Doppler Radar - A Weatherman's best friend

Apr 29, 02:43 PM

With spring time here in the Midwest, we all know that this means the threat of severe weather is possible when thunderstorms come rolling on through. Fortunately, meteorologists have a very sophisticated tool at hand (or in the air!) that can be very valuable when the atmosphere becomes active. We are very fortunate in the United States to have an impressive network of radar sites across the country to keep an eye to the sky. I would like to explain how Doppler Radars work and how meteorologists like myself use its beneficial information.

Think of radar as X-ray vision that atmospheric scientists use to examine the inside of clouds and storms. Doppler Radars essentially consist of a transmitter that sends out short, powerful microwave pulses and a receiver. When these pulses of energy encounter an object in the air (like rain, snow and hail), some of that energy is reflected back towards the radar site and is received. The amount of time it takes for the signal to bounce back to the receiver determines the distance of the object.

Once these pulses are received, the information is processed by very complex computer programs so that it can be viewed and studied. Doppler Radar is so sensitive that it can detect birds in flight and even bugs! So obviously when it’s raining cats and dogs, the Doppler can detect if it’s a labrador pooch or a spaniel falling from the sky.

Depending on the size of the object in the air, the amount of the energy pulse that is reflected back to the radar will correlate to the displayed value. For example, large, heavy raindrops will have more “reflectivity” back to the radar than smaller, light raindrops. This will show on the display as heavier rainfall, especially in thunderstorms, as yellow, orange and red values versus lighter values of blue and green. So the radar not only shows where the precipitation is occurring but also the intensity.

Not only does the Doppler Radar detect the location and intensity of precipitation, it can also be used to measure wind direction and strength. This is especially vital when looking for rotation within thunderstorms to determine if it is tornadic. Take a look at the example below of a tornado on radar from May 1999 in Oklahoma:

As you can see, on the left is the reflectivity mode that shows precipitation and on the right is storm velocity, showing wind direction. If you look at the velocity side on the right half, you’ll notice some of the storm is shaded green and other parts red. Green indicates the wind is moving toward the radar and in the areas of red, the wind is moving away from the radar site. This is key because on this example, there is a distinct area where the green and red areas are touching each other directly. This indicates the winds are quickly moving away and toward the radar and that this storm has a strong rotation.

So with severe weather a potential for us here in the Midwest as we move through spring into summer, I thought it might be a good idea to give a little insight on a very powerful tool. There are many tools we use as meteorologists to forecast, but the Doppler Radar shines when it comes to severe weather and keeping the public alert and safe.

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