PS106/1 - Weekly Report No. 4 | 13 - 21 June 2017
Week 2 at the ice floe
The TROPOS aerosol container, with online and off-line instrumentation (high volume and size-resolved samplers), was installed on Polarstern on the upper deck to measure in-situ atmospheric aerosol physical-chemical properties.
The measurements of total number concentration (Condensation Particle Counter), number size distribution in a range from 2 nm to 20 micrometre (Neutral Air Ion Spectrometer, Mobility Particle Size Spectrometer and Aerodynamic Particle Sizer), black carbon mass concentration (Multi Angle Absorption Photometer), light scattering coefficient at three different wavelengths (Nephelometer), and volatility/hygroscopicity properties (Volatility and Hygroscopicity Tandem Differential Mobility Analyser) started on 25th of May, 2017 and are performed continuously throughout the cruise. Regular calibration and check procedures are performed on weekly bases to ensure high data quality and inter-comparability. Ice Nuclei Spectrometer and Cloud Condensation Nuclei Counter were also deployed on Aerosol container to measure number concentration of ice nucleating and cloud active particle number, respectively. Later analysis of the chemical composition and the information from backward trajectories will show us to what extend aerosol is locally produced and long-range transported. These in-situ data are also quite valuable for active and passive aerosol remote sensing.
After setting up and final characterization, which was only possible after final set up on the ship, the FTIR (Fourier Transform Infrared Radiation) instruments perform as expected. We had altogether four clear days to perform solar absorption measurements and many days to use the emission in the thermal infrared to perform a characterization of the thin cloud cover in the Arctic. Shown are two spectra, one measured during cloudy conditions, one at a clear day.
In Figure 1, one can clearly see the effect of a thin cloud on the observed infrared radiation. The upper envelope is caused by fully saturated emission and is basically black body radiation. The upward pointing lines are emissions from H2O, CO2 and Ozone. One can clearly see that a cloud has two effects; it attenuates the emission lines and adds additional broadband radiation. The form of the additional radiation depends on the properties of the cloud itself, notably mean radius of the droplets in a water cloud and the optical thickness, two properties, which are accessible in this wavelength region.
While drifting, attached to the floe, we conducted a variety of measurements from the helicopter and on the ice to characterise properties of melt ponds and the surrounding sea ice. Since the ponds alter the albedo of the sea ice, which means the amount of reflected sunlight, they tend to warm up the sea ice system. Conducting many measurements at and in melt ponds, we aim to learn more about how they alter the surface properties of the sea ice during the melting season. We have been lucky because melt ponds started to appear quite in time when we were occupying the floe. During a survey flight with the heli we found an area with some small pond forming; we therefore decided to conquer our “lake district” (see Fig. 2) for further measurement. We conducted airborne measurements using different cameras, i.e. a RGB camera and a hyperspectral sensor. On June 10, we had clear sky conditions perfect for remote sensing acquisitions and accompanying field measurements. To gather validation data for airborne and satellite data we measured radiance and irradiance above the pond surface and water depth. Since the ice bottom strongly influences the amount and spectral distribution of radiation they scatter back towards the atmosphere we also conduct drillings for assessing pond bottom depth. Additionally we took water samples; these samples are pre-processed in one of the laboratories of the ship, shock-frozen and then stored in a freezer until Polarstern arrives at Bremerhaven in October. To assess water constituents such as chlorophyll, particulate and suspended matter the samples will then be brought to a lab.
In the framework of our physical observations of the sea ice and its snow load we have focused our investigations on the changes with respect to snow and ice melting during the last weeks. How does the energy (mostly solar irradiance) effect and change the snow and sea ice? To this end the thickness of snow and ice was measured along kilometre wide profiles along the entire floe. The additional probing of snow and sea ice with respect to aerosols and their interaction with the atmosphere was an essential part of these works. As we are in the middle of the melt season, strong changes could be observed within the past two weeks that could also be sorted into different types of ice. Especially the snow load on the sea ice did melt away drastically and the sea ice became significantly warmer and thinner. A large part of the investigations with respect to energy and mass balance was performed by means of a diving robot as this allows mapping the sea ice and irradiances from below. The platform carries a large variety of bio-physical sensors to investigate the interactions between sea ice and ocean. Already the high-resolution videos of the sea ice from below show characteristic properties of the sea ice and how those dominate the habitat below. In order to continue the observations best, also an autonomous observatory containing different devices has been installed on the ice and was left behind. These devices now trace the coming development until we recover it during the expedition PS106.2.
The oceanographic group has continuously carried out measurements with the ship Conductivity Temperature Depth (CTD) profiler. This instrument provides high resolution data of temperature, salinity, chlorophyll and oxygen throughout the water column which helps us recognize which different types of water that are present at each site, and how their position in the water column changes from place to place or from time to time. With the 24 bottles attached on the steel frame, we can also collect water from any depth for further analyses. Most profiles have been made all way down to bottom depth, which often has been greater than 1 000 meters during our journey. During our weeks at sea we have followed the changes of Atlantic water and seen the seasonal melting of sea ice affect the surface waters.
During the ice floe drift, a 200 meter deep mooring line was deployed from the ice equipped with Acoustic Doppler Current Profilers and temperature/salinity sensors. The main information supplied by this mooring are the prevailing water velocities but it is also possible to analyze e.g. the daily patterns for vertically migrating organisms. If such patterns can be identified, we have a good chance of understanding what kind of organisms we see, by using the information collected by the biological groups during the cruise.
A particle analyzer (LISST) was several times deployed together with the CTD and separately under the ice (see Fig. 3). LISST determines the sizes of the particles present in the water. This can be used together with other measurements to understand processes such as vertical migration and sedimentation.
Best regards from Scientists and Crew,
Andreas Macke, chief scientist
With contributions from Simonas Kecorius, Marcel Nicolaus, Anna Nicolopoulos, Mathias Palm, Natascha Oppelt