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Author Topic: Dual Polarization Radar Guide  (Read 44802 times)

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Memphis Weather

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Dual Polarization Radar Guide
« on: December 15, 2011, 04:16:18 PM »
The following is a series of posts explaining the basics of dual polarization radar, the various products, and their potential applications. This will be far from exhaustive – I highly recommend you check out the NWS Training which was linked in the other thread if you want a more detailed (and technical) interpretation of dual pol data. I’ll be using many of the points and examples here directly from the training. Link: http://www.wdtb.noaa.gov/courses/dualpol/index.html

Another Important note: operational use of dual pol has so far been very limited. Most historical data is still based out of Oklahoma so what was useful there may not be applicable to the Southeast, Northeast US, etc. New and different techniques and applications will come out of dual pol as it is expanded across the country over the next 2 years. Any guidelines and applications are very ‘rough’ and should not be an expectation in every case.
So…what is dual-pol exactly?

Basically, dual polarization radar employs sending out two simultaneous “pulses” of energy (one horizontal, one vertical) which makes for a more complete assessment (3D in a sense) of any returns in the atmosphere that the radar detects. Previously, you just had the horizontal pulse which did a good enough job of detecting precip areas and their intensity, but you couldn’t say with much certainty if that precip was a rain drop, snow flake, hail stone, etc. Adding the vertical pulse will help fill in those gaps immensely and provide many operational applications.

Standard Doppler Radar - which sends out just a single horizontal pulse of energy

Dual Polarization Radar - which sends out both a horizontal and vertical pulse of energy
This allows for the radar to better identify specific characteristics of radar returns - shape, size, type, etc.

What are the potential advantages?

•   Improved detection of non-weather echoes (ground clutter, birds, insects, chaff)
•   Ability to determine echo type/character at radar beam-level (NOT at ground-level)
•   Much improved accuracy of precipitation estimates – better flooding assessment
•   Identification of the melting layer with much more certainty
•   Better severe weather signatures including hail and most importantly –tornado debris

Remember that dual polarization will change none of the limitations inherent with radar. As a radar beam moves further out from its origin, the height of the beam increases. On average, 50 miles out from the radar, the beam is already looking 4-5,000 feet. So, what the radar shows will not necessarily represent ground conditions at that location. Dual polarization will NOT revolutionize surface precipitation typing in the winter, but it will help. As always, observations and reports from those at the surface will be more valuable than most anything dual-pol can provide.

Another major limitation – as the beam moves farther out in range, it also broadens. A beam 100 miles out may be detecting returns in an area up to a mile across. A lot of different things can be happening in that mile-long area, but it will only show up as one “pixel” to you. When you add in dual polarization trying to characterize a bunch of “different things” into one pixel, it can and often will be noisy and unreliable. In general, these products will be VERY noisy and more difficult to interpret – the learning curve will be much steeper.

With these general things to keep in mind, we’ll now go over the major new products in more detail.

Memphis Weather

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Re: Dual Polarization Radar Guide
« Reply #1 on: December 15, 2011, 04:20:14 PM »
Correlation Coefficient (CC or RHO)

Correlation Coefficient is a product that measures how similarly the two radar pulses (horizontal and vertical) are behaving. What does that mean? Basically, it looks for how homogenous the returns are in each pulse. If returns “look” the same in each pulse, the CC is high. If they are vastly different, the CC is low. 

How does this apply meteorologically? If you have a very high CC, confidence is pretty high that whatever is occurring in that area, it’s probably uniform. In other words, if its rain, it’s probably all rain. If it’s snow, it’s probably all snow.  As CC decreases, that means there is likely a mixture of returns. It could be a mixture of melting snow, rain, ice or even hail. If CC gets very low, its means the returns are so diverse, it likely not even weather-related, it must be something biological.

What values should you look for?

CC can range from 0 to 1.05…the higher the number…the more uniformity. In terms of meteorological significance, the range of values you want is much narrower though.

For uniform precipitation, look for values of .95 or greater. This applies for all precipitation types.  As CC decreases to .80 to .95…there is likely some mixture going on. It could be melting snow, it could be rain-coated hail, or it could be “big drops” of rain. Big drops usually occur on the outer edge of convection and are low-rate rainfall. In hail, values closer to .80 may be more suggestive of very large hail (because it typically becomes more jagged in shape). In snow, values closer to .80 are usually more suggestive of very large, wet flakes. Dry snow will always be closer to 1.

If CC is less than .80, it’s very rarely something weather-related. It could be clutter, biological targets, chaff, or debris.

Applications (Applications will be covered in more detail later):

•   Differentiation of weather vs. non-weather returns
•   Melting layer identification
•   Precip transitions
•   Large hail detection
•   Tornadic debris detection

Memphis Weather

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Re: Dual Polarization Radar Guide
« Reply #2 on: December 15, 2011, 04:22:06 PM »
Differential Reflectivity (ZDR)

Differential Reflectivity measures the absolute difference between the horizontal and vertical reflectivity pulse, as opposed to CC which looked at their correlation value.

In general, if ZDR is near zero, it means the difference is small, so the return is more spherical in shape.  As ZDR becomes increasingly positive, the return is more dominantly horizontal. As ZDR becomes increasingly negative, the return is more dominantly vertical.

What values should you look for?

ZDR can range from -8 to 8, though values rarely get that extreme on either side. If in a rain situation, as ZDR increases, drop size increases. ZDR over 3 is indicative of big drops. In hail, though it is usually not spherical in shape, it actually appears that way to the radar, so ZDR values drop to near zero. However, if hail is small and/or water coated, ZDR will usually be very high upwards of 5 or 6. This hail will almost never be severe in size by the time it reaches the surface. Conversely, negative ZDR can be indicative of very large hail. For snow/ice, ZDR can vary significantly. In general, dry snow is closer to 0 ZDR, with wet snow increasing in amount. Non-weather returns can have a ZDR of literally almost anything.

Applications (Applications will be covered in more detail later):

•   Hail identification
•   Melting layer identification (not as good as CC)
•   Updraft detection
•   Tornadic debris detection (not quite as good as CC)
•   Precip transitions

Memphis Weather

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Re: Dual Polarization Radar Guide
« Reply #3 on: December 15, 2011, 04:24:13 PM »
Specific Differential Phase (KDP)

First, don’t confuse this product with PHI, or Standard Differential Phase, which may be viewable in some radar software (mainly GR2Analyst). Without going into detail, this is a useless product for meteorological interpretation and doesn’t need to be covered.

KDP is a very hard product to explain in basic terms, so I’m going straight to the values/applications section.

What values should you look for?

KDP can range from -2 to 10. KDP is most useful in rain/heavy rain events and will be the main reason precipitation estimations will vastly improve in dual-pol. As KDP increases, rain rate increases. KDP over 1 is very heavy rain though like in ZDR extreme values may be indicative of water-coated, small hail.  Pure hail and snow have near 0 KDP. KDP will not compute for non-weather returns as it will be far too noisy, but this is a drawback as it may mistakenly not compute for legitimate weather echoes and that can have an impact on precip estimations.

Applications (Applications will be covered in more detail later):

•   Heavy rain detection
•   Will better differentiate standard rain processes from others (like tropical rainfall)

Memphis Weather

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Re: Dual Polarization Radar Guide
« Reply #4 on: December 15, 2011, 04:28:57 PM »
Hydrometeor Classification Algorithm (HCA)

The HCA is a computer algorithm that looks at the various standard and dual polarization products, and based on “fuzzy” math techniques, determines the most likely classification type for each radar return. Again, this is a computer algorithm, so it will be subject to error (significant at times).

There are 11 classification types possible…

•   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)
•   Graupel (Dark Pink)
•   Hail/Hail Mixed with Rain (Red)
•   Unknown (Light Purple)

Though this will likely be product most will want to look at first, it should be used as a safety net only after looking at the other standard and dual-pol products to quickly determine and/or confirm areas of interest on a large-scale. It is an algorithm subject to error, gives no indication on uncertainty of the given classification (other types are ruled out based on simple probabilities) and, again, most important of all, this does NOT serve as a surface-based precipitation type product.

For example, it could be 90 degrees in the middle of summer in a strong thunderstorm situation, and if the radar is sampling an area above the freezing level, even in the middle of a severe thunderstorm, it will classify it as a winter precip type (or hail).

Memphis Weather

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Re: Dual Polarization Radar Guide
« Reply #5 on: December 15, 2011, 04:43:44 PM »

Like with the standard radar products, the best applications of dual-pol data will be when used in combination with all the various products (including the standard ones). With very few exceptions, you won’t determine something out of the data from looking solely at one specific product. Also with very few to no exceptions, dual-pol data won’t replace looking at the standard products. They will remain as essential as before.

1) Winter Weather

By far the most important application in winter weather will be identification of the melting layer. For years, standard Doppler has been able to detect the melting layer in some situations through “bright banding” or an area (usually circular) of enhanced reflectivity which is detecting the melting snow as it falls into the above freezing air. Dual polarization will greatly enhance the ability to detect this, especially through Correlation Coefficient. As explained, CC lowers to between .8 and .95 when returns are mixed. Of course, this is what is happening in the melting layer. So, CC is an excellent means to find the melting layer. An algorithm has also been developed which plots the CC-based melting layer on the map, and this will be available eventually in some radar software (most notably GRLevel3, it’s not available in Radarscope as of yet). Because the CC data is so good, this will actually be a fairly reliable algorithm though not 100% foolproof. The top of the melting layer (the point furthest out from the radar site) will be the wet bulb zero height and getting a 4-6 minute update of this information will be incredibly helpful in the future.

How can the melting layer be helpful in precipitation-typing? Obviously, if a melting layer is present, once precipitation falls below it, snow is no longer an option. It will either be rain, freezing rain, or sleet. Unfortunately, there’s no real way to differentiate between those three on dual-pol, so you’ll have to refer to environmental conditions.  In VERY rare cases, and only very near the radar site, you may see CC lower once again below the melting layer. If this is happening, this may be an indication of refreezing which is indicative of sleet. Again, this will be very rare – don’t expect to see this happening. Also keep in mind CC will lower near the radar site anyway sometimes due to clutter which makes it even harder to point out.

Remember for snow, CC is usually high and ZDR low, closer to zero. Snow near the melting layer will obviously be wetter, with lower CC and higher ZDR. In transitions from rain to snow, you will see ZDR lower quickly and CC increase, even in a wet snow transition (though the change may not be as extreme). Mixed rain/snow will keep an increased ZDR and decreased CC.  Graupel/Ice crystals will usually only be easily detected within or above the melting layer

Limitations: Again, this can’t be emphasized enough, winter precip typing by dual-pol will, at best, be only applicable at the radar beam height. While this will help tremendously, it won’t replace surface obs/reports. Additionally, with the melting layer detection, keep in mind there may sometimes be more than one melting layer. And, the melting layer may not always be perfectly circular and thus can “blend” in with the other data. This is especially true in transition situations. As always, assessing and knowing environmental conditions first will be most important.

Memphis Weather

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Re: Dual Polarization Radar Guide
« Reply #6 on: December 15, 2011, 04:51:17 PM »
2) Hail

Hail will be much better detected in dual-pol than with standard radar and should outright replace any need for VIL or other similar products. Standard base reflectivity will remain essential though.

As discussed above, ZDR in hail will decrease notably to closer to zero. When these values are coincident with reflectivity values above 55dbZ, hail is occurring. That is a guarantee, though again keep in mind that’s only at the level being sampled, not necessarily at the ground. If you have 70dbZ and no lowering in ZDR, you either have no hail or its very water-coated (more on this below).

CC will also lower, but the lowering will tend to be more local/minor in value than with ZDR, so it’s not quite as good of a detection. That said, CC may lower dramatically to less than .85 where there are very large and pure (not water-coated) hailstones occurring. KDP will generally be a non-factor in hail as values usually stay close to 0.

ZDR will run higher in water-coated hail. If you have very high ZDR in hail, as also earlier discussed, hail is already well into the process of melting and will very likely NOT be severe in size. KDP will be higher in water-coated hail as well.

The three-body scatter spike (TBSS) will remain an important reflectivity signature to look for (and another guarantee of hail) but it can be detected in dual-pol too. ZDR will be extreme (5+) with CC very low (.80 or lower). These may be more apparent with dual-pol in situations where rainfall may mask the signature on base reflectivity.

Finally, remember that thunderstorms with rotation will have a better chance of producing large and especially very large hail. Even if the storm is non-tornadic, check velocity products for circulation that would enhance that potential.

Limitations: Other than these very general guidelines, dual-pol will not be a strong indicator of exact hail sizes. Its main advantage is determining the presence of hail at all. This is also one application in particular where guidelines may be vastly different in different areas of the country. We’ll just have to see what happens as we see the coverage expand. And again, the presence of hail on radar does not mean that hail will be occurring at the surface, though the stronger the indications above, the better the chances it is.
« Last Edit: December 15, 2011, 05:03:40 PM by Kevin »

Memphis Weather

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Re: Dual Polarization Radar Guide
« Reply #7 on: December 15, 2011, 05:03:11 PM »
3) Tornadic Debris Signature (TDS)

This may be the biggest operational advantage in all of dual polarization. Dual-pol products greatly enhance the ability to detect debris lofted by tornadoes, and several cases in OK have shown detection in even weaker tornadoes (as low as EF1).

Differentiating between weather and non-weather related echoes is a definite selling point of dual-pol. The signatures between such are greatly different, and since debris is non-weather related, if those signatures begin showing up when surrounded by legitimate weather, and coincident with a strong velocity signature suggestive of a tornado, there’s no doubt whatsoever that debris is present, and thus, a tornado occurring.

What to look for? High reflectivity and the strong velocity signature are how the TDS has been identified previously, and that won’t change. But, reflectivity won’t need to be nearly as high especially in weaker tornadoes. If the dual-pol signature is there w/velocity, that’s all you need. CC will go into the non-weather range, less than .80. ZDR will also lower, generally to values near or just below 0. If you have those signatures but there is not a velocity couplet, it’s NOT debris. Simple as that.

A properly identified TDS is 100% confirmation of a damaging tornado. NWS is already suggesting that WFOs relay TDS information in warning products, and they are also advising that they should be LSR’d and then surveyed because there will be no question about it.

The TDS was seen in several of the tornadoes associated with Hurricane Irene via the Morehead City, NC that was beta testing. The TDS was noted in weaker cases and in situations where reflectivity was not really enhanced at all. The fact it picked them up in low-topped, shallow, tropical supercells is very encouraging overall.

Limitations: The TDS obviously has no impact on lead-time. Once you see it, the tornado is there. These will NOT improve initial tornado warnings, but can greatly help in communicating a serious and confirmed threat to the public. TDS also has no direct correlation to tornado strength, as mentioned; it’s been detected in weak tornado cases.  It will be important not to assume too much now from debris detection now that they will be more easily seen. All that said, it will NOT be detected in every tornado. Obviously, something has to be there, and weak tornadoes will be hit and miss for sure. Finally, ability for detection will significantly decrease as you go further out from the radar site.

Memphis Weather

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Re: Dual Polarization Radar Guide
« Reply #8 on: December 15, 2011, 05:06:44 PM »
4) Heavy Rain

Heavy rain is another big advantage with dual-pol. KDP is vastly better at detecting areas of heavy rainfall and their resulting rate and accumulation. Remember that hail is a non-factor in KDP in most situations. This is a big deal, because it means hail contamination will no longer be the problem it used to be too. All together, radar-based rainfall amounts will be much, much improved with dual pol.

The better estimations also result from dual-pol better understanding what types of rain processes are occurring. In case you didn’t know, tropical rainfall is a much different situation in terms or rain rates/amounts than standard (continental) rainfall. The NWS tries to account for this by changing up the “Z-R relationship” of the radar for each event to make estimations more accurate, but it doesn’t always work well. Dual-pol will do this much better too (and automatically), and again that makes its estimations much better.

In general, heavy rain occurs with KDP greater than 1. As it further increases, rate increases too. Extreme KDP (greater than 5) can be an indication though of small, water-coated hail as earlier noted.

Some KDP to rain-rate (per hour) conversions:

•   0.2 -- .5”/hr
•   0.5 -- 1”/hr
•   1.2 -- 2”/hr
•   2.0 -- 3”/hr
•   2.8 -- 4”/hr
•   3.6 -- 5”/hr
•   4.5 -- 6”/hr

With the new rain estimations, there will be entirely new rainfall products. A one-hour and storm total are the most prevalent, but also for the first time there will be a true instantaneous rain rate product. So far, these aren’t available with any software yet, but GRLevel3 is planning support. There will also be “difference” products between the standard and dual-pol versions for comparison reasons. There will still be some use to using the old rainfall products.

Limitations: The big drawback as mentioned in the KDP post is that when the radar detects non-weather related echoes, it does not compute KDP at all. This will create “blank” areas on the product. The problem: sometimes KDP will not compute for legitimate precipitation. It’s not a frequent occurrence, but if this happens, the dual-pol rainfall products will obviously suffer. This is the main reason for keeping the standard products available. NWS is hoping to rectify this in future algorithms/radar software releases.

Offline Thundersnow

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Dual Polarization Radar Guide
« Reply #9 on: December 15, 2011, 10:08:04 PM »
Nice work, Kevin!  I need to take some time to read through this more thoroughly.

But, some of the details sound like a real breakthrough in weather radar... probably the most significant since the advent of Doppler.
"To say you have no choice... is a failure of imagination."
                                                           - Jean Luc Picard

Offline WillT

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Re: Dual Polarization Radar Guide
« Reply #10 on: March 21, 2012, 07:34:19 PM »
Hey Kevin, do you have any objections to me printing this series of posts for my own future reference?

Memphis Weather

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Re: Dual Polarization Radar Guide
« Reply #11 on: March 21, 2012, 07:48:57 PM »
Hey Kevin, do you have any objections to me printing this series of posts for my own future reference?

Not at all! Feel free! Glad to know its of use to you!

Memphis Weather

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Re: Dual Polarization Radar Guide
« Reply #12 on: May 29, 2012, 02:10:35 PM »
Just a bump since there was a question about interpretation...here's a realtime example of hail in dual-pol in NY State...

On the left, 0.5* Base Reflectivity (BR); the right, 0.5* Differential Reflectivity (ZDR). The area of interest is circled, being sampled at an elevation of about 5500ft. Reflectivity values are at a maximum of 60dbZ or greater, and ZDR is at a minimum of 0.5 to 1.5db. When co-located, that is a guarantee of hail in that part of the storm. We are also below the melting layer (thin gray line) so we're sure there's no winter-type precipitation possible other than hail.

Ideally, you'd want ZDR closer to zero, but one thing to take note of is the region of the country this is in - the Northeast. As noted in the original posts, as dual-polarization is expanded nationwide, we may learn that exact dp values may vary from place to place, climate to climate on when to expect hail or other specific weather phenomena. We *may* be seeing a situation where ZDR might trend higher for hail in that portion of the country. Far from certain, research on multiple events/storms would have to be done over time but you can be sure that's happening as we speak.

Of course, the other thing to note, at 5500ft we're guaranteed hail, but there's still ample room for some melting. I should note Specific Differential Phase (KDP) was quite high at around 3.75, and that indicates the hail was already becoming water coated. The water coated hail may also be contributing to the higher-than-ideal ZDR. The hail would at least be a smaller size when reaching the ground, and might have melted completely. If KDP was hovering much closer to zero, chances are much higher that any large hail could have made it.

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Re: Dual Polarization Radar Guide
« Reply #13 on: October 12, 2012, 04:01:30 PM »
Hi Kevin, I living in Turkey and ask some questions about dual pole radar.
1.We have 4 dual pole radar but 3 of them built on the mountains. Their heights approximately 1000 m. So we see the precipitiations on high levels. In my opinion we dont need dual pole radars becouse of their height but our met office want to take more. I wonder what is your oponion about this subject?

2.Why dual pole radars need zero degree height. Why dont they classify precipitation without 0 degre height. Everybody who know 0 degree height can classify precipitation with single radars.

3.Some of the case we can see melting hail high Dbz and high Zdr. High zdr also show heavy precipitation. How can we understand  it is shower or hail?

Memphis Weather

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Re: Dual Polarization Radar Guide
« Reply #14 on: October 12, 2012, 05:34:50 PM »
Hi Kevin, I living in Turkey and ask some questions about dual pole radar.
1.We have 4 dual pole radar but 3 of them built on the mountains. Their heights approximately 1000 m. So we see the precipitiations on high levels. In my opinion we dont need dual pole radars becouse of their height but our met office want to take more. I wonder what is your oponion about this subject?

2.Why dual pole radars need zero degree height. Why dont they classify precipitation without 0 degre height. Everybody who know 0 degree height can classify precipitation with single radars.

3.Some of the case we can see melting hail high Dbz and high Zdr. High zdr also show heavy precipitation. How can we understand  it is shower or hail?

Hey there! Welcome to the forum, first of all. Nice to have somebody from Turkey! Appreciate the questions.

1) The high-level radars probably don't have the usefulness of dp as they would at a lower altitude but having dp never hurts either. It seems they want as much coverage as possible, and in a terrain environment like Turkey, you need more radars because of the amount of beam blocking taking place. If they're wanting to build more radars in a lower altitude, that's probably a good idea. I would think higher altitude coverage is sufficient unless there's a significant population area still not being served well.

2) If you're referring to the melting layer information you get with dp, that can be a big help in determining precipitation type. You do have models and soundings that will always assist but a dp radar brings the added advantage of having a real-time update of that information at the point of the radar site every few minutes, which models and soundings can NOT give you.

In a specific case, if you're in a situation where mixed or transitioning types are expected, you'll be getting much better data with dual-pol than without it. Obviously, if it's an all rain or all snow situation, then yes there's not much need. I'm not sure how often this may apply to your area, but it is definitely beneficial in many parts of the US. The melting layer can also be somewhat helpful in a hail case, as that image just above your post indicates.

3) You can get hail in a high dbZ/high ZDR situation, but it's much less common, at least what I've seen in the US. If that's happening though, its probably going to be pretty small and pretty melted. In that situation unfortunately it's going to be much more difficult to diagnose, but again probabilities would point to just heavy rain in a good majority of cases.

I hope that helps you a bit! Thanks for the post!


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