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KRAX WSR-88D Dual Polarization Upgrade

View of the KRAX Raleigh Radar - click to enlarge

The NWS Raleigh WSR-88D Doppler radar, KRAX, located in Clayton, NC was upgraded to dual polarization technology on Saturday, 10 November 2012. This is a very exciting development which will result in improved products and services provided by the National Weather Service and other meteorologists. More details on the dual polarization upgrade are shown below.

Potential Benefits of Dual Pol

  • 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.
  • Help confirm that a tornado has touched down near the radar and is causing damage.

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.
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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.
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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).
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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.


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 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 identify the location of the various winter precipitation types including the rain/snow line, adding value to our winter weather products. Finally, the upgrade can allow us to better identify when a tornado has touched down near the radar and is causing damage. Overall, this new radar technology will have many benefits to meteorologists, especially as we develop our local expertise.

Additional Information and Reference Material:

Thanks to the National Weather Service office in Burlington Vermont for providing most of this material.

National Weather Service
Raleigh Forecast Office
1005 Capability Drive, Suite 300
Centennial Campus
Raleigh, North Carolina 27606-5226
(919) 515-8209
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Page Last Modified: 13 November 2012 21:20:03 UTC
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