Half-mast flag in commemoration to Dr Soenke Neben on 17 November 2009 (photo: M. Ettlin)
ANT-XXVI/1, Weekly Report No. 5
17 November - 23 November 2009
This reporting week begins with a sad moment. Polarstern is flying the flag on Tuesday from sunrise to sunset in commemoration to our late colleague Dr. Sönke Neben. At 10:15 board time, at the time of the obsequies in Germany, the typhoon is blown, the flag is half-masted (Fig. 1), and a minute's silence is taken.
After the two stations at the Vema-Channel Polarstern is on direct route to Punta Arenas. Whenever the cloudiness allows, we stop at noon for our daily subsurface light measurements. At 40°S a last station for plankton sampling was performed. Two longer stations for testing a deep-ocean CO2 measurement device have been carried out as well. At the end of this reporting week the air- and sea surface temperatures become single-digits. Probably still much to warm for polar researchers, but during the past four weeks we have got used to tropical and subtropical conditions, and are freezing now. On Sunday the mail of the polar(stern) philatelists have been marked with the expedition and ships stamp, and have been forwarded to the post office.
This is also the day where we perform the last station on this cruise. We can see our breath as we do the last measurements of subsurface light fluctuations from board the zodiac, accompanied by albatrosses and other less spectacular birds. On Monday, all measurement equipment is stored. Tuesday is booked for cleaning all laboratories. On Wednesday we get a last lunch on board. It is planned to disembark all scientists and luggage by boat.
Fig. 2: Energy fluxes at the sea surface based on one-minute averages of OCEANET-Atmosphere. (diagram: K. Bumke)
The central topic of this last report is the energetics at the sea surface, i.e. the balance between downwelling and reflected solar radiation, upwelling thermal emission from surface, downwelling thermal emission from the atmosphere and the fluxes of sensible and latent heat. The energy budget at the surface is the most relevant property in understanding the climate processes on this planet. Especially clouds have a strong effect on the energy budget which is still insufficiently known because of their complexity. The laboratory container OCEANET-Atmosphere has been especially designed to simultaneously measure the full energy budget and the state of the cloudy atmosphere.
Figure 2 summarizes the above mentioned components of the energy budget for most part of this cruise. Red symbols denote solar downwelling radiation, blue positive values are downwelling atmospheric thermal radiation, blue negative thermal emission from the surface, and cyan and green values stand for sensible and latent heat flux, respectively. The daily averages are given by Magenta coloured circles. Although the individual components vary strongly, the overall budget is rather balanced. Depending on cloudiness, wind and humidity (not shown here) the balance can be positive or negative, i.e. the sea surface can gain energy from the atmosphere or loose it to the atmosphere. The analysis of our data will contribute to provide climate models with realistic numbers for this interaction between ocean and atmosphere.
Fig. 3: Diurnal cycle of the solar radiation measured with the board system and with OCEANET-Atmosphere. (diagram: H. Kleta)
If one looks more closely at the solar radiation values shown in the diagram above, one detects situations where the radiation at the surface is larger than the maximum available radiation at the top of the atmosphere. This value amounts to 1365 Wm-2. The fact that we observe more at the ground is not a measurement error but a well known phenomena caused by the downward scattering at the white clouds outside the solar disk which is stronger than that from the blue sky. In fact, on 9. 11. 2009 we observe with 1647 Wm-2 the largest value since the beginning of our measurement series on Polarstern in 2007.
Figure 3 shows the diurnal cycle of the solar radiative flux at this day, Fig. 4 the sky image at the very moment of the largest flux. Although or rather because the sky was nearly overcast, the sum of direct undisturbed solar radiative flux that shines through a cloud gap and the additional diffuse radiative flux from nearly the entire hemisphere is exceptionally large. Such situations only last for several seconds. On average, the shadowing by clouds is much larger than this radiation enhancement.
Full sky image at the time of the largest solar irradiance. (photo: J. Kalisch)
Besides devices to obtain the energy budget OCEANET-Atmosphere also contains an autonomous system developed by the German Weather Service to monitor the standard meteorological quantities. The system is applied for the first time under marine conditions during this cruise of Polarstern. The comparison of this so called SCalable Automatic Weather Station (SCAWS) with the meteorological data from the ships system looks very good. This marks an important mile stone in our future plans to operate OCEANET-Atmosphere on cargo and other research vessels which do not operate an own weather station. The SCAWS data are directly send into the data stream of the DWD via the Meteosat satellite and are available to the user in nearly real time.
We are very curious to see how measurements and telemetry function as soon as we disembark in Punta Arenas, when SCAWS performs its first solo part. Unfortunately, the transmission range is limited to the field of view of the Meteosat satellite so that we cannot directly receive data from the polar regions or from outside the Atlantic. But all measurements are stored on board and become available at the end of the journey.
A group photograph collage of the scientists of the second part of expedition ANT-XXVI/1 from Las Palmas to Punta Areas. (photos: D. Stronzek, collage: J. Lampel)
For the weather: We just missed a strong storm track at the beginning of this reporting week. Instead, we are influenced by a ridge of high pressure with few clouds. However, on Thursday a frontal cloud system from a lee deep pressure system originating east of the Andes over Uruguay arrives over the ship. Until Monday we stayed in the intensifying ridge of high pressure with mostly sunny weather. Except for a rainy Thursday the sea is calm with 2.5 m sea swell and wind strengths around 5. On the weather maps we can see pronounced lows to the southwest and northeast of us. Retrospective the weather was good to us over most part of the cruise.
This ends the first section of the Polarstern expedition ANT-XXVI/1. We gather together for at the bow for a group picture (Fig. 5). We wish crew and scientists much success for the next section and thank Master Pahl and his crew for there tireless service and the nice working atmosphere on board.
Best regards on behalf of all,
Andreas Macke