An international team of scientists spent time earlier this year measuring precipitation above the Canadian province of Ontario. The aim: To improve satellite estimates of falling snow and test ground validation capabilities before a planned launch of the Global Precipitation Measurement (GPM) core satellite in mid-2014.

Measuring snowfall from space is no easy task. NASA is working with counterparts in Canada and Japan to prepare for GPM, which will include cutting-edge technology that detects falling snow from space, and provide worldwide data on rain and snowfall every three hours.

The GPM core satellite will be used to unify other satellite-based rainfall sensors from space agencies around the world, said Gail Skofronick Jackson, deputy project scientist at NASA’s Goddard Space Flight Center in Maryland.

“Right now we’re able to get measurements every three hours, but we don’t get snow, just rain rates,” Jackson said.
Data from GPM will help improve rain and snow models for weather forecasting, she said. “We also can put it into climate change models.”

The Experiment

During January and February, NASA flew a science lab 33,000 feet above snowstorms in preparation for GPM, NASA’s first mission designed to detect falling snow from space.

NASA worked with Environment Canada for the GPM Cold-season Precipitation Experiment (GCPEx), using a DC-8 plane carrying radar and a radiometer to simulate measurements that will be taken by GPM. The plane passed over a ground network for snow gauges and sensors at Environment Canada’s Center for Atmospheric Research Experiments, located north of Toronto. On the ground, pictures were taken of snowflakes, using high-speed photographic equipment.

Two other aircraft, from Canada and the University of North Dakota, also flew in the experiment, measuring the microphysical properties of raindrops and snowflakes.

“What goes up on the satellite is radar with the same capabilities as this radar (on the DC-8),” said David Hudak, a research scientist with Environment Canada. “It will have the same frequencies and the capabilities. You want to have some measurements in advance so you can develop the algorithms.”

Goddard’s Mathew Schwaller, GPM ground validation project manager, and Manuel Vega, research engineer, recently co-authored a paper with others at Colorado State University on a dual-frequency, dual-polarized, Doppler radar system (D3R) tested in connection with GPM, and recent observations from GCPEx.

“During the GCPEx campaign, the field worthiness of the D3R system was demonstrated through continuous and automated scans at a secure site in ‘tough’ weather conditions,” according to the paper, submitted for publication at the 2012 IEEE International Geoscience and Remote Sensing Symposium (IGARSS). “The real time images were continuously archived on the NASA GCPEx portal. Often, the radar was operated remotely.”

The D3R operations began on Jan. 17 and continued until March 1, without any reported system crashes, including at temperatures reaching minus 20 degrees Celsius (minus 4 degrees Fahrenheit).
“The robustness of D3R, including remote monitoring of system health were essentially proven during the GCPEx campaign,” the authors wrote.

The GPM Technology

Snowflakes can contain different amounts of air and water. Instruments aboard GPM will help scientists to measure the overall water content of falling snow. That’s vital data, especially when it comes to forecasting storms and water levels for freshwater systems like the Great Lakes.

“Rain and snow are the water cycle, and it’s a global water cycle,” Jackson said. “If it’s not falling in one state, maybe it’s falling in another. We need to have a global view of precipitation and that’s what the satellite mission gives us. When you look at it from space, you get a uniform picture of what’s happening.”

The GPM satellite will be the largest spacecraft that Goddard has put together in-house, Jackson said. Instruments installed on the satellite will be provided by NASA and the Japanese Aerospace and Exploration Agency (JAXA).

GPM will deploy a core satellite that carries a Dual-frequency Precipitation Radar (DPR) and a multi-channel Global Precipitation Measurement (GPM) Microwave Imager (GMI). This advanced system is an upgrade from the Tropical Rainfall Measuring Mission (TRMM), launched in 1997.
The new GPM core observatory system will measure precipitation from space and serve as a reference standard to unify precipitation measurements from a network of other research and operational satellites, according to NASA.

The Japanese are building the radar that will be housed on GPM and will be launching the spacecraft into orbit from Tanegashima Island.
The DPR consists of Ku- and Ka-band precipitation radars, allowing for differentiation between rain and snow. The Ku-band is an updated version of a unit used with success during the TRMM mission, NASA officials note. The shorter wavelength of the Ka-band radar extends the sensitivity of the DPR to enable discrimination between lighter rain and snow. Compared to technology used during TRMM, the DPR is expected to be more sensitive, particularly with regard to light rainfall and snow in high-latitude regions.
The Ku- and Ka-band radars will be configured to link up with the same location on Earth, gathering data that provides a 3-D look at rain, and more accurate estimates of rainfall rate, according to Dr. Walter Petersen, NASA GPM ground validation science manager.

“It is the addition of the Ka band that really makes the DPR a unique platform,” Petersen said. “The addition of that second, higher frequency enables us to retrieve more information on light precipitation rates than TRMM permits.”

By combining data from the GPM sensors, scientists will be able to retrieve drop-size distributions, and even discriminate between particle types (layers of rain drops v. snow or ice particles).

“When you combine the GMI and DPR on a single core satellite platform, you improve the quality of your precipitation retrieval because the combined measurements from the different and independent instrument types are complementary, forcing what amounts to a consistent ‘answer’ in terms of estimating the precipitation contents being observed,” Petersen added.

Why It’s Important

The GPM mission will be an important step forward for several reasons.
“From the Canadian perspective, so much of the precipitation we get here is solid precipitation,” Hudak said. “Knowing how much snow is falling and when the snow melts in the spring has a major impact on water levels and water resources.”

In Canada, the radar network that observes precipitation only covers the southern 10 percent of the country. “We just don’t know where it’s precipitating and how much with very good accuracy,” he said.

“What you have now is some of the weather satellites that take pictures. But the technology on GPM will be able to far more accurately characterize and quantify precipitation. It’s revolutionary.”
The GPM will orbit the Earth from the Antarctic Circle to the Arctic Circle, at a 65-degree inclination.

The mission is designed to improve global measurements of precipitation, help advance knowledge of the Earth’s water and energy cycle, and improve forecasting of extreme events that cause natural hazards and disasters, NASA officials say. The GPM also is expected to improve understanding of how precipitation processes may be affected by human activities.

The winter of early 2012 wasn’t a banner one for snowfall, but localized snow events and squalls from the Great Lakes helped, Hudak said. About a dozen light-to-moderate snowfall events were measured during the experiment.

“The instruments are working very well,” Jackson said during the campaign earlier this year. “The data we’re going to get from the field campaign is just fabulous. It’s like this dream scenario of all the measurements that you would ever need.”

While the field worthiness of the D3R system was demonstrated during the experiment, work is still needed to evaluate the system performance in warm and hot climates, the IGARRS paper notes. The platform will get its chance to perform when it is deployed in warm-season experiments in Iowa and North Carolina, and at its future home base at Wallops Island, Virginia.