Forecasting whether it will rain is relatively easy. Forecasting how much rain will fall? That’s the hard part.
Weather radar does an excellent job detecting rain and intensity, but it isn’t as accurate showing the actual rain totals. Along the way, some raindrops evaporate before ever hitting the ground and that can lead to rain amounts that are too high, especially in areas farther away from radar sites.
Researchers at the University of Missouri think they’ve found a way to fix the rain totals problem.
Weather radar mostly tracks raindrops when they’re still in the air. But Neil Fox, an associate professor of atmospheric science at MU, puts it, “We haven’t been paying enough attention to what happens after radar “sees” the raindrop.”
Evaporation matters a lot!
After a rain event you’ll see the KPRC2 Weather Team give rainfall estimates of the storm. But those numbers are almost always too high.
That’s where this research comes in.
The study used dual-polarization radar, which sends out both horizontal and vertical radar beams. This allows scientists to estimate the shape and size of raindrops and size matters. Smaller drops evaporate quickly, but also encounter less air resistance. Larger drops fall more efficiently, but friction and gravity will flatten them, increasing evaporation around the edges if they are moving through dry air.
By combining raindrop size with measurements of atmospheric humidity, the researchers were able to track individual raindrops from the moment radar detected them all the way to the ground, calculating exactly how much evaporation occurred along the way.
The result? A noticeable improvement in rainfall estimates, particularly in areas farther from radar. That’s important because as radar beams travel, they rise higher into the atmosphere. The farther you are from the radar, the more likely it is that the system is “seeing” rain that won’t fully survive the trip to the ground.
And those farther-out areas are often agricultural regions. Farmers rely heavily on rainfall estimates to make decisions about irrigation and crop management. The more accurate the rain totals, the more useful it becomes for the people who depend on it most. The Houston radar tower is in League City. Farms and ranches start around 50 miles west of the radar beam.
Better rainfall estimates don’t just improve rain totals, they improve the entire forecast. Rainfall influences how storms evolve, how temperatures change, even air quality. So when we get the rain right, everything else downstream in the forecast gets better too.
The next step in this research is to pair these improved rainfall estimates with weather model data. That’s not an easy task, but it is doable. If successful, it would sharpen forecasts on both ends, improving rainfall predictions before a storm arrives and making rainfall estimates after the storm more accurate as well.
There are 159 National Weather Service radar sites across the United States feeding data into weather models. That network is a big reason this kind of improvement is even possible.
Below are the five radar sites closest to Houston. These sites are the ones that play a major role in how our KPRC 2 Weather Team “sees” storms coming and tells you what to expect.