As we head into the 2018 Atlantic hurricane season, now is a good time to reflect on the accomplishments achieved by CYGNSS since its launch in December 2016. Early mission operations focused on engineering commissioning of the satellites and of the constellation as a whole. One achievement in particular is noteworthy. The satellites have no active means of propulsion, yet their relative spacing is important for achieving the required spatial and temporal sampling. The desired spacing is achieved by individually adjusting a spacecraft’s orientation and, as a result, the atmospheric drag it experiences. This technique is referred to as “differential drag”. An increase in drag will lower a satellite’s altitude, thereby changing its orbital velocity. We adjust the distance between spacecraft by adjusting their relative velocities. This is a new way of managing the spacing between a constellation of satellites, and one that can be significantly less risky and lower in cost than using traditional active propulsion. As a result, we were able to afford more satellites for the same price, which ultimately led to better, more frequent, sampling of short lived, extreme weather events like tropical cyclones.
Here is a figure, provided by CYGNSS team member Kyle Nave of ADS, illustrating the change in relative speed between two of the CYGNSS spacecraft that occurred the first time a differential drag maneuver was performed, on February 23, 2017.
The orbital phase rate between the two spacecraft is shown before, during and after the higher of the two had its orientation changed to maximize atmospheric drag. Phase rate measures how quickly the angle between two satellites changes. By increasing the drag on the higher one, it lowers to an altitude and orbital velocity closer to the lower one, thus reducing the phase rate. This was an important first confirmation of our ability to perform the maneuver. Since then, there have been many more drag maneuvers. Five of the eight satellites are now properly positioned relative to one another at a common altitude, and the remaining three are expected to have their drag maneuvers completed later this year.
The primary science objective of the CYGNSS mission is measurement of near surface wind speed over the ocean in and near the inner core of tropical cyclones. In an earlier NASA blog, (15 Dec 2017), I reported on our measurements of Hurricane Maria made in September 2017. Since that time, we have been examining the quality of our measurements both within and away from major storms. Measurements at ocean wind speeds below 20 m/s (44 mph) were found to have an RMS uncertainty of 1.4 m/s (3 mph). Measurements of storm force winds during the 2017 Atlantic hurricane season were found to have an uncertainty of 17% of the wind speed. The analysis that produced these results is reported in Ruf et al. (2018). DOI: 10.1109/JSTARS.2018.2825948.
CYGNSS operates continuously, over both ocean and land, and the land data have been another focus of recent investigations. The quality of some of those measurements, in particular regarding its spatial resolution, has come as something of a pleasant surprise. Here is one example of CYGNSS land imagery, of the Amazon River basin in South America, provided by Dr. Clara Chew of UCAR.
In the image, inland water bodies are prominently visible. This includes not only the major arms of the Amazon River but also its quite narrow minor tributaries. Careful examination of this and similar CYGNSS images suggests that the spatial resolution is markedly better here than it is over typical open ocean areas. The explanation lies in a transition of the electromagnetic scattering from an incoherent, rough surface regime over ocean to a largely coherent, near specular regime over inland waters. The fact that coherently scattered signals have inherently better spatial resolution is a well known phenomenon. What was unexpected is the widespread, global extent to which land surface conditions support coherent scattering. It requires the height of the surface roughness to be significantly below the wavelength of the radiowave signal, which in our case is 19 cm. This is apparently a ubiquitous property of wetland regions. It is a very fortuitous property for us, as it should enable an entirely new direction in scientific applications of CYGNSS measurements over land. NASA has recently added new investigators to the CYGNSS team specifically to study these new and exciting land applications.
A recent article summarizing these and other CYGNSS achievements, as well as some of the future applications of its measurements, is available at <www.nature.com/articles/s41598-018-27127-4>. The mission has demonstrated that smaller, more cost-efficient satellites are able to make important contributions to the advancement of science. In the months and years ahead, CYGNSS will hopefully be able to demonstrate that those advances can lead to practical scientific applications, such as extreme weather monitoring and prediction, that will benefit humankind.
CYGNSS was launched into low Earth orbit on December 15, 2016 at 08:37 EST and today is its first anniversary. The mission has had a very busy first year on orbit, transitioning from an early engineering commissioning phase into the science observing phase in time for the very active 2017 Atlantic hurricane season. The mission was supported during the hurricane season by the NOAA Airborne Operations Center (AOC), which operates a fleet of P-3 “hurricane hunter” airplanes that make reconnaissance flights into tropical storms and hurricanes to observe wind speed and other weather conditions first hand. We worked closely with AOC to coordinate many of their flight campaigns with overpasses of the storms by CYGNSS. They were able to time many of their eyewall penetrations to align closely in both time and space with our overpasses, which helps us train and evaluate our own wind speed measurements As a result, we now have dozens of coincident tracks of wind speed observations through the inner core regions of Hurricanes Harvey, Irma, Jose and Maria. The collaboration with NOAA this summer and fall has been incredibly fruitful, and I and the CYGNSS project team are very grateful for their generous support.
As the 2017 hurricane season winded down, we turned our attention to processing the coincident overpass data and characterizing and evaluating the performance of our wind speed measurements. One example is shown here. On September 24, 2017 at 18:13-18:21 UTC, the CYGNSS FM#2 spacecraft flew across Hurricane Maria.
The red line in the figure shows the track of the specular reflection from transmissions by the GPS PRN#13 satellite. CYGNSS makes its wind measurements along this track. The black line in the figure shows the flight path of the P-3 hurricane hunter that day. A distinctive cloverleaf pattern can be seen that results from the plane making multiple eyewall penetrations. The colored portion of the P-3 flight path is the leg closest in time and space to the CYGNSS specular point track. The color scale represents the difference in time between the CYGNSS overpass and the P-3 observing time. With such close coincidence in time and space, we hope and expect that the two measurements of wind speed will be consistent.
The next figure shows the wind speed measured by CYGNSS (blue), measured by the P-3 airplane using its Stepped Frequency Microwave Radiometer (SFMR) wind speed sensor (red), and produced by the ECMWF and GDAS numerical weather prediction (NWP) models (black) along the CYGNSS specular point track.
Away from the storm center at lower wind speeds, CYGNSS and NWP measurements agree well. Near the storm center, CYGNSS responds to the much higher wind speeds. In general, NWP models tend to underestimate peak winds in large storms and this is likely the case here. While NWP models generate winds everywhere, SFMR winds are only available along the portion of the satellite track where the P-3 airplane flew. In the region where coincident measurements were made, CYGNSS and SFMR winds can be seen to agree fairly well. It should be noted that the scatter present in the CYGNSS measurements can be seen to increase as the wind speed increases. This is probably a result of the decrease in GPS signal strength scattered in the specular direction when the sea surface is significantly roughened by high winds. How best to handle this characteristic of the CYGNSS wind speed retrievals will be an important topic of upcoming investigations.
Happy Birthday, CYGNSS!
p.s. and just in time for the first year anniversary of CYGNSS on orbit, new science data files using the latest (v2.0) engineering calibration and science retrieval algorithms have just been posted at the NASA PO.DAAC web site. Access the data by going to
https://podaac.jpl.nasa.gov/dataset/CYGNSS_L1_V2.0
https://podaac.jpl.nasa.gov/dataset/CYGNSS_L2_V2.0
https://podaac.jpl.nasa.gov/dataset/CYGNSS_L3_V2.0
and selecting the ‘Data Access’ tab to reach an FTP link to the data files.
By Eric Lindstrom
Your blogger, Eric Lindstrom, sending his daughter Kelsey a birthday balloon.
SPURS-2 on R/V Revelle has been a balloon bonanza. We have been profiling the atmosphere every six hours with radiosondes (AKA; sondes) attached to helium filled weather balloons. The sondes measure the temperature and humidity profiles in the atmosphere are transmitted via radio signal back to a receiver on the ship. They are tracked by GPS so also provide height and wind (via their changing position). The sondes are lost after the balloon pops in the upper atmosphere (over 12.4 miles –20 kilometers– up). The profile data of the atmosphere are key to studying convection, clouds, rain, and stability in the tropical atmosphere. Modern meteorology depends on hundreds of balloon launches worldwide every 12 hours. Such observations over the ocean are much less common and still provide for scientific discovery.
Kelsey’s balloon, floating away.
The regular weather balloon launches have also been a great way to send our birthday wishes home to friends and family. Today’s blog and balloon launch come with a hearty “Happy Birthday” to my daughter, Kelsey, in Everett, Washington! Kelsey, your birthday balloon made it to more than 21,000 meters (far into the stratosphere) and an atmospheric pressure of 44 millibars (1011.7 millibars at sea level here).
The Lighter Than Air Infrared System, just after launch.
To study sea surface temperature variation on small scales, we have deployed the Lighter-Than-Air InfraRed System (LTAIRS). LTAIRS is a large, re-usable helium-filled balloon tethered to the ship with an instrument package hanging below it. The APL team lead by Andy Jessup has suspended a camera package that images the temperature patterns on the sea surface. The LTAIRS package is deployed when there are relatively steady light winds. It is quite a drama if the LTAIRS changes height or direction rapidly. This can happen, for example, if it is launched in the heat of the day and quickly loses lift in the rapid cooling after sunset (the balloon loses lift as the helium cools). Or, if the wind shifts rapidly during a rain shower the balloon can quickly drift back over the ship bringing its tether into the ship (think tangles!). We want no drama with an expensive ($50,000 worth) camera sitting just below the balloon. However, it is oceanography and any deployed equipment is at risk of loss. Innovative observations with potential for discovery (a bonanza!) are the reward for taking and managing the risk of the balloon acrobatics. So far there are very interesting variations and patterns in surface temperature during rain events. It will take considerable post-expedition analysis to pull the whole story together, but these results look extremely promising.
For ballooning we have quite a supply of helium aboard. For many years the world’s supply of helium has been under pressure, with usage depleting national reserves of this noble gas. Recently new supplies have been discovered in Africa, but helium is another one of those resources like oil, fresh water, or fish that have come under global management for human uses. Its supply closely parallels that of natural gas from which it is separated. Conservation activities and rising cost seems the logical outcome for the long term. Helium, many other resources, will see the times of bonanza and bust as we find and use our way through the reserves and discoveries.
Unrelated to the balloons, but here’s a photo of a beautiful sunset at sea.
Overall, the ballooning on R/V Revelle for SPURS-2 has been very successful and highly informative – a real bonanza of scientific information. Raymond Graham will get his Masters degree puzzling over the atmospheric profiles. Andy Jessup will spend weeks glued to the computer puzzling over the patterns in the LTAIRS IR images. And every one had fun at some point in the expedition filling or launching a balloon. Very uplifting all around!
I have always wanted to visit Greenland, the “ground zero” for climate change. Its ice cap rises more than 3,200 meters (2 miles!) above sea level and it holds so much ice that if it all melted, the height of sea level would rise by 7 meters (23 feet). The landscape is stark and inhospitable for most plants and animals. Greenland’s location makes it very sensitive to climate change, and it has a dynamic geological history as a result. It is undergoing rapid change now, as shown in the chart below. Greenland is “ground zero” for climate change.
The loss of ice mass by the Greenland Ice Sheet from 2002 to 2015, measured by NASA’s Gravity Recovery and Climate Experiment (GRACE) and by the DC8 in NASA’s Operation IceBridge. Each gigaton of ice lost adds about 1 cubic kilometer of water to the oceans. The ice sheet has lost significant mass every year since measurements began in 2002.
For our flight to Greenland, the DC8 took off from Lajes air base in the Açores and headed directly to the ice sheet, traversing its length along the western edge. Our destination was the Canadian scientific base at Eureka (80o N), off the northern coast on Ellesmere Island’s Fosheim Peninsula. Instruments at this station record the total amount of greenhouse gases and pollutants between the Earth’s surface and the Sun. Our goal was to measure these gases at each altitude in the atmosphere, in order to check the accuracy of these data. We would then head south and land at Kangerlussuaq, on Greenland’s west coast.
The ice cap was mostly covered in clouds until we reached the northern part of Greenland. When we saw the ice surface, it showed many dark features, including melt ponds and dark hills, especially at the edge of the ice sheet.
A lonely iceberg floats off the eastern coast of Ellesmere Island, Canada. Satellite data show that areas around Ellesmere Island that formerly retained floating ice all year round are now ice free, like the waters in this image.
While flying over Ellesmere Island, I snapped a photo that dramatically illustrates how dark surfaces put heat into the atmosphere. The picture shows currents of air being warmed by dark, ice-free rock, rising up along the slopes of small hills and producing the little cloud right at the top of each one. This is the way that heat absorbed by the rocky surface is distributed throughout the lowest layer of the atmosphere.
The rocky hills of Ellesmere Island absorb energy from sunlight and from heat radiated by the atmosphere (“longwave radiation”). This unusual photo shows the warm, dark rocky surface heating the air, causing warm plumes to rise and forming small cloud caps over each hill. The process efficiently transfers heat from the surface to the atmosphere (Photo: Steve Wofsy).
When we flew out over the sea, we could see that the waters off shore were almost completely free of ice. Occasionally we saw a few icebergs, or small amounts of ice near shore, but otherwise we saw very little floating ice.
The floating ice in the Arctic does not store a great mass of water like the Greenland’s land-based ice sheet, but it does have a very strong influence on regional climate. When the ATom mission started in early August, we flew over the Beaufort Sea north of Alaska, up to 80oN, and saw ice floes covered with dark melt ponds and interspersed with dark areas of open water. Ice started to break up in this area especially early this year, and with all that dark water absorbing heat from the sun and from the atmosphere, I wondered if most of the Beaufort Sea might become ice free by summer’s end. Now we were back!
Satellite data show that the Arctic Ocean did not lose all of its ice. But the Beaufort Sea did lose its floating ice, as far north as 75o N, and 30 to 70% was lost at 80oN. Water temperatures became quite warm off the Arctic coast of Alaska, maybe just about swimmable for a New Englander like me (5 – 10 C, 40 – 50 F). This large expanse of warm water will tend to make the fall season warmer and provide more snowfall in the Arctic than in former times, when the ice persisted more extensively on the ocean surface. The loss of floating ice amplifies itself, a process called “positive feedback.” As open water and melt ponds cover more area, the ice and surrounding water absorb more energy, leading to more ice loss, more energy absorbed, etc.
This year the overall loss of floating ice was among the greatest ever observed, although it did not quite match the record set in 2012. Indeed, as we were flying over Ellesmere, the large luxury cruise ship Crystal Serenity was reportedly starting the first crossing of the fabled Northwest Passage by a ship of its type, with 1000 paying passengers.
I also photographed the calving edge of a huge tidewater glacier, where huge pieces of ice (bigger than a football field, weighing 1 million tons or more) break off the glacier and drop into the sea. In this picture, you can see how dark the surface of the glacier has become as it travels to the sea, in contrast to the clean blue ice below. The darkening accelerates the melting of the glacier; it is due to deposition of some of the pollutants (soot) that we measure, and also to algae that grow on the ice. Scientists don’t yet know how much of this dark material is “natural” versus human-caused, and they are studying the ice to understand how much darkening is caused by deposition in the form of soot from fires or by pollution-related nutrients that stimulate the growth of algae.
The end of the line: calving front of a tidewater glacier in western Greenland. As ice flows down from the Greenland ice cap to the sea, the surface darkens due to deposition of atmospheric pollutants and growth of algae. The picture shows the boundary where the moving glacier enters the ocean. Huge pieces of ice break off (“calving”) the leading edge and drop into the water—the origin of icebergs. (Photo: Steve Wofsy)
After flying over Eureka and the west coast of Greenland, we landed in Kangerlussuaq, just a short distance south of the Arctic Circle. The area around Kangerlussuaq is stark and beautiful. The town is very reminiscent of many communities across the Arctic. It lies in the midst of a deglaciated area of bare rock mountains and a thin layer of tundra, in a lovely fjord. The river roars like a freight train as it goes under the bridge, carrying meltwater from the ice cap to the sea. The weather was very warm and dry. My afternoon run was interrupted by an encounter with a musk ox. The Sunday dinner at the Boat Club may have been the best of my month long trip around the world, featuring at least eight types of smoked or cured fish. This country is really breathtaking. But it was also a rather sad place, very, very quiet, with dozens of buildings left empty after the closing of the extensive military facilities when the Cold War ended, and the summer had been much warmer than average, drying up soils and plants.
After two days in Kangerlussuaq, we set out for home across Arctic Canada, arriving for a brief overnight in Minneapolis. We stayed at a hotel just across the road from the Mall of America, a vast place with reportedly 30-40 million visitors per year. The contrast with Kangerlussuaq was very dramatic—scenery, culture, sounds, smells, climate, air quality.
This transition was in many ways so typical of the ATom experience, and ATom science. We went from the high desert, with its fires and searing heat, directly to the floating ice near the North Pole, then Hawaii and American Samoa in the subtropics, then suddenly to winter in New Zealand and Chile, back to the subtropics on Ascension Island and the Açores (Portugal), and then again to a land of ice caps and glaciers in Greenland – all in the space of three weeks.
We measured the chemicals in the atmosphere, from bottom to top and along the whole route, and all in the space of just over three weeks. We saw some stunningly dirty air even in the middle of the ocean, in the subtropics and in the Arctic, with a lot of pollution coming from biomass fires, and also some very clean air. Our instruments are extremely sensitive, though, and in even the remote region of the Antarctic polar vortex, we appear to have detected traces of pollution.
After traveling on the DC8 across both the Atlantic and the Pacific, the world seemed smaller than ever before, and the atmosphere, not at all infinite or inexhaustible.
The goal of ATom is to learn about how the most remote parts of the atmosphere are affected by pollutants emitted on land. We have a lot to work with! It will take months to analyze our data and really understand what we have measured, and to assess what we have learned about humanity’s impact on air quality and climate. We also have a lot of memories and impressions (and some sleep to catch up on). I know it will take me months, or years, to fully process what I have experienced and, along with the wonderful ATom team, to extract the quantitative scientific information we need to understand the environment.
This image shows a deglaciated region in western Greenland, with the ice cap in the background. (Inset: Map of Greenland, showing location of our landing site at Kangerlussuaq at 67o N, 50.7 W). According to Richard Alley of Penn State University, this area around Kangerlussuaq was deglaciated at the end of the last ice age, 8-10,000 years ago, but ice returned and scraped the ridges bare during the Little Ice Age, a period between about 1300 and 1870 AD during which Europe and North America had colder winters than in the 20th century.
The NASA DC8 Flying Laboratory in Kangerlussuaq, Greenland. Inset: Our flight paths during ATom-1. We overnighted in 11 different time zones at 10 sites (one clock shift) in 23 days, covering 65,000 km (40,500 mi, equal to 1.6 times around the earth). We made 160 vertical soundings, and measured more than 300 chemical and aerosol parameters.
As we prepare for our last flight in ATom-1, I’ve been reflecting on what I should do differently next time around as we begin preparation for ATom-2, which will start uploading in December 2016 for flights starting in January 2017.
I’m a bit of a girly girl who likes her comforts. But after hauling bags for miles as we get on and off the aircraft, I’m seriously considering becoming a minimalist! The weight of the instrument laptop seems to have increased as the project has gone on, so what I consider “essential” for each stop has been streamlined to a very short list! Washing clothes has also proved easier than I thought so I could have managed with less clothes (I can’t believe I’m admitting that!) but I did wear everything I brought at some stage so nothing was a waste of space. In fact, the list of things I wish I had brought is quite short: my hiking boots! I had assumed that taking off from and landing into warm areas meant I could wear light clothes – which was true (to a point). While on each flight, I ended up wearing my thermal vest, long johns, jeans, and my ski jacket as the aircraft is so cold. My toes were particularly cold by the end of each flight! With the ATom-2 happening in Northern Hemisphere winter, we are still trying to figure out if we can borrow suitable clothes to wear when we arrive in Alaska and Greenland. But, as I’m barely 5 feet tall, with child size feet, I’ll probably end up needing to bring my own gear anyway, so maybe my snow boots can work as my aircraft slippers next time?
Róisín Commane hauling her bags off the DC8 for the final time on Atom-1.
It’s really easy for me to put on weight during projects like ATom. So I’ve been trying to eat small portions of healthy food, as well as be active on days we don’t fly. My running is slower than some people’s walking but at least it’s something! For lunch on the aircraft, I have mostly eaten ham and cheese sandwiches, with some peanut butter sandwiches when we couldn’t get meat and cheese. My lunches have been functional but uninspiring. And then Shuka Schwartz wanders by with the most amazing smelling food and I get food envy! I’m still trying to figure out how he makes such good lunches. I’m open to suggestions on this as I’m not a particularly inventive chef. Maybe we can bring a few more ingredients to help construct more imaginative lunches. Or maybe I’ll just start ordering extra at dinner the night before a flight for lack of better options.
Not the most inspiring lunch… Photo by Róisín Commane
We knew internet access would be difficult in some locations but it turned out to work quite well in most locations in the Pacific. I think this gave me a false sense of confidence in internet infrastructure and I didn’t arrange to not contact people while in the Atlantic. The internet was down while we were on Ascension Island, but I did manage to send a text, from the one working phone, telling my family I arrived safely. Internet access was really expensive in Kangerlussuaq, but I have to admit that I was happy to have a break from answering emails for a few days – and I don’t think I was the only one! I think I will just plan on a complete contact black out for those locations next time so my family is not concerned.
Róisín in Ascension Island.
This is the first time I’ve been involved in where I’m one of the ‘media people’. Like most people, I’m used to taking photos (to show my family and friends) but videos have proved to be more useful to the NASA media reps. They are doing are doing a wonderful job given how little I’m giving them to work with! We’ve also done some discussion of the science on video. It took a lot of encouragement, but I have started to not worry too much about how awful I think I look on camera or how weird my voice sounds. Instead, I’m just not looking at it 😉
Róisín Commane at Russel Glacier, Greenland.
As part of doing the media outreach I’ve been asking the ATom scientists and crew to think of their favorite ATom memory. Most people have great memories of places and events. I will always remember the strikingly blue melt ponds on the Greenland ice sheet and how little sea ice there was north of Alaska. But I think my favorite memory from ATom will be the people. We’ve travelled around the world on an aircraft with 42 people on board: some people left us in Christchurch while more arrived to join the adventure. In such close quarters, I was aware that little things could have mushroomed into major conflicts. But people worked really well together. Friendships were formed and the science benefited greatly from a fantastic team effort. In particular, we owe a huge thank you to the crew of the DC-8 who brought us safely around the world and made all the measurements possible.
ATom Scientists and crew are happy to be home! Photo by Michael Prather.
