How Doppler radar helps detect severe storms

Published 7:00 am Tuesday, March 21, 2017

By Skip Rigney

The forecast for the next few days is for nice spring weather. Afternoons will be mostly sunny through Thursday.

Nights will be mostly clear, but high humidity coupled with nearly calm winds will allow fog and low clouds to form that could persist in the mornings.

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Temperatures will range from near 60 in the mornings to the lower or middle 80s each afternoon through Thursday.

On Friday, winds will pick up from the south as low pressure develops over Kansas. Those winds will bring in more clouds and possibly some isolated showers.

Our best chance this week for showers and thunderstorms will come either Friday night or Saturday. As the low pressure center moves east from Kansas toward Tennessee a cool front will be trailing southward from it.

Depending on how fast the system moves, the rain ahead of the front could move through our area as early as Friday night or as late as sometime Saturday.

There is the potential for severe thunderstorms in our area with this weather system, but some indications are that the highest risk may be to our north.

Meteorologists at the National Weather Service (NWS) Forecast Office in Slidell will monitor their weather radar for signs that any of the thunderstorms are becoming severe. Depending on what they observe with the radar, they can issue warnings for severe thunderstorms or tornadoes.

Two weeks ago I discussed in this column how weather radar works. One aspect I did not discuss was the Doppler capability and how it is used to detect severe weather and especially tornadoes.

The pulses of microwave energy being transmitted by the radar have a wavelength of about 4 inches.

The wavelength of the microwaves scattered back to the radar by raindrops will be slightly longer if the raindrops are being blown by the wind away from the radar and slightly shorter if the raindrops are being blown toward the radar.

This effect is known in physics as a Doppler frequency shift. Christian Doppler, an Austrian physicist, described the phenomenon in 1842.

We’ve all experienced it with sound waves from sirens.

As an emergency vehicle is moving toward you, the pitch of the siren sounds higher than it does after it has passed your location and is moving away from you.

Computer programs at the radar site compute the Doppler shift of the radar’s energy that gets scattered back to the radar receiver by raindrops.

From that information the computer software calculates the speed of the raindrops and the wind blowing them toward or away from the radar.

Tornadoes, because they have a tight circulation, show up on the Doppler wind map as a small area of high speed winds blowing toward the radar adjacent to high speed winds blowing away from the radar.

The ability to derive wind speeds using Doppler shifts was a major improvement when the NWS deployed its network of 160 Next-Generation Radars, also known as NEXRAD, in the 1990s.

Hopefully none of the storms this Friday night or Saturday will become severe.

However, if they do, it’s good to know that meteorologists at the NWS in Slidell will be using Doppler radar to help them decide whether to issue warnings.