May 3 – May 9, 2008

The third cruise report is in the sign of the sun. The solar light is the energy source for nearly all atmospheric, oceanographic and biological activities on this planet. On this POLARSTERN cruise the availability of light at the sea surface and in the deep is measured by the Phyto-Optics group of Astrid Bracher (on board represented by Bettina Schmitt) on a daily basis. The radiation data are studied with co-located vertical profiles of phytoplankton concentration in order to establish the relation between light availability and phytoplankton species.

The Cloud-and-Radiation group of Andreas Macke investigates the influence of clouds on the radiation budget at the sea surface with a special focus on small scale-fluctuations of the light availability. It makes a difference in the heating of the ocean and in photo-biological and photo-chemical processes that depend non-linear on light fluctuations, whether the same amount of light is available homogeneously or fluctuating. The reasons for light fluctuations are the changing cloudiness in the atmosphere, and to the larger extend sea surface waves. Both working groups add to each other excellently and combine marine physical and biological scientific questions.

The vertical profiles of the down- and upwelling spectral irradiance (solar energy at a specific wavelength per time that falls on a horizontal plane) are measured with a RAMSES spectrometer down to a depth of 200 m. Figure 1 shows the measurement device shortly before operation. For the light profiling POLARSTERN turns the working deck towards the sun. Optimal conditions prevail when the wind sea moves the device away from the ship so that shadowing by the ship is minimized.

Figure 2 shows a vertical profile of the downward spectral irradiance at the red, green, and blue spectral regime. Because of the material properties of water the red part is absorbed very quickly and the green part, further down the blue part dominates. The latter is also caused by the stronger Rayleigh scattering at water molecules in the blue. Down to 100 m depth both parts are clearly visible. As expected, the light variations decrease with increasing depth, but are still detectable at a depth of 40 m.

In order to quantify the spatial and temporal characteristics of these light fluctuations the brightness distribution on a white diffuse reflecting plate (Fig. 3) is filmed at various depths and will be spectrally analyzed later on. From this the characteristic space and time scales of the mostly surface wave induced light fluctuations can be obtained as a function of sea condition and depth. At the same time the sea surface tilts are determined by means of a light float. Back at the institute the data will be fed into a Monte-Carlo radiative transfer code to reproduce the observed light fluctuations in the model.

On May 5 we had a day with so far strongest pronounced cloud induced fluctuations of the solar and thermal downwelling irradiance at the sea surface. With 1420 Wm^-2 at 13:23 UTC we have most likely measured the strongest solar irradiance for the entire cruise. At that time the sun was nearly in zenith with a top-of-atmosphere irradiance of 1360 Wm^-2. The fact that we observe a surplus of radiation at the surface results from the additional diffuse radiation, especially around the vicinity of the direction of the sun. Figure 5 shows a full-sky image taken at the time of the strongest excess radiation. The additional diffuse radiation from the sun's vicinity is clearly shown.

The conditions in the tropical atmosphere was mostly determined by shallow cumulus clouds. Only few high convective cumulus towers that are usually characteristic for the ITCZ could be observed at the horizon. North of the thermal equator we came into the region of Sahara air masses. The corresponding dry air at higher altitudes might be a reason for the fact that tropical convection was rarely observed. An equator baptism has been celebrated.

On the occasion of the death of Prof. Dr. Friedrich Schott, a former Director of the Kiel Institute for Marine Sciences POLARSTERN was flagged half-mast  and the ship’s horn was blown exactly at 13:30 MEST when the funeral service began in Kiel. All German and many foreign research vessels participated in this action to honour a leading scientist in Physical Oceanography.

Best regards on behalf of all,

Andreas Macke