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A thin layer of silvery-white ice clouds exists on the edge of our atmosphere. Known as noctilucent clouds or polar mesospheric clouds (PMCs), they form at 83 kilometres above the poles of our Earth during summer. A recent NASA long-duration balloon mission carrying an instrument developed by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) on board was able to observe these clouds over the course of almost six days at their place of origin in the mesosphere. The results will help scientists better understand turbulence in the atmosphere, as well as in oceans, lakes and other planetary atmospheres, and may even improve weather forecasting.

On 8 July 2018, NASA's PMC-Turbo Mission launched a giant balloon to study PMCs. For almost six days, the balloon travelled floated through the stratosphere at a height of 38 kilometres, from its launch site in Esrange, Sweden, across the Arctic to Western Nunavut, Canada. During its flight, cameras aboard the balloon captured six million high-resolution images with a total data volume of 120 terabytes, with most images showing the PMCs in different stages. Among others, these images reveal the processes that lead to turbulence.

"From what we have seen so far, we expect to have a really spectacular dataset from this mission," said Dave Fritts, principal investigator of the PMC-Turbo Mission at GATS in Boulder, Colorado (USA). "Our cameras have most probably captured some really interesting events, which we hope will provide new insights into these complex dynamics."

Noctilucent clouds are formed from ice crystals, which condense on tiny meteor particles in the upper atmosphere. They appear as bright silvery-light blue rippling clouds that are visible from the edge of polar regions just after the Sun has set during summer. These clouds are affected by what are known as atmospheric gravity waves. These are formed, for example, by convection in the atmosphere or when wind pushes air upwards on account of mountain ranges. The resulting gravity waves play a major role in transferring energy from the lower atmosphere up to the mesosphere.

"This is the first time we have been able to directly visualise the flow of energy from the larger gravity waves to smaller flow instabilities and turbulence in the upper atmosphere," Fritts said. "At these altitudes, you can literally see the gravity waves breaking – like ocean waves on the beach – and observe the transition to turbulence."

The PMC-Turbo balloon payload was equipped with seven specially designed imaging systems to observe the clouds. Each included a high-resolution camera, a computer control and communications system, and 32 terabytes of data storage. The imaging systems were arranged to create both a mosaic of wide angle images with a field of view 160 kilometres across, as well as using more narrow views to image turbulence structures as small as 20 metres across. For the first time, a lidar – or laser radar– was on board to measure the precise altitudes of the PMCs, as well as the temperature fluctuations above and below the PMCs, caused by gravity waves.

"We know the 2D structure of the clouds from camera images, but in order to fully describe the waves in the clouds we need to accurately measure the height as well," said Bernd Kaifler, the DLR researcher working at the DLR site in Oberpfaffenhofen, who designed the balloon's lidar experiment. "From the lidar measurements we can visualise the vertical structure of the waves, thus collecting important data, which we would have not been able to derive from the images alone."

As opposed to the cameras, the balloon lidar is an active measuring instrument which sends hundreds of short laser pulses upwards every second and detects laser light reflected off the clouds and air. Until now, similar instruments have only been used from the ground. Given the balloon's flying altitude of 38 kilometres, the balloon lidar is roughly halfway up to the noctilucent clouds, resulting in a much better signal quality and thus higher resolution. The movement of the balloon allows assessment of the spatial structure as well. "The dataset collected during the PMC-Turbo Mission is extremely valuable, because never before has anyone been able to see such tiny structures caused by the breaking of gravity waves," says Natalie Kaifler, a scientist at DLR's Institute of Atmospheric Physics. "The movements of the clouds, often in multiple thin layers, exhibit an enormous variability down to very small scales of a few metres."

Developing a compact and lightweight lidar instrument capable of operating aboard a balloon gondola was challenging. Because of the thin air at flying altitude, the laser, detectors and electronics needed to be housed in a pressurised container. Furthermore, since there was not a sufficient amount of air for cooling, a large radiator had to be developed to radiate waste heat produced by the laser and electronics into space. For controlling and monitoring the instrument, commands and data were relayed through NASA’s communication satellites. "The balloon lidar was essentially similar to a satellite experiment, yet carried out on a much smaller budget," says Bernd Kaifler.

Learning about the causes and effects of turbulence will help scientists understand more than only the structure and variability of the upper atmosphere. Turbulence occurs in fluids across the universe and the results will help scientists improve modelling of all these systems. Of course, this is also the case for weather forecast models on Earth.

NASA also studies noctilucent clouds with the Aeronomy of Ice in the Mesosphere (AIM) satellites, which were launched in 2007 into a Low Earth Orbit. AIM tracks large scale structures in the clouds across a global scale, but can only analyse structures a few kilometres across. PMC-Turbo helps fill in the details, explaining what happens at smaller scales where turbulence occurs.

The PMC-Turbo payload was successfully recovered from its landing site in the Canadian Arctic. The recovered instruments will partially be reused and deployed in future missions, including a planned flight over the Antarctic this December.