With the steady trade wind we had mostly good weather with only scattered rain showers (Fig. 1). Under the trade wind inversion clouds can only reach a vertical extent of 3km, therefore heavy rain is unlikely. During the next days, by approaching the ITCZ (Innertropical Convergence Zone) this might change, however.

The en-route measurements were running according to schedule. On the way, we deployed five floats (Fig. 2) in the frame of the Argo program. They have a lifespan of 4-5 years and have to be replaced regularly. These autonomous floats measure temperature and salinity profiles in the oceans and drift in 1000 m below surface. Currently, there are nearly 4000 active floats around the world and provide valuable data for understanding and modelling ocean circulation.

One of the main objectives of this cruise is the continuation of the OCEANET program for observing atmospheric profiles over the oceans. This cruise is already the fifteenth over the Atlantic in the frame of OCEANET, which was launched in 2007. The measurements focus on the lower troposphere and especially exchange processes between ocean and atmosphere.

The main instrumentation is installed in a container which has been set up on the observation deck of Polarstern (Fig. 3). The instruments include a multi wavelength-polarization-Raman lidar (Polly^XT, Fig. 4). It emits a laser beam in three wavelengths (355, 532 and 1064 nm). From the received signal, profiles of backscatter and extinction can be obtained which is then used to determine aerosol profiles (concentration, properties) as well as water vapor profiles. By using an additional near-range telescope, also the marine boundary layer can be analyzed.

Another main instrument of OCEANET is the microwave radiometer HATPRO (bottom right in Fig. 3). From this instrument, we obtain profiles of temperature as well as integrated water vapor and cloud liquid water path. It is especially well suited to determine temperature inversions and low marine boundary layers. In Fig. 5, the temperature profile along our course is shown. In addition, the inversion strength for the lowest 400 m can be seen in the upper part of Fig. 5. Different air masses and ocean currents lead to very specific conditions. In the area of the Falkland current (50-45°S), cold sea water causes a strong inversion below relatively warm air masses from the northwest. More inversions were present around 35°S at the transition to subtropical air masses.

Furthermore, several broadband radiation measurements are performed in order to determine the full radiation balance. In addition, we installed spectral radiance and irradiance measurements from which cloud properties can be determined. A high-resolution cloud camera complements the set of instruments.

One special focus of the OCEANET campaign is the validation of satellites for atmospheric research, such as CALIPSO or CloudSat). Intensive measurements, including radiosondes, are performed when these satellites cross our track. Ground-based remote sensing measurements can provide valuable complementary information and are nearly inexistent over oceans. These satellite overpasses have already been performed at the cruise ANT-XXIX/10 two years ago, and is planned five times at the current cruise, of which two were already successful.

Since we have 20 unbaptized fellows here on board, who are about to cross the equator, Neptun’s spies have already been seen in the corridors of Polarstern. A personal visit by the God of the Seas is scheduled for Tuesday. More details of this audience might be given in the next weekly report.

On behalf of all cruise participants, best regards

Bernhard Pospichal

24. 04. 2016, 5°S, 26°W