top of page

Don't trust those excessive RCM tops!

In some cases if you are looking at the Radar Coded Message (RCM) tops on the Aviation Weather Center website (https://www.aviationweather.gov/radar/rcm) and see tops of 60K or more depicted in black, they are likely bogus. For example, for the depiction below you can see tops showing at 680 (68,000 feet MSL) and 640 (64,000 feet MSL) that are indeed bogus.

First, some background...


This radar summary chart is created from what is called the Radar Coded Messages (RCMs). The original plan was to have the RCM output manually checked/corrected/modified. However, that plan was changed even before the original NEXRAD network was operational. The RCMs are sent as the radar product generators (RPGs) generate them. I also believe little, if any, modifications or enhancements have been made to the RCM software since that time.


The Federal Meteorological Handbook Number 11 (FMH-11 Part B, page 2-112), provides some background information on the echo tops on the chart:


“The Echo Tops algorithm estimates echo (non-zero reflectivity category) top heights for 4 x 4 km (2.2 x 2.2 nm) grid boxes. A grid box represents the area on the Earth where a single value of reflectivity is assigned. The height of the echo top is measured from ground level to the mid-point of the radar radial beam for the highest non-zero reflectivity category per grid box.”


Thus, the Echo Tops algorithm is the source for the RCM echo top information on this image.