Dual Polarization Radar Guide

[Memphis Weather]

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

[Memphis Weather]

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

[Memphis Weather]

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):

[Memphis Weather]

Specific Differential Phase (KDP)

First, don

[Memphis Weather]

Hydrometeor Classification Algorithm (HCA)

The HCA is a computer algorithm that looks at the various standard and dual polarization products, and based on

[Memphis Weather]

Applications

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

[Memphis Weather]

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

[Memphis Weather]

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

[Memphis Weather]

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

[Thundersnow]

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.

[WillT]

Hey Kevin, do you have any objections to me printing this series of posts for my own future reference?

[Memphis Weather]

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]

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.

[storm_chaser]

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]

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!