Investigation of the dynamics of the North Water polynya for 1996-2010 using satellite data

The North Water polynya (NOW) forms recurrently between Ellesmere Island and northwest Greenland and is one of the largest and therefore most important polynyas in the northern hemisphere. It is considered to have a huge impact on atmospheric, biological and ocean processes. An investigation of polynya dynamics between 1996 and 2009/2010 has been performed using sea ice concentration (SIC) products from the passive microwave sensors SSM/I and AMSR-E. From SIC data we derived the total polynya area (POLA, SIC threshold of <70%), and the open water area (OWA). In June the mean POLA of the NOW reaches its maximum with ca. 101000 km2 (SSM/I) and 79000 km2 (AMSR-E), while the mean OWA has values of 81000 km2 (SSM/I) and 73000 km2 (AMSR-E), respectively. The number of days without the occurrence of the NOW decreases slightly during the period 1996-2010. The pattern of ice formation and decay shows a tendency to earlier melt of sea ice in the northern Baffin Bay in summer compared to previous studies. The dynamics of the ice bridge located at Smith Sound has a large influence on the formation of the NOW. Time series for regions north and south of the bridge are used to identify the ice bridge characteristics associated with its influence on ice export through the Nares Strait. In addition to the long-term study, a case study for a polynya event on 19 March 2009 has been carried out using MODIS data and a surface energy model. MODIS ice surface temperatures and NCEP reanalysis data are used to derive the thermal ice thickness distribution and ice production. A mean ice production of 0.35 cm/h is calculated for this case study

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

Airborne investigations of summertime katabatic winds and boundary layer structures over the NOW polynya near Greenland

The aircraft based experiment IKAPOS (Investigation of Katabatic winds and POlynyas during Summer) was performed in June 2010 over 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). IKAPOS is mainly based on measurements using 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. 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 under 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. 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 14 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

An aircraft-based investigation of the turbulent stable boundary layer over the North Water polynya (Greenland)

Atmosphere/sea-ice/ocean exchange processes over the NOW (North Water) polynya in northwest Greenland were studied during the aircraft-based experiment IKAPOS (Investigation of Katabatic winds and Polynyas during Summer) 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, two laser altimeters, and video and digital cameras. In order to determine turbulent heat and momentum fluxes, POLAR 5 was instrumented with a turbulence measurement system collecting data on a nose boom with a sampling rate of 100 Hz. While a polynya is associated with a convective boundary layer during winter, the situation in early summer is quite different. With the surface temperature at the melting point of ocean water, warm air advection leads to the formation of a stable boundary layer. Over the NOW in June, a stable, but fully turbulent boundary layer with strong winds of 15 m s-1 to 20 m s-1 was found during conditions of relatively warm synoptically induced northerly winds through the Nares Strait. Strong surface inversions were found in the lowest 100 m to 200 m. As a consequence of channeling effects at Smith Sound a well-pronounced low-level jet system was documented. These channeling effects lead to an increased wind-induced sea-ice export from the Nares Strait through Smith Sound. Cross-sections of mean quantities over the polynya are presented. The multiresolution decomposition is used to identify the spectral gap for the computation of turbulent fluxes. For the NOW polynya flights gap scales are between 500 m – 1000 m. Sensible heat fluxes are around -30W/m2 in the area of highest wind speed at Smith Sound. Accordingly, the momentum flux shows also a maximum caused by the channeling, which is an important factor for the process of NOW formation. The experimental data for the fully turbulent SBL are used to test parameterizations of exchange coefficients. The results of IKAPOS are valuable for the validation of numerical models (including climate models) and will contribute to the understanding of the exchange processes over summertime Arctic polynyas and the formation of the NOW polynya

Bildung und Stabilität von Phasen aus Li, Sn, Si und C

Phasenbildungsprozesse und Phasenstabilitäten wurden im binären System Li-C sowie in den ternären Systemen Li-C-Si, Li-C-Sn und Li-Si-Sn untersucht. Im binären Li-C-System lagen die Schwerpunkte der Untersuchungen auf der Charakterisierung der batterierelevanten Li-Graphit-Interkalationsphasen (Li-GICs) sowie auf der Untersuchung der thermodynamischen Stabilität der in diesem System bekannten Phasen LiC6 und Li2C2. Mit Hilfe von Wärmebehandlungsexperimenten und dynamischer Differenzkalorimetrie konnte gezeigt werden, dass die Phase Li2C2 thermodynamisch stabil ist, wohingegen es sich bei der Li-Graphit-Interkalationsphase LiC6 um eine metastabile Phase handelt. Erstmals wurden Gefügeuntersuchungen in Verbindung mit Röntgendiffraktometrie eingesetzt, um Li-GICs mit verschiedenen Li-Gehalten zu charakterisieren. Es wurde gezeigt, dass die Bildung der Li-GICs während des Ein- und Ausbaus von Li nicht in allen Aspekten symmetrisch verläuft. Die Untersuchungen zeigten außerdem, dass die Phase LiC12 einen variablen Li-Gehalt aufweisen kann und somit nicht stöchiometrisch ist. Die bislang komplett unbeschriebenen ternären Systeme Li-Si-C, Li-Sn-C und Li-Si-Sn wurden mittels Wärmebehandlungsexperimenten und dynamischer Differenzkalorimetrie hinsichtlich ihrer Phasengleichgewichte untersucht. Hierfür wurden gezielt bestimmte Zusammensetzungen ausgewählt, welche mittels binärer Phasen aus den Randsystemen präzise eingestellt wurden. Die bei der Untersuchung der ternären Systeme detektierten Phasen weichen deutlich von denen ab, die durch Extrapolation aus den binären Randsystemen vorhergesagt werden. Im System Li-Si-C wurde ein Dreiphasengleichgewicht, welches die Phasen Li7Si3, Li2C2 und SiC enthält, eindeutig identifiziert. Für eine abschließende Aussage über die im thermodynamischen Gleichgewicht vorliegenden Phasen in den Systemen Li-Sn-C und Li-Sn-Si bedarf es einer weiteren, detaillierten Untersuchung. Ternäre Phasen wurden in keinem der betrachteten Systeme beobachtet

Characteristics of intermittent turbulence in the upper stable boundary layer over Greenland

The experiment IGLOS (Investigation of the Greenland Boundary Layer Over Summit) was conducted in June and July 2002 in the central plateau of the Greenland inland ice. The German research aircraft Polar2, equipped with the turbulence measurement system Meteopod, was used to investigate turbulence and radiation flux profiles near research station “Summit Camp”. Aircraft measurements are combined with measurements of radiation fluxes and turbulent quantities made from a 50 m tower at Summit Camp operated by Eidgenössische Technische Hochschule (ETH) Zürich. During all six flight missions, well-developed stable boundary layers were found. Even in high-wind conditions, the surface inversion thickness did not exceed roughly 100 m. The turbulent height of the stable boundary layer (SBL) was found to be much smaller than the surface inversion thickness. Above the surface layer, significant turbulent fluxes occurred only intermittently in intervals on the order of a few kilometres. Turbulent event fraction in the upper SBL shows the same dependence on gradient Richardson number as reported for near-surface measurements. Clear-air longwave radiation divergence was always found to contribute significantly to the SBL heat budget. In low-wind cases, radiative cooling even turned out to be dominant

The surface energy balance of early summer land-fast sea ice in Atka Bay, Antarctica

In-situ measurements of the land-fast sea ice energy balance are scarce. We present a data set that comprises eddy-covariance measurements of sensible and latent heat as well as measurements of the sea-ice temperature gradient, long-wave and short-wave radiation measurements over land-fast sea ice in Atka Bay, Antarctica. With this setup we are able to monitor all components of the sea-ice energy budget. Additionally, we also measured the turbulent flux of CO2 over sea ice. This 37 day-long data set is evaluated for the transition period from austral winter to summer (November to December 2012) with regard to atmospheric stability and the general weather conditions. Results for the eddy-covariance measurements show an average sensible heat flux of 6.45+-10.72 W/m2 and a latent heat flux of 12.71+-9.48 W/m2 (with one standard deviation respectively) for low pressure/high wind-speed conditions. The average net radiation is 44.37+-41.54 W/m2 and for the CO2 flux an average of -3.35+-3.37μmol/m2 was measured. During high pressure/low wind-speed conditions an average of -3.03+-10.48 W/m2 and 10.76+-10.52 W/m2 was recorded for the sensible and latent heat flux, while the average net radiation and the CO2 flux are 35.63+-56.70 W/m2 and -1.95+-1.72μmol/m2 respectively. The fast ice is therefore found as a sink of CO2 for both situations

Aircraft-based investigation of boundary layer structures over the North Water Polynya and in summertime katabatic winds over northwest Greenland

The experiment IKAPOS (Investigation of Katabatic winds and Polynyas during Summer) was performed in June 2010. It was targeted at the summertime katabatic wind system in coastal areas of northwest Greenland and at atmosphere/sea-ice/ocean exchange processes over the North Water (NOW) Polynya. The katabatic-driven near-surface wind regime plays 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 less than during winter, but strong winds can occur under appropriate synoptic forcing. On the other hand, the NOW Polynya represents one of the largest polynyas of the Arctic, and the air-sea interaction over the NOW has a significant impact on ocean processes, ice formation, gas exchange and biology. IKAPOS is mainly based on measurements with the research aircraft POLAR 5 of Alfred Wegener Institute (AWI, Bremerhaven). In order to determine turbulent heat and momentum fluxes, POLAR 5 was instrumented with a turbulence measurement system collecting data on a nose boom with a sampling rate of 100 Hz. Besides navigational and further basic meteorological instrumentation, the aircraft was equipped with radiation and surface temperature sensors, laser altimeters, and video and digital cameras. A total of six research flights have been performed, two of them were katabatic wind flights (over Humboldt and Steenstrup Glacier, respectively). The katabatic wind flights capture conditions of weak and strong synoptically forced katabatic wind with up to 14 m s-1 wind speed. Over the NOW, a stable, but fully turbulent boundary layer with strong winds of 15 m s-1 to 20 m s-1 was present during conditions of relatively warm synoptically induced northerly winds. Strong surface inversions were found in the lowest 100 m to 200 m. As a consequence of channeling effects at Smith Sound a well-pronounced low-level jet system was documented. These channeling effects lead to an increased wind-induced sea-ice export from the Nares Strait through Smith Sound. Three-dimensional (3D) structures of mean quantities will be presented as well as turbulent fluxes and energy budgets in the ABL over the ice sheet of Greenland and the NOW Polynya. The findings of IKAPOS are valuable for the validation of numerical models (including climate models) and will contribute to the understanding of the exchange processes over summertime Arctic polynyas and the Greenland ice sheet