Tuesday, March 24, 2009
This is lots of talk about Doppler radar...but the interesting thing is that most of you NEVER see the Doppler part. That will end right now! Normally, the only radar images you see show you where it is precipitating. More technically, the normal radar images show you reflectivity...how much of the microwave radar beam is scattered back to the radar. Heavier precipitation generally has a higher reflectivity...so that is why radar is useful.
But you don't need a Doppler radar to see precipitation. A Doppler radar has an additional capability...it can measure the velocity of the precipitation particles towards or away from the radar. And since precipitation is pushed around by the winds, that give you information about the winds. TV stations tell you about their super Doppler radars, their pinpoint Dopplers, their Storm-Tracker Dopplers, and for a short time a decade ago...their Wappler Doppler radar. At one point, there was a pinpoint, severe-weather center, storm tracker Doppler radar! But they never ever show the Doppler imagery on TV. I should correct myself..when KING 5 got their own radar, Jeff Renner showed the Doppler velocities for a few days. It was taken off VERY quickly, never to return. OK, want to see what you are missing?
Tonight there is a very nice convergence zone over central Puget Sound (see image). A nice example...and most of you probably recognize the band of cloud extending across the Sound (see image). Next, take a look at the Doppler velocity image (image). Both are for the lowest radar angle (.5 degree above the horizontal). Enough to drive you to drink?
Cool colors (e.g, blue and green) show velocites towards the radar and warm colds (yellow, red) indicate velocities moving away. The radar is located in the center of image at Camano Island. So air is moving towards the radar in the Strait of Juan de Fuca (since the colors are green and blue) and away from the radar in the very north Sound (see the small area of yellow). Those are northerlies. And you see the gray area over the Sound? There is no velocity towards or away from the radar....this is the center of the convergence zone. Anyway, it is lots of fun and not a little challenging sometime to figure out the wind directions from the Doppler image...since it is only giving you one component of the wind (towards or away from the radar). I bet you now understand why this is not shown on TV...my colleagues at the TV stations would spend all their time explaining the images!
Posted by Cliff Mass at 7:22 PM