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Anything outdoors!  Tornadoes, eagles, blizzards, or auroras.  If it is in the sky, running through the woods or swimming in the water, I'm there!

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Thursday, March 19, 2009

NEXRAD and migratory birds

Seems to be some confusion about the difference between sundown inversions actual migratory birds showing up on NWS radar sites. There are a couple of birder blogs I like to check on from time to time as these guys usually have some cool photography to check out. Apparently there seems to be some confusion on reading radar returns on being either simple inversions at sundown (aka ground clutter). Bottom line is there seems to be this line of thought where anything blue on the national composite radar which expands out from the radar site is millions of migrating birds. NOT COMPLETELY TRUE. Chris Gullickson replied over on ST where the birds usually move at approximately 30mph more than the winds and the returns from the reflectivity would be rather simple to to validate by comparing to the velocity data from the same radar site.

The USGS isn't helping the matter by posting this page:

http://www.fort.usgs.gov/radar/

Some of it is accurate and some of it is way off.

Note in the top left corner the reference to the May 16 1999 migration event in Buffalo, NY. Here is the complete set of radar images from that study as a powerpoint presentation as prepared by John E Black of Brock University. This would have been a great opportunity to compare not just clear air mode radar returns versus base velocity data, but also for ground truth observations in such a narrow geographic corridor. I would still argue some of the "lake avoidance" claim is only partially accurate as the air over that large of a body of water will create a stable layer which will not show up as clutter.

The other thing to keeping in mind when trying to interpret this sundown radar anomaly is the mode the radar is set in at the time people are claiming is a mass migration. Trying to keep this simple so I'll only use clear air mode and storm mode. When you look at the height of the radar beam in relation to distance from the radar site and taking the height at which birds would be migrating, it would be a very small sample very close to the radar site. Since FCC regs do not allow the beam to be transmitted at a 0 degree height, the lowest scan would by 5 degrees above the horizon. There is too much clutter within 20km of the radar site and 70km is too far out as the beam is too high at that point.

I'm going with the results of this study the USFWS did out at ABR.

http://www.fort.usgs.gov/Products/Publications/21596/21596.pdf

Bottom line is none of the flocks are dense enough or large enough to create a return. Now I know some of the hard core birders out there will not want to believe it (since this study pertains to shorebirds), but after getting the opinions of several doppler experts (If John Wetter reads this, please add additional info) and the USFWS study, I have a hard time believing the radars are in fact returning anything much more than false returns. Besides, wouldn't a large flock of birds with bodies at least the size of a baseball cause a TBSS to show on the radar??

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