Foul Weather Friend

Satellites that detect subtle geological ripples in the landscape also can espy patches of water vapor in the air that might unleash storms, a new study shows. The radar technique, described in today's Science, offers a promising way to predict rainfall patterns more precisely.

The method, called satellite radar interferometry, is just a few years old but has won many converts among geophysicists. Two satellite images of the same strip of land taken days or months apart can expose seismic deformation along a fault, bulges in a volcano, or other tiny shifts in elevation that are hard to spot on the ground. Radar waves from the satellites travel different distances if the ground rises or falls just a few millimeters, creating an interference pattern when computers combine the two images. Water vapor in the air delays the radar signals slightly, but geologists usually subtract that delay as noise.

Now, a team led by Dutch scientists has mined watery gold from that noise. Geodetic engineer Ramon Hanssen of the Delft University of Technology and his colleagues analyzed three sets of satellite images of the terrain near IJssel Lake in the Netherlands. The images in each pair were taken one day apart to eliminate possible ground motions. The delays from water vapor stood out as the main difference from one image to the next. By comparing the satellite signals to ground-based records of rainfall, Hanssen's team found that the biggest radar delays mirrored the places where the most rain had fallen.

Moreover, the satellite images revealed bands of water vapor in regions where heavy clouds would soon form. Such features are invisible to radar stations on the ground, Hanssen notes. "Meteorologists need to know how the distribution of water vapor in the atmosphere changes with time," he says. "Radar interferometry offers much better resolution than weather satellites, which cannot measure the properties of individual clouds."

The technique could be ideal for microscale forecasting of storms near airports, for instance, says atmospheric scientist Randolph Ware of the University Corporation for Atmospheric Research in Boulder, Colorado. "It's a very exciting application," Ware says. The interferograms can be made from an aircraft if, using precise navigation techniques, it flies over the same cloud twice in the exact same path, he notes. But Hanssen and Ware agree that widespread use of the method in general weather forecasting is far in the future, as it would require many special low-flying radar satellites.