New Satellite Will Help Scientists Gauge Global Rainfall as the Earth Warms
Photo Credit: Vladislav Gurfinkel / Shutterstock
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Climate scientists trying to figure out how fast the world is warming have a hard task on their hands. But, says meteorologist Brian Soden at the University of Miami's Rosenstiel School for Marine and Atmospheric Science, rainfall presents an even tougher nut to crack. "You can be driving down the highway and at one stoplight you’re in sunshine and at the next you’re at a downpour," he sighs. "Temperature isn’t like that."
The vast variability in precipitation over space and time has made it one of the most difficult climate variables to measure. The UN’s Intergovernmental Panel on Climate Change has concluded that a warmer world should overall be a rainier one, with wet areas destined to get wetter and dry areas drier. But the certainty in rainfall predictions is frustratingly low. Researchers still don’t know what sorts of clouds lead to what sorts of rain; when and where and how much water evaporates into clouds, thereby moving energy around the globe; or if rainfall really will increase as theory predicts.
Some of that is now set to change thanks to the launch of the joint Japanese-NASA Global Precipitation Measurement (GPM) observatory in February. The satellite is the first designed to detect harder-to-spot precipitation, including drizzle and snow, and greatly expands the well-watched part of the planet from the tropics north to Iceland and south to the tip of the Antarctic Peninsula.
GPM marks a huge step for rainfall measurement, says Robert Adler, an atmospheric scientist at the University of Maryland and one of NASA’s Precipitation Measurement Missions principal investigators. The resulting data will be made available to disaster relief agencies within 1-3 hours, which should help to get help to areas hit by flash floods. The information will be plugged into basic science questions like what sorts of aerosols make for rainier clouds. And it will extend the currently-short satellite record, says Soden, giving modelers the data they need to validate their predictions. "Modelers better be excited about this," says Gail Skofronick-Jackson, deputy project scientist for GPM at NASA’s Goddard Space Flight Center. "I’m going to be mad if they’re not."
Measuring rainfall is one of those things that seems deceptively simple. "It sounds really easy, like just putting a bucket out," says Adler. "It is more complicated than that. Globally, it’s been a struggle for a long time to understand how precipitation is distributed."
The gold standard for rainfall measurements are rain gauges, which simply look at the height or weight of water collected in a container. Today there are about 6,000 of these around the world, contributing data to international databases like the Global Precipitation Climatology Project , with some national compilations dating back to the 1860s or earlier. But these records still present problems. Like temperature sensors that are sometimes installed accidentally in the cool shade or near hot slabs of concrete, rain sensors are sometimes placed comically too close to trees that grow and eventually shelter the buckets. Windy days can blow water out of the gauges. And, most importantly, the spatial coverage is extremely limited.
There are no rain gauges at sea, for example. And the developing world often has few stations or isn’t inclined to share its data for geopolitical reasons. For instance, where conflicts rage about access to river water passing from one nation to another — as from to India to Pakistan, for example — there may be reasons to keep information about alternative access to freshwater resources from rain under wraps. With gauges alone, says Adler, "we didn’t have good information" for more than 75-80 percent of the planet.