Published: 2007-10-13 10:04:02
Updated: 2007-10-13 10:04:02
Posted October 13, 2007 10:04 a.m. EDT
By Pete Smullen
MIKE MOSS SAYS: Pete, Dual doppler is our name for dual-polarization (also known as polarimateric) doppler radar, in which the radar simultaneously transmits radio energy pulses that are horizontally and vertically polarized, rather than the single horizontally polarized beam that has been traditionally used in most weather radar. This is done to take advantage of the fact that hydrometeors (raindrops, hailstones, snow flakes, etc) in some cases have different apparent radar cross sectional areas depending upon whether they are struck by a horizontally polarized beam or a vertically polarized one. The classic example of this is the raindrop, which when very small is almost a perfect sphere that scatters similar amounts of energy at either polarization, versus very large raindrops, which take on an oblate shape (on average) that has been likened to that of a hamburger bun. This shape makes the large droplets more reflective of horizontally polarized radiation than vertically, so that a differential measurement of the two polarizations can easily distinguish between an area with predominantly small droplets and an area with much larger ones. More importantly, since large hailstones, even through they can have irregular, nonspherical shapes indivisually, tend to tumble as they fall, their average shape is much closer to spherical and therefore they can be discriminated from heavy rain that might produce a similar reflectivity signature on single polarization radar by the fact that, unlike the large raindrops, the horizontal and vertical polarization reflectivities are almost the same.
I've answered a few other questions about the new systems that may provide more of the kind of information you're interested in. Here are some addresses for those earlier posts:
Also, here are a couple of more general discussions of dual polarization technology that you might find useful: