A primer on dual polarization radar – coming soon to the Mid-South!

You’re probably well aware of the use of “Doppler” radar technology in the Mid-South, and across the country, to detect and track areas of precipitation - whether rain, snow, or severe thunderstorms. This technology has been in widespread use since the 1990s and has helped to vastly improve the forecast and warning process for all types of weather phenomena. Now, nearly 20 years later, the National Weather Service is undertaking the most comprehensive and significant upgrade to its Doppler radar network since its inception. This upgrade, known as dual polarization (or polarimetric radar), will provide new and more accurate information to meteorologists about various precipitation areas detected by radar, allowing for even further improvements in the forecast and warning process.

So what exactly is dual-polarization and how does it differ from the conventional Doppler radar that we’ve used for years now? All weather radars, including Doppler, send out pulses of energy into the atmosphere in order to detect precipitation. When a pulse of energy hits a raindrop or a snowflake, for example, some of that energy is “reflected” back to the radar, which can then be mapped through computer software for display to meteorologists and general public. If you've seen or heard the term “radar reflectivity” before on TV or the internet, this is where that comes from! The radar is simply measuring the amount of reflected energy it’s received back. The more reflected energy that’s received back, in general, the heavier the  precipitation that is being detected. This is how we can differentiate between areas of lighter and heavier precipitation.

Up until recently, weather radars have sent out these pulses of energy oriented in only one direction - horizontally (see animation below). While this is sufficient for the general detection and overall intensity of precipitation, characteristics of the precipitation droplets, such as their size, shape, dimensions, and composition, can not be determined using only radar pulses oriented horizontally. In meteorology, knowing these details can be crucial in identifying whether precipitation is in the form of rain, snow, or even hail.

Conventional Doppler radar transmits and receives pulses of energy in a horizontally-oriented direction. This provides good detail on where precipitation areas are and how heavy they might be, but with little other information available. Image courtesy National Weather Service.

That is where the new technology comes into play. With dual polarization employed, the radar will now send and receive back pulses of energy oriented both horizontally and vertically (see animation below). With the vertical pulse added, we now are sent back some of those crucial details about precipitation returns that can help better identify some of their specific characteristics.

With dual polarization technology, the radar now transmits and receives pulses of energy in both a horizontal and vertical orientation. New details about precipitation returns are available, that can more accurately identify precipitation types and other characteristics. Image courtesy National Weather Service.

Among the benefits dual polarization may provide, based on National Weather Service research:

• Better estimation of overall precipitation amounts
• Improved detection of areas of heavy rainfall and flooding potential, improving the warning process
• Improved detection and mitigation of non-weather echoes (removal of false returns such as ground clutter from the radar display)
• Ability to classify possible precipitation types (which will improve even more with the subsequent software updates following the initial deployment of dual polarization)
• New severe thunderstorm signatures, including better detection of hail and even tornado debris, aiding in the warning process

These potential benefits will be found through the addition of 14 new radar products that will become available once dual polarization is activated at each radar site. Even more benefits currently unknown may be found as the technology is deployed and used operationally across the country. However, with that said, it should be noted that there will remain limitations and drawbacks. Radar remains just one tool of a vast array of information available to meteorologists, and none will ever be more valuable than actual reports and observations by the “eyes and ears” at the surface - NWS severe storm spotters and the general public!

One of 14 new radar products available with dual polarization: Differential Reflectivity (DR). DR compares the intensity of energy reflected back in the horizontal pulse vs. the vertical pulse, which helps determine the shape and size of a precipitation return. In this example, cooler colors (blues/greens) are areas of snowfall; warmer colors (yellows/reds) are areas of ground clutter and other non-weather related returns

The National Weather Service is in the early stages of its transition to dual polarization technology, with its entire network of radars not expected to be completely upgraded until 2013. We are fortunate here in the Memphis area however, as we will become one of the first locations in the nation to receive the upgrade! The NWS Doppler radar in Millington, TN will receive dual polarization during the month of December. The upgrade consists not of replacing the entire radar itself, but rather replacing hardware within the already existing radar. Because of that, the radar must be taken offline during the installation process. The process takes about two weeks and is scheduled to be unavailable from December 1st-15th.

During this time, data from the NWS radar in Millington, which also powers MemphisWeather.Net’s StormView Radar, will not be available. Surrounding regional radars should remain online to help monitor the weather conditions in the Memphis metro and Mid-South area, and MWN provides links to those surrounding radar sites in the “Radar/Satellite” menu on our website. We will also provide links as needed on our Facebook and Twitter feeds during the upgrade.

If you want more information on the impending radar upgrade to dual polarization and other general information about it, visit the links below.

• Latest from the NWS in Memphis on the Dual Polarization Upgrade
• Frequently Asked Questions about Dual Polarization, from the National Severe Storms Laboratory
• Basic Training on Dual Polarization for Non-Meteorologists, from the NWS Warning Decision Training Branch

--Kevin Terry, MemphisWeather.net

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