved understanding and quantification of polynya effects in the Laptev Sea can be achieved by hi...

Sea-ice concentrations in the Laptev Sea simulated by the coupled North Atlantic - Arctic Ocean - Sea-Ice Model (NAOSIM) and Finite Element Sea-Ice Ocean Model (FESOM) are verified using sea-ice concentrations from AMSR-E satellite data and a polynya classification method for winter 2007/08. Simulated sea-ice fields from different model runs are co...

Previous versions of the numerical weather prediction model COSMO (Consortium for Small-Scale Modeling) have used a constant sea ice surface temperature, but observations show a high degree of variability on sub-daily time-scales. To account for this, we have implemented a thermodynamic sea ice module in COSMO and performed simulations at a resolut...

Processes of the exchange of energy and momentum at the sea ice-ocean-atmosphere interface are key processes for the polar climate system. Heat and moisture fluxes are strongly modulated by open water fractions associated with polynyas, having important consequences for the atmosphere, ocean processes, ice formation, brine release, gas exchange and...

Wind-driven coastal polynyas in the polar oceans are recognized as regions of extensive new ice formation in the cold season. Hence, they may play an increasing role in the uncertain future of the sea-ice budget in the polar oceans. The Laptev Sea polynyas in the Siberian Arctic are well recognized as being significant ice producers and might gain...

Observations of snow properties, superimposed ice, and atmospheric heat fluxes have been performed on first-year and second-year sea ice in the western Weddell Sea, Antarctica. Snow in this region is particular as it does usually survive summer ablation. Measurements were performed during Ice Station Polarstern (ISPOL), a 5-week drift station of th...

In this paper first results of a sensitivity study using dual polarimetric TerraSAR-X data for ice thickness estimation are presented. The sea ice thickness reference data set was measured, coincident to the SAR data take, by means of a helicopter-borne EM sounding device on April 28, 2008 in the Russian Arctic. For some of the signatures, namely t...

Temporal snowmelt dynamics on fast ice in Kongsfjorden/Svalbard are studied for the period 1990–2003, using visible and near-infrared channels of the Advanced Very High Resolution Radiometer (AVHRR). Long-term radiation data from an adjacent Baseline Surface Radiation Network station, as well as extensive glaciological and meteorological field meas...

The winter net sea-ice production (NSIP) over the Laptev Sea shelf is inferred from continuous summer-to-winter historical salinity records of 1960s-1990s. While the NSIP strongly depends on the assumed salinity of newly formed ice, the NSIP quasi-decadal variability can be linked to the wind-driven circulation anomalies in the Laptev Sea region. T...