Just before midnight Eastern Daylight Time on June 15, 2015, a fireball appeared over central Africa, streaked across Madagascar, and tracked across the uninhabited Southern Indian Ocean. This was the fiery end of the joint NASA/Japan Aerospace Exploration Agency (JAXA) Tropical Rainfall Measuring Mission (TRMM). TRMM’s homecoming after more than 17 years in orbit also marked the end of the first major satellite mission specifically designed to gather data about tropical precipitation and its contributions to global circulation.
The data collection started by TRMM continues with the joint NASA/JAXA Global Precipitation Measurement (GPM) mission, which launched in February 2014. From the launch of TRMM in 1997 to the ongoing GPM mission, remotely-sensed hydrometeorological data have evolved greatly. This is a reflection of not only better instrumentation, but also a better understanding of the water cycle and how this cycle interconnects with the energy, carbon, and other cycles driving our planet.
The Importance of Collecting Precipitation Data from Space
If you gathered all of the world’s rain gauges currently in use in one place they would cover an area only about the size of two basketball courts. Since you can’t have a rain gauge or sensor everywhere rain can fall, trying to collect global precipitation data from the ground is an almost impossible task. You need to set your sights higher. Much higher.
“You have to be able to observe where and when rain is falling around the world all the time,” says Dalia Kirschbaum, GPM Deputy Project Scientist for Applications. “By being able to observe rain remotely, from a satellite, you are able to track or at least estimate where all the water is going.”
And knowing where this water is going is critical information not only for analyzing global weather and climate, but also for managing life on Earth. “Knowing where and when our water is falling tells us about how our society operates,” Kirschbaum says. “You know where we might have transportation issues, where we might have water issues that lead to crop failures, where we might have flooding that requires calling in disaster response agencies. Having this vantage point from space allows you to look at the globe and see the big picture.”
From TRMM to GPM
The overall objective of TRMM was to use active sensors (a Precipitation Radar, the first of its kind in space) and passive sensors (the Visible and Infrared Scanner [VIRS] and the TRMM Microwave Imager [TMI]) to quantitatively describe tropical rainfall. Along with these three primary instruments, TRMM also carried a Lightning Imaging Sensor (LIS) to observe the distribution and variability of lightning and the Clouds and Earth’s Radiant Energy System (CERES), a broadband scanning radiometer to measure emitted and reflected radiative energy from Earth’s surface and the atmosphere.
TRMM data were used to obtain multiyear sets of tropical and subtropical rainfall observations; develop a better understanding of the interactions between sea, air, and land masses and their influence on global rainfall and climate; improve the modeling of tropical rainfall processes; and enhance satellite rainfall measurement techniques.
TRMM’s success and unique combination of data products were prime factors in extending the mission far beyond its original three-year mission length. “When the TRMM precipitation data came in, its combination of instruments provided better data than anything before it; this was powerful stuff,” says George Huffman, the GPM Deputy Project Scientist. “This allowed us to pull in other satellites and routinely feed fine-scale rainfall information to flood forecasts. Suddenly you can track the swath of rainfall laid down by a hurricane. There’s real societal impact with the TRMM data.”
With the success of TRMM, planning for the next precipitation mission, GPM, began in the early 2000s. GPM was developed to provide not only data continuity, but an improvement on the TRMM data. “Scientifically, we’re learning more from GPM because of its advanced instrumentation building off what we learned from TRMM,” says Kirschbaum.
GPM improves on TRMM’s capabilities in numerous ways. Although GPM has only two instruments versus the five instruments on TRMM, these two instruments (a Dual-frequency Precipitation Radar [DPR] and a radiometer called GPM Microwave Imager [GMI]) are some of the most advanced ever developed to measure precipitation from space. The DPR is the only dual frequency radar in space, and is capable of creating 3-D profiles and intensity estimates of precipitation ranging from rain to snow and ice. The GMI has a greater frequency range than TRMM’s (13 channels versus 9 channels), which allows GPM to measure precipitation intensity and type through all cloud layers using a wider data swath.
One of the most significant evolutions in GPM data is its broader global coverage. While TRMM collected data in tropical and subtropical regions between roughly 35˚ north and south latitude, GPM collects data between approximately 65˚ north and south latitude. This allows GPM’s instruments to collect data on storms as they form in the tropics and move into the middle and high latitudes.
Another important GPM data enhancement over TRMM is its design as a Core Observatory that coordinates data collection from a constellation of partner satellites, rather than as a single satellite. The GPM Core Observatory calibrates the data from almost a dozen orbiting U.S. and international satellites that observe precipitation, and helps ensure that the data collected from these satellites has a uniform structure. The number of partner satellites in the constellation will change over time as new satellites are launched and older satellites are decommissioned. “The benefit to the data users is that when you knit all of these data together, it gives you all these looks around the day and you get much finer time resolution,” Huffman says.
Kirschbaum likens the role of the GPM Core Observatory to that of an oboe tuning an orchestra. “The oboist plays a note and everyone tunes to this note,” she says. “Even though the instruments may be different, they all need to tune to one standard to get a single harmonious piece. Because GPM can sense 13 channels of microwave energy, it is able to cross the channels of all the different radiometers in the constellation of satellites. We’re able to use GPM to serve as the reference, as the tuning center, to unify the global constellation of satellites.”
TRMM and GPM Data and Data Products
The greatest legacy of TRMM and GPM is a continuous data record reaching back to the start of the TRMM era in 1998 (the first year for which processed TRMM data products are available). TRMM data originally were processed and archived in the TRMM Science Data and Information System (TSDIS). In preparation for the GPM mission, this organization evolved into NASA's Precipitation Processing System (PPS).
Located at NASA’s Goddard Space Flight Center in Greenbelt, MD, the PPS processes data from the TRMM and GPM missions as well as data from GPM constellation satellites. These processed data are transferred to the Goddard Earth Sciences Data and Information Services Center (GES DISC) for archiving and distribution. GES DISC is one of NASA's Earth Observing System Data and Information System (EOSDIS) Distributed Active Archive Centers (DAACs) that are responsible for discipline-specific Earth observing data from satellite, airborne, and field campaigns as part of the Earth Science Data Systems (ESDS) Program.
Huffman notes that even though TRMM and GPM currently provide nearly 20 years of continuous precipitation data (including the brief period when TRMM and GPM collected data simultaneously), they are two different missions that use different algorithms to produce their data collections—it’s not a simple apples to apples comparison. In order to allow for easier long-term studies using these data and create a more integrated and standardized data collection, the U.S. GPM science team developed the Integrated Multi-satellite Retrievals for GPM (IMERG). A similar system, the Global Satellite Mapping of Precipitation (GSMaP), was developed by JAXA under the sponsorship of the Japan Science and Technology Agency.
Through IMERG, precipitation data from TRMM, GPM, and more than 20 current and past precipitation sensor missions are being converted into a uniformly processed data set by PPS. Where data users formerly had to work with two different types of precipitation numbers in one data set (one for TRMM and one for GPM), IMERG allows users to work with one number. When fully developed, IMERG will give data users the ability to seamlessly use TRMM and GPM data collections for long-term studies. “The plan is to take all the TRMM data and apply the IMERG algorithm all the way back to 1998,” says Kirschbaum. “You’ll have one consistent algorithm that’s processing all of the data. Everything will be in the same resolution.”
TRMM to GPM to …
Tropical Rainfall Measuring Mission (TRMM) was the first step in an evolving process of collecting global precipitation data from space, and helped reveal questions about precipitation measurement that are being addressed by Global Precipitation Measurement (GPM). Huffman, Kirschbaum, and their colleagues already are looking past GPM to see what the next phase in the evolution of this science may be. As Kirschbaum notes, this next phase will continue to rely on space-based platforms. “Having remote perspectives of all of the components of the water cycle is instrumental to understanding not only how our water is circulating, but how this might change in a changing climate,” she says.
Kirschbaum also notes that having a multi-satellite view is an important aspect in furthering the exploration of the water cycle and its interconnection with the planet’s other cycles. Several new satellite missions have been launched or are nearing their scheduled launch date that will continue these explorations and complement TRMM and GPM data. These include NASA’s recently-launched Soil Moisture Active Passive (SMAP), which collects global soil moisture and freeze/thaw data to help better predict and understand global flood, drought, and other groundwater-related processes; and the upcoming joint NASA/German Research Centre for Geosciences Gravity Recovery and Climate Experiment Follow-on (GRACE-FO), which is scheduled to launch in 2017 to collect data about water movement and storage and how these processes affect Earth’s mass and gravity. Other upcoming missions that will complement TRMM and GPM include the Ice, Cloud,and land Elevation Satellite (ICESat-2) and the joint NASA/French Centre national d'études spatiales (cnes)/Canadian Space Agency Surface Water and Ocean Topography (SWOT) mission, which are scheduled for launch in 2017 and 2020, respectively.
Just as GPM is addressing questions raised by TRMM data, data from GPM and other ongoing missions are revealing new areas to examine about precipitation and the water cycle. The overall question remains—what is the next step in this evolution? “In GPM it was pretty clear that our next step [from TRMM] was to go to higher latitudes and get radar looking at snowy regions,” Huffman says. “The next step from GPM is a little harder. What won’t we know about precipitation 10 years from now that we’ll need to explore?”