The Riddle of the Green Beam
“Why are there 10 ÜWIs this year? And what’s the deal with the green laser?” Surely, some of you have already asked yourselves these questions. With the Antarctic summer just around the corner, it’s high time to shed some light on this riddle.
The end – or more accurately, the source – of the green laser beam can be found in a container roughly 400 m south of the Neumayer Station. Beside the handline to the SPUSO, the container rests atop the former platform for the EDEN greenhouse. The laser is part of a so-called lidar system, which uses pulses of laser light to reveal even the smallest particles in the atmosphere. These tiny particles can be cloud droplets, ice crystals or aerosols, i.e., minuscule atmospheric pollutants. A small percentage of the laser light is scattered back by these particles, captured by a telescope and linked to a respective elevation in the atmosphere. Certain characteristics of the signal received, e.g. the signal strength, wavelength and direction of polarisation, can in turn be used to determine where above our heads these aerosols, liquid cloud droplets, and ice crystals can be found. Further, a cloud radar system provides more detailed information on the properties of clouds and ice crystals. Similar to the lidar approach, it emits bursts of radio waves and analyses what is reflected back. Clouds are semi-transparent for radio waves, which means that radar – unlike lidar – allows us to peer deep inside them. When the data from the lidar, radar and further instruments are combined, the outcome is a detailed image of the atmosphere above us. The goal of these observations is to gain a better grasp of how clouds and precipitation are formed.
In order for a cloud to form at all, it takes not just water vapour but also pollutants, i.e., aerosols that cling to and accumulate on the water vapour. Even at temperatures below freezing, namely, down to -40°C, clouds are composed of liquid water droplets. That may seem strange, since in our kitchen freezers, we can generally rely on water to freeze at 0°C. But what’s often missing in the atmosphere is an “anchor point” where the ice can attach and then accumulate. In our freezers, the water can “anchor” on e.g. the inside wall of a bottle. In the atmosphere, aerosols perform the same function. Depending on how “good” they are as anchors, the higher the temperature at which water droplets can freeze. Relatively coarse mineral dust, swept up from deserts, can lead to freezing at an atmospheric temperature of -15°C. At temperatures higher than that, it would seem that only the smallest biological particles – e.g. the spores of certain fungi, algae, and proteins produced by bacteria – have qualities conducive to ice formation. In the atmosphere, even under “polluted” conditions, ice-forming aerosol particles are extremely rare; in the clean setting of the Antarctic and the Southern Ocean, they’re virtually non-existent. Nevertheless, they represent an important puzzle piece in the climate system. In a cloud of supercooled water (i.e., between -40 and 0°C), on average only one out of 200,000 water droplets freezes. But after that, things quickly speed up. Water vapour begins to preferentially accumulate on the ice crystals, causing them to rapidly grow to between 20 and 100 times their original size – until, at some point, they become heavy enough to drift to the ground as snowflakes.
There is very little high-quality observational data on these processes within clouds from the Antarctic. Above all, there’s virtually none available on the eight-month-long Antarctic winter. To help close this gap in our knowledge, the Leibniz Institute for Tropospheric Research (TROPOS) mobile monitoring platform “OCEANET-Atmosphere” is currently here at Neumayer for a year-long stay.
To make sure all of the instruments are running as they should, the container also has a dedicated ÜWI. Affectionately known as “Laser Martin”, depending on the time of day or night, type of problem, and weather conditions, he regularly tromps out to the container – sometimes in a wonderful mood, sometimes in a less-than-wonderful one – to restart, tune or dig out (when snowed over) some device or another. In the meantime, the rest of us ÜWIs enjoy the never-ending, free-admission laser light show, not to mention the source of orientation it offers in the dark.
