64 NC counties and 1 VA county are under alert, including Wake, Cumberland, Durham, Johnston, and Orange counties. Details
Published: 2007-09-19 10:23:33
Updated: 2007-09-19 10:23:33
Posted September 19, 2007
MIKE MOSS SAYS: Anggie, Both stations have replaced older radars with new models that have more powerful transmitters, in each case capable of producing one million watts of peak power. Both radars operate at the same wavelength (5 centimeters, also known as C-band) and have similar sized radar dishes that produce a narrowly-focused, highly detailed radar beam.
The principle difference with the new DUALDoppler5000 is the addition of Polarimetric capability, also known as Dual-Polarization. This means we have the ability to simultaneously transmit radar pulses that are horizontally polarized (the traditional method of transmitting weather radar pulses) and vertically polarized. This property allows the radar and its analysis software suite to measure a number of new parameters that can not be measured using a single polarization system, such as the difference in radar reflectivity between the vertical and horizontal signals, the difference in phase change along the path of the beam in the two signals, the ratio of depolarization of each signal, and the cross-correlation coefficient of the two different polarizations. These are not the kinds of signals that you would show on the air directly, but they can be combined in sophisticated algortithms based on years of research carried out by the National Severe Storms Laboratory and several universities, in order to assign a most probable hydrometeor class to any given portion of a precipitation echo. This can give us a very high confidence, for example, as to whether a high reflectivity area in a thunderstorm is composed of heavy rain or hail, and can also give us a reasonably good estimate as to whether the precipitation particles within the beam are principally rain, wet snow, dry snow, snow pellets (also called graupel) or whether the echoes are most likely non-meteorological targets such as birds, bugs, ground returns or in extreme cases, tornado debris. One still has to use some meteorological reasoning and additional data to infer whether the precipitation types observed at the altitude of the radar beam will remain the same as the precipitation falls the rest of the way to the ground (for example, snow in the beam could melt and become rain at surface, rain in the beam could freeze and become sleet at the surface, etc), especially at large ranges from the radar site. However, the dual polarization gives us an added data point that we would not otherwise have. This feature is already implemented in our new system, and is the only radar of the kind active in North Carolina.
In addition, recent research has developed techniques using Polarimatric capabilities that result in notably more accurate radar estimates of accumulated rainfall than are available through single polarization alone. This capability has not been implemented in our system, but we anticipate rolling it out in the not-to-distant future.
Likewise, one drawback of any radar operating at C-band wavelength is that no matter how much power you add, the signal is subject to strong blockage and attentuation by areas of heavy rain. This can lead to underestimates of rainfall intensity in other cells that are downrange of the intense cell that blocks the beam. However, the measurement of differential phase shifts in dual-polarization mode provides information that can be used to correct those attenuated intensities and display them as if the beam were not being blocked. Again, this is a cutting-edge research area and we do not have this feature implemented yet, but anticipate that it should become available wihtin just a few months.