Bjorn Kjellstrand woke up one July morning in Esrange, Sweden, 200 km north of the Arctic Circle ecstatic to find that finally, he had the highest resolution images of Polar Mesospheric Clouds (PMCs) to date. After multiple all-nighters and several failed balloon launch attempts, he finally had the pictures which held the potential to completely revolutionize the world's current climate models. As the data on these rare clouds arrived from cameras aboard the high-altitude balloon launched by NASA, Kjellstrand recalled, "Waking up to PMCs was an excellent start to the day!"
As a physics graduate student at Columbia University, Kjellstrand knew that high-quality pictures of PMCs were important for climate scientists because they would lead to a deeper understanding of atmospheric gravity waves (GWs), an important factor in climate models. Atmospheric GWs are waves of air pockets thatriseand fall through layers of the atmosphere . A GW usually originates in the troposphere, the lowest layer of the atmosphere, from a thunderstorm or from wind flow over a mountain range. Air pockets from the troposphere rise into the mesosphere layer, 50-100 km above the ground and fall back down due to gravity. Because GWs move heat through different layers of the atmosphere, they have a critical impact on weather and climate.
The best place to study the effect of atmospheric GWs is in the mesopause region of the mesosphere, 70-100 km above the ground, because the mesopause experiences frequent and large-amplitude GWs . PMCs also form in the mesopause. Similar to an ocean wave, the GWs cause ripples in the PMCs when a GW breaks . The ripples in the PMCs can be used to deduce information about GWs, and the GW information can then be incorporated in weather and climate models.
Until now, only ground-based cameras have been used to observe PMCs, and the data has therefore been limited for a few reasons. PMCs are only visible about an hour after sunset and only form near the poles around the time of their respective solstices . Therefore, using a ground-based camera, PMCs can only be observed at a time when the sun is below the horizon relative to the camera and when they are backlit by the sun. This means that the camera must be located several hundred kilometers from the PMC and that the PMC can only be observed for a short time window when the lighting conditions are optimal. Because of the poor quality and limited quantity of current PMC images, current weather and climate models only grossly account for atmospheric GWs. The spatial resolution of ground-based images is too poor to see details of PMC ripples, and the low quantity of images does not provide a wide representation of GWs breaking. Most weather and climate models use different approximations of GWs, however, because of the poor approximations, the models frequently disagree and are inaccurate.
Capturing images of PMCs from NASA’s high-altitude balloon would hopefully solve both of these problems. By observing from high altitudes, the balloon would both be closer to the PMCs and be able to capture images for the entire duration of the balloon flight. Dr. Glenn Jones, a researcher involved in the project explained, "The balloon images will have 10 times better spatial resolution, and a much higher volume of images, because we don't have to get lucky with lighting conditions."
The inspiration to observe PMCs by balloon was serendipitous. In December 2012, a high-altitude balloon was launched from Antarctica for an unrelated cosmological experiment co-led by Kjellstrand's advisor, Dr. Amber Miller. The balloon included telescopes to discover remnants of the Big Bang. Additionally, the balloon included cameras to orient the telescopes. While analyzing data from the cameras, researchers spotted wispy traces moving across images, and soon learned they were PMCs . Dr. Miller got in touch with Dr. David Fritts at GATS Inc., who specializes in atmospheric modeling. Dr. Fritts said, "When I saw the exquisite detail they had, I was just blown away." The scientists then began collaborating on a dedicated balloon to image PMCs.
The path from serendipitous discovery of PMCs in early 2013 to the launch of a dedicated balloon in July 2018 was bumpy. The scientists had initially planned to include a camera on a balloon which was to be launched from Antarctica in December 2017 as part of an astronomy experiment named SuperTIGER. By piggybacking on the SuperTIGER balloon, the PMC researchers hoped to collect pilot data to further refine their instrumentation for the July 2018 launch. However, after sixteen launch attempts failed due to poor weather conditions, the SuperTIGER launch was scrubbed for the season.
When summer arrived, Kjellstrand recalled, "I believed that the largest risk for us with the summer 2018 launch would be inclement weather. We tried to reduce this risk by arriving in Sweden as early as possible to ensure we didn't miss an opportunity." Kjellstrand spent several weeks in Sweden and said, "When I first arrived, the ground was still covered in snow, and nights lasted an hour or two, but within a few weeks, the snow melted, and night disappeared."
Despite arriving early in Sweden, there were seven failed launch attempts due to poor weather, and Kjellstrand said, "because of the many failed attempts, I didn't become very excited until inflation started!"
There were still nerve-wracking moments even after a successful launch. Immediately after, Kjellstrand said, "We received extremely interesting images that I initially thought were clouds, but quickly realized that the images weren't moving.We were worried that these were reflections and thought our experiment was going to fail." However, as the balloon continued to rise, Kjellstrand said, "The bright features slowly faded, and we began to see stars. We now think that some ice formed on the outside of our pressure vessels during ascent." Kjellstrand recalled, "The hours when we thought our experiment had failed were stressful, but because we had attempted three launches in the last few days, we were running on little sleep, and I had been awake for 36 hours - I was too tired to worry!" The researchers took shifts monitoring the balloon's ascent, and Kjellstrand happily woke up to images of PMCs.
Kjellstrand said the initial data, recovered with the balloon payload in the tundra of northern Canada, appears promising, noting that "We have seen many interesting dynamics at high spatial resolution." Dr. Fritts explained, "Weather prediction models, such as hurricane track forecasts, and climate models approximate GWs to get a better forecast and are still a long way from reality because the approximations are not very good." He continued, "To the extent that our observations will help us better understand GW dynamics, they will contribute to better approximations, and, as a consequence, hopefully better weather and climate predictions."
Hopefully, what began 6 years ago from an accidental observation of PMCs from ancillary equipment in an astronomy experiment will soon yield more information about PMCs and atmospheric GWs to shape more accurate weather and climate models.
Guest Contributor, Signal to Noise Magazine
PhD Biomedical Engineering, University of Southern California, 2012
 Atmospheric gravity waves. https://www.atoptics.co.uk/highsky/hgrav.htm.
 Miller, A.D. et al. Stratospheric imaging of polar mesospheric clouds: a new window on small-scale atmospheric dynamics. Geophys. Res. Let. 42, 6058-6065 (2015).
 Noctilucent clouds, NLCs. https://www.atoptics.co.uk/highsky/nlc1.htm.