Published: 2007-10-01 09:55:35
Updated: 2007-10-01 09:55:35
Posted October 1, 2007
MIKE MOSS SAYS: Aaron, The classification of precipitation types using dual polarization technology is accomplished by measuring several different parameters that can be detected by the use of both vertically and horizontally polarized signals, and then assesing the value of each of those signals using an algorithm type known as "fuzzy logic" to determine which type, or class, of hydrometeor most likely produced that set of values. The principle measurements used in classifying hydrometeors are reflectivity Z (the same "intensity" measurements made by all weather radars), differential reflectivity Zdr (the difference in reflectivity measure by the horizontal and vertical beam components - a group of spherical scatterers like small raindrops will have a nearly zero value, while large "flattened" drops will have a positive value because the horizontal radar cross section is greater than the vertical, many insects will have a very high positive value, and many ground clutter targets will have a small negative value due to a larger vertical radar cross section than horizontal), and the cross correlation coefficient RHOhv (a measure of how closely the returned vertically and horizontally polarized signals track with each other at a given time - for a uniform group of scatterers like small raindrops, this value is close to 1, whereas it drops to slightly lower values for mixed precipitation types and even lower for some other targets like ground clutter and insects).
Discriminating dry aggregated snow (i.e. snowflakes, which are composed of a number of snow crystals that are entangled or stuck to each other) from wet snow (usually melting flakes) is based in part on a tendency for wet snow to have a lower RHOhv than dry snow, along with a typically higher reflectivity Z due to the liquid water coating the melting flakes. On the other hand, individual dry snow crystals tend to have a much lower reflectivity Z and a fairly high differential reflectivity Zdr, so that they can be readily classified as a dry snow type. In addition, a measured or modeled freezing level is entered into the radar system as a consistency check on the selection of snow class, with the expectation that above that altitude dry snow should be the predominant hydrometeor type. Of course, once this selection is made, it remains important for the meteorolgist to consider the thermal structure of the lower atmosphere between the height of the radar beam and the ground, because the radar may correctly identify rain, wet snow or dry snow within the beam, but the type could change in the few hundred (near the radar) to several thousand (farther from the radar) feet that the precipitation falls before reaching the surface due to additional melting (with snow possibly becoming rain) while falling into warmer air below, and/or to falling into a subfreezing layer near the surface that could result in sleet or freezing rain.