Size matters, if you’re a raindrop

Weather services require more accurate data on raindrop size so that they can tell farmers exactly how much rain fell

It’s perplexing that the fiercest of thunderstorms with pounding rain sometimes leave very little water on the ground.

It’s a question that’s puzzled researchers since the birth of climatology, and farmers for thousands of years before that. It’s a question Neil Fox wanted answered.

To get those answers, the University of Missouri scientist turned to a relatively new research tool called dual-polarization radar.

Fox said farmers beyond a 50 kilometre radius of the nearest U.S. National Weather Service radar have no idea how much rain falls on fields where they don’t have a conventional rain gauge.

Farmers within that 50 km radar radius can access accurate reports online for rainfall on their fields, providing them a useful management tool.

As farms become more spread out geographically, it has become more difficult for a grower to know about moisture levels across the entire farm. It’s not practical to put rain gauges everywhere. Nobody has the time to check them, anyway.

Although weather stations do an excellent job, their cost prevents coverage of the entire farm, Fox said.

“When you have these localized convective thunderstorms, they can miss your gauges,” he said, adding that one field may have received an inch or two while a field on the other side of the road is nearly dry.

“That’s why radar can be such a useful tool. Within the coverage area, radar gives farmers a good reading on how much rain fell on each field, but outside the coverage area, radar is of little value.”

Fox said there is a difference between raindrops shaped the way we think they should be shaped and raindrops shaped like tiny little balls.

Smaller drops remain spherical and are subject to evaporation when they leave the moist, saturated surroundings of the mother cloud and drop into the warm dry air where they evaporate. These tiny spheres seldom live long enough to reach ground. The moist muggy air we encounter after a thunderstorm is the result of these small spheres evaporating.

The larger, teardrop-shaped raindrops are the ones that contribute to our soil moisture. They have enough volume that evaporation from their surface will not cause their demise on the trip down.

The challenge for Fox was to figure out a way to identify each type of drops as it leaves the cloud and then track it until it totally evaporates or hits the soil.

“We’re mostly interested in what happens to a raindrop after it leaves the base of the cloud,” he said.

“While the drop is still in the cloud, it’s saturated, it’s surrounded by moisture, so it won’t evaporate or deteriorate in any way. But once it leaves the cloud, it finds itself in a warmer drier environment. It’s no longer in a protective saturated environment, so it starts to evaporate.”

Using dual-polarization radar, Fox is able to determine if the horizontal measurement of a drop is the same as the vertical measurement of the drop. If they’re the same, then he knows it’s a round raindrop, which in all likelihood will evaporate before hitting the soil.

That’s why we can experience a mighty thunderstorm with almost no rain hitting the ground. More of the items measured by dual-polarization are small round droplets.

Dual-polarization also tells Fox if he has drops that are taller than they are wide. These larger raindrops are shaped more like what we typically think of as a raindrop shape, and they will likely survive to reach the ground.

“Dual-polarization radar is hard to explain over the phone because normally I have to wave my hands around all over the place to explain it,” he said.

“Dual-polarization sends out two radar beams. One beam is oriented horizontally, as in conventional radar. The other beam is vertical, so it’s 90 degrees to the horizontal beam. Signals coming back from the raindrops can tell us the size and shape of each drop.”

Smaller drops encounter less air resistance, so they fall faster and evaporate faster.

By combining this speed information with a model that assessed the atmospheric humidity, researchers develop a tracing method that follows both large and small raindrops from the point first observed by the radar to the point when they hit the ground.

This formula precisely determines how much evaporation would occur for any given raindrop. From there, it’s simply a matter for the researchers to count the raindrops.

“We’re still in the research stage with this,” he said.

“We’re concentrating on mid-Missouri because there aren’t many on-farm weather stations around here. Urban areas get better radar coverage because of airports and protection of dense population concentrations.

“It’s not a predictive tool yet, but it will become one in the future. It’ll give us a better understanding of what’s coming in a storm.

“It’ll help us know what to expect. The U.S. National Weather Service began converting all their stations to dual-polarization about five years ago. I believe Environment Canada has begun converting also.”

The spherical experimental radar tower is located on the University of Missouri campus.

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