I don’t know about the FA display, but here’s some info on the NEXRAD:
I worked for the NWS from 2007 to 2012. Our Science and Operations Officer told me that
can be caused by flying insects. (real bugs, vice the software kind) Sounds pretty bizarre, but the NEXRAD has a decent MDS level and uses DSP to enhance the target display. He said if the targets appear in the early morning or early evening hours, most of the time it’s insects, especially if the display has a “grainy” appearance.
The NEXRAD I maintain (Frederick, OK) is displaying similar targets, i.e. green near the radar changing to dark blue. then light blue as distance from the radar increases.
This is a “composite” reflectivity display:
This is a “base” reflectivity display:
The main difference is composite reflectivity shows the strongest reflected energy at all elevation scans, not just the reflected energy at a single elevation scan. Elevation is antenna elevation, i.e the angle above the horizon the antenna is positioned at.
More info on the NEXRAD at: https://www.weather.gov/iwx/wsr_88d