High-altitude noctilucent clouds are a rare and beautiful treat for summer visitors to Antarctica. They are also a sign of something peculiar about the polar atmosphere. They occur in the mesopause, about 85km above Earth’s surface, in very low temperatures — about minus 130°C. Yet these low temperatures occur only in summer when the polar atmosphere is bathed in sunlight. How is this possible?
One theory is that as air moves upward toward the mesopause, it expands and cools as its thermal energy shares itself over a bigger volume and that this cooling effect more than counteracts the warming effect of the sun. This upward motion, the theory goes, is driven by poleward movement of air in the mesosphere (the atmospheric level just below the mesopause) on a planetary scale. In Antarctica, these southward winds come together at the pole with nowhere else to go but up.
To find out how these southward winds are sustained throughout the summer we need to know more about mesosphere-mesopause wind speeds. It is here that the atmospheric radar installed at Australia’s Davis research station comes into play. This radar works by sending out radio pulses and measuring the tiny signal reflected back to the ground by turbulent wind eddies common in this part of the atmosphere. By recording the motion of these eddies, the speed of the wind that carries them along can be measured at 2km intervals every two minutes throughout a height range from 50km to above 100km. In the seeming chaos of the winds at these heights, various physical phenomena can be identified, ranging from minute–to–minute turbulence to daily and sub-daily (12- and 8-hour) movements of the air known as tides. In between these time scales are a class of waves that hold the key to this puzzle, called atmospheric gravity waves.
Atmospheric gravity waves begin their life cycles in our near-ground weather systems, travelling up through the atmosphere and taking some of our weather’s energy with them. They grow as they move upward and their energy is spread over the increasingly rarefied air. Sometimes the wind from atmospheric tides brings about their destruction well below the mesosphere, but many grow until they become unstable, breaking like waves on a beach. In breaking, they transfer their energy which drives the circulation responsible for the cold summer mesopause. Data from the Davis radar allows Australian physicists to record the motions due to turbulence, gravity waves and tides to better understand their nature and how they interact. The system can operate automatically, sending data via the web to research centres around the world.
- Tides in the atmosphere (Recent research into atmospheric tides, June 2007)