Tuesday, May 5, 2015

Unusually Strong Clear-Air Radar Returns

Some of the strongest and most widespread clear-air returns that I've seen from the Salt Lake City KMTX NEXRAD radar were evident in radar loops this morning.  Note in the loop below the widespread coverage of echoes with reflectivities of 5–20 dBZ prior to sunrise, followed by their rapid decay after about 1130 UTC (0530 MDT).  

Clear-air returns are not produced by precipitation, but instead by other meteorological or non-meteorological phenomenon including insects, birds, aerosols and something known as Bragg Scatter, which is produced when there are large variations in atmospheric density on scales comparable to or smaller than the wavelength of the radar.  They are frequently found in the eastern United States.  Just check out all the blotches of clear air returns at 0400 UTC this morning in the composite radar analysis below.

I've got a busy day ahead, so I leave it to some of the radar wonks out there to comment on the factors contributing to some of this morning's strong returns, especially since we see them less commonly than the eastern U.S.


  1. Bragg scattering. Given dewpoints in the mid-40s this morning and a relatively moist layer aloft in today's sounding, I would owe the density differences to all the moisture in the atmosphere, similar to the eastern U.S.

  2. Birds. Most species migrate at night and put down at dawn. There are a lot more songbirds in the eastern US so their radar signatures are much more pronounced than out west. You will also notice fall migration will see even stronger bird migration returns than spring when all the young birds have been added to the population. -Colby