National Weather Service Weather Forecast Office

The dual-polarization radar upgrade to the NWS Green Bay Doppler radar is complete.

This much-anticipated upgrade uses a new technology called dual polarization, or "dual-pol," that is part of the NWS vision to build a Weather-Ready Nation to better protect lives and livelihoods.  This technology will result in 14 new radar products that will enable us to better identify the type and intensity of precipitation, improve rainfall estimate accuracy, and more accurately identify non-meteorological echoes.

The previous Doppler radar transmitted and received pulses of radio waves in a horizontal orientation.  As a result, the radar only measured the horizontal dimensions of targets (e.g., cloud and precipitation droplets).  Dual-polarimetric radar transmits and receives pulses in a horizontal and vertical orientation.  Adding in the vertical component will result in better estimates of the size, shape, and variety of sampled 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), and the identification of non-meteorological returns.

Previous NWS Doppler radar	Dual-polarimetric radar

• Improved accuracy of precipitation estimates, leading to better flash flood detection
• Ability to discern between heavy rain, hail, snow, and sleet
• Improved detection of non-meteorological echoes (ground clutter, chaff, anomalous propagation, birds and insects)
• Identification of the melting layer
• New severe storm signatures (hail detection, updraft detection, tornadic debris)

Dual-Pol will not improve tornado lead times or be able to provide exact precipitation type on the ground.

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: 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. 

• Differential Reflectivity (ZDR): basically, the difference between the reflected horizontal and vertical power returns. Among other things, it is a good indicator of drop shape. In turn the shape is a good estimate of average drop size.
• Correlation Coefficient (CC): a statistical 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 (KDP): 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. The specific differential phase is a "propagation effect." It is a very good estimator of rain rate.

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

• Dual-Pol Training for NWS Partners (also includes training for non-NWS partners and non-meteorologists)
• National Severe Storms Laboratory Dual-Pol Page