NEXRAD, Roost Rings over NW Houston


For the month of August I have been noticing most mornings on the NOAA weather radar a persistent "roost ring" located in northwest Houston in the vicinity of the Willowbrook Mall. The ring appears between 6:30 and 6:45 am and grows until around 7:15 or so. Early in the month the ring was centered somewhat east of the mall and by late Aug has moved a few miles west southwest of the mall.

I created the following animated gif showing a beautiful "textbook perfect" example of the roost ring developing the morning of 27 August, 2016. The ring, centered over Houston's Willowbrook Mall area, is due to Purple Martins in taking off from their roost for their daily foraging. The first indication on the radar starts about 6:37 am CDT and grows until 7:22 (when I stopped tracking). Range circles every 10 Miles.

27 Aug 2016 base reflectivity

The next animated gif shows the radial velocity for the same event. Here one can readily see the birds moving both towards (green) as well as away (red) from the radar site located in SE Houston. Note the general direction of cloud movement is from NNE to SSW.

27 Aug 2016 relative velocity

Elevation calculations for the signature:
Nominal elevation angle is 0.55 deg
Radar elevation 18 ft

6:37 am
Distance to center 44 mi.; Elev 2250 ft
7:27 am
Inner ring distance 25 mi.; Elev 1300 ft
Outer ring distance 65 mi.; Elev 3300 ft

The above calculations suggest most of the reflection returns are from Martins flying at an average elevation of 2250 ft, ranging between 1300 and 3300 ft. The ring diameter grows to 40 miles (20 mi radius) in 50 minutes time, or 24 mph average radial flight velocity. As the birds do not fly exactly radial from the origin, they must be flying considerably faster than this.


Purple Martins are common in Houston and the Willowbrook area roost site is well known. The local Audubon Chapter hosts an annual Purple Martin Migration Madness Party several Saturday nights at the site each August.


kjs


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