19 March 2006
Storm-relative Motion Radar Image Pair (Del Rio TX)
These two images were transmitted 19 minutes apart and represent the 0.5 (half) degree elevation angle for the beam. This is the lowest slice of the radar beam, which is about 3,000 to 3,500 feet above ground level where the supercell begins to encounter the southwestern arc of bats. The colors depict the motion of every echo return toward or away from the radar, with the speed and direction of the storm factored into the calculation. This means it shows the motions inside the supercell as if the storm were stationary -- a handy way for meteorologists to judge the storm's internal behavior -- including both rotation and horizontal surges of wind. Being made mostly of water, with some solid material, bats make good reflectors of radar energy too. Since a flock of bats represents a cloud of large reflectors for the radar beam, these images also indicate the movement of the bats relative to the storm.
Warm colors represent motion away from the radar, and cold colors indicate motion toward the radar. Using the scale in the middle, the bright red bat-echoes next to the supercell indicate that the storm and the animals were closing in on each other at speeds sometimes breaking 50 knots (58 mph). Bats can't fly quite this fast on their own, so it is also evident that they were moving with the air currents flowing into the mesocyclone.
Since we can only see the motion of the bats compared to the storm, other imagery (such as the reflectivity loop), must be used to guess the ground-relative movement of the bats as a group. What became of these storm-menaced animals? Did they try to turn around and fly away from the storm as it closed in? Were they fighting against the inflow winds? It's hard to say; but if they were, the efforts of the bats in that red band were not successful. The ones that were pulled into the mesocyclone probably had a horrendous -- maybe even fatal -- experience inside that storm, dealing with heavy rain, frequent lightning, large hail, and extreme turbulence -- including updrafts rocketing skyward at a hundred feet per second or more. I have seen dead birds falling from the sky in the vicinity of supercells, and can guess that the same fate befell many of those bats.
The reflectivity images at the corresponding times can be seen by opening up that image loop, then stopping the loop at the 2349Z and 0008Z images (for reference, time stamp is at bottom).
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