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April 25th Update: The National Weather Service Doppler Radar in Brownsville is back on the air! We have completed our installation of Dual Pol hardware and software. Please bear with us as we iron out any "kinks" through the end of April. Check out how NOAA celebrated the final installation at Brownsville.

The most significant upgrade to the National Weather Service (NWS) network of Doppler radars (WSR-88D) is in the latter stages of completion. Nearly all of the Doppler radars nationwide have already been upgraded, and Brownsville/Rio Grande Valley’s upgrade is now underway! This upgrade incorporates a new technology called dual–polarization, or "Dual–Pol" for short. The upgrade for the radar at the Brownsville/South Padre Island International Airport began on April 15th, and was completed on April 20th. 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 upgrade, please contact Warning Coordination Meteorologist, Barry Goldsmith at (956) 504–1631 extension 223 (e-mail barry.goldsmith@noaa.gov), or Science and Operations Officer Douglas Butts at (956) 504–1631 extension 224 (e-mail douglas.butts@noaa.gov).

What is Dual–Pol?

Standard 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. We expect this will lead to 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.

Standard horizontal pulse radar signal transmission Horizontal and Vertical pulse radar signal transmission

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

  • Differential Reflectivity (ZDR): The differential reflectivity is a ratio of the reflected horizontal and vertical power returns, and is a good indicator of drop shape. In turn, the shape is a good estimate of average drop size.
  • Correlation Coefficient (CC): The correlation coefficient is a correlation between the reflected horizontal and vertical power returns, and is a good indicator of regions where there is a mixture of precipitation types, such as rain and snow.
  • Specific Differential Phase (KDP): 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 (wavelengths) along the propagation path for horizontal and vertically polarized waves. It should not to be confused with the Doppler frequency shift, caused by the motion of the cloud and precipitation particles. Unlike Differential Reflectivity and Correlation Coefficient, which are dependent on reflected power, the specific differential phase is a "propagation effect", and 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 arroyo or creek flooding
  • Ability to discern between heavy rain, hail, snow, and sleet, and 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/insects, and tornado debris) and reduce their effects on radar data displays
  • Detection of aircraft icing conditions
  • Identification of the melting layer, helpful in determining precipitation type in winter and summer storms

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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