Doppler What?

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Doppler Radar Dome | Photo by 4warnweatherteam.com

I’m pretty sure most people have heard of Doppler radar, but how many actually know what it is and how it works? I will give you its history and why it is better than the old radar. Before I can explain how the radar works we need to the definition of the term “Doppler Effect.” We have to go back to 1842 in Prague, what is now the capital of the Czech Republic, and an Austrian Physicist named Christian Doppler who first proposed the effect or shift. The “Doppler Effect” is the change in frequency of a wave for an observer moving relative to its source. What the heck does that mean?

The Doppler Effect With Moving Car Example | Image by www.scienceadda.com
The Doppler Effect With Moving Car Example | Image by www.scienceadda.com

One of the simplest demonstrations of the “Doppler Effect” is the variation in sound when you hear a car or railroad train that is far away and as it gets closer the sound gets louder and the pitch changes to the loudest as it is nearest you and then decreases again as it move away from you. That forms the basis for why doppler radar is so different from conventional radar.

Aircraft Radar Installation | Photo from Wikipedia
Aircraft Radar Installation | Photo from Wikipedia

Next we have to delve into how regular radar works. As you might remember the acronym “Radar”  stands for RAdio Detection And Ranging. It was developed as a tool to find the enemy back in WWII. Conventional radar sends out a beam of radio waves that bounces off solid objects and shows them as a blip on a screen. They found that occasionally something interfered with the return echo of the radar beam. That something was rain and it gave the meteorologists the idea of using radar to find where rain was falling and to follow its motion.

Doppler Radar is a more recent invention that dramatically increases the effectiveness of radar. The main difference with Doppler radar is that it not only sends a microwave beam out, but it also measures the frequency of the wave as it leaves and again while it is returning. It gives the meteorologist a three-dimentional look at the sky similar to a slice of pie reveals what is inside the pie itself. The Doppler radar development project was given the acronym NEXRAD standing for  NEXt Generation RADar.

Pulse-Doppler Radar Tower At The National Severe Storms Laboratory | Photo by NSSL
Pulse-Doppler Radar Tower At The National Severe Storms Laboratory | Photo by NSSL

Rather than just using citations from the internet for this explanation I contacted a scientist I met back in 1975 at the first ever Tornado Seminar for Television Weather Personnel held at NOAA’s National Severe Storms Laboratory in Norman, Oklahoma. His name is Dr. Richard Doviak and he was the expert who showed us one of the first Doppler Radar units ever made. His team along with many graduate students put in hundreds of man-hours developing the system from a used Air Force radar unit and a computer to interface with. Here is his explanation of the difference between the two types of Doppler radar. “Doppler weather radars typically transmit microwave radiation in bursts of short duration. These radars are called pulsed-Doppler radars to distinguish them from those that emit continuous waves such as radars used by police to detect the speed of automobiles.

Equation For Pulse-Doppler Radar | Image by www.knmi.nl
Simplest Equation For Pulse-Doppler Radar | Image by www.knmi.nl

Pulsed-Dopler radars can measure both the range (r) and radial velocity (v) of scatters.” ” The US National Weather Service (NWS) network of Doppler weather radars emit..pulses of 10-cm wavelength radiation similar to that used in microwave ovens.. These radars are called WSR-88Ds, where WSR is an acronym of Weather Surveillance Radar. The number 88 denotes the year the contract was issued for a limited production and testing; the first unit for routine operation was installed in late 1992 and the last radar in a network of about 160 radars was installed in 1997; the letter D represents the Doppler capability; polarimetric upgrades were completed in 2013.”

Dr. Doviak’s presentation goes into great detail explaining the formulas and computer algorithms used in making Doppler radar function the way it does, but for this article those details would be much too complicated for any but the most experienced scientist. He sums up the future of Doppler radar by saying: “Through the combination of the various signatures of the polarimetric data, it is possible to greatly improve the performance of tornado detection algorithms and hydrometeor classification techniques.

National Weather Service Portland Doppler Radar from 2/16/14 | Image NWS Portland
National Weather Service Portland Doppler Radar from 2/16/14 | Image NWS Portland

After extensive research with polarimetric radar, operational WSR-88Ds have been recently upgraded to have polarimetric capability. With this advent, new scientific applications are sure to be developed over the coming years.” To simplify his statement we will have the ability to see more and do more with Doppler radar than we have up to this time. The Doppler radar image above indicates the intensity of rainfall in color codes.

Let me know what you would like me to talk about or explain. You can comment below or email me at: [email protected].

 

Tim Chuey is a Member of the American Meteorological Society and the National Weather Association and has been Awarded Seals of Approval for television weathercasting from both organizations.

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