This weather event happened before and was featured on a unsolved mystery program on the Science Channel. They interviewed some experts and probable causes were brought forward. In the end there was no clear answer to the mystery donut shape seen on the Doppler radar image.
I just googled the effect and there is one case where it was caused by a swarm of birds. Your image seems to be too large for it to be birds.
Roost Ring Donut
https://chicago.cbslocal.com/2014/07/29/roost-ring-donut-image-on-weather-radar-formed-by-birds/