Aqua Mission (EOS/PM-1)
The Aqua mission is a part of the NASA's international Earth Observing System (EOS). Aqua was formerly named EOS/PM-1, signifying its afternoon equatorial crossing time. NASA renamed the EOS/PM-1 satellite to Aqua on Oct. 18, 1999. The Aqua mission is part of NASA's ESE (Earth Science Enterprise) program. 1) 2) 3)
The focus of the Aqua mission is the multi-disciplinary study of the Earth's water cycle, including the interrelated processes (atmosphere, oceans, and land surface) and their relationship to Earth system changes. The data sets of Aqua provide information on cloud formation, precipitation, and radiative properties, air-sea fluxes of energy, carbon, and moisture (AIRS, AMSU, AMSR-E, HSB, CERES, MODIS); and sea ice concentrations and extents (AMSR-E).
Figure 1: Illustration of the Aqua satellite (image credit: NASA)
The Aqua spacecraft is based on TRW's modular, standardized AB1200 bus design (also referred to as T-330 platform) with common subsystems (Note: Northrop Grumman purchased TRW in Dec. 2002). The satellite dimensions are: 2.68 m x 2.47 m x 6.49 m (stowed) and 4.81 m x 16.70 m x 8.04 m (deployed). Aqua is three-axis stabilized, with a total mass of 2,934 kg at launch, S/C mass of 1,750 kg, payload mass =1,082 kg, propellant mass = 102 kg; power = 4.86 kW (EOL). Propulsion: hydrazine blow-down system; 4 pairs of thrusters. The design life is six years.
RF communications: X-band, S-band (TDRSS and Deep Space Network/Ground Network compatible). All communications are based on CCSDS protocols. Like the Terra mission, Aqua provides various means of payload data downlinks, among them Direct Broadcast (DB).
Figure 2: The Aqua spacecraft in launch preparation at VAFB (image credit: NASA)
Launch: The Aqua spacecraft was launched on May 4, 2002 with a Delta-2 7920-10L vehicle from VAFB, CA. Aqua is the second satellite in NASA's series of EOS spacecraft. - Aura, the third of the three large satellites in the EOS series, was launched in July 2004 and is lined up behind Aqua, in the same orbit.
Orbit: Sun-synchronous circular orbit, altitude = 705 km (nominal), inclination = 98.2º, local equator crossing at 13:30 (1:30 PM) on ascending node, period = 98.8 minutes, the repeat cycle is 16 days (233 orbits).
The Aqua spacecraft is part of the "A-train" (Aqua in the lead and Aura at the tail, the nominal separation between Aqua and Aura is about 15 minutes) or "afternoon constellation" (a loose formation flight which started sometime after the Aura launch July 15, 2004). The objective is to coordinate observations and to provide a coincident set of data on aerosol and cloud properties, radiative fluxes and atmospheric state essential for accurate quantification of aerosol and cloud radiative effects.
The PARASOL spacecraft of CNES (launch on Dec. 18, 2004) is part of the A-train as of February 2005. The OCO mission (launch in 2009) will be the newest member of the A-train. Once completed, the A-train will be led by OCO, followed by Aqua, then CloudSat, CALIPSO, PARASOL, and, in the rear, Aura. 4)
Note: The OCO (Orbiting Carbon Observatory) spacecraft experienced a launch failure on Feb. 24, 2009 - hence, it is not part of the A-train.
Figure 3: Illustration of Aqua in the A-train (image credit: NASA)
• April 20, 2017: From space, the Strait of Gibraltar appears tiny compared to the continents it separates. At the strait's narrowest point, Africa stands just 14 km from Europe. But the narrow waterway is a complex environment that gives rise to striking phytoplankton blooms when conditions are right (Figure 4). 5)
- Water conditions and circulation near the strait produced a bloom with colorful tendrils visible in this image, acquired on March 8, 2017. The image is composed from data acquired with VIIRS (Visible Infrared Imaging Radiometer Suite) on Suomi NPP, and MODIS (Moderate Resolution Imaging Spectroradiometer) on NASA's Aqua satellite. A series of processing steps were applied to highlight color differences and bring out the bloom's subtler features.
- The intricate swirls of phytoplankton trace the patterns of water flow, which in this region can become quite turbulent. For example, water moving east from the North Atlantic into the Mediterranean Sea has created turbulence in the form of internal waves. These waves—sometimes with heights up to 100 meters—occur primarily deep within the ocean, with just a mere crest poking through the surface. At the same time, water flowing west helps stir up water in the North Atlantic, including the Golfo de Cádiz.
- While most of the swirls of color are phytoplankton, ocean scientist Norman Kuring of NASA's Goddard Space Flight Center notes that some of the color near coastal areas could be due to sediment suspended in the water, particularly near the mouths of rivers. Some of the yellow-green plume near the Guadalquivir River, for example, could be due to CDOM (Colored Dissolved Organic Matter). "My guess is that there is less suspended sediment along the Iberian and African coastlines than you might expect to find in eastern U.S. coastal waters, which overlay a broader continental shelf than what is found around Iberia," Kuring said.
Figure 4: MODIS and VIIRS observations on Aqua and Suomi NPP, respectively, showing water conditions near the Strait of Gibraltar, acquired on March 8, 2017 (image credit: NASA Earth Observatory, image by Norman Kuring, text by Kathryn Hansen)
• March 16, 2017: The first global, long-term satellite study of airborne ammonia gas has revealed "hotspots" of the pollutant over four of the world's most productive agricultural regions. The results of the study, conducted using data from NASA's AIRS (Atmospheric Infrared Sounder) instrument on NASA's Aqua satellite, could inform the development of strategies to control pollution from ammonia and ammonia byproducts in Earth's agricultural areas (Figure 5). 6)
- A University of Maryland-led team discovered steadily increasing ammonia concentrations from 2002 to 2016 over agricultural centers in the United States, Europe, China and India. Increased concentrations of atmospheric ammonia are linked to poor air and water quality. The NASA-funded study, published March 16 in Geophysical Research Letters, describes probable causes for the observed increased airborne ammonia concentrations in each region. Although specifics vary between areas, the increases are broadly tied to crop fertilizers, livestock animal wastes, changes to atmospheric chemistry, and warming soils that retain less ammonia. 7)
- "Measuring ammonia from the ground is difficult, but the satellite-based method we have developed allows us to track ammonia efficiently and accurately, said Juying Warner, UoM (University of Maryland) associate research scientist in atmospheric and oceanic science. "We hope that our results will help guide better management of ammonia emissions."
- AIRS, in conjunction with the AMSU (Advanced Microwave Sounding Unit) also on Aqua, senses emitted infrared and microwave radiation from Earth to provide a 3D look at our planet's weather and climate. Working in tandem, the instruments make simultaneous observations down to Earth's surface, even in the presence of heavy clouds. With more than 2,000 channels sensing different regions of the atmosphere, the system creates a global, 3D map of atmospheric temperature and humidity, cloud amounts and heights, concentrations of selected greenhouse and other trace gases, and many other atmospheric phenomena.
- "AIRS wasn't designed to observe ammonia (NH3), but the instrument sensitivity and stability have allowed us to monitor ammonia trends," said AIRS Project Scientist Eric Fetzer of NASA/JPL (Jet Propulsion Laboratory), Pasadena, California. "The unexpected large ammonia increase is one example of rapid atmospheric changes from human activities that AIRS is observing."
- Gaseous ammonia is a natural part of Earth's nitrogen cycle, but excess ammonia is harmful to plants and reduces air and water quality. In the troposphere — the lowest, most dense part of the atmosphere where all weather takes place and where people live — ammonia gas reacts with nitric and sulfuric acids to form nitrate-containing particles. Those particles contribute to aerosol pollution that is damaging to human health. Ammonia gas can also fall back to Earth and enter lakes, streams and oceans, where it contributes to harmful algal blooms and "dead zones" with dangerously low oxygen levels.
- "Little ammonia comes from tailpipes or smokestacks. It's mainly agricultural, from fertilizer and animal husbandry," said co-author and University of Maryland professor Russell Dickerson. "It has a profound effect on air and water quality — and ecosystems."
- Each major agricultural region highlighted in the study experienced a slightly different combination of factors that correlate with increased ammonia in the air from 2002 to 2016. The United States, for example, has not experienced a dramatic increase in fertilizer use or major changes in fertilizer application practices. But the study authors found that successful legislation to reduce acid rain in the early 1990s most likely had the unintended effect of increasing gaseous ammonia. The acids that cause acid rain also scrub ammonia gas from the atmosphere, and so the sharp decrease in these acids in the atmosphere is the most plausible explanation for the increase in ammonia over the same time frame.
- Europe experienced the least dramatic increase in atmospheric ammonia of the major agricultural areas studied. The researchers suggest this is due in part to successful limits on ammonia-rich fertilizers and improved practices for treating animal waste. Much like the United States, a major potential cause for increased ammonia traces back to reductions in atmospheric acids that would normally remove ammonia from the atmosphere.
- "The decrease in acid rain is a good thing. Aerosol loading has plummeted — a substantial benefit to us all," Dickerson said. "But it has also increased gaseous ammonia loading, which we can see from space."
- In China, a complex interaction of factors is tied to increased atmospheric ammonia. The authors suggest efforts to limit sulfur dioxide — a key precursor of sulfuric acid, one of the acids that scrubs ammonia from the atmosphere — could be partially responsible. But China has also greatly expanded agricultural activities since 2002, widening its use of ammonia-containing fertilizers and increasing ammonia emissions from animal waste. Warming of agricultural soils, due at least in part to global climate change, could also contribute.
- "The increase in ammonia has spiked aerosol loading in China. This is a major contributor to the thick haze seen in Beijing during the winter, for example," Warner said. "Also, meat is becoming a more popular component of the Chinese diet. As people shift from a vegetarian to a meat-based diet, ammonia emissions will continue to go up."
- In India, a broad increase in fertilizer use coupled with large contributions from livestock waste have resulted in the world's highest concentrations of atmospheric ammonia. But the researchers note that ammonia concentrations have not increased nearly as quickly as over other regions. The study authors suggest that this is most likely due to increased emissions of acid rain precursors and, consequently, some increased scrubbing of ammonia from the atmosphere. This leads to increased levels of haze, a dangerous trend confirmed by other NASA satellite instruments, Dickerson said.
- In all regions, the researchers attributed some of the increase in atmospheric ammonia to climate change, reflected in warmer air and soil temperatures. Ammonia vaporizes more readily from warmer soil, so as the soils in each region have warmed year by year, their contributions to atmospheric ammonia have also increased since 2002. The study also ascribes some ammonia fluctuations to wildfires, but these events are sporadic and unpredictable. As such, the authors excluded wildfires in their current analysis.
- "This analysis has provided the first evidence for some processes we suspected were happening in the atmosphere for some time," Warner said. "We would like to incorporate data from other sources in future studies to build a clearer picture."
Figure 5: Global atmospheric ammonia trends measured from space from 2002 to 2016. The hot colors represent increases from a combination of increased fertilizer application, reduced scavenging by acid aerosols and climate warming. The cool colors show decreases due to reduced agricultural burning or fewer wildfires (image credit: Juying Warner and UoM study team, GRL)
• February 24, 2017: It is rare for satellites to get a clear view of the whole Antarctic Peninsula, the northernmost arm of the continent and one of the largest contributors to sea level rise during the past half-century. In the winter, polar darkness hides this rocky, ice-covered strip; in the summer, clouds usually block the view. 8)
- But every now and then, usually in January or February, there is enough of a break in the clouds to get a good glimpse. That is what happened on January 8, 2016, when MODIS (Moderate Resolution Imaging Spectroradiometer) on NASA's Aqua satellite captured this remarkably clear view of the peninsula (Figure 6).
- The ice-covered and mountainous peninsula stretches 1,300 km into the Southern Ocean. Some geologists think that millions of years ago the mountains may have connected to the Andes. An arcing underwater ridge connects the tip of the peninsula with several small islands (South Georgia, South Sandwich, and South Orkney) then continues to Tierra del Fuego, the southernmost point of South America.
- As with most of Antarctica, the feature that dominates the peninsula and the ocean surrounding it is ice. There is ice on land: the glaciers and ice sheets. There are thick slabs of floating ice that fan out along the coasts: the ice shelves. There are blocky chunks of floating ice that break off, or calve, into the sea—icebergs. Finally, there is often a thin crust of sea ice that forms on the ocean surface in cold weather.
- Just a few percent of the peninsula's land area is ice-free, and in some places the ice is as much as 500 m thick. The rare exposed areas (brown) are mainly isolated crags and mountain peaks—nunataks—that poke up through the ice layer.
- In recent decades, weather stations have measured fluctuating temperatures on the Antarctic Peninsula. Between 1951 and 2000, temperatures rose by 2.8°C, faster than anywhere else in the world. However, air temperatures then began an equally rapid swing in the opposite direction, dropping by roughly 1°C between 2000 and 2014. Meanwhile, surrounding ocean temperatures have been warming since the 1990s, particularly on the western side of the peninsula opposite the Larsen C Ice Shelf.
- While changing air temperatures may have had some effects on the ice, scientists increasingly believe that warming waters have changed the peninsula's ice shelves and glaciers the most. Alison Cook and David Vaughan of the British Antarctic Survey analyzed satellite images and aerial photographs dating back to the 1960s and concluded that seven of the twelve ice shelves along the coasts had gotten smaller between the 1960s and 2010.
- Larsen A (3,736 km2 in the 1960s), Larsen B (11,958 km2 in the 1960s), and Wordie (1,917 km2 in the 1960s) have changed the most, losing much of their ice. Note that when this image was captured sea ice—not the thick ice of an ice shelf—was present in Wordie Bay. George VI had lost some ice, but it remained about 92 percent of the size it was in 1947. Larsen C was relatively stable in 2010, but it is now poised to calve an iceberg the size of Delaware. Larsen D, which expanded by 4 percent, was the only ice shelf that grew larger, according to Cook and Vaughan's analysis.
- A similar story played out on the glaciers. Of the 860 glaciers on the peninsula, Cook and Vaughan found that 90 percent of them had retreated between the time they were first photographed roughly a half-century ago and 2010. Of these, 30 glaciers lost more than 10 km2 of ice, 190 lost between 1 and 10 km2, and 558 lost 1 km2 or less. Eighty-two glaciers advanced, most by less than 1 kilometer.
- The Fleming, Hariot, and Prospect glaciers—which flow off the peninsula into Wordie Bay—retreated the most during that time period. Hektoria, Crane, and Jorum — glaciers that flow into the Larsen B embayment — also retreated significantly. All of these glaciers saw rates of ice loss accelerate significantly when adjacent ice shelves, which helped hold them in place, broke up.
• January 3, 2017: It may not be obvious to the naked eye, but events on Earth's surface — such as a burning fire or a floating iceberg — can affect the development and shape of clouds in the sky. The connection stems from how these events influence the rise and fall of air masses. 9)
- In general, clouds form where the air is ascending. Air cools as it rises, and because cold air holds less water, it quickly saturates and reaches the point of condensation as it cools in the atmosphere. The most common way to get moist air masses to rise is to heat the ground with sunlight. But the energy to lift air can come from other sources too, such as from the heat of a volcano or a fire (Figure 7).
Figure 7: Energy from the Sun-warmed ground is not the only way to get air masses to lift, cool, and form clouds. That energy can also come from the heat of a volcano or, in this case, fire. NASA's Aqua satellite captured this image of a pyrocumulonimbus cloud on August 5, 2014, south of Yellowknife, Canada (image credit: NASA Earth Observatory, images by Joshua Stevens and Jeff Schmaltz, using MODIS data from LANCE/EOSDIS Rapid Response)
Legend to Figure 7: Pyrocumulonimbus and pyrocumulus clouds — sometimes called "fire clouds" — are tall, cauliflower-shaped clouds that show up in satellite imagery as opaque white patches hovering over darker smoke. With the exception of their fiery origin, pyrocumulus clouds are similar to cumulus clouds in structure. The image of Figure 7 is taken from the feature: "A Celebration of Clouds". 10)
- While cloud growth favors ascending air, the converse also applies. Clouds generally fail to form where the air is descending. On June 1, 2016, VIIRS (Visible Infrared Imaging Radiometer Suite) on the Suomi NPP satellite captured this image (Figure 8) of a void in the low stratus clouds over iceberg A-56 as it drifted in the South Atlantic Ocean (Ref. 9).
- The exact reason for the hole is somewhat of a mystery. It could have happened by chance, although imagery from the days before and after this date suggest something else was at work. Steve Palm, a research meteorologist at NASA/GSFC (Goddard Space Flight Center), thinks the iceberg could have disrupted the air flow or modified the atmosphere in such a way to cause the clouds nearby to dissipate. If an obstacle is large enough, it can divert the flow of low-level air around it. At the same time, air above and downstream of the obstacle converges and sinks.
- "The sinking motion warms and dries out the air, causing a hole in the clouds," Palm said. "It is a common phenomenon often caused by islands."
• December 20, 2016: In the middle of most ocean basins, far from nutrient-laden coastal upwelling and river outflows, life is sparse. Few fish reside here, and even the smaller lifeforms like phytoplankton are few and far between. — Coral islands, however, buzz with aquatic activity. Particularly in the vast Pacific Ocean, it is as though these islands have a life-sustaining halo extending dozens of kilometers and invigorating otherwise barren waters. A recent study published in Nature Communications shows that coral reefs and atolls seem to provide enough nutrients to sustain biological hot spots far beyond their immediate vicinity. 11) 12) 13)
- Scientists have long known of the "island mass effect," (IME)—when the qualities of ecologically productive islands extend into nutrient-depleted waters. But they did not know just how prevalent the effect was, or just how much credit to give phytoplankton for injecting life into surrounding areas. The study found that the vast majority (more than 90 percent) of coral reef islands and atolls raise biological diversity in the ocean around them. In fact, there is an average of 86 percent more phytoplankton on the outer fringes of these reefs than would otherwise be expected.
- "There's been sort of a longstanding paradox in how island reef ecosystems are so productive when their surrounding environment isn't," said Jamison Gove, lead author of the paper and research oceanographer in NOAA's Ecosystems and Oceanography Division. The field of study is roughly 60 years old, "But this is the first research to show that IME is a ubiquitous phenomenon."
- The swath of green at the equator (Figure 9) is an area of ocean upwelling known as the equatorial cold tongue, and it brings colder, nutrient-rich water from the depths to the surface. At the northeastern tip of Australia, there is another thick lip of green. However, the strong signal here may be due in part to bottom reflectance in shallow waters, which appears similar to chlorophyll by satellites. (Authors accounted for reflectance in the study area.)
- In many ways, the island mass effect resembles larger ocean upwelling events. When ocean currents and waves encounter an island, they flow around it, and deep, nutrient-rich water is jolted upward in currents. This stirring of the water column fosters biological activity. Such hot spots (shown in green) appear around the majority of islands in the Pacific, indicating high chlorophyll values in these places.
- Activity around the Gilbert Islands stands out. The islands themselves are far smaller than the green splotches surrounding them. The abundance of phytoplankton indirectly indicates the presence of other aquatic life as well—including squid, small fish, and larger predators like tuna—that consume phytoplankton or each other.
- By extension, people rely on those microscopic plankton for their food, Gove said. "The more we understand productivity in these ecosystems, the more we can think about future changes and how these communities stand to benefit or not as the climate changes."
Figure 9: The image depicts sea surface chlorophyll in the Pacific Ocean between July 2002 and June 2012. The map was made using data acquired by the MODIS (Moderate Resolution Imaging Spectroradiometer) on NASA's Aqua satellite. Green areas correlate to higher chlorophyll (in milligram/m3), an indication of blooming phytoplankton. Blue areas indicate lower concentrations of chlorophyll and phytoplankton (image credit: NASA Earth Observatory map by Joshua Stevens, using chlorophyll data courtesy of Jamison Gove/Pacific Islands Fisheries Science Center and Gove, Jamison M., et al. (2016), caption by Pola Lem)
• On December 12, 2016, the MODIS (Moderate Resolution Imaging Spectroradiometer) on NASA's Aqua satellite captured this natural-color image of dust over the Arabian Sea. The vortex of clouds and dust is rotating in a direction dictated by Earth's rotation. In the Northern Hemisphere, this cyclonic rotation is counter-clockwise when looking down from space. 14)
- The dust arrived over the sea with a mass of warm desert air—a condition known to suppress cloud formation. It is possible that the warm, dry center of the vortex had not mixed much with the moist marine air surrounding it. The edges of the vortex may have mixed more with the marine air, giving rise to shallow, isolated cumulus clouds.
Figure 10: MODIS natural color image of dust over the Arabian Sea, acquired on Dec. 12, 2016 [image credit: Jeff Schmaltz, MODIS Rapid Response Team at NASA/GSFC, caption by Kathryn Hansen with image interpretation by Andrew Ackerman (NASA/GISS) and Toshihisa Matsui (NASA/GSFC)]
• November 1, 2016: There is more than one way for iron to get into the central Gulf of Alaska, but none rival the visual spectacle of wind-blown plumes of dust from from Alaska's Copper River Valley. MODIS (Moderate Resolution Imaging Spectroradiometer) on Aqua and Terra captured the image of Figure 11 in October 2016. 15)
- Much of the dust is comprised of "glacial flour" or "rock flour"—a silty powder with grains finer than sand. This iron- and feldspar-rich substance forms when glaciers grind against underlying bedrock. Winds tend to loft it in the fall when river levels are low and snow has not yet fallen.
- While coastal waters in this part of Alaska have relatively high levels of iron, the micronutrient tends to be in short supply in deeper waters in the middle of the Gulf of Alaska. Soluble iron is an essential nutrient in marine ecosystems because phytoplankton—which sit at the center of marine food webs—depend on it to develop. These floating microscopic plant-like organisms are eaten, in turn, by everything from zooplankton to whales. So an ocean without much phytoplankton is often an ocean without much life at all.
- Dust storms play a key role in fueling phytoplankton blooms by delivering iron to the Gulf of Alaska. Even a small addition of iron there can spur a bloom, explained Santiago Gassó, a scientist at NASA Goddard Space Flight Center who is monitoring the current dust storm.
- While Gassó and other scientists will be using satellites to keep an eye out for phytoplankton blooms in the coming days, there is no guarantee that they will see one. "It is early winter in Alaska, so there may not be enough phytoplankton still alive (or not yet dormant) to trigger a response," Gassó said. It is also possible that a bloom may occur, but that satellites will miss it because of cloud cover.
- While the relationship between iron and phytoplankton is understood in broad terms, there are many details about the process in the Gulf of Alaska that are still unknown. For instance, what happens to iron that does not get used quickly? It may remain near the surface all winter and promote phytoplankton blooms in the spring, when more light is available. Or it may just sink.
- To help clarify the role of dust storms, a small team of NASA-funded scientists, including Gassó, have been studying the area since 2011 with satellites and with ground-based air monitors on Middleton Island. The island sits right in the middle of many dust plumes streaming from the Copper River delta. "I don't think people realize that Middleton Island is one of the best places in the world to study the natural ocean fertilization by wind-blown dust," said Gassó.
• Sept. 24, 2016: In mid-September 2016, wildfires continued to burn in Siberia (Figure 12) amid what has been an active fire season for the region. These fires are the latest in an active fire season. In late June 2016, satellites showed wildfires between the Russian cities of Miryuga and Koyumba, roughly 160 km east of the current flames. 16)
- The map of Figure 13 shows the concentration of aerosols over Russia on September 18 as detected by the OMPS (Ozone Mapping Profiler Suite) on the Suomi-NPP satellite. High concentrations are represented with shades of deep red; the lowest concentrations are shades of light yellow.
- Other data revealed that smoke from the fires was lofted high into the atmosphere. On September 20, the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) satellite, which can detect the altitude of ash clouds, indicated that the tops of the smoke plumes reached an altitude of 9 km.
- An image of the same region acquired a few days earlier shows the smoke moving toward the southwest near the town of Ust'-Kut, Russia. A smoky haze filled the sky, and flakes of ash from the fire fell over the town, according to a local news report. The report also noted that employees of the Eastern Siberia-Pacific Ocean oil pipeline (which passes through the area) had to be evacuated.
Figure 12: The natural-color image, acquired on Sept. 18, 2016 by the MODIS instrument on NASA's Aqua satellite, shows huge plumes of smoke streaming toward the northwest. Areas in red show where MODIS detected unusually warm temperatures associated with fire (image credit: NASA Earth Observatory, image by Jeff Schmaltz)
Figure 13: Aerosol concentrations over Russia acquired on Sept. 18, 2016 by the OMPS instrument on Suomi-NPP. High concentrations are represented with shades of deep red; the lowest concentrations are shades of light yellow (image credit: NASA Earth Observatory, image by Jeff Schmaltz)
• July 26, 2016: Like the Aral Sea (of Kazakhstan and Uzbekistan), Iran's salty Lake Urmia has shrunk rapidly during the past few decades. As it grows smaller, the lake grows saltier. And as it grows saltier, microscopic organisms are periodically turning the water striking shades of red and orange (Figure 14). 17)
- The color changes have become common in the spring and early summer due to seasonal precipitation and climate patterns. Spring is the wettest season in northwestern Iran, with rainfall usually peaking in April. Snow on nearby mountains within the watershed also melts in the spring. The combination of rain and snowmelt sends a surge of fresh water into Lake Urmia in April and May. By July, the influx of fresh water has tapered off and lake levels begin to drop.
- The fresh water in the spring drives salinity levels down, but the lake generally becomes saltier as summer heat and dryness take hold. That's when the microorganisms show their colors, too. Careful sampling of the water would be required to determine which organisms transformed the lake in 2016, but scientists say there are likely two main groups of organisms involved: a family of algae called Dunaliella and an archaic family of bacteria known as Halobacteriaceae.
- "Previous research suggests that Dunaliella salina is responsible for reddening of Lake Urmia," explained Mohammad Tourian, a scientist at the University of Stuttgart. "In the marine environment, Dunaliella salina appears green; however, in conditions of high salinity and light intensity, the microalgae turns red due to the production of protective carotenoids in the cells."
- Other scientists emphasize the role of Halobacteriaceae, a group of bacteria found in water that is saturated or nearly saturated with salt. These bacteria release a red pigment called bacteriorhodopsin that absorbs light and converts it into energy for the bacteria. When populations of the bacteria are large enough, they can stain bodies of water.
- Note that in July 2016, the color of the lake (and thus salinity levels) appeared to be relatively constant, despite the presence of a highway causeway. While there has been concern that the causeway would make it difficult for water to circulate between the northern and southern arms of the lake—as is the case in the Great Salt Lake of Utah—the effect in Lake Urmia is not nearly as noticeable. The images of Figure 15, captured by the Operational Land Imager (OLI) on Landsat-8 on April 20 and July 9, 2016, show part of the causeway before and after the lake's color changed.
- While Lake Urmia has shifted from green to red and back several times in recent years, trends suggest that a red Urmia could become increasingly common. Drought and intensive water diversion for agriculture has been limiting the amount of fresh water reaching the lake. "The lake volume has been decreasing at an alarming rate of 1.03 km3 per year," noted Tourian, who recently analyzed data from several satellites to track how Urmia has changed. "The results from satellite imagery revealed a loss of water extent at an average rate of 220 km2 per year, which indicates that the lake has lost about 70 percent of its surface area over the last 14 years."
Figure 14: The MODIS instrument on Aqua recently captured a transition in the color of Lake Urmia between April and July 2016. On April 23, (left image) the water was green; by July 18, it was the color of wine. The shoreline is encrusted with salt deposits and appears white. Note that the ring of salt is especially noticeable in July, when water levels were lower (image credit: NASA Earth Observatory, images by Joshua Stevens)
Figure 15: Detail image of Lake Urmia, captured with OLI (Operational Land Imager) of Landsat-8 on April 20 and July 9, 2016 (image credit: NASA Earth Observatory, images by Joshua Stevens using USGS data)
• June 18, 2016: Dust storms over the Red Sea are not uncommon. This sea, after all, is surrounded by deserts. But sometime atmospheric conditions and topography combine to produce a storm that appears extraordinary in satellite imagery (Figure 16). 18)
- Winds appear to be blowing east-northeast out of Africa. "Although we see transport from the wide coastal area, the plume is especially dense over the Tokar Delta," said Georgiy Stenchikov of King Abdullah University of Science and Technology of Thuwal, Saudi Arabia.
- Gaps in near-coastal mountain ranges become pathways through which winds can carry dust and sand from inland areas toward the sea. For example Tokar Gap—located about 50 km inland—funnels winds toward the southeast from June to September. These winds spread dust from the Tokar delta out over the Red Sea and toward the Arabian Peninsula.
- According to Stenchikov, the wind gusts that caused the dust outbreak on June 15 were due to a cold front moving southeast. The front was related to a cyclone centered near the Persian Gulf, and it caused turbulent mixing of air and a series of associated haboobs.
- Scientists have been studying the dust in this area for a number of reasons. In general, dust in cloud-free conditions reflects sunlight and causes radiative cooling of the land and atmosphere. But according to Stenchikov, the effects on the energy balance of the Red Sea have not been well quantified.
- In addition, dust generated over the coastal area is often deposited in the sea. This provides an important nutrient supply to the Red Sea, which otherwise has very low nutrient levels, particularly in its more northern reaches.
• March 2016: For a chemical compound that shows up nearly everywhere on the planet, methane still surprises us. It is one of the most potent greenhouse gases, and yet the reasons for why and where it shows up are often a mystery. What we know for sure is that a lot more methane (CH4) has made its way into the atmosphere since the beginning of the Industrial Revolution. Less understood is why the ebb and flow of this gas has changed in recent decades. 19)
- One can find the odorless, transparent gas miles below Earth's surface and miles above it. Methane bubbles up from swamps and rivers, belches from volcanoes, rises from wildfires, and seeps from the guts of cows and termites (where is it made by microbes). Human settlements are awash with the gas. Methane leaks silently from natural gas and oil wells and pipelines, as well as coal mines. It stews in landfills, sewage treatment plants, and rice paddies.
- AIRS (Atmospheric Infrared Sounder) aboard NASA's Aqua satellite offers one spaceborne perspective on the methane in Earth's atmosphere. The map of Figure 17 shows global methane concentrations in January 2016 at a pressure of 400 hPa (hectopascal), or roughly 6 km above Earth's surface. Methane concentrations are higher in the northern hemisphere because both natural- and human-caused sources of methane are more abundant there. Since AIRS observed the methane fairly high in the atmosphere, winds may have transported plumes of gas considerable distances from their sources.
Figure 17: The methane data are from the AIRS (Atmospheric Infrared Sounder) on the Aqua mission acquired in the period January 1-31, 2016 and from in situ measurements (image credit: NASA Earth Observatory, Joshua Stephens)
• March 2, 2016: The Korean peninsula is an ideal laboratory for understanding air quality. Home to about 75 million people, the peninsula sits downwind from several major pollution sources (Figure 18), and also has a few of its own. 20)
- The peninsula lies a few hundred kilometers east of several large, industrialized cities in China that emit pollutants such as sulfur dioxide and nitrogen dioxide. Winds routinely send plumes of dust from the Taklimakan and Gobi deserts toward the Koreas. And outbreaks of wildfires and crop fires in China and Russia can produce plumes of smoke that make their way into Korean airspace. Meanwhile, the city of Seoul (population 10 million) is a major source of urban pollution. Crop fires are a seasonal occurrence in North Korea. As a result, the air Koreans breathe contains a complex and seasonally changing mixture of gases and particles that can pose problems for human health.
- To gain a better understanding of air pollution in this part of the world, NASA and the Republic of Korea are developing plans for a cooperative field study of air quality in May and June 2016. The KORUS-AQ (Korea U.S.–Air Quality study) will assess air quality across urban, rural, and coastal areas of South Korea using combined observations from aircraft, ground sites, ships, and satellites. The findings are crucial for the development of ground- and space-based instruments, as well as computer models that can provide more accurate air quality assessments for decision-makers.
- "KORUS-AQ is a step forward in an international effort to develop a global air quality observing system," said James Crawford, a lead U.S. scientist on the project from NASA's Langley Research Center. "Both of our countries will be launching geostationary satellites that will join other satellites in a system that includes surface networks, air quality models, and targeted airborne sampling."
- Key science goals include: developing a better understanding of the conditions affecting the vertical distribution of air pollutants; how that distribution relates to cloud cover; what factors govern ozone photochemistry and the formation of aerosols; and how well models represent real atmospheric composition over the Korean peninsula.
- NASA will work with South Korea's National Institute of Environmental Research using a King Air aircraft from Hanseo University, a NASA DC-8 flying laboratory, and a NASA Beechcraft UC-12B King Air. Five South Korean instruments will fly on the DC-8, and one NASA instrument will be onboard the Hanseo aircraft. — The KORUS-AQ team planned the field campaign for spring because April to June is when the Korean peninsula gets its strongest influx of pollution from upwind sources. 21)
• Feb. 23, 2016: The Aqua spacecraft and its sensor complement (AIRS, AMSU, CERES, MODIS) are operating nominally (in their 14th year on orbit) with a life expectancy into the early 2020s. 22)
• December 7, 2015: JAXA is reporting that it has ended the operation of the AMSR-E (Advanced Microwave Scanning Radiometer-EOS) on December 4, 2015. AMSR-E has been operated for over nine years as an onboard device installed on Aqua, after its launch on May 4, 2002. 23)
- On Oct. 4, 2011, the AMSR-E reached its limit to maintain the antenna rotation speed necessary for regular observations (40 rotations per minute), and the radiometer automatically halted its observation and rotation. Further operation of AMSR-E was suspended. JAXA prepared a recovery plan with NASA engineers, and the AMSR-E restarted its observations in a slow rotation mode (2 rpm) on December 4, 2012.
- Although the AMSR-E observation data in slow rotation mode limited to observe sparse areas, it was used for cross-calibration with the AMSR-2 (successor of the AMSR-E) onboard the GCOM-W (Global Change Observation Mission-Water) of JAXA since its launch on May 18, 2012, in order to produce and provide a consistent and long-term dataset between the AMSR-E and AMSR-2, by correcting their differences in sensor properties.
- Now, the AMSR-E instrument reached its limit to maintain the antenna rotation speed necessary for the slow rotation mode of 2 rpm and it automatically halted its observation and rotation on December 4, 2015. As of December, this marks just three years of simultaneous AMSR-E and AMSR-2 operation. Since the project obtained sufficient data necessary for cross-calibration, JAXA decided to complete operation of the AMSR-E at this time.
- The AMSR-2 on GCOM-W has been operating as the successor of the AMSR-E in the same orbit. The AMSR-2 continues the long-term, high-resolution observation of global water cycle variation by the AMSR-E and related operational utilization, which are new fields exploited by the AMSR-E. Moreover, the AMSR-2 contributes on an ongoing basis to both fields of practical application and water cycle/and climate variation research.
Table 1: AMSR-E achievements on Aqua
• November 2015: In late October 2015, a 3,726 m volcano on the Indonesian island of Lombok began erupting. In the days that followed, ash from Mount Rinjani blanketed towns and farmland across three Indonesian islands and shut down air traffic to a number of airports. The MODIS instrument of the NASA Aqua and Terra satellites acquired these natural-color images of the plume. Ash drifted westward from Lombok toward Bali and Java. A small segment of eastern Java's coast is visible to the southwest of Bali. 24)
Figure 19: The Aqua MODIS instrument captured this image of the eruption of Mount Rinjani, Indonesia on Nov. 4, 2015 (image credit: NASA, Jeff Schmaltz)
Table 2: Aqua status summary (as of November 5, 2015) 25)
• In June 2015, the NASA Earth Science Senior Review 2015 was submitted to Michael Freilich of NASA. A total of 10 NASA missions were evaluated in extended operations: Aqua, Aquarius, Aura, CALIPSO, CloudSat, EO-1, GRACE, OSTM (Jason-2), SORCE, and Terra. The Aqua missions was recommended to continue through FY 16-17 and FY 18-19. 26)
- The Aqua spacecraft is still going strong after 13 years, and four of its instruments (AIRS, AMSU, CERES, and MODIS) continue to collect valuable data about the atmosphere, oceans, land, and ice. The Panel ranked this mission as the first among those missions reviewed. Based upon Aqua's high quality climate data records, the continuity of this time series is critical for the scientific community, governmental agencies and the international operational user community. Therefore, the Panel found that Aqua mission should be continued as currently baselined.
• The image of Figure 20 was released on March 24, 2015 in NASA's Earth Observatory series. According to ocean color expert Menghua Wang of NOAA, The region of Bohai Sea, Yellow Sea, and East China is one of the most turbid and dynamic ocean areas in the world. 27)
Legend to Figure 20: In the image, the brown area along China's Subei Shoal is turbid water commonly seen in coastal regions. According to Wang, shallow water depths, tidal currents, and strong winter winds likely contributed to the mixing of sediment through the water. Some of the swirls in the image might be due to the Yellow Sea Warm Current, which intrudes into the Yellow Sea in wintertime. This branch of the Kuroshio Current changes the temperature of the sea surface and brings instability that could be the cause of the relatively dark swirls in the lower-middle part of the image.
• On Feb. 26, 2015, the MODIS instrument on NASA's Aqua satellite observed some of that dust starting a trans-Atlantic journey. In Figure 21, vast amounts of dust rise up from Senegal, Mauritania, and Gambia. The plumes are thick and brown, suggesting that he dust is still compact and that it probably arose close to the coast—not from a more distant location in the North African interior. Some of the dust also appears to be settling into the waters just offshore, adding to the darkening effect in the satellite view. A bit farther offshore, the water surface is brightened by sunglint, the reflection of sunlight directly back at the camera from a relatively smooth surface. 28)
Hundreds of millions of tons of sand and dust particles are lifted from North African deserts each year and carried across the Atlantic Ocean. So much dust is kicked up that the microscopic particles amass into sweeping tan plumes that are visible to satellites.
In a new paper published on February 24, 2015, scientists using a NASA satellite announced that they had quantified in three dimensions how much dust makes the trans-Atlantic journey from the Sahara Desert to South America. Scientists not only measured the volume of dust, but they also calculated how much phosphorus—remnant in Saharan sands from the desert's ancient past as a lake bed—gets carried from one of the planet's most desolate places to one of its most fertile. 29)
• August 5, 2014: Algae bloom on Lake Erie (Figure 22). For at least fifty years, phytoplankton and algae blooms have been a regular occurrence in summer on Lake Erie. The microscopic, floating plants generally start to flourish in June and July as the water warms and stratifies, and their numbers typically peak in August and September. But it's not every year that a bloom leads to the shutdown of water supplies in an American or Canadian city. 30)
The dominant organism in the Lake Erie bloom is Microcystis spp., a type of freshwater blue-green algae that produces a toxin harmful to humans. If consumed, Microcystis can cause numbness, nausea, dizziness, and vomiting and lead to liver damage. (In rare cases, it can be deadly.) On August 2, 2014, environmental monitors for Toledo and surrounding towns in northwestern Ohio determined that public water supplies had levels of microcystin toxin that were higher than recommended by the World Health Organization (1.0 parts per billion). They warned residents not to drink or cook with tap water; boiling is not effective against the toxin. Though the bloom has continued, treatment facilities have since added extra filtering steps (including activated carbon), and public water sources were declared safe again on August 4.
• July 2, 2014: NASA's Earth Observatory series released two images of the Leeuwin Current, flowing south along Australia's western shore, depicting SST (Sea Surface Temperature) and Chlorophyll Concentration of the region (Figure 23). 31)
Legend to Figure 23: The Leeuwin Current is an oddity, because it flows away from the equator to the pole. Though invisible to the naked eye, the warm current stands out in measurements of the temperature of the ocean's surface (SST), as shown in the left image acquired by the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument on NASA's Aqua satellite on June 6, 2014. Warmer water is orange and pink, while cooler water is purple.
The current hugs the coast, curving south and then east with the coastline. It is the world's longest coastal current, extending 5,500 km from the North West Cape to the west coast of Tasmania—roughly equivalent to the distance between San Francisco and Miami.
The heat the current transfers to southern Australia moderates the climate, making it hospitable to marine species normally found much closer to the equator. Its warmth also encourages rain to form and fall over western Australia, saving it from the extreme dryness found on the southwestern shores of the other southern continents. Without the current, western Australia might resemble South America's Atacama desert or southern Africa's Namib desert.
While the current makes southern Australia hospitable, it normally turns the ocean into a desert. The warm water contains limited nutrients to sustain plant life, and it represses upwelling, so surface waters seldom get recharged with nutrients from the ocean floor. Without nutrients, few phytoplankton grow. Since plankton are the base of the food chain, only small populations of fish are able to live in the warm waters.
In the right image of ocean chlorophyll, however, the ocean is clearly in full bloom. Acquired on June 6, 2014, by the MODIS sensor on the Aqua satellite, the image shows high concentrations of chlorophyll in yellow and lower concentrations in blue. The highest concentrations are aligned with the warmest parts of the current.
If warm water represses phytoplankton growth, why are plankton growing so well here? The answer is related to the current's swirling flow. The Leeuwin Current is extremely prone to eddies and meanders. In the fall, the current intensifies, flowing faster. The stronger eddies stir the water, allowing nutrients to reach the surface and fueling plankton blooms. In the winter, the blooms continue due to cooler temperatures and storms that agitate the water.
• May 17, 2014: The 2014 wildfire season got off to a ferocious start in southern California and northwestern Mexico when record-breaking temperatures and powerful Santa Ana winds fueled at least nine fires between May 14–16. Cal Fire estimated that by the morning of May 16, more than 7,700 hectare had burned, and news reports said that more than 100,000 people were forced to evacuate at various points over the past few days. 32)
- The MODIS instrument on Aqua detected several fires in San Diego County on May 14, 2014 (Figure 24). MODIS also observed large fires burning in the Baja California region of Mexico. Red outlines indicate hot spots where the sensor detected unusually warm surface temperatures associated with fires. Winds blew thick plumes of smoke west over the Pacific Ocean.
- Drought has plagued the western United States—especially central and southern California—for months, priming vegetation for wildfires. By mid-May, the entire state was classified as being in some level of drought (ranging from severe to exceptional), according to the U.S. Drought Monitor. To break the drought, most of the state would need 9 to 15 inches (23 to 38 centimeters) of precipitation to fall in one month, Weather Underground meteorologist Jeff Masters estimated. That would amount to more than a half-year's worth of precipitation for most of the state.
Legend to Figure 24: Red outlines indicate hot spots where the satellite's MODIS sensor detected unusually warm surface temperatures associated with fires. Winds blew thick plumes of smoke west over the Pacific Ocean.
• On April 25, 2014, the MODIS instrument on Aqua observed dozens of fires burning in North Korea. Actively burning areas, detected by the thermal bands on MODIS, are outlined in red. Fields and grasslands appear light brown. Forests at lower elevations appear green; at higher elevations, forests are still brown at this time of year. Collectively, the fires produced enough smoke to send plumes of haze drifting east over the Sea of Japan. 33)
- Many fires appear in farming areas along rivers. While North Korea's best agricultural land is located on the coastal plain in the western part of the country, many people farm marginal land along rivers in the mountainous areas. They use fire to clear debris from last year's crops and to help fertilize the soil for the coming season.
- However, some of the fires were burning in heavily forested areas, suggesting that they might be wildfires. Drooping wires on aging power lines are a common cause of wildfires in North Korea, according to a report published in the Asia-Pacific Journal.
Figure 25: Actively burning areas, as viewed by NASA's Aqua satellite on April 25, 2014, are outlined in red. Fields and grasslands appear light brown. Forests at lower elevations appear green; at higher elevations, forests are still brown at this time of year (image credit: NASA Earth Observatory)
• NASA released Figure 26 on April 30, 2014 showing a MODIS image on the Aqua satellite. Off the coast of southwest Africa, ocean currents, winds, and the underwater shelf interact to create compelling biology and chemistry. Plant-like phytoplankton often bloom in the nutrient-rich surface waters, while bacteria on the seafloor consume decaying plant and animal matter and occasionally release gas that bubbles to the surface. 34)
Just off the coast of Namibia, the Benguela Current flows along the ocean surface. It moves north and west along the coast from South Africa and is enriched by iron and other nutrients from the Southern Ocean and from dust blowing off African coastal deserts. Easterly winds push surface waters offshore and promote upwelling near the coast, which brings up cold, nutrient-rich waters from the deeper ocean. These interactions can make the ocean come alive with color.
Near the shore, yellow-green features in the water suggest the presence of sulfur. Studies have described how bacteria in oxygen-depleted bottom waters consume organic matter and produce prodigious amounts of hydrogen sulfide. As that gas bubbles up into more oxygen-rich water, the sulfur precipitates out and floats near the surface. It can give off a potent rotten-egg smell and pose a toxic threat to fish.
Further offshore, milky green water may be a bloom of one or several species of phytoplankton. As countless microscopic, plant-like organisms consume sunlight and nutrients, they also consume oxygen. Oxygen depletion can sometimes become so complete that it creates a "dead zone" that can suffocate other marine species. At the same time, the oxygen-depleted waters help sulfur-producing bacteria to thrive.
• Feb. 2014: The Great Lakes Region of North America is experiencing a bitter cold winter. The true color image of Figure 27 shows the mostly frozen state of the Great Lakes on Feb. 19, 2014. On that date, ice spanned 80.3% of the lakes, according to NOAA's Great Lakes Environmental Research Laboratory in Ann Arbor, Michigan. 35)
The ice reached an even greater extent on Feb. 13, when it covered about 88% of the Great Lakes – coverage not achieved since 1994, when ice spanned over 90 %. In addition to this year, ice has covered more than 80% of the lakes in only five other years since 1973. The average annual maximum ice extent in that time period is just over 50%. The smallest maximum ice cover occurred in 2002, when only 9.5% of the lakes froze over.
Figure 27: This image, acquired with MODIS on the Aqua satellite, shows the Great Lakes on Feb. 19, 2014, when ice covered 80.3% of the lakes (image credit: Jeff Schmaltz, LANCE/EOSDIS MODIS Rapid Response Team, NASA)
• The Aqua spacecraft and its payload (except for AMSR-E which operates in a reduced mode) are operating nominally in 2014.
Figure 28: The Aqua satellite acquired this natural-color satellite image of a plankton bloom on Dec. 30, 2013. The eddy is centered about 600 km off the coast of Australia in the southeastern Indian Ocean (image credit: NASA Earth Observatory) 36)
Legend to Figure 28: In this Aqua/MODIS image, an eddy is outlined by a milky green phytoplankton bloom. Eddies are masses of water that typically spin off of larger currents and rotate in whirlpool-like fashion. They can stretch for hundreds of kilometers and last for months.
While the northern latitudes are bathed in the dull colors and light of mid-winter, the waters of the southern hemisphere are alive with mid-summer blooms. The eddy is centered at roughly 40º South latitude and 120º East longitude, about 600 km off the coast of Australia in the southeastern Indian Ocean.
• Nov. 8, 2013: The typhoon Haiyan was located over the central Philippines and was quickly heading towards the west at 22 knots (25 mph) when Aqua observed the region. 37)
Legend to Figure 29: The lowest temperatures, in dark purple, are associated with the high, cold cloud tops of powerful thunderstorms with heavy rainfall potential. The Philippine islands stretch from the center of the image to the northwest. Northern Indonesia is at the bottom of the image, and northeastern Malaysia is at the lower left of the image. Some of the Philippine regions being pounded by the storm, in the area with purple coloring, are the Visayas, Bicol, National Capital, Central Luzon, Calabarzon, Northern Mindanao, and Mimaropa regions.
• June 2013: The 2013 Senior Review evaluated 13 NASA satellite missions in extended operations: ACRIMSAT, Aqua, Aura, CALIPSO, CloudSat, EO-1, GRACE, Jason-1, OSTM, QuikSCAT, SORCE, Terra, and TRMM. The Senior Review was tasked with reviewing proposals submitted by each mission team for extended operations and funding for FY14-FY15, and FY16-FY17. Since CloudSat, GRACE, QuikSCAT and SORCE have shown evidence of aging issues, they received baseline funding for extension through 2015. 38)
- The Aqua mission is now 5 years into its extended mission of producing a wide array of measurements in support of addressing NASA's Earth Science mission both from the perspective of creating climate data records necessary to evaluate climate change and from the perspective of products needed to better understand fundamental Earth science processes. The Aqua mission has been extremely successful and produces a large number of critical products that are very widely used by scientists, government agencies and operational groups.
- The AMSR-E instrument, which suffered a major anomaly in 2011, now operates in a reduced mode that provides data for cross-calibration with other AMSR instruments. All other instruments on Aqua are still operating nominally and the spacecraft is in excellent health and has enough fuel to operate through 2022.
- Of the Aqua sensor complement, MODIS and AIRS are making extremely unique and popular measurements for science and operational applications. The continuity of these data products is highly desirable for the scientific community and the broader user community.
• In February 2013, Aqua is over four and a half years beyond its prime mission, and yet the spacecraft and four of its instruments continue to operate well. HSB (from Brazil) failed in February 2003, and AMSR-E (from Japan) ceased science operations much more recently, in October 2011. As of December 2012, AMSR-E is again turned on, but at a much slower rotation rate (2 rpm versus 40 rpm) than desired for science data. The AMSR-E data being collected now are largely intended for cross-calibration with data from the AMSR-2 instrument (launch on May 17, 2012) flown on Japan's GCOM-W (= Shizuku) mission. The MODIS, AIRS, CERES, and AMSU (all from the U.S.) instruments on Aqua continue to work well. It's projected that the mission could continue until 2022. 39)
The retrieved atmospheric parameters using the observations from AMSR-E on Aqua are used primarily in climate research as well as in atmospheric models used in weather forecasting. This JAXA instrument performed exceptionally well for more than three times its design lifetime. 40)
• In mid-August 2012, an intense wildfire broke out on the Greek island of Chios, sending a thick plume of smoke southward toward the island of Crete. 41)
Legend to Figure 30: The image shows part of the Aegean Sea dotted with many Greek islands between the mainlands of Greece and Turkey. Greece typically sees little rain between April and September and experiences some of its highest temperatures in late July and early August. Wildfires are fairly common in the hot, dry days of August.
• July 2012: Aqua is operational and has now exceeded 10 years of on-orbit operations. It has collected a wealth of data that have been used for a variety of scientific and practical purposes. Well over 2,000 scientific papers have been published using Aqua data. An example of the many Aqua results deals with the the global energy budget. 42)
Legend to Figure 31: CERES measurements allow the derivation of the solar radiation reflected from the Earth/atmosphere system back to space and the Earth's longwave radiation emitted to space. The CERES data from Aqua and Terra have been used with incoming solar radiation data from the TIM (Total Irradiance Monitor) on the SORCE (Solar Radiation and Climate Experiment) mission to calculate that the Earth has been accumulating energy at a rate of approximately 0.50 ± 0.43 Wm-2 over the course of the 10 year period 2001-2010. 43) This slight imbalance at the top of the atmosphere means that more energy is entering than leaving the Earth system, resulting in overall warming.
• In May 2012, Aqua marked its 10th year on-orbit, delivering unprecedented data about the Earth's climate, water cycle and much more. The mission demonstrates the considerable benefits of long-term, space-based environmental monitoring. 44) 45) 46)
Figure 32: Image of stratocumulus clouds over the Pacific Ocean observed by MODIS on June 20, 2012 (image credit: NASA) 47)
Legend to Figure 32: A layer of stratocumulus clouds over the Pacific Ocean served as the backdrop for this rainbow-like optical phenomenon known as a glory. Glories generally appear as concentric rings of color in front of mist or fog. They form when water droplets within clouds scatter sunlight back toward a source of illumination (in this case the Sun). - Although glories may look similar to rainbows, the way light is scattered to produce them is different. Rainbows are formed by refraction and reflection; glories are formed by backward diffraction. The most vivid glories form when an observer looks down on thin clouds with droplets that are between 10 -30 µm in diameter. The brightest and most colorful glories also form when droplets are roughly the same size.
Another notable feature in this image are the swirling von Karman vortices that are visible to the right of the glory. The alternating double row of vortices form in the wake of an obstacle, in this instance the eastern Pacific island of Guadalupe, located ~ 240 km off the west coast of Mexico's Baja California peninsula.
• In early 2012, the Aqua spacecraft and its instruments (AIRS, AMSU, CERES and MODIS) are in nominal operation. - In June 2011, the NASA Earth Science Senior Review recommended an extension of the Aqua mission to 2015. 48)
• The AMSR-E instrument operations ended on October 4, 2011. The AMSR-E instrument of JAXA (built by Mitsubishi Electric Company) continued its operation for more than 9 years (design life of 3 years). However, since the end of August, 2011, a continuous increase of relatively large antenna rotation friction was detected twice; as a consequence, JAXA has been monitoring the condition. On October 4, 2011, the AMSR-E reached its limit to maintain the rotation speed necessary for regular observations (40 rpm), and the radiometer automatically halted its observations and rotation. Although, JAXA continued to analyze this problem, and take necessary measures to correct the situation in cooperation with NASA, the AMSR-E mission came to an end. The cause of the failure is most likely due to aging lubricant in the bearing mechanism. 49)
The good news is that AMSR2, a slightly modified and improved version of AMSR-E, will be launched in 2012 on JAXA's GCOM-W1 satellite, and will join Aqua and the other satellites in NASA's A-Train constellation of Earth observation satellites. - The Aqua project had hoped that AMSR-E would provide at least one year of data overlap with the new AMSR2 instrument on GCOM-W1. 50)
• The Aqua spacecraft and its payload are operating nominally in 2011 with five of the six original Earth-observing instruments still operating well (these are: AIRS, AMSU, AMSR-E, MODIS, and CERES). It now looks like there is a good chance that the mission can continue at least to 2020. 51)
Legend to Figure 33: Brilliant shades of blue and green explode across the Barents Sea in this natural-color image taken on August 14, 2011, by MODIS on the Aqua spacecraft. Phytoplankton are tiny, microscopic plant-like organisms, but when they get together and start growing they can cover hundreds of square kilometers and be easily visible in satellite images. When conditions are right, phytoplankton populations can grow explosively, a phenomenon known as a bloom. A bloom may last several weeks, but the life span of any individual phytoplankton is rarely more than a few days. The area in this image is immediately north of the Scandinavian peninsula. Blooms spanning hundreds or even thousands of kilometers occur across the North Atlantic and Arctic Oceans every year. 52)
• The Aqua spacecraft and its payload are operating nominally in 2010. The Aqua sensors contain much synergy with each other and with other sensors and satellite platforms (e.g. Terra), and global climate model simulations. Many of these synergies have been explored, resulting in improved accuracy of core and new bio and geophysical products, and new understanding of the environment. - NASA hopes to continue the Aqua mission until at least the NPP mission is going to be launched in late 2011.
• The prime mission of Aqua was completed in September 2008. Five of the original six Aqua instruments are still operational and in good health, and should continue to operate successfully over the next four years (FY10-FY13) of the proposed continuation and beyond. In 2009, Aqua has adequate propellant for at least eight more years of normal operations. 53)
Scientific accomplishments and current merits of the Aqua platform are excellent. These merits include data and discoveries from approximately 100 data products that address each of NASA's six interdisciplinary Earth science focus areas and 12 Applied Science Program Elements. The Aqua data are considered to be critical for the activities associated with the current or upcoming IPCC Working Group 1 Assessment Report 5 (AR5), 2009–2012, for regional to global climate change assessment and forecasting studies.
• Aqua is operating nominally in 2005. 54)
• The HSB (Humidity Sounder for Brazil) instrument of INPE ceased operating in February 2003.
• The AIRS instrument, the first high-spectral-resolution infrared sounder developed by NASA/JPL, has provided the most significant increase in forecast improvement in this time range of any other single instrument.
• Nominal Aqua mission operations began on September 1, 2002.
Sensor complement: (AIRS, AMSU/HSB, AMSR-E, CERES, MODIS)
Aqua has six Earth-observing instruments on board, collecting a variety of global data sets. 55)
Note: The descriptions of CERES and MODIS can be found under Terra.
Table 3: Overview of sensor complement on the Aqua spacecraft
AIRS (Atmospheric Infrared Sounder):
AIRS is a NASA/JPL instrument, PI: M. T. Chahine; prime contractor is BAE Systems (Infrared and Imaging Systems Division (LMIRIS) of BAE Systems, in Lexington, MA). AIRS, along with AMSU and HSB, is of HIRS and MSU heritage flown on the NOAA POES series. Objective: High-spectral-resolution measurement of global temperature/humidity profiles in the atmosphere in support of operational weather forecasting by NOAA. Measurement of the Earth's upwelling infrared radiances in the spectral range of 3.74 - 15.4 µm, simultaneously at 2378 frequencies (bands). Four visible wavelength channels are also present. 56) 57) 58) 59) 60) 61) 62)
Figure 34: Photo of the AIRS instrument (image credit: NASA)
The AIRS spectrometer is a pupil imaging, multi-aperture echelle grating design that utilizes a coarse 13 lines/mm grating at high orders (3-11) to disperse infrared energy across a series of detector arrays. The typical entrance slit of a spectrometer is subdivided into a series of eleven apertures, each of which is imaged onto the focal plane. The grating serves to spectrally disperse each image, which in turn is overlaid onto a HgCdTe detector array with each detector in the array viewing a unique wavelength by virtue of the grating dispersion. Rejection of overlapping grating orders and background photon suppression is provided by a series of IR bandpass filters located within the spectrometer and directly on the focal plane. Use of the grating in combination with the filter set provides a two-dimensional color map on the focal plane with a high degree of design flexibility in terms of color arrangement and spacing. Cooling of the spectrometer to 155 K is provided by a two stage passive radiator assembly with 10 Watt cooling capacity at 155 K.
Figure 35: Isometric view of the AIRS instrument (image credit: NASA/JPL)
Dispersed energy exiting the spectrometer is imaged onto a state-of-the-art hybrid PV/PC: HgCdTe focal plane assembly (FPA) consisting of a series of multi-linear arrays each associated with a specific entrance aperture. The assembly consists of 17 arrays arranged in 12 modules with each module individually optimized for wavelength and photon flux. The module set includes 10 photovoltaic (PV) modules covering the 3.7 - 13.7 µm region and 2 photoconductive (PC) modules for the 13.7 - 15.4 µm region. The more advanced PV modules include on-focal plane signal processing via a custom CMOS Readout IC (ROIC) specifically designed for AIRS temperature, photon flux and radiation conditions. The ROIC provides the first stage of signal integration at a 1.4 ms subsample rate, which are summed off focal plane in groups of 16 to meet full footprint dwell time requirements. The IR FPA provides simultaneous measurement of 2378 spectral samples across the 3.7 - 15.4 µm region with two samples per resolution element. Additionally, each PV sample is further divided by two in the cross-dispersed direction to provide increased yield and a measure of spectral redundancy. As a consequence, the IR FPA contains a total of 4482 active detectors. The complex FPA is packaged in a vacuum dewar maintained at the 155 K spectrometer operating temperature, with the IR FPA cooled to 58 K via a redundant, 1.5 W capacity Split Stirling pulse tube cryocooler.
Figure 36: Illustration of the FPA (Focal Plane Assembly), image credit: NASA/JPL)
Figure 37: The AIRS spectrometer assembly (image credit: NASA/JPL)
Figure 38: AIRS scan assembly (image credit: NASA/JPL)
Figure 39: Illustration of the cryocooler assembly (image credit: NASA/JPL)
The infrared region of 3.74-15.4 µm has a spectral resolution of 1200 (lambda/ delta lambda). The high spectral resolution permits the separation of unwanted spectral emissions and, in particular, provides spectrally clean "super windows," ideal for surface observations. - This is supplemented by a VNIR photometer of four bands in the range between 0.4 and 1.0 µm. The VNIR channels are used to discriminate between low-level clouds and different terrain and surface covers, including snow and ice. The AIRS infrared bands have an IFOV of 1.1º and FOV = ± 49.5º scanning capability perpendicular to the spacecraft ground track (swath width = 1650 km, 13.5 km horizontal resolution in nadir, 1 km vertical). It takes 22.41 ms for each footprint of 1.1º in diameter (or 13.5 km). Each IR scan produces 90 footprints across the flight track and takes 2.67 s (see Figure 40). The VNIR channels have a footprint of 0.185º or about 2.3 km on the ground, nine VNIR footprints are within a 40 km swath. The VNIR photometer is boresighted to the spectrometer to allow simultaneous VNIR observations.
The VNIR photometer uses optical filters to define the four spectral bands. It operates at ambient temperatures (293-300 K). Inflight calibration is performed during each scan period. In addition, AIRS uses four independent cold-space views.
The major data products derived from AIRS are atmospheric temperature profiles, humidity profiles (from channels in the 6.3 µm water vapor band and the 11 µm windows, sensitive to the water vapor continuum), and land skin surface temperature.
AIRS is flown on the Aqua satellite with two operational microwave sounders: NOAA's AMSU and Brazil's HSB (Humidity Sounder Brazil). Together, the three sensors constitute constitute a possible advanced operational sounding system for future NOAA missions - offering increased accuracy of short-term weather predictions, improved tracking of severe weather events like hurricanes, and advances in climate research.
Table 4: Overview of some AIRS parameters
Some AIRS results in 2010:
The excellent sensitivity and stability of the AIRS instrument has recently allowed the AIRS team to successfully retrieve Carbon Dioxide (CO2) concentrations in the mid-troposphere (8-10 km) with a horizontal resolution of 100 km and an accuracy of better than 2 ppm. 63)
Originally designed to retrieve temperature and water vapor profiles for weather forecast improvement, the AIRS (Atmospheric Infrared Sounder) has become a valuable tool for the measurement and mapping of mid-tropospheric carbon dioxide concentrations. Several researchers have demonstrated the ability to retrieve mid-tropospheric CO2 from AIRS by different methods. The retrieval method selected for processing and distribution is called the method of "Vanishing Partial Derivatives" and results in over 15,000 CO2 retrievals per 24-hour period with global coverage and an accuracy better than 2 ppm.
The AIRS CO2 accuracy has been validated against a variety of mid-tropospheric aircraft measurements as well as upward looking interferometers (FTIR) from the ground.
Mid-tropospheric CO2 concentrations are an indicator for atmospheric transport and several interesting findings have resulted from analysis of the data.
- First is the non-uniformity of CO2, primarily caused by weather.
- Second is the ability to identify stratospheric-tropospheric exchange during a sudden stratospheric warming event.
- Third is the presence of a seasonally varying belt of enhanced CO2 concentrations in the Southern Hemisphere.
Figure 41: AIRS yields about 15,000 mid-tropospheric CO2 measurements per day (image credit: NASA/JPL)
Carbon dioxide turns out to be an excellent tracer gas since it does not react with other gases in the atmosphere. The project is finding that the AIRS mid-tropospheric CO2 is a good indicator of vertical motion in the atmosphere. It is a known fact that the majority of atmospheric CO2 is produced and absorbed near the surface and that there are no sources or sinks in the free troposphere. Thus elevated levels of mid-tropospheric CO2 are the result of airflow into the mid-troposphere from the near surface.
The most obvious finding from the AIRS retrievals is that the distribution of CO2 is not uniform as indicated in the models. Strong latitudinal and longitudinal gradients exist particularly over the large land masses in the Northern Hemisphere. This phenomenon is referred to as "CO2 weather". The large variability in atmospheric circulation due to convection and global and mesoscale transport is responsible for most of the variability seen in the AIRS data. This implies that the AIRS CO2 data will be extremely useful for validating global scale transport in GCMs (Global Circulation Models).
Figure 42: AIRS mid-tropospheric CO2 is a tracer for atmospheric motion particularly in the vertical direction. July, 2010 monthly average (image credit: NASA/JPL)
AMSU/HSB (Advanced Microwave Sounding Unit (NASA Instrument)/ (Humidity Sounder for Brazil), provided by INPE. Both instruments operate in conjunction.
AMSU was designed and developed by Aerojet of Azusa, CA (a GenCorp company). AMSU primarily provides temperature soundings, whereas HSB provides humidity soundings. AMSU is a 15-channel microwave radiometer. AMSU and HSB have a total of 19 channels, 15 are assigned to AMSU, each having a 3.3º beamwidth, and four are assigned to HSB, each having a beamwidth of 1.1º. AMSU comprises two separate units: AMSU-A1 (channels 3-15), and AMSU-A2 (channels 1 and 2). Channels 3 - 14 use the 50 to 60 GHz oxygen band to provide data for vertical temperature profiles up to 50 km. The "window" channels (1, 2, and 15) provide data to enhance the temperature sounding by correcting for surface emissivity, atmospheric liquid water, and total precipitable water. HSB channels 17 - 20 use the 183.3 GHz water vapor absorption line to provide data for the humidity profile. 64) 65)
AMSU-A1 measures temperature profiles from the surface up to 50 km in 15 channels. Temperature resolution: 0.25 - 1.2 K. The AMSU-A1 instrument has two 15 cm diameter antennas (reflectors with momentum compensation), each with a 3.3º nominal IFOV at the half power points or FWHM (Full width Half maximum). Each antenna provides a cross-track scan of ±49.5º from nadir with a total of 30 Earth views (scan positions) per scan line. The total scan period is eight seconds. The footprint (resolution) at nadir is 40 km. The swath width is approximately 1690 km. Internal calibration is performed with internal warm loads and cold space.
AMSU-A2 has a single 28 cm diameter antenna (reflector without momentum compensation) with a 3.3º nominal IFOV. All other instrument/observation parameters are the same as those of AMSU-A1.
AMSU parameters: mass = 91 kg (49 kg for AMSU-A1, 42 kg for AMSU-A2); power = 101 W; data rate = 2.0 kbit/s; thermal control by heater, central thermal bus, radiator; thermal operating range= 0-20º C.
Table 5: Spectral parameters of the AMSU-A and HSB instruments
Figure 43: View of AMSU-A1 (left) and AMSU-A2 (right), image credit: Aerojet
Table 6: Summary of AMSU instrument parameters
HSB (Humidity Sounder for Brazil):
HSB is an INPE-provided instrument of AMSU-B heritage (built by MMS (Matra Marconi Space) of Bristol, UK (now EADS Astrium Ltd) with participation of Equatorial Sistemas of Brazil), and sponsored by AEB (Brazilian Space Agency). HSB is a microwave radiometer with the objective to measure atmospheric radiation, to obtain atmospheric water vapor profile measurements and to detect precipitation under clouds with 13.5 km horizontal nadir resolution (humidity profiles for weather foresting). 66) 67) 68)
HSB is a four-channel self-calibrating instrument (passive sounder) providing a humidity profiling capability in the frequency range of 150 - 190 GHz, spanning the height from surface to about 42 km. The measured signals are also sensitive to a) liquid water in clouds (cloud liquid water content) and b) graupel and large water droplets in precipitating clouds (qualitative estimate of precipitation rate). HSB scans in the cross-track direction at a rate of 2.67 seconds in continuous mode. The instrument features a momentum-compensated scan mirror system. HSB is operated in combination with AMSU-A, they have a total of 19 channels: 15 are assigned to AMSU-A, each having a 3.3º beamwidth, and four assigned to HSB, each having a 1.1º beamwidth. The HSB receiver channels are configured to operate in DSB (Double Sideband).
The HSB collected valuable data for the first nine months of the mission but ceased operating in February 2003 (scanner anomaly).
Table 7: Specification of the HSB instrument
Figure 44: Photo of the HSB instrument (image credit: NASA)
AMSR-E (Advanced Microwave Scanning Radiometer-EOS):
AMSR-E is a JAXA/NASA cooperative instrument, of AMSR heritage, built by Mitsubishi Electronics Corporation (PIs: A. Shibata, R. W. Spencer). The objective is the measurement of geophysical parameters such as: cloud properties, radiative energy flux, precipitation, land surface wetness (moisture), sea ice, snow cover, sea surface temperature (SST), and sea surface wind fields. AMSR-E is a modified design of AMSR on ADEOS-II (Japan).
The AMSR-E instrument is a conically scanning total power passive microwave radiometer sensing microwave radiation (brightness temperatures) at 12 channels and 6 frequencies ranging from 6.9 to 89.0 GHz (6.925, 10.65, 18.7, 23.8, 36.5, and 89.0 GHz). Horizontally and vertically polarized radiation are measured separately at each frequency. 69) 70) 71)
AMSR-E consists of an offset parabolic reflector 1.6 m in diameter, fed by an array of six feedhorns. The reflector and feedhorn arrays are mounted on a drum which contains the radiometers, digital data subsystem, mechanical scanning subsystem, and power subsystem. The reflector/feed/drum assembly is rotated about the axis of the drum by a coaxially mounted bearing and power transfer assembly. All data, commands, timing and telemetry signals, and power pass through the assembly on slip ring connectors to the rotating assembly. The AMSR-E instrument has a mass of 314 kg, power = 350 W, a duty cycle of 100%, and an average data rate of 87.4 kbit/s.
Table 8: Performance parameters of AMSR-E
Figure 45: Schematic view of the AMSR-E instrument (image credit: NASA)
The AMSR-E instrument rotates continuously about an axis parallel to the local spacecraft vertical at 40 rpm. At an altitude of 705 km, it measures the upwelling scene brightness temperatures over an angular sector of ± 61º about the subsatellite track, resulting in a swath width of 1445 km. During a period of 1.5 seconds the S/C subsatellite point travels 10 km. Even though the IFOV for each channel is different, active scene measurements are recorded at equal intervals of 10 km (5 km for the 89 GHz channels) along the scan. The half cone angle at which the reflector is fixed is 47.4º which results in an Earth incidence angle of 55.0º.
Figure 46: Line drawing of the AMSR-E instrument (image credit: NASA)
Instrument calibration. The radiometer calibration accuracy budget, exclusive of antenna pattern correction effects, is composed of three major contributors: warm load reference error, cold load reference error, radiometer electronics nonlinearities and errors.
Some data products from AMSR-E are:
• Level 2A brightness temperatures
• Level 2 rainfall
• Level 3 rainfall
• Columnar cloud water over the oceans
• Columnar water vapor over the oceans
• Sea surface temperature (SST)
• Sea surface wind speed
• Sea ice concentration
• Sea ice temperature
• Snow depth on sea ice
• Snow-water equivalent on land
• Surface soil moisture
Figure 47: The Aqua spacecraft and instrument accommodations (image credit: NASA, JAXA)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).