Raindrops Fall Faster Than Doppler Radar Says

September 28, 2009

Researchers have discovered that smaller raindrops can fall faster than bigger drops—and drop faster than a so-called terminal velocity. Moreover, by interpreting small, fast drops as larger ones, scientists relying on Doppler radar have been overestimating the amount of rainfall by as much as 20%.

Alexander Kostinski and Raymond Shaw (Michigan Technological University) and Guillermo Montero-Martinez and Fernando Garcia-Garcia (Universidad Nacional Autónoma de México) used a collecting system and a complex algorithm to analyze the speeds of 64,000 raindrops that fell in three years over Mexico City during calm conditions. Discovering that clusters of raindrops can exceed a terminal speed, the researchers surmise that "superterminal" drops develop from the breakup of larger drops, which produces smaller fragments moving at the same speed as their parent raindrops—and faster than the terminal speed predicted by their size.

When a large drop falling at full speed breaks up because it's unstable or collides with another drop, the resulting droplets continue at the same speed—too fast for their diminutive size. After a few milliseconds, air resistance slows each drop to its terminal velocity.

"People have seen indications of faster-than-terminal drops, but they always attributed it to splashing on the instruments," Shaw said. "If we want to forecast weather or rain, we need to understand the rain formation processes and be able to accurately measure the amount of rain."

Details of the research appeared in the journal Geophysical Research Letters in an article titled "Do All Raindrops Fall at Terminal Speed?"

Source: American Geophysical Union, June 10, 2009; Natural History Magazine, Sept. 14, 2009.