Antarctic ice clouds
Instruments measuring atmospheric processes and temperatures in the Antarctic atmosphere, some 80 to 100 km above the Earth’s surface, have recorded some of the lowest temperatures and the highest charged ice clouds ever observed in the Earth’s atmosphere.
The measurements were made at Davis station between November 2010 and November 2012 using an iron resonance ‘LIDAR’ (light detection and ranging) instrument from the Leibniz-Institute of Atmospheric Physics in Germany, and the Australian Antarctic Division’s atmospheric radar.
The iron resonance LIDAR can operate in daylight and measures, amongst other things, vertical winds and temperatures in the iron layer (approximately 80-100 km), as well as ‘noctilucent’ (night-shining) clouds. Noctilucent clouds appear in the summer mesosphere, near 84 km, and are visible at sub-polar latitudes when the sun sets 6 to 16 degrees below the horizon.
Noctilucent clouds, together with sub-visual Polar Mesosphere Summer Echoes (PMSE) – which are radar echoes related to charged ice-particles – can be used to detect climate change, as they are very sensitive to temperature changes. Noctilucent clouds and PMSE have been observed for more than 100 years and 30 years, respectively, in the Arctic, and since the early 1960s and 2000s, respectively, in the Antarctic. In the Arctic, these ice clouds are occurring more frequently and over a greater area than in the past, and this change is hypothesised to be linked to climate change.
To learn more about the processes involved in ice-particle cloud formation and dynamics we monitored the mesosphere over two summers with the iron LIDAR and radar operating at 55 MHz. The iron LIDAR achieved over 2700 hours of operation, providing the largest, nearly continuous mesosphere temperature record in Antarctica.
Measurements near the summer solstice (21 December) revealed extremely low temperatures, sometimes below - 160°C at 90 to 98 km altitude; possibly the lowest temperature ever observed in the Earth's atmosphere, and at unexpectedly high altitudes. The radar also recorded low intensity charged ice clouds up to 94 km, which are among the highest ever recorded. Initial published results show that the Antarctic mesopause altitude changes throughout the summer season by several kilometers, which is significantly different from the Arctic. Temperatures at Davis near 86 km are similar to the northern hemisphere, but they are much colder at Davis at higher altitudes.
We found that the thermal structure around the mesopause above Davis is closely related to the general wind circulation in the stratosphere (10–50 km) and the break-down of the polar vortex (an annual stratosphere wind system that circulates around Antarctica from winter to early summer). Both the strength of the polar vortex wind flow and the timing of its annual break-down are linked to atmospheric wave activity in the southern hemisphere, which is also important for the Antarctic ozone hole.
In contrast to theoretical expectations, we occasionally find the mesopause region to be significantly higher and colder than it is in the northern hemisphere. We also find large thermal atmospheric tides in the summer months at Davis, with amplitudes of up to 6–7 degrees Kelvin, which is much larger than expected from models. These thermal atmospheric tides result from the daily variation of solar radiation heating the Earth’s atmosphere. Current models predict that tides are basically absent at high altitudes in the polar summer.
Middle atmosphere processes may affect weather in the troposphere (0–10 km) and possibly long-term climate; therefore climate simulations are improved by incorporating these processes. The expansion of international weather and climate models to higher altitudes requires us to understand whole-of-atmosphere dynamics. The findings from this Australian–German collaboration will be incorporated in the Kühlungsborn Mechanistic general Circulation Model to better understand the role of mesospheric ice particles for the physics of the middle atmosphere. Australian and German scientists are currently working to explain our new and unexpected observations, which will impact our basic understanding of this part of our atmosphere.
RAY MORRIS1 and FRANZ-JOSEF LUBKEN2
1Australian Antarctic Division
2Director, Leibniz Institute of Atmospheric Physics