When climate alters the amount of rainfall on land, bringing drought or flood, it gets a lot of attention. But an often overlooked factor in the global climate system is rainfall at sea. Rain clouds dump large amounts of freshwater into the ocean’s top layer, and as it mixes into the depths, it can change the ocean’s temperature and salinity profiles and the circulation of deep water and can even affect global climate events like El Niño.
Furthermore, the water cycle doesn’t just move water from the ground into the air—it also moves heat. The heat that causes water to evaporate is released into the atmosphere when that water condenses. All of these factors mean that understanding how much rain falls in the ocean is crucial to our understanding of Earth’s climate.
Unfortunately, when rain falls at sea, there’s usually no one there to see it. Ground-based radar has limited range, and gauges on islands or buoys can only give scattered readings. It’s very difficult to estimate the overall extent of rainfall on the ocean’s surface.
One proposed solution is to use satellite-based radar that can detect the ripples and splashes caused by the falling raindrops. To evaluate this method, Liu et al. analyzed radar observations from the European Space Agency’s ENVISAT satellite—in particular, a 2006 storm in the Luzon Strait that was also in range of ground-based radar in Taiwan. They found that when the rainfall increased, as measured by the ground radar, so did the backscattering radar signature from space—but only to a point. When the rainfall increased to monsoon-like levels—specifically, at a rate of 2.4 centimeters per hour—its radar signature as seen from space actually shrank.
To determine exactly what it was that the radar was detecting, the authors also captured super-slow-motion footage of water droplets impacting a tank of water. They found that the biggest features weren’t the ripples or the craters from the falling raindrops, but rather the rebounding “stalks” that form as the impact ring of water collapses back in on itself.
The authors hope these results will help scientists interpret space-based radar data in order to better measure the effects of rainfall at sea and improve our understanding of its place in the broader climate system. (Journal of Geophysical Research: Oceans, doi:10.1002/2015JC010975, 2016)
—Mark Zastrow, Freelance Writer
Citation: Zastrow, M. (2016), The unique radar signature of rain falling on water, Eos, 97, doi:10.1029/2016EO046315. Published on 19 February 2016.