Birds, bugs, or rain? What the radar is really seeing
If you checked the ABC 17 Stormtrack Doppler Radar early this morning, you probably saw what looked like a pop-up rain shower moving over the Kansas City and St. Joseph area. But if you were actually driving through that region, your windshield would have been completely dry. So, what was the radar actually picking up?
The answer lies in understanding the physics behind the data. Not everything that shows up as a bright green or yellow blob on our weather maps is actually precipitation. Often, the radar captures non-meteorological targets, and this morning was a perfect example: a massive flock of birds taking flight around sunrise, or debris being kicked into the air by the incoming cold front.
Specifically with birds, when they leave their roost for the morning, they take off in a massive, expanding circle. On our standard base reflectivity radar, which measures the intensity of energy bouncing back to the radar dish, thousands of birds look exactly like a sudden, expanding downpour. To the untrained eye, it is an easy meteorological illusion.
So, how do we filter out the fakes? We look inside the radar beam using a highly advanced dual-polarization tool called the correlation coefficient.
Standard radar only sends out a horizontal pulse of energy. Dual-polarization radar sends out both horizontal and vertical pulses, giving us a true, three-dimensional look at whatever the beam hits in the sky. When we switch the radar over to the correlation coefficient layer, the picture completely changes. Instead of a smooth, uniform green circle, the St. Joseph area suddenly lit up with a chaotic, noisy mix of purples, blues, and yellows.
This happens because the correlation coefficient measures how uniform objects are. Falling raindrops are generally all the exact same shape and size, flattened on the bottom like a hamburger bun, so they return a solid, uniform color on this product. A massive flock of birds, however, is a chaotic mess of different shapes, sizes, and flight patterns. The radar instantly recognizes that this target is non-meteorological and flags it with a messy, pixelated signature.
While filtering out a morning bird migration is a neat radar trick, this exact same atmospheric physics is actually one of our most critical tools as we head into the colder months here in Mid-Missouri.
When we are inevitably going to be tracking a messy storm moving across the region, we will specifically be looking for the melting layer. Because liquid rain, melting sleet, and giant snowflakes all have drastically different physical shapes, the correlation coefficient allows us to see exactly where precipitation is freezing or melting hundreds of feet above our heads. It is the ultimate tool for finding the exact rain-snow line.
By analyzing this dual-polarization data, we can track precisely when the precipitation is changing over in your neighborhood, even when surface temperatures are hovering right around that freezing mark. It is just one more way to look past the standard weather maps, using deeper data to keep you and your family a step ahead of any storms this winter.
