Climatologist explains the fickle nature of thunderstorms

Why do fierce thunderstorms sometimes leave very little moisture on the ground? It’s a riddle that’s puzzled humankind for thousands of years.

University of Missouri climatologist Neil Fox wanted to solve the riddle. To get at those answers, he turned to a relatively new research tool called dual-polarization radar.

Farmers beyond a 30-mile radius (48 kilometres) of the nearest United States National Weather Service radar have no idea how much rain falls on their fields. Farmers within that 30-mile radar radius can access accurate reports on-line for rainfall on their fields, providing them a useful management tool.

As farms become spread out geographically, it’s become more difficult for growers to assess moisture levels across the entire farm. It’s not practical to put rain gauges everywhere because nobody has the time to check them. Although digital weather stations do an excellent job, their cost prevents coverage of the entire farm, said Fox.

“When you have these localized convective thunderstorms, they can miss your gauges,” says Fox, adding that one end of the field may have received an inch or two, while the other end is 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.”

Fox explains the 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, they quickly 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 vaporizing.

The larger oddly-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 it hits the soil.

“We’re mostly interested in what happens to a raindrop after it leaves the base of the cloud. 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 change shape and maybe evaporate.”

Using dual-polarization radar, Fox determines 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 those dorps 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 closely to what we typically think of as a raindrop, and they will likely survive to reach the ground.

“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 entering this speed information into a model that assesses the atmospheric humidity, researchers develop a tracking 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 occurs for every raindrop. From there, it’s simply a matter for grad students to count the raindrops.

“We’re still in the research stage with this. We’re concentrating on mid-Missouri because there aren’t many on-farm weather stations around here.

“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.”

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