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.


My opinion is that the “few isolated pixels” of high reflectivity are “real” data that have made it through the clutter suppression algorithm. More specifically, I think the pixels you sometimes see are due to aircraft returns that have made it through the Point Target Detection and a removal program run in the radar data acquisition (RDA) signal processor. Then the algorithm falsely determines the tops to be way above what we see as normal.


One last point, the RCM reflectivity data has a very poor temporal and spatial resolution of about 12 km (very poor) and is updated every 30 minutes (also pretty poor), respectively. Why again are we using this? Don't have a clue, but evidently there are enough pilots out there that still do.


By the way, there are times where you actually might have a correct RCM top. For example, on April 28, 2021, there were reports of hail four inches in diameter just to the west of San Antonio, Texas. That was from an intense storm with tops estimated to 64,000 feet MSL! And below you'll see the RCM nearby this storm has a top for this storm depicted at 630 (63,000 feet MSL). This one is likely real given the weather that was occurring at the time.

In most cases, it is rare to see a storm with tops that exceed about 55,000 feet. It's not impossible, but those kind of storms will typically produce severe hail, strong winds and possibly tornadoes. In most cases, RCM tops of 60K or more are likely bogus.


Most pilots are weatherwise, but some are otherwise™


Dr. Scott Dennstaedt

Weather Systems Engineer

Founder, EZWxBrief

CFI & former NWS meteorologist


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