ANT-XXVI/4, Weekly Report No. 6
Fig. 1: K. Lonitz on duty with the measurement of optical thicknesss using a sun photometer. (photo: T. Hanschmann)
11 May - 17 May 2010
In the early morning of May 17: Polarstern arrives on time – after 40 days at see, 15.000 kilometers, 35 stations, several thousands of cubic meters of filtered air, 10.000 liters of sampled water, more than 1.000.000 liters of seawater measured in flow-through systems, and other uncounted statistical events – at her berth in Bremerhaven. The final week of our cruise ANT-XXVI/4 had started with a short port call to Las Palmas on Gran Canaria where Polarstern remained only for a few hours to take onboard a group of eight further participants. Everybody had been looking forward to the arrival of eight-year-old Katja, who had come with her mother Ute Lange and Claudia Klages of AWI as her personal guide. Katja had won her cruise participation through the „Expeditionspass“ program, a national activity for school kids by the German Federal Ministry of Education and Research. From then on Katja moved freely on the ship so that she could bring home a full diary and many stories to tell her class mates. Another not less interested and curious new participant was Gerlind Rüve from the German Council of Science and Humanities (Wissenschaftsrat) in Cologne, who had accepted an invitation by Professor Karin Lochte, Director of the Alfred-Wegener-Institute. For about a week she took every opportunity to hear about the scientific facettes of our current cruise and gain insight into logistical and organisational aspects of marine research in Germany.
Fig 2: T. Kanitz on the roof of the OCEANET container for atmospheric observations. (photo: A. Körtzinger)
Besides the resumption of our measurement and station routine an important instrument test was due after departure from Las Palmas. On May 10, Saad El-Naggar and Peter Gerchow from AWI, who had come to Las Palmas specifically for this purpose, carried out successful tests of the vessel-mounted Posidonia-System for underwater positioning of acoustical targets. This system had caused considerable headaches, but the tests finally openend a way forward.
My 6th and final weekly report is dedicated to the atmospheric working group of the Institute for Tropospheric Research in Leipzig (Thomas Kanitz, Timo Hanschmann) and the Max-Planck-Institute for Meteorology in Hamburg (Katrin Lonitz). This group employs a wide range of instruments for the remote sensing of atmospheric properties. A good example is the sun photometer for measurement of the optical thickness of the atmosphere (Fig. 1). This is a a measure of how opaque a medium is to radiation passing through it and in the case of the atmosphere provides important information about aerosol loading. The small hand-held sun photometer has to be directed towards the sun and can thus only be used on sunny days – a pleasant prerequisite that was met very often during our cruise. Between 8°N and 11°N increased values of the aerosol optical depths indicated the presence of a dust event which had already been forcasted by aerosol models several days in advance.
Fig. 3: The OCEANET container for atmospheric observations on the compass platform of RV Polarstern under spectacular Mammatus clouds. (photo: T. Kanitz)
The OCEANET container for atmospheric observations is the centrepiece of the group’s equipment (Fig. 2+3) and had successfully completed its maiden voyage during the preceding Polarstern transit. The container is set up on the compass plattform and equipped with a suite of sensors and instruments for remote sensing of the atmosphere and for determining the energy balance at the surface of the ocean. The mobile Raman-LIDAR Polly XT of the IfT Leipzig sends a bright green laser beam into the sky which allows to measure reflection by particles up to a height of 20 kilometers. As the LIDAR system measures at five different wavelengths it not only provides information about the amout of particles but also about their size and properties. The dust event which had been picked up clearly with the sun photometer also came out nicely in the LIDAR data as a marked dust layer at 1000 und 4000 meters elevation. Even the fine structure with a dust-free layer interspersed at approx. 2000 meters height was clearly resolved.
But the group’s work is not only about dust. Other measurements are performed in order to close the energy budget at the surface the ocean. Pyranometer and pyrgeometer provide measurements of the solar and thermal radiation which are major components of the energy budget. An automatic cloud camera system which images the full sky from horizon to horizon at 15-second intervals allows determination of clouds cover and type. Besides the standard meteorological sensors the container also features a microwave radiometer which provides vertical profiles of temperature and humidity up to 10 kilometres height – properties that otherwise would only be accessible by weather ballons. The data also provide an estimate of the total amount of liquid water present in the air column, the so-called liquid water path. Despite all this high-end instrumentation nothing can keep the group members in the container when spectacular atmospheric phenomena like rainbows, halos, cloudsbursts or Mammatus clouds (Fig. 3) are offered outside. It is these special moments that – besides all scientific fascination – make every research cruise a special experience.
Having said this, I would like – also in the name of all members of the scientific party – to express my hearty thanks to captain Uwe Pahl and the entire crew of R/V Polarstern for highly professional and always friendly cooperation during the very successful cruise,
Arne Körtzinger