Missing the rain doesn't happen only during heat waves. Meteorologists are missing rain even in the middle of a storm.
All along the U.S. West Coast, in-coming winter storms deposit substantial rainfall that goes undetected by the current generation of scanning radars. From coastal northern Washington to southern California, researchers now estimate that as much as a third of the seasonal rainfall is missed.
The previously unknown type of rain goes by the odd name "non-bright-band rain," which describes not what it is, but what it isn't. Think of it as shallow, more finely textured than your average rain, but much more than a mist.
From low-lying clouds rolling in off the Pacific Ocean, this rain forms and falls from a height that is below the scanning beams of modern NEXRAD radars. More than that, it forms by way of a process known as "collision and coalescence" that is different than the snow-melting that produces the "bright band" that modern radar picks up.
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The first clue to the rain's existence came in the late 1990s when researcher David W. Reynolds, now chief Weather Service meteorologist in Monterey, Calif., noticed that NEXRAD radar had estimated a half-inch of rainfall over the Russian River, north of San Francisco, but, in fact, 10 inches fell.
So yes, on those rainy days it was supposed to be, not so rainy? Blame the rain that fell under the radar.
It may seem strange that weather science is still grappling with such a fundamental process, but meteorologist Marty Ralph, chief of the Water Cycle Branch of NOAA's Earth System Research Laboratory in Boulder, Colo., observes that the formation of rain is "a very messy science problem."
"It's a fundamentally difficult thing to measure," he told Discovery News, "and the physics of it happens at the microscopic level." At the same time, all of the larger-scale processes that affect weather such as air temperature and winds come into play.
The conventional view of rain formation, developed by Norwegian researchers, recognizes two mechanisms within different cloud layers -- high-altitude "seeder" clouds and warmer, low-level "feeders." High in the atmosphere, condensing, supercooled water vapor attaches to a bit of dust or aerosol to form ice. It begins falling as snow and finally becomes a raindrop as it falls through the melting layer. Thousands of feet below the seeder cloud, the rain often encounters a warmer "feeder" cloud where the raindrops can pick up the mass of more droplets on the way to the ground.
"What we've discovered is, you can take away the seeder cloud altogether on the West Coast and the feeder cloud can produce just as much rainfall as if the seeder cloud were present," said Ralph.
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Ground truthing forecasts by monitoring how much rainfall actually hits the ground was key in realizing the existence of the mystery rain. To catch the process in action, National Weather Service engineer James Jordan invented a new vertically-pointing radar. Now instead of falling under the radar, the rain is showing up as "non-bright-band rain" - and meteorologists are estimating the amount of rainfall it represents: heaviest along the coast, up to 35 percent of a season's total, but as much as 15-20 percent in the Cascades and the northern Sierra Nevada.
Ralph and his team at NOAA are now working on how to integrate detection of this missing rain into the scanning radars most weather service teams use. They are also tackling with the problem of improving numerical forecasting models, which are currently not adequately capturing the formation processes of the shallow rain.
While researchers haven't yet investigated this idea, Ralph speculates that he would not be surprised to find this shallow under-the-radar rain on every continent with a western coastline exposed to westerly winds carrying ocean storms.
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IMAGE 1: Spectators watch from a soggy grandstand the third round of the 140th Open Championship at Royal St George's on July 16, 2011 in Sandwich, England. (Photo by David Cannon/R&A/R&A via Getty Images)
IMAGE 2: An experimental vertically-oriented radar captures rainfall at Cazadero, a weather station in San Francisco, where half the rainfall of the three-inch storm in May 2009 was not represented in conventional radar images. CREDIT: Courtesy NOAA/Martin Ralph
Tags: Meteorology, Physics
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