ANT-XXVI/1, Weekly Report No. 2

27 October - 2 November 2009

Polarstern has arrived in time at Gran Canaria in Las Palmas on the 27th of October. 24 employees from AWI disembarked here. They took advantage of this first part of our Atlantic crossing to check various equipment on board. Two scientists joined the group and we continued our journey at 22:00. The day at the harbour has also been used to fix the lidar device. For this reason an employee from the Institute for Tropospheric Research has arrived from Leipzig bringing replacement parts. Now one of the most important instruments for aerosol remote sensing is fully functional just in time before we cross the Saharan dust region.
We are steaming in south-westerly direction to arrive at 23°W and 20°N. From there the course continuous in exactly southerly direction right through the Cape Verde islands until we reach 20°S. Along this 23W-section we are passing several moorings and add our surface near observations to these deep oceanographic measurements. To this end Polarstern's ADCP has been switched on and measures by acoustic means the current profile down to 100 m depth along the entire section.
The main task of ANT-XXI/1 is to continuously monitor the cloudy atmosphere and the aerosol, the biological and chemical properties of the upper ocean, and the energy and material fluxes between ocean and atmosphere along the steaming boat, what is usually termed as “underway-measurements”. But there are also CTD-profiles and measurements of the subsurface light regime at fixed stations. More to that later on. The underway-measurements are carried out in the framework of the joint project OCEANET with contributions from the Leibniz-Institutes IFM-GEOMAR and IfT as well as AWI and the GKSS Research Center. OCEANET is funded by the Leibniz Society.
For the first time the atmosphere observatory OCEANET-Atmosphere is on board, a 20 feet sea container equipped with devices to remotely sense the atmosphere and to determine the energy balance at the sea surface. Part of OCEANET-Atmosphere is the above mentioned lidar, that sends out a laser beam up to 30 km height and determines the size and type of atmospheric particle from the returned signal. Of course, the laser beam is only visible to us during night, but than it's a spectacular view, as figure 1 shows.

Indeed, the aerosol – which includes the Saharan dust – is the highlight of this section along the coast of West Africa. Besides the lidar we have two sun-photometer on board, one from NASA and one from the Institute for Space Sciences in Berlin. The instruments determine the aerosol load and size in the atmospheric column. The devices are pointed directly into the direction of the sun and measure the attenuation of the direct solar radiation due to scattering at the aerosol particles. Furthermore, a so called MAX-DOAS radiometer is on board, which retrieves trace gases and aerosol from the spectral signature of scattered sun light at different viewing directions. With this massive equipment for aerosol remote sensing we are very lucky to pass a Saharan dust event.

The spatial dimension of the dust plume is best visible from the lidar observations as figure 2 shows quite impressively. One can see that on October 31 the dust cloud ranges from about 1000 to 2000 m height and that it slowly sinks down towards the end. For our understanding of transport and sedimentation of dust into the Atlantic this measurement is a precious puzzle stone that we picked up on our voyage.

While the lidar reveals a lot about the structure of a dust cloud, a sun-photometer can more directly determine the total amount of aerosol in the atmospheric column, the so called optical thickness.

Figure 3 shows the temporal change in optical thickness for the same day. Here we see almost six-times the value of a normal clear sky day. This “golden day” will keep the dust scientists busy for some time.

For the fifth time the microwave radiometer HATPRO (Humidity And Temperature PROfiler) is on board Polarstern, this time sheltered in the OCEANET-container. Nevertheless, we have to note that one microwave channel (out of 14) broke down, which slightly hampers our humidity measurements. The company that provided the radiometer helps as good as it can and delivers new software that works with the reduced data stream.

Further instruments of the OCEANET-Container are so called pyranometer and pyrgeometer to determine the solar and thermal down-welling radiation at the surface, a full-sky-imager to determine cloud cover and cloud type, a meteorological observation mast to determine turbulent fluxes, a further mast for standard meteorological observations, and a UV-radiometer from the Institute of Medical Climatology at the University of Kiel. Figure  4 shows the measurement container with its devices on the roof as seen from the crow's nest.