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GEORGE ELLIOTT: Advanced Doppler Radar
Submitted by George Elliott on Sat, 09/07/2013 - 6:45am.
It’s been a number of years since Doppler radar has been in use across the country. Hard to believe, seeing in how it is routine now to see Doppler radar images on television, but at one time, this technology, as all new technology, was cutting-edge, and used exclusively by The National Weather Service, FAA, and other government, university, and research facilities.
Well, the original Doppler radar, now routinely used is beginning to be displaced by the latest in Doppler radar technology, developed by NOAA researchers. The new systems are called “dual-polarization” Doppler radars, or dual-pol, technology, which will provide forecasters at each NWS office around the cuntry with new, more-detailed images of atmospheric conditions and of course, storms.
The dual-pol upgrade is the most significant enhancement made to the nation’s radar network since Doppler radar was first installed in the early 1990s. These radars, which are now fully installed across the country in NWS offices, provide better information about the type of precipitation in the atmosphere and its intensity, size and location.
The dual-pol upgrade includes new software and a hardware attachment to the radar dish that sends and receives both horizontal and vertical pulses of energy, providing a much more informative two-dimensional picture. Conventional Doppler radars only send out a horizontal pulse of energy that gives forecasters a one-dimensional picture of whatever is in the air, precipitation or non-precipitation. Dual-pol radar helps forecasters clearly identify rain, hail, snow or ice pellets, and other flying objects.
Another important benefit: dual-pol more clearly detects airborne tornado debris — allowing forecasters to confirm a tornado is on the ground and causing damage so they can more confidently warn communities in its path. This is especially helpful in the dark when ground spotters are unable to see the tornado.
In an example of how effective the new systems are, in August 2011, the NWS Doppler radar in the Newport/Moorehead City NWS office, the dual-pol radar provided forecasters with unprecedented views of Hurricane Irene as it moved over the Outer Banks. It was the first time a hurricane had been scanned by a dual-pol radar. The dual-pol precipitation estimates for Irene in eastern North Carolina performed better than the existing precipitation estimations, which are based on radar reflectivity alone. As a matter of fact, a post storm analysis indicated that in some areas the rainfall estimates were as much as several inches more accurate than the older radar estimates.
An extremely heavy rainfall over southeastern Virginia and northeastern North Carolina on August 25, 2012 developed very quickly and wasn’t forecast well in advance. However, once the storm system developed, the new radar system quickly latched onto the potential significance and accurately was able to estimate extreme rainfall from the storm, leading to superior flood warnings as compared to the original Doppler radar rainfall estimates.
Obviously, dual-polarization radar has the ability to far better determine the type and intensity of precipitation than its conventional Doppler radar counterpart, which leads to better estimates of rainfall amounts, which in turn can lead to more accurate flood and flash flood warnings.
In addition to superior rainfall estimates, the dual-pol radars can very accurately delineate types of precipitation within a storm system. In many cases, there is a fine line between where heavy rain, ice, and snow fall out of winter storms. It can literally be snowing heavily in one area of town, while rain or freezing rain and sleet is falling elsewhere. Well, the new radar systems are much more precise in identifying precipitation types. As a matter of fact, in November of 2012, the Boston NWS was able to determine the transition zone from heavy coastal rain, to sleet, rain and snow, to all heavy snow, across southern New England.
On January 21, 2012, a winter storm brought a mixture of snow, sleet, and freezing rain over the Northeast. Portions of Connecticut received over 10 inches of snow. Across Long Island, much lower snowfall totals were reported, with 2 to 3 inches falling across western Long Island and 4 to 6 inches reported across eastern sections of the island. The lower totals in western Long Island were largely a result of the snow transitioning to sleet and freezing rain earlier in the day. Without the new radar systems to more finely define these precipitation patterns as the storm evolved, the total impact of the storm would not have been as accurately predicted.
The technology has come a long way, and one can see that in everything from snowstorm forecasting, thunderstorm and tornado development, rainfall monitoring, whether in hurricane situations or non-tropical systems, and even forecasting the movement of dust that create major blinding dust storms at times over the west.
By: George Elliott