Monday, April 9, 2012

Dual Polarization radar and why you should care.

Hi it's Rebecca again, I thought I would take the time to talk about a major weather radar upgrade. This upgrade is being installed in all National Weather Service (NWS) radars across the nation. Starting next week it's being installed at the Albany radar site. The new radar is known as Dual-Polarization technology (Dual-Pol). This will significantly enhance the conventional WSR-88D doppler radars currently in use. This post will cover the basics of what Dual-Pol is, some of its advantages over current WSR-88D doppler radar, and some of its limitations.

Here's a link to a blog post I did, that talks about conventional doppler radar.

Normal Doppler radar sends out a horizontal pulse of energy which is then returned to the radar antenna. The data is sent to a computer which analyses it; precipitation is determined by measuring the strength of the echoes received by the radar antenna. The computer then displays this information. Unlike traditional doppler radar, Dual-Pol is polarized. This means it sends out two pulses at the same time, one horizontal and the other vertical. In essence conventional dopper sees in one dimension whereas Dual-Pol sees in two dimensions. Which gives a much better idea of the shape and how large something is, when the pulse is returned.  Before the upgrade, The radar could see areas of and the intensity of precipitation in a storm very well, but it had extremely hard time telling if it was seeing a rain drop, snow flake, hail stone, bird, exc. Adding the vertical pulse will greatly help fill in the missing pieces and provide other applications that I will go into later.

What advantages does it have?

Dual-Pol will have several advantages over conventional doppler radar.  Here are the major improvements. These will be covered in a little more detail later. However, I would be happy to answer any questions you have on these applications, either on this blog post or via e-mail. Just click on my name (highlighted web link) at the bottom of this page. This will take you to my profile / about me page.
1)  Improved accuracy of precipitation estimates, leading to better flash flood detection.
2)  Ability to distinguish between heavy rain, hail, snow, and sleet.
3)  Differentiation of weather vs. non-weather returns.
4)  Identification of the melting layer.
5)  New severe storm signatures (hail detection, updraft detection, tornadic debris detection )

One thing I would like to say about these applications. Dual-Pol radar products will be used alongside standard radar products.  Most of the time, you won’t be able to determine something on one  data product alone.  

Improved the accuracy of precipitation estimates.
Conventional radar has to rely on how reflective the return is when it assigns rainfall rates.(reflectivity is the amount of transmitted power returned to the radar receiver)  This can be a big problem if there is hail present in the storm cell. Hail is a good reflector of energy and will return very high dBZ values. Because of this hail can cause the accumulated rainfall estimates to be higher than what is actually occurring.  Dual-Pol radar can see the hail; so it does a much better job of accounting for areas of rain that have hail mixed in. Therefore, it can give a much better assessment on how much rain has fallen over a given area. This is crucial since flooding is the #1 weather killer. Better rainfall estimation leads to more reliable flood warnings and helps people get out of harm’s way.

Ability to distinguish between heavy rain, hail, snow, and sleet.
With conventional doppler radar  you can see the presence and movement of precipitation. However, it's almost impossible to tell the difference between various types of winter precipitation. Like I said above, Dual-Pol radar transmits and receives pulses in both a horizontal and vertical orientation.  Consequently it can measure the width and height of objects in its path. the ability to detect the presence and movement of winter precipitation, but it is very difficult to infer whether it is rain, snow, or a mixture of both. Because Dual-Pol radar can see individual shapes it solves this problem.  As a result, precipitation types at all levels of the lower atmosphere can be seen (with-in the radar beam). This will allow forecasters to see which atmospheric zones have rain, snow, or mixed precipitation and how fast these zones are changing.  Therefore, based on environmental conditions from the radar beam to the surface, forecasters have a great deal of  certainty as to where rain, snow, or mixed precipitation is falling. Standard doppler radar has detected the melting layer in some situations, through bright banding (a circular or arcing band of higher reflectivity around the radar site). This area of bright banding is where the snow is melting. Dual-Pol radar is very good at identifying where this melting layer is and thus indicate where  snow is actually falling through a warmer layer.

                                                                 An example of bright banding

The ability to see shapes doesn't only help during wintertime. Hail can be a big issue in severe thunderstorms. Because Dual-Pol radar can see horizontally and vertically; it won't only be able to tell where hail is falling, but it will give a better idea where there might be large hail. A  pulse thunderstorm can form within a few minutes, develop It's core of hail and wind, and dissipate shortly thereafter. Weather forecasters need to be able to quickly see into these cells, in order to see which cells contain hail (sometimes large hail) and damaging winds.  So they can issue warning before any of that hail or damaging wind reaches the ground.

The radar upgrade will also be better at discriminating between precipitation types (snow, sleet, hail, and rain), and  non-meteorological types (Ground clutter, chaff, birds, smoke plumes from wildfires)
Tornadic Debris Detection
This is where the Dual-Pol upgrade will shine. Tornadoes often have debris whirling around. In fact, lighter debris such as insulation, leaves, and so forth  can be pulled to heights  of over 10,000 feet. Dual-Pol greatly increases our ability to see this debris. So when there's a strong velocity couplet showing a tornado vortex signature along with debris detection, there will be little doubt that a damaging tornado is on the ground. This will be particularly valuable at night or when the tornado is rain wrapped; when it's extreamly difficult for spotters and chasers to visibly see tornadoes. 

Dual-Pol WILL NOT improve initial tornado warnings or increase warning lead times. Also while it will significantly improve our ability to see and track large destructive tornado; it won't do nearly as good a job on smaller weaker tornadoes. While it has detected tornadoes in the EF1 won't catch as many of them as it does EF3's thru EF5's. Also the farther away the tornado is from the radar site the harder it will be to identify.  

There are 11 classification types possible on the radar return

 Biological (Light Gray)
 Clutter (Dark Gray)
 Ice Crystals (Pink)
 Wet Snow (Dark Blue)
 Dry Snow (Light Blue)
 Rain (Light Green)
 Heavy Rain (Dark Green)
 Big Drops (Gold/yellow)
 Graupel (Dark Pink)
 Hail/Hail Mixed with Rain (Red)
 Unknown (Light Purple)

Limitations of the Dual-Pol upgrade:

As I said, while Dual- Pol radar provides specific information about the location of a tornado. At least for now, it doesn't provide added information about where a tornado will form ahead of time.
During the wintertime Dual-Pol's  ability to see precipitation type will be limited to the beam height.  Also while it's a good tool at finding the melting layer. There can often be more than one melting layer; in which case melting layer data can blend in with other radar data. Therefore forecasters will still have to rely on surface readings, reports, and sample environmental conditions.

It also has the same limitations as standard radar, as the beam moves farther away from the radar site, it also broadens.  This means at long range, the radar is sampling all the things in a much larger area. However, it will only show up as a pixel or two. which can lead to a noisy image.

You will remember, I said Dual-Pol radar is good at detecting non weather related echoes, this can lead to blank areas. Sometimes the radar algorithm won't calculate the legitimate precipitation because of it.  while it doesn't happen all the time it does happen.

Dual-Pol radar has a very steep learning curve.  

A few links for more info

A lesson by the NWS

Video that briefly goes into Dual-Pol

I guess that's about it. You now have a working knowledge of Dual-Pol radar. While it won't replace conventional WSR-88D doppler radar, it does give the forecaster a vast array of additional tools that will greatly increase forecast accuracy in many areas, particularly in the area of storm tracking.  As always, I hope you enjoyed this post and learned a thing or two.


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