Crossing the equator

We were only hit by minor showers, but in close proximity to the ship we could see quite threatening clouds (Fig. 1). On Friday, we passed the Cape Verde Islands (Fig. 2) which marked the onset of the trade wind zone with frequent stratocumulus clouds. Today we have rather cool temperatures around 20°C and wind with 6-7 Beaufort from north-east, at a position about 200 nm west of Cap Blanc.

We successfully continued all meteorological profile observations along the route, as they were described in the previous weekly report. Additionally, we could perform four measurements during satellite overpasses of Cloudsat and Calipso at different meteorological conditions, which will allow further comparison and validation between satellite products and ship-based measurements.

Everyone here on board is doing fine. We already got used to the constant daily rhythm with fixed worktimes and mealtimes. Last Tuesday, however, shortly after the equator crossing we had a special program: Neptune himself welcomed the newbies on board and allowed them to cross to the northern hemisphere only after a baptism ritual. This exciting day ended with a tasty barbecue on the working deck. Thanks to the tropical temperatures, the party went on late into the night.

This week, we would like to present the scientific contribution of the Max-Planck-Institute for Meteorology (MPI-M, Hamburg, Germany). The work consists of aerosol extinction measurements in several visible and near infrared spectral bands with a handheld Microtops sun photometer (Fig.3). After corrections for molecular and gaseous extinction, these measurements yield the aerosol optical depth, an indicator of particle size and the atmospheric water vapor column.

These measurements are part of the maritime component of AERONET, a NASA coordinated worldwide network of ground-based remote sensing activities. Stefan Kinne from the MPI-M organized since long such aerosol measurements on German research vehicles. The meteorology student Christiane Duscha and Stephan Bakan of the MPI-M (Fig.4) perform these observations during the PS98 cruise. The resulting data will add to the constantly growing aerosol data set over global oceans.

After initial difficulties due to adverse weather conditions, the group was able to submit almost daily valid observations to the data center at NASA GSFC (Fig.5). After a preliminary quality check the data are provided to the public within very short time to the public under the web address „ aeronet.gsfc.nasa.gov/new_web/cruises_now/Polarstern_16_0.html “. Comparison with results from a successful cruise during April/May 2008 shows general similarities as well as remarkable differences in details (Fig.6). The aerosol load off the African coast is found to be considerably smaller this time and the stronger reduction with wavelength hints at substantially smaller particles.

In addition, the group is testing a combined system of thermal infrared and visible wide-angle cameras for day and night control of cloudiness. During the cruise, further development and optimization of an available automatic cloudiness extraction software is intended.  Furthermore, special attention is given to the performance of a precipitation disdrometer deployed to the Polarstern by the OceanRAIN project of the University of Hamburg. These data are supposed to serve as long-term ground reference for satellite based precipitation estimates.

On behalf of all cruise participants, best regards

Bernhard Pospichal

01.05.2016, 22°N, 21°W