Identifying false weather returns (anomalous propagation) using dual-polarization radar data

In the wake of rain and storms earlier tonight, a radar phenomena known as "beam ducting" created anomalous propagation (AP, or in plain language, false weather returns).  This is somewhat common in rain-cooled air that is found behind departing precipitation.  One advantage of the brand-new dual polarization radar data is the ability to tell the difference between these false weather returns, or AP, and real precipitation.  The correlation coefficient product returns certain values when the precipitation is real and other values when it is not. See the image below.

On the left is base reflectivity, the product most people are familiar with seeing that shows location of precipitation.  On the right, the corresponding correlation coefficient (CC) product.  The area that is circled in green has a high CC value (reds are near the far right of the color scale on the right panel), indicating that the returns are likely precipitation.  In the larger white circled area, what appears to be precipitation on the left has much lower CC values that are random and speckled.  In this case, the radar returns are not actually precipitation, but AP.  If you could see a loop of the precipitation product, you would see the real precipitation moving smoothly across the screen, whereas the AP jumps around irregularly - another clue that it is not rain.  Hence, we have one benefit of the new correlation coefficient product from dual-polarization radar!

For additional information on how radar beams behave in the atmosphere, visit this page from the NWS Jetstream program.

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StormView Radar to be unavailable December 1-15 due to radar upgrade

Beginning Thursday morning, December 1, MemphisWeather.net's StormView Radar will be unavailable for two weeks.  It is expected to return to service on December 15.  Alternate radar links will be provided on the StormView Radar webpages, as well as on our mobile site (m.memphisweather.net).  On the MWN Android and iPhone apps, radar data will NOT be available during the outage due to programming constraints.

The reason for the outage is a significant upgrade to the National Weather Service (NWS) Doppler radar in Millington, TN.  The Millington radar (KNQA) provides the raw data to power StormView Radar. The upgrade, known as "dual polarization" (or dual-pol), was described in a previous blog post and represents the largest advancement in technology applied to the nationwide Doppler radar network since it's initial installment in the 1990s.  There are many benefits that meteorologists with access to the radar data (especially those who provide warning services to the public from the NWS) will reap.  Those are also described in the previous post linked above.

We apologize in advance for the inconvenience this will likely cause, though we ask you to understand that this is completely beyond our control and is a necessary and highly beneficial upgrade to the weather community.

To learn more about dual polarization, or to take online training designed for non-meteorologists on the changes, visit this MWN blog post or the NWS-Memphis dual-pol overview page.

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For weather information for Memphis and the Mid-South, where and when you need it, visit MemphisWeather.net on the web, m.memphisweather.net on your mobile phone, download our iPhone or Android apps, or visit us on Facebook or Twitter.

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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For weather information for Memphis and the Mid-South, where and when you need it, visit MemphisWeather.net on the web, m.memphisweather.net on your mobile phone, download our iPhone or Android apps, or visit us on Facebook or Twitter.

Climate trends, El Nino, and waterfowl

I have seen several interesting weather items of note in the past couple of days that I thought I would touch on today. Here they are, in decreasing scale order (globally to locally):

October Climate Stats
While the U.S. experienced its wettest, and third coolest, October in recorded history, that didn't bear out across the rest of the globe. NCDC indicates, and Dr. Jeff Masters of Weather Underground reports, that last month was the 6th warmest October on record globally. In the U.S., temperatures averaged 4.0F below normal, while precipitation was almost double the typical October average. It appears from the graphic above that outside of the continental U.S., northern Europe was the only other region where temps averaged below normal. In addition, U.S. drought and fire activity both decreased in October.


El Nino Strengthens
According to the Climate Prediction Center's (CPC) weekly El Nino report issued this morning, El Nino conditions have strengthened across the central and eastern Pacific in the past month, with sea surface temperatures now averaging 1.0-2.0 degrees above normal. In fact, the most watched region of the Pacific (dubbed "El Nino 3.4") has now crossed a threshold that allows this El Nino event to be classified a "strong" event. Model forecasts call for moderate to strong El Nino conditions to continue through the 2009-2010 Northern Hemisphere winter. Three-month forecasts for the Mid-South reflect semi-typical El Nino considerations, including slightly below normal temperatures and below normal precipitation. More on the current El Nino conditions can be found here.


Another Winter Forecast
My blogosphere friend Paul Yeager of CloudyandCool.com wrote yesterday about a computer model called the NCEP coupled forecast system (CFS) that has advantages over typical day-to-day models in forecasting long-range (read Paul's blog for a great explanation). In the FWIW category, the CFS output agrees with the CPC and most other winter forecasts that have taken into consideration the effects of El Nino and forecasted cool and dry winter months for the Mid-South. For a look at U.S. temperature forecast maps from the CFS, click here, and for precipitation, click here.


Doppler Ducks
Ryan Vaughan with KAIT-TV in Jonesboro woke up Saturday morning to reports of large flocks of waterfowl (likely geese and some ducks) flying south over northeast AR. As he checked NWS Doppler Radar, he noticed what appeared to be light rain showers headed south over the area (while other returns were moving northeast through north MS). Putting two and two together, it was determined that the migratory birds were showing up on radar! (Read more on Ryan's blog, including a radar loop of the occurrence.)

While a very cool thing to witness, birds on radar is certainly not unprecedented (see my previous blog, "The Birds"). In fact, during certain times of year, at sunrise, "expanding donuts" of radar echoes appear near Reelfoot Lake and other areas around the region known to be excellent sleeping spots for our feathered friends. As the waterfowl take off in all directions, they show up as an ever-expanding ring of echoes.

So hunters, not only will checking the early morning forecast on MemphisWeather.net be helpful in determining the weather conditions for your stakeout, but accessing Doppler radar on MWN Mobile might net you a heads-up to an approaching flock!

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Reno NEXRAD failure due to high wind and lee waves

High wind in Reno, NV during the early morning hours on December 19th left the NWS NEXRAD radar in... well... "less than ideal" condition. This would be deemed "radar failure." Check out the photos below of the aftermath of an estimated 140 mph wind gust! More information can be found here. Some very interesting meteorology went into the conditions that caused the destruction of this radar, which are described briefly in the link above. There were "lee waves" occurring downwind of the Sierra Nevadas that morning which caused intense vertical motion that was picked up on satellite. The lee waves were the likely cause of the extreme gusts. The fact that the radar is located at almost 8,300 feet above sea level contributed.

It will take quite a while for this radar site to be back in working order, but in the meantime, forecasters at the Reno NWS office will have to resort to using surrounding radar sites for their radar data. I've seen pictures of one other such radar failure in the past decade - the Laughlin AFB radar near Del Rio, TX radar that was hit by a presumed downburst with 90-100 mph wind on May 24, 2001.