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Observing Insects in a Boundary

David Moran
Aug 1, 2018 2:40:35 PM

Severe thunderstorms developed across portions of western Kansas and eastern Colorado on Friday, July 27, 2018. In the midst of these thunderstorms, a persistent line developed, extending from northeast of Stratton, Colorado to southeast of Goodland, Kansas. 

Dual-polarization products can help determine what the radar might be observing. The loop below shows a very thin line of reflectivity that persisted with this storm for hours.

Loop of  Line of Trapped Insects
Line of Trapped Insects

When determining what a radar echo might represent, Correlation Coefficient can be used to determine whether it is meteorological or non-meteorological. Correlation Coefficient values less than 0.8 are generally associated with non-meteorological echoes. 

Loop of Correlation Coefficient 7/27/18
Loop of Correlation Coefficient

Looking at the Correlation Coefficient in the area of the line observed in the reflectivity loop, we can see that values are low (less than 0.8), indicating that these echoes are, in fact, not meteorological. Given the relatively low values of reflectivity and correlation coefficient in this boundary, the radar could be observing insects, but let's check additional data first.

Next we look at the velocity of these echoes. Red shades in the image below indicate echoes are moving away from the radar whereas green shades indicate those that are moving toward it.  

Velocity Image 6:25pm CDT 7/27/18
Velocity near Boundary

Analyzing the velocity image in the location of the boundary, we can see that the echoes moving away from the radar are reasonably uniformly within this line. In addition to looking at the velocity, we can also look at how it is changing within the radar beam using Spectrum Width. Higher Spectrum Width values show a more significant variation in the velocity being observed. Here is the Spectrum Width at the same time of the velocity image above.

Spectrum Width 7/27/18 6:25pm CDT
Spectrum Width

Given the low spectrum width values, the velocities are fairly uniform near the boundary. Brighter oranges and pink shades over northeastern Colorado indicate higher spectrum width values (and a more extensive variety of velocities being observed by the radar). These higher spectrum width values are associated with higher elevations in the thunderstorms in that area. The low reflectivity values, coupled with the velocity and low spectrum width, suggest that the radar is observing insects within this boundary.

While weather radar is most often used to observe meteorological phenomena, non-meteorological echoes can also be viewed. By using dual-polarized data, users can be more confident about what they see on RadarScope.

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