Meereis-Produktion und Eistransport

Das Fach Umweltmeteorologie an der Universität Trier führt seit 2007 Kooperationsprojekte mit Russland durch. Diese Kooperation kann in den kommenden drei Jahren (2013-2016) im Rahmen eines durch das Bundesministerium für Bildung und Forschung (BMBF) geförderten Projekts für Wissenschaftlich-Technische Zusammenarbeit (WTZ) zwischen Deutschland und Russland fortgesetzt werden

Aufbruch in den sibirischen Winter – Klimamessungen in der Laptev-See

Wie wirken sich Prozesse der Wechselwirkung zwischen Atmosphäre, Meereis und Ozean entlang der Küstenränder der Laptev-See aus? Was passiert, wenn Meereis durch Wind wegtransportiert wird? Von der Expeditionsbasis in der sibirischen Hafenstadt Tiksi, starteten Wissenschaftler mit dem Hubschrauber in die Eiscamps auf dem Meereis. Vom Festeis am Rand der Polynja wurden zahlreiche meteorologische und ozeanographische Messungen durchgeführt sowie hydrologische und biologische Proben aus verschiedenen Wassertiefen der Laptev-See genommen. Kurzzeit-Meeres- bodenobservationen nahmen während der gesamten Dauer der Expedition kontinuierlich ozeano- graphische Daten auf

An Aircraft-Based Study of Strong Gap Flows in Nares Strait, Greenland

Gap flows and the stable boundary layer were studied in northwest Greenland during the aircraft-based Investigation of Katabatic Winds and Polynyas during Summer (IKAPOS) experiment in June 2010. The measurements were performed using the research aircraft POLAR 5 of Alfred Wegener Institute (AWI; Bremerhaven). Besides navigational and basic meteorological instrumentation, the aircraft was equipped with radiation and surface temperature sensors and a turbulence measurement system. In the area of Smith Sound at the southern end of the Nares Strait, a stable, but fully turbulent, boundary layer with strong winds of up to 22 m s−1 was found during conditions of synoptically induced northerly winds through the Nares Strait. Strong surface inversions were present in the lowest 100–200 m. As a consequence of channeling effects, a well-pronounced low-level jet system was documented for each of four flights. The wind maximum is located at 20–50-km distance from the exit of Smith Sound. The 3D boundary layer structure past this gap is studied in detail. The channeling process is consistent with gap flow theory. The flow through the gap and over the surrounding mountains leads to the lowering of isotropic surfaces and the acceleration of the flow. The orographically channeled flow through Smith Sound plays a key role for the formation of the North Water polynya being the largest ice-producing polynya in the Arctic

Improvement and Sensitivity Analysis of Thermal Thin-Ice Thickness Retrievals

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 model to derive TITs with MODIS ice surface temperatures $(T_{s})$ and NCEP/DOE Reanalysis II in the Laptev Sea for two winter seasons. Improvements of the algorithm mainly concern the implementation of an iterative approach to calculate the atmospheric flux components taking the atmospheric stratification into account. Furthermore, a sensitivity study is performed to analyze the errors of the ice thickness. The results are the following: 1) 2-m air temperatures $(T_{a})$ and $T_{s}$ have the highest impact on the retrieved ice thickness; 2) an overestimation of $T_{a}$ yields smaller ice thickness errors as an underestimation of $T_{a}$; 3) NCEP $T_{a}$ shows often a warm bias; and 4) the mean absolute error for ice thicknesses up to 20 cm is $pm$4.7 cm. Based on these results, we conclude that, despite the shortcomings of the NCEP data (coarse spatial resolution and no polynyas), this data set is appropriate in combination with MODIS $T_{s}$ for the retrieval of TITs up to 20 cm in the Laptev Sea region. The TIT algorithm can be applied to other polynya regions and to past and future time periods. Our TIT product is a valuable data set for verification of other model and remote sensing ice thickness data

Tracking of synoptic weather systems in the Siberian Arctic and their impact on the Laptev Sea polynya

Synoptic weather systems in the Arctic are studied using a track algorithm based on a feature-tracking method. The tracking is performed with the 850 hPa relative vorticity field derived from the NCEP/NCAR reanalysis dataset for the winter seasons 1978-2007. A climatology of synoptic systems is calculated for the whole Arctic. A low cyclone track density is found for eastern Siberia and the Pacific part of the Arctic. Cyclonic and anticyclonic track density decreases from the first half to the second half of the 30-year investigation period in large parts of the Arctic. A detailed study is performed for the Laptev Sea area of the Siberian Arctic, whis is an active area for sea ice production in flaw polynyas. The reaction of the Laptev polynya system to tracks of cyclones and anticyclones is investigated by selecting favourate track directions. The Anabar-Lena Polynya is affected by cyclones moving eastwards across the Laptev Sea. These cyclones cause an opening of the polynya on the day before the cyclone passage and a closing on the day after. The West-New-Siberian Polynya (WNS) is affected by cyclone tracks moving northwards along the western flank of the Laptev Sea. The cyclones mainly have an opening impact one and two days before the cyclone passage. For the WNS polynya, anticyclones passing from the east have the largest effect

Pan-Arctic lead detection from MODIS thermal infrared imagery

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 of data at a higher spatial resolution. We apply and evaluate different lead segmentation techniques based on sea-ice surface temperatures as measured by the Moderate Resolution Imaging Spectroradiometer (MODIS). Daily lead composite maps indicate the presence of cloud artifacts that arise from ambiguities in the segmentation process and shortcomings in the MODIS cloud mask. A fuzzy cloud artifact filter is hence implemented to mitigate these effects and the associated potential misclassification of leads. The filter is adjusted with reference data from thermal infrared image sequences, and applied to daily MODIS data from January to April 2008. The daily lead product can be used to deduct the structure and dynamics of wintertime sea-ice leads and to assess seasonal divergence patterns of the Arctic Ocean

Polynya dynamics and associated atmospheric forcing at the Ronne Ice Shelf

The Ronne Ice Shelf is known as one of the most active regions of polynya developments around the Antarctic continent. Low temperatures are prevailing throughout the whole year, particularly in winter. It is generally recognized that polynya formations are primarily forced by offshore winds and secondarily by ocean currents. Many authors have addressed this issue previously at the Ross Ice Shelf and Adélie Coast and connected polynya dynamics to strong katabatic surge events. Such investigations of atmospheric dynamics and simultaneous polynya occurrence are still severely underrepresented for the southwestern part of the Weddell Sea and especially for the Ronne Ice Shelf. Due to the very flat terrain gradients of the ice shelf katabatic winds are of minor importance in that area. Other atmospheric processes must therefore play a crucial role for polynya developments at the Ronne Ice Shelf. High-resolution simulations have been carried out for the Weddell Sea region using the non-hydrostatic NWP model COSMO from the German Meteorological Service (DWD). For the austral autumn and winter (March to August) 2008 daily forecast simulations were conducted with the consideration of daily sea-ice coverage deduced from the passive microwave system AMSR-E. These simulations are used to analyze the synoptic and mesoscale atmospheric dynamics of the Weddell Sea region and find linkages to polynya occurrence at the Ronne Ice Shelf. For that reason, the relation between the surface wind speed, the synoptic pressure gradient in the free atmosphere and polynya area is investigated. Seven significant polynya events are identified for the simulation period, three in the autumn and four in the winter season. It can be shown that in almost all cases synoptic cyclones are the primary polynya forcing systems. In most cases the timely interaction of several passing cyclones in the northern and central Weddell Sea leads to maintenance of a strong synoptic pressure gradient above the Ronne Ice Shelf. This strong synoptic forcing results in a moderate to strong offshore surface wind. It turned out that these synoptic depressions lead to strong barrier winds above the northwestern Ronne Ice Shelf and along the eastern flank of the Antarctic Peninsula. The fact, that these barrier winds often appear prior or during the initial break up of sea ice at the shelf ice edge, suggest that this mesoscale wind phenomenon plays a crucial role for polynya development. Furthermore, even mesoscale cyclogenesis above the Ronne Ice Shelf and the following northeastward passage of such a system can break up sea-ice cover under large-scale stationary weather conditions

Sea-Ice Wintertime Lead Frequencies and Regional Characteristics in the Arctic, 2003–2015

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 time scale. The method separates identified leads into two uncertainty categories, with the high uncertainty being attributed to artifacts that arise from warm signatures of unrecognized clouds. Based on the obtained lead detections, we compute quasi-daily pan-Arctic lead maps for the months of January to April, 2003–2015. Our results highlight the marginal ice zone in the Fram Strait and Barents Sea as the primary region for lead activity. The spatial distribution of the average pan-Arctic lead frequencies reveals, moreover, distinct patterns of predominant fracture zones in the Beaufort Sea and along the shelf-breaks, mainly in the Siberian sector of the Arctic Ocean as well as the well-known polynya and fast-ice locations. Additionally, a substantial inter-annual variability of lead occurrences in the Arctic is indicated

A Workshop on Polar Lows

European Polar Low Working Group This workshop summarized the current state of PL research in the Arctic and Antarctic. A couple of related projects are in the planning phase or already funded. The creation of a PL database for the Norwegian Sea in the frame of the Sea Surface Temperature and Altimeter Synergy (STARS) project (http://projects.met.no stars) will provide a valuable resource for future research and, potentially, predictability improvements. The maintenance of this database and the creation of similar databases for other polar areas including satellite and NWP data are highly recommended. There is also a need for free and timely access to satellite data, in particular to SAR data to fill the gap caused by the mission end of Envisat. With the increasing resolution of climate models, mesoscale processes such as polar MCs will have to be considered in international research programs such as the World Climate Research Programme (WCRP) Polar Climate Predictability Initiative and the World Weather Research Programme (WWRP) Polar Predictability Project

Aircraft-based studies of katabatic winds and boundary layer structures over the NOW polynya near Greenland during summer

During June 2010, the aircraft based experiment IKAPOS was performed in northwestern Greenland. The main goals were studies of the summertime katabatic wind system and of the atmospheric boundary layer (ABL) over the North Water Polynya (NOW). Katabatic winds play a key role in exchange processes of energy and momentum between the atmosphere and the underlying surface over the ice sheet of Greenland. During summer, cooling of the boundary layer and hence the katabatic forcing is much less than during winter, but strong winds can occur during appropriate synoptic forcing. On the other hand, the NOW represents one of the largest polynyas of the Arctic, and the air-sea interaction over the NOW has important consequences for ocean processes, ice formation, gas exchange and biology. The present study is based on aicraft measurements in the ABL using the research aircraft POLAR 5 of Alfred-Wegner-Institute (AWI, Bremerhaven). In order to study the turbulence structure and 3D spatial structures of mean quantities POLAR 5 was instrumented with a turbulence measurement system collecting data on a nose boom sampling at a rate of 100 Hz, additional basic meteorological equipment, radiation and surface temperature sensors, laser altimeter, and photo and video cameras. For different synoptic situations four flights over the NOW and one flight each over the Humboldt and the Steenstrup Glaciers were performed. Over the glaciers, katabatic winds with up to 16 m/s wind speed were found. Over the NOW, a stable, but fully turbulent ABL was present during conditions of strong and relatively warm synoptically induced northerly winds. Strong surface inversions were found in the lowest 100 m – 200 m agl. As a consequence of channeling effects at Smith Sound a well-pronounced low-level jet with wind speed maxima of more than 20 m/s was detected. Thus wind-induced sea-ice export from the Nares Strait is considerably increased