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Dual Polarization Schedule: KTYX (July 9th thru 22 July 2012) and KCXX (July 23rd thru 5 August 2012)
On 9 July 2012 the upgrade to dual polarization (Dual Pol) radar technology will begin at KTYX radar in Montague, New York on the east end of Lake Ontario. The installation is expected to be completed by July 22nd. The work will shift to KCXX radar in Colchester, Vermont on 23 July 2012, with an expected completion date of August 5th. During the two week upgrade process each radar will be off-line until work is completed. During these periods, limited radar coverage will be provided by the surrounding radars from Albany, Binghamton, and Buffalo, New York as well as from Gray, Maine and Taunton, Massachusetts. All but the Gray, Maine site are already upgraded to Dual Pol.


Potential Benefits of Dual Pol
The Dual Pol technology will add an additional 14 products to the suite of radar data available to National Weather Service meteorologists, for further improvement to our watch, warning, and advisory decision support services across the North Country. This new radar technology will benefit forecasters in the following ways:
  • Help confirm that a tornado has touched down and is causing damage.
  • Help categorize the type of precipitation that is falling.
  • Improve estimates of total precipitation amount.
  • Better estimate of the size distribution of hydrometeors.
  • Upgrade the ability to identify areas of heavy rainfall rates for flash flood potential.
  • Better detection and mitigation of non-weather echoes.
  • Easier identification of the melting layer for enhanced forecasting of snow levels and detection of icing for the aviation community.
  • New hail signatures for severe thunderstorms.


How does Dual Polarization Work?
Current NWS Doppler radars transmit and receive pulses of radio waves in a horizontal orientation (see Figure 1. below). As a result, the radar only measures the horizontal dimensions of targets (e.g. cloud and precipitation droplets). Dual-polarimetric radar transmits and receives pulses in both a horizontal and vertical orientation (see Figure 2 below). Therefore, the radar measures both the horizontal and vertical dimensions of targets. Since the radar receives energy from both the horizontal and vertical pulses, we can obtain better estimates of the size, shape, and variety of targets. It is expected that this will result in significant improvements in the estimation of precipitation rates, the ability to discriminate between precipitation types (e.g. hail vs. rain or rain vs. wet snow), and the identification of non-meteorological returns, such as chaff, ground clutter, migrating birds/insects and smoke plumes from wildfires that are not uncommonly detected by weather radar systems such as WSR-88D.
Current KXX Doppler Radar Dual Polarization Radar

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New Products Associated with Dual Polarization Radar:
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Differential Reflectivity:

Is a good indicator of drop shape and a good estimate of average drop size. The differential reflectivity is a ratio of the reflected horizontal and vertical power returns. For example, see Figure 3 to the right.
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Correlation Coefficient:

Is a good indicator of regions where there is a mixture of precipitation types, such as rain and snow a statistical correlation between the reflected horizontal and vertical power returns. For an example, see Figure 4 to the right.
Click to enlarge
Specific Differential Phase:

is a very good estimator of rain rate. The specific differential phase is a comparison of the returned phase difference between the horizontal and vertical pulses. This phase difference is caused by the difference in the number of wave cycles (or wavelengths) along the propagation path for horizontal and vertically polarized waves. It should not be confused with the Doppler frequency shift, which is caused by the motion of the cloud and precipitation particles. Unlike the differential reflectivity, correlation coefficient and linear depolarization ratio, which are all dependent on reflected power, the specific differential phase is a "propagation effect." For an example, see Figure 5 to the right.
Click to enlarge
Melting Layer:

Better identification of the melting layer will help determine snow levels in the wintertime, lead to less hail contamination in the precipitation estimates during heavy rain events associated with summertime thunderstorms, and help in the hydrometeor classification of precipitation returns. Figure 6 shows the melting layer at 0.5° elevation slice overlaid on the hydrometeor classification algorithm. The first dotted line closes to the KPBZ radar is where the top of the beam enters the bottom of the melting layer, while the two solid lines indicate the middle of the beam intersection of the melting layer, and the final dotted line is where the bottom of the radar beam exits the top of the melting layer at the 0.5° elevation slice. The melting layer algorithm uses the characteristics of the correlation coefficient, differential reflectivity, and reflectivity products to determine the height of the 0° Celsius line. This algorithm uses differential reflectivity values between 0.8 to 2.2 dB, correlation coefficient values greater than 0.85, and reflectivity between 30 and 47 dBZ. If no data is available, then the algorithm uses the most recent RUC 0° Celsius height and 500 meters below for bottom of the beam intersection.
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Hydrometeor Classification Algorithm (HCA):

The HCA can tell the difference between ten types of radar echoes using different radar variables: These ten different types of radar echoes include: ground clutter / anomalous propagation (AP occurs when the radar beam is bent downward towards the earth due to inversions or a rapid change in dewpoint) and produces false echoes, biological scatters (insects and birds),dry snow, wet snow, crystals (horizontally or vertically oriented), graupel (soft hail), big rain drops, light and moderate rain, heavy rain, and rain / hail mixture. The key to the HCA is detecting the melting layer using polarimetric measurements. Once the melting layer is determined, the algorithm utilizes five radar variables in a fuzzy logic classification scheme to differentiate between the echoes. See Figure 7 for an example of the hydrometeor classification algorithm (HCA).
Click to enlarge
Quantitative Precipitation Estimates:

The dual pol radar technology will have several new quantitative precipitation estimate products, to help in better detection of areas of heavy rainfall. Figure 8 below shows a four panel of quantitative precipitation estimates: Legacy One Hour Precipitation (OHP) upper left, Legacy Storm Total (STP) upper right, Dual Pol 1hr Accumulation (OHA) lower left, and Dual Pol Storm Total Accumulation (STA) lower right. From the image below you can clearly see the better detection of individual rain bands and the associated heavier quantitative precipitation amounts.



Conclusion
There will be many benefits to the new Dual Pol radar technology, which will improve the products and services provided by the National Weather Service and other meteorologists. This radar technology will be able to better identify when a tornado has touched down and is causing damage. It will also help estimate the size distribution of hydrometeors, such as raindrops, snowflakes, hailstones, and drizzle, which will result in better estimates of precipitation amounts. This improvement will also help in the detection of extremely heavy rainfall rates and amounts, leading to better warning services of flash flooding events. Also, with enhanced detection of the melting layer and better classification of hydrometeors, forecasters will be able to recognize hail in summer thunderstorms and snow levels in mountainous terrain, adding value to our winter weather products. Overall, this new radar technology will have many benefits to meteorologists, especially as we develop our local expertise.



Additional Information and Reference Material:
Figure 1. Current KCXX Doppler Radar transmission of horizontal pulse only.
Figure 2. Dual Polarization Radar with transmission of horizontal and vertical pulses.
Figure 3. Differential Reflectivity in dB.
Figure 4. Correlation Coefficient (CC).
Figure 5. Specific Differential Phase (deg/km).
Figure 6. Melting Layer (ML) at 0.5° and detection of the melting layer at the bottom, middle and top of the radar beam.
Figure 7. Hydrometeor Classification Algorithm (HCA).
Figure 8: Legacy One Hour Precipitation (OHP) upper left, Legacy Storm Total (STP) upper right, Dual Pol 1hr Accumulation (OHA) lower left, and Dual Pol Storm Total Accumulation (STA) lower right.


National Weather Service
Burlington
1200 Airport Drive
S. Burlington VT 05403
(802)862-2475

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