The National Weather Service radar in New Braunfels, TX (which serves the Austin and San Antonio Metro areas) was just put back online about an hour ago after the "dual polarization upgrade" was complete. It will be interesting to see how the radar performs during the severe weather event that is expected to take place this afternoon.
With this and many other sites coming online recently, I thought it would be appropriate to put the following post regarding Dual Polarization Radar back up to the top of the blog. The original post from July 9, 2011 follows below:
The first major upgrade to the National Weather Service's WSR-88D radar network in over 20 years is getting underway across the country. The latest technology being rolled out is called "Dual Polarization".
"What the heck is that???" and perhaps more importantly, "what's in it for me???" - you ask?
In a nutshell, the conventional WSR-88D radar sends out a single pulse of radar energy into a storm, which captures a generally horizontal snapshot of raindrops, hailstones, snow flakes, etc. (as shown on the left half of the illustration below):
The right half of the same illustration shows what Dual Polarization technology will do. The radar will not only send out a horizontal pulse of energy (red) but also a vertical pulse (blue), which will give us a far greater depiction of the size, shape and density of raindrops, hailstones, snow flakes and other targets.
Dual Polarization (also known as Dual-Pol for short) radar technology has been installed and tested for a number of years at the NSSL in Norman, OK. Within the last few months the rollout to operational NWS radar sites has begun. The first site to be installed was at Vance AFB, OK, followed by Phoenix, AZ and Morehead City, NC. As this is being written, Dual-Pol is currently being installed at radar sites near Pittsburgh, PA and Wichita, KS. A complete schedule of the rollout, which will continue into early 2013, is located here. Each installation takes about 2 weeks to complete, during which time the radar unit will be out of service to allow for both the upgrade and training at the local NWS office.
Now that we know what Dual-Pol is, what will it do for us? Perhaps the greatest initial impact will come in the form of vastly improved precipitation estimates. With the current WSR-88D technology, a thunderstorm must be in existence for about 1 hour before the radar is able to estimate how much rain is being produced. With Dual-Pol technology, we'll receive a nearly instantaneous read on rainfall rates. This will obviously have enormous potential not only in regard to flash flooding & other severe rainfall situations, but also in dealing with snowfall, ice accumulations, etc.
Dual-Pol rainfall estimate (left) vs. WSR-88D estimate (right)
Current technology on the WSR-88D sometimes makes it difficult for the radar to differentiate between hailstones and raindrops, particularly at greater distances from the radar site. As a result, the 88D often over estimates the rainfall rate with severe storms (mis-identifying hail as heavy rain). The Dual-Pol technology will make the estimates more accurate in these situations (see the above image for just one example).
Dual-Pol technology will also aid in the tornado warning process. Right now, it takes a fairly large debris field (which usually means a fairly large tornado) to produce a debris signature on the WSR-88D. A Tornadic Debris Signature (or TDS) can be detected by Dual-Pol radar in association with a much smaller debris field (which means a smaller tornado). Please note that I am talking about debris being detected by the radar here, not rotation. In the database that was used to develop some of the algorithms for the new Dual-Pol radar system, debris signatures were observed with tornadoes as weak as EF2 intensity (winds of 111-135 mph). Today we would normally only see a debris signature on the WSR-88D in association with a tornado of high-end EF3 to EF4 intensity or greater (roughly 150+ mph surface wind speeds).
The above image was taken from the Dual-Pol radar at the NSSL radar testbed in Norman, OK. The white and pink colorations within the white circled area on the image shows where the Dual-Pol is detecting tornadic debris. This debris field was not as easily identified on conventional radar. Of course, you must also keep in mind that the tornado, regardless of strength, would have to be in an area where debris could be picked up before the radar would be able to see it.
Dual-Pol technology is likely to be of particular help in the tornado warning process in 2 specific areas: (1). when the circulation is rain wrapped and not easily visible by storm spotters and (2). when the circulation is occurring at night and not easily visible to storm spotters.
The system will also be a big help with non-precipitation events as well. Because it takes a more detailed "slice" through the atmosphere, Dual-Pol technology will be able to forewarn of local hazards near the radar site, like dust storms for example. Below is an image taken by the Dual-Pol radar near Phoenix, Arizona this past Tuesday, July 5th:
The red dashed line notes the leading edge of a wall of dust that was advancing toward the city from the South. Here is what that advancing dust cloud looked like outside the National Weather Service office:
The enhanced scanning of the atmosphere very near the earth's surface by the Dual-Pol radar will allow it to detect very small particles, like the dust particles above, as well as insects, birds, etc., close to the radar site. The WSR-88D is already able to do this to some extent, but the resolution and detail of such scans will be even greater with the new Dual-Pol technology.
Now that we have a few Dual-Pol radars online and others being added about every 2 weeks, I'll be posting some images of the new technology in action, and comparing it to the old where possible. Stay tuned!
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