Getting the drop on 'giant' rain.

Getting the drop on 'giant' rain

You can get a world-class tan in Hawaii, but if you move a little off the island's eastern shore you can also get soaked by world-class rainfall. Each year, 300 inches of rain drop from a band of clouds near Hilo. Now these clouds have set a new kind of rain record. Atmospheric scientists report in the October GEOPHYSICAL RESEARCH LETTERS the discovery of the world's largest raindrops from warm clouds.

During the 1985 Joint Hawaiian Warm Rain Project, Kenneth V. Beard of the Illinois State Water Survey in Champaign and his colleagues found many drops measuring 4 and 5 millimeters in diameter, as well as one 8-mm drop. Drops up to 6 mm in size are known to form when cloud ice melts, but the conventional view of warm, tropical raindrops has held that they seldom get larger than 2.5 mm; earlier laboratory studies, modeling and field observations suggested that larger drops are routinely shattered when they collide with smaller drops.

Apart from showing that conventional thinking about the formation and survival of warm raindrops is all wet, the new finding is important to scientists who use radar to monitor rain clouds and estimate rainfall rates. To calculate these rates, meteorologists need an accurate account of the size distribution of drops in a cloud because the strength of radar signals reflected from a drop is strongly influenced by the drop's size. In addition, radar signals are affected by the shape of a drop, and the larger a drop, the more distorted it becomes (SN: 3/2/85, p. 136).

Recent laboratory and modeling results have convinced Beard, David B. Johnson at the Bureau of Reclamation in Denver and Darrel Baumgardner at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., that their finding of large drops in the Hawaiian clouds makes sense on a theoretical basis. Using a drop-size distribution commonly used in radar measurements, they calculate that 5-mm raindrops are destroyed once every 42 seconds. But using their actual observations, in which they found a much higher proportion of large drops than that standard model had predicted, the researchers obtain a destruction rate of only one 5-mm drop every 23 minutes.

"Because earlier observations suggested that these clouds don't have big drops, nobody was looking for them," says Beard. "And no one was doing any calculations to see if they were possible. Nature is often more interesting than what we theoreticians can invent."

Beard's group thinks the high water content of the cloud band near Hilo contributes to the unexpectedly high number of large drops. The clouds' large updrafts are also key because they suspend drops, enabling them to grow large before they fall. In addition, "relatively low numbers of small drops seem to be a necessary condition," says Beard, so that few collisions occur.

The researchers also suspect that the updraft may be tilted so that large drops fall out of the cloud, avoiding the cloud's rising small drops. Or, it may be that large drops are the first to fall from a cloud. "We certainly find large drops in the absence of significant numbers of small raindrops," says Beard. But, he adds, "we don't really understand how this part of it occurs."

In order to investigate the wind patterns that give rise to these clouds, Roy Rasmussen at the Bureau of Reclamation in Denver and Piotr Smolarkiewicz at NCAR simulated the formation of the cloud band with a three-dimensional computer model. They found that the trade winds approaching the island are deflected back and down the slopes of two 13,000-foot-tall volcanoes. The deflected winds then run head-on with low-level trade winds, the converging air rises and clouds form. In other rainy areas, such as the island of Maui, clouds form because the air is pushed up by mountains. Having the winds turn back on themselves is "is fairly unusual," says Rasmussen. "No one's reported on this phenomenon before."