Tornado Debris: As Seen By Radar

One difficult challenge for a severe weather forecaster is the potential for tornadoes along a line of thunderstorms. These spin-ups can be short-lived, lasting only a few minutes. Such was the case in Harveyville, Kan. on Feb. 28, 2012.

The first image below shows conventional radar reflectivity (left) and storm-relative velocity (right) at 8:57 pm on Feb. 28. A line of severe thunderstorms was pushing through northeast Kansas that evening. In the storm-relative velocity image (right), there is no sign of a tight tornadic circulation.

Base reflectivity (left) and storm-relative velocity (right) near Harveyville, Kan. at 8:57 pm CT, Feb. 28, 2012. Harveyville, is underlined in yellow in both images. Images courtesy: NWS-Topeka, Kan.

Unfortunately for the city of Harveyville, this changed in a matter of minutes. A circulation tightened up along the squall line as it moved into Harveyville, as you can see in the storm-relative velocity image below.

Storm-relative velocity (right) near Harveyville, Kan. at 9:02 pm CT, Feb. 28, 2012. Tornadic circulation shown by red circle near Harveyville. Image courtesy: NWS-Topeka, Kan.

The very next radar "volume scan" to come in four minutes later showed what is referred to as atornado debris signature (TDS) in the dual-polarization data. This is circled in red in the image below at right.

What the dual-polarization radar is picking up below in the "correlation coefficient" or CC product is the presence of irregularly-shaped targets oriented randomly. Translation: tornado debris lofted after the tornado hit Harveyville.

Storm-relative velocity (left) and dual-polarization "correlation coefficient" (right) northeast of Harveyville, Kan. at 9:06 pm CT, Feb. 28, 2012. The Images courtesy: NWS-Topeka, Kan.

Note the storm-relative velocity image above does not show nearly the strong couplet the earlier image did.

In post-analysis, the National Weather Service in Topeka concluded the Harveyville tornado,rated EF2 on the Enhanced-Fujita scale, was only on the ground for a total of four minutes, first touching down only 1 mile southwest of Harveyville...only 1 minute before hitting the town.

In this case, dual-polarization radar confirmed the presence of tornadic debris.

http://www.weather.com/outlook/weather-news/news/articles/tornado-debris-radar_2012-03-08?page=2