Dual-Pol

The National Weather Service Doppler Radar in Corpus Christi will be turned off from March 29 through April 12 for a major upgrade. However backup coverage will be provided by radars in Brownsville, Del Rio, New Braunfels, and Houston.

The most significant upgrade to the National Weather Service (NWS) network of Doppler radars [WSR-88D] is in the latter stages of completion. Over 120 of the 155 Doppler radars nationwide have already been upgraded, and Corpus Christi’s upgrade is coming soon. This upgrade incorporates a new technology called dual-polarization, or dual-pol. The upgrade for the Corpus Christi Doppler radar is scheduled to occur during the first two weeks of April 2013 at the Corpus Christi International Airport. This new technology will result in 14 new radar products to the suite of radar data already available to NWS forecasters. These tools will assist forecasters in the warning and forecast process. This new technology and data primarily will help forecasters improve rainfall estimates along with identifying the type of precipitation that is falling.

We strongly encourage all users of these new products to complete a series of online training modules that were recorded by the Warning Decision Training Branch (WDTB).  Modules are available for non-NWS meteorologists and non-meteorologists through this website. Additional information on dual-pol can be accessed from the National Severe Storms Laboratory website.

For more information about this outage and upgrade, please contact Warning Coordination Meteorologist, John Metz at 361-299-1353 or e-mail John.Metz@noaa.gov.

What is Dual-Pol?

The current Doppler radars transmit and receive pulses of radio waves in a horizontal orientation. As a result, the radar only measures the horizontal dimensions of targets such as cloud and precipitation droplets. Dual-polarimetric radar transmits and receives pulses in both a horizontal and vertical orientation. Therefore, the radar measures both the horizontal and vertical dimensions of targets. Since the radar receives energy from 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 (hail vs. rain), and the identification of non-meteorological returns.

Some of the fundamental variables measured by polarimetric radars, and a short description of each, are listed below:

  • Differential Reflectivity - The differential reflectivity is a ratio of the reflected horizontal and vertical power returns. Amongst other things, it is a good indicator of drop shape. In turn, the shape is a good estimate of average drop size.
  • Correlation Coefficient - The correlation coefficient is a correlation between the reflected horizontal and vertical power returns. It is a good indicator of regions where there is a mixture of precipitation types, such as rain and snow.
  • Specific Differential Phase - 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 to be confused with the Doppler frequency shift, which is caused by the motion of the cloud and precipitation particles. Unlike the differential reflectivity and correlation coefficient, which are all dependent on reflected power, the specific differential phase is a "propagation effect". It is a very good estimator of rain rate.

Dual-Pol Technology Benefits

  • Improve the accuracy of precipitation estimates, leading to better flash flood detection and improved stream flow forecasts during river flooding
  • Ability to discern between heavy rain, hail, snow, and sleet, especially helpful discerning locations of precipitation type during winter weather events
  • Improve the detection of non-meteorological echoes (such as ground clutter, anomalous propagation, birds, and tornado debris) and reduce their effects on radar data displays
  • Detection of aircraft icing conditions
  • Identification of the melting layer

Dual-Pol will not improve tornado warning lead times but will have the ability to identify debris from a damaging tornado, providing confirmation on the location and danger of an ongoing tornado, even when storm spotter reports are not possible due to night time or low visibility near the tornado

Dual-Pol Products & Applications

The base radar products that have been available to users are base reflectivity, base velocity, and spectrum width.  Three new base products will be available after the dual-pol upgrade, including differential reflectivity (ZDR), correlation coefficient (CC), and specific differential phase (KDP).  In addition to these three new base products, there will be several derived products.  These include a melting layer (ML) product, a hydrometeor classification algorithm (HCA) product, and eight new precipitation products.

ZDR CC
Differential Reflectivity Correlation Coefficient
KDP HCA
Specific Differential Phase Hydrometeor Classification Algorithm

Images are courtesy of the Warning Decision Training Branch.

Applications

Using differential reflectivity to identify a hail core
Using correlation coefficient to identify the melting layer and thunderstorm updrafts
Using specific differential phase to identify the melting layer and thunderstorm updrafts

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