For many people, the arrival of spring is signaled by daffodils pushing out of the ground, blossoms erupting on cherry trees, and increasingly ragged lawns.
But for meteorologists, another sign often comes to mind:
the return of birds on our weather radars as our winged friends migrate northward into and across our region.
As I have described before, weather radars are quite capable of seeing targets other than precipitation. Stationary targets (like hills and mountains) are removed by clutter and terrain suppression algorithms. Birds are actually quite good targets since they are so much bigger than raindrops or snowflakes; the amount of radiation scattered by a target goes with the fifth power of its diameter, so a bird that is say 2 inches across (roughly 59 mm) scatters radar far more effectively than a raindrop (typically a few mm across).
Let's go back to Thursday evening at 6:47. Not long after sunset, there was very little visible in the Langley Hill coastal radar (you will notice a little bit of return from the ocean surface off of Hoquiam).
But as the skies darkened, the radar blossomed with echoes...echoes that stayed over land...these are the birds. They like to fly at night, a time with less predators and a period during which they can use celestial navigation.
Here are the Doppler velocities from the radar at the same time. Remember, radar shows you the velocity of the "target" towards or away from the radar. Green indicates approaching, yellow little velocity towards or away, red indicates moving away. With green to the south and red to the north, this implies our feathered targets are moving north! Consistent with the bird theory.
Now let me show you something really neat. NOAA has a special vertically pointing radar on the coast at Westport that gives the strength of the radar return and the Doppler velocity for the volume above the radar (see below). The plot shows the signal to noise radar, with red being the most signal. You will also see the Doppler velocities, plotted in the normal meteorological style (barb pointing in the direction the target is going). You notice the "signal" gets large around 03 UTC (7 PM) 8 March (calendar day 7 March here) and then dies around 14 UTC (6 AM). The birds like to fly at night, with most around 2000-7000 ft. The Doppler winds show southerlies and southeasterlies, consistent with northward migration.
With dry weather the next few days, the birds should be pretty obvious in the radar imagery.
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