The Global Precipitation Measurement Core Observatory (GPM) will be the first probe to measure snow in the atmosphere from space. Credit: NASA/JAXA

Researchers who study inputs to the Earth’s water cycle are eagerly awaiting the launch this week of a satellite that should provide the most detailed global measurements yet of rain and snow. The data collected by the US$933-million Global Precipitation Measurement Core Observatory (GPM) could improve severe weather forecasts, estimates of freshwater supplies and projections of future climate change.

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The satellite, a joint effort of NASA and the Japan Aerospace Exploration Agency (JAXA), is set to launch on 27 February from the Tanegashima Space Center in southwestern Japan.

Once GPM settles into orbit above Earth, it will use radar and passive microwave instruments to map precipitation every three hours from the Arctic Circle to the Antarctic Circle — roughly 65 degrees north to 65 degrees south latitude. Its dual-band radar, developed by JAXA, will feed scientists information on the size of raindrops and snowflakes, and their concentration in different layers of the atmosphere. The NASA-built passive microwave imager will monitor the intensity of rain and snow in a given location.

“I like to describe it as a CT scan of the atmosphere,” says Gail Skofronick-Jackson, GPM project scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. 

Improved coverage

Similar data is collected now by an existing satellite, the Tropical Rainfall Measuring Mission (TRMM), which NASA and JAXA launched in 1997. But TRMM covers a much smaller swath of the globe — a band from 35 degrees north to 35 degrees south latitude, or roughly from the Mediterranean Sea to the southern tip of South Africa. It also carries less-sophisticated instruments: TRMM’s single-band radar cannot detect as wide a range of precipitation particle sizes as GPM’s will, nor can it see snowflakes. “We’re only telling part of the story right now, because we’re only talking about rainfall,” says Marshall Shepherd, a research meteorologist at the University of Georgia in Athens and a member of the GPM science team.

GPM’s expanded coverage area will allow scientists to track conditions in poorly monitored areas, such as the world’s oceans and mountain ranges, as well as much of Africa, Asia and South America. Researchers also hope to use GPM data to study the evolution of major storms outside of the tropics, and to develop real-time warnings of landslides and floods. Such capabilities would have proved invaluable this month in southern England, which has suffered severe flooding after an unusually stormy winter. The area affected sits well north of the tropical band monitored by TRMM, but it is within the zone that GPM will track.

Although GPM’s official design life is just three years, the satellite will carry enough fuel to operate for 5–12 years, depending on solar weather conditions. There is precedent: TRMM, launched in 1997 and designed to operate for three years, is still chugging along 17 years later, although it is finally nearing the end of its useful life. “We’ve been very lucky,” says Robert Adler, an atmospheric scientist at the University of Maryland in College Park who serves on the science team for the TRMM and GPM missions.