11 October 2009

How To Interpret Weather Radar (A short course with no math!)

Posted by Dan Satterfield

A SHORT HISTORY FIRST

Weather radar is now a common site on any TV weathercast, and radar images are all over the Internet.

It wasn’t really meant to be that way though….

Hurricane Carla in September 1961. The eye is visible on the Galveston WSR 57 radar. This was the first hurricane on radar that TV viewers had seen.

Hurricane Carla in September 1961. The eye is visible on the Galveston WSR 57 radar. This was the first hurricane on radar that TV viewers had seen.

Apparently, the first weather radar image of a dangerous storm shown on TV live was back in 1961. A Houston TV station sent a young reporter to Galveston to cover the approach of Hurricane Carla. It was a mean category 4 storm and heading right toward the city. The reporter showed up at the local weather bureau office and saw an image of the hurricane on the newly installed radar.The Galveston WSR 57 radar had been installed just the year before.

It was a scary picture and he wanted to show it on air. This was unheard of at the time. It had never been done before!

This request required permission from the Weather Bureau headquarters in Washington. It was supposedly given very reluctantly, but it had the desired effect. Most people got out of town before Carla stormed ashore. That young TV news guy from Houston was named Dan Rather by the way. Betcha didn’t know that!

It was not much of a radar by today’s standards. It had no Doppler capability, and was built on World War 2 technology. They were the main weather radar used by the National Weather Service  (Successor to the Weather Bureau), until the last one was decommissioned in 1996.

The replacements are called WSR 88D. This stands for Weather Service Radar 1988 Doppler. It was, however, not up and running in most places until the mid 1990′s.

DOPPLER RADAR

Doppler Radar image of winds showing the Greensburg Kansas EF5 tornado. The blue colour near Greensburg is strong wind blowing toward the radar. The Pink is very strong wind going away from the radar. The tornado wiped out the town of Greensburg.

Doppler Radar image of winds showing the Greensburg Kansas EF5 tornado. The blue colour near Greensburg is strong wind blowing toward the radar. The Pink is very strong wind going away from the radar. The tornado wiped out the town of Greensburg.

Doppler Radar has the ability to detect rain drops (and anything else in the beam) moving toward or away from the radar. This is based on the Doppler effect. It’s the same affect that you hear when a train passes by and the tone or frequency of the horn changes. This effect happens with ALL electromagnetic radiation as well. This includes light and radio waves (Which are basically the same thing!).

Astronomers use it to determine how fast stars that are millions of light years away are moving away from us. It’s called the red shift, because the light from objects moving away is shifted toward the red light end of the spectrum.

Is this real rain on the radar? NO! Read on! (NWS Jacksonville WSR88D)

Is this real rain on the radar? NO! Read on! (NWS Jacksonville WSR88D)

Tornadoes are spotted on Doppler radar when rain drops are rotated around an intense low pressure in a thunderstorm. This intense low is called a meso (small scale) cyclone. These mesocyclones are the parent circulation of a tornado. Only a few mesocyclone produce tornadoes, and for this reason, false alarm rates on tornado warnings remain unacceptably high. Keep in mind the radar can only detect the motion of objects moving toward or away from it. Not side to side.

This same technology is very possibly responsible for your last speeding ticket!

THINGS YOU SHOULD REMEMBER

Radars do not see rain!

They just detect radiation that is reflected back as the narrow beam travels outward. Bugs, dust, hail stones, birds and even clouds reflect the beam as well! Weather radars operate mainly at a wave length between 3 and 10cm. This minimises reflections from objects that are much bigger, or smaller than rain drops. A chunk of ice with a coating of water is a very good reflector at these wavelengths, and many times what you may think is heavy rain is actually hail.

Rain intensity is measured in dbz which is based on the power returned to the radar. 20 dbz is a very light rain. 40 dbz is very heavy. 50 dbz is blinding rain or may be hail. Most online or on TV radars use red and yellow for the higher values. These values are vaild only if the returns on the radar are from rain drops!

Radar Beams Don’t Travel long The Ground!

Radar beams get higher as they go away from the radar dish. This can cause storms to not show up at all! Image from NOAA

Radar beams get higher as they go away from the radar dish. This can cause storms to not show up at all! Image from NOAA

The radar beam travels in a basically straight line away from the radar. It is usually pointed at 0.5 to 2 degrees above the horizon. Since the Earth is curved, a radar beam is usually nearly  25 km high when it hits an object 500 km away! This is above all but the most violent thunderstorms. Tornadoes are low level phenomenon.  A radar is unlikely to detect a circulation around one more than 150 km away. Even 100 km is really stretching it.

Radar Beams Sometimes Hit The Ground!

Temperature inversions can cause the radar beam to hit the ground (NOAA img.)

Temperature inversions can cause the radar beam to hit the ground (NOAA image)

On clear, calm nights, a strong temperature inversion may develop. This means the air warms with height. These inversions cause a density difference in the atmosphere that bends the beam to the ground. The radar may look like it is detecting strong storms. It’s just dirt!

Meteorologists call this Anomalous Propagation. Our top news anchor at the station keeps telling me that he had a case of that once, but the doctor gave him a cream that cleared it right up!

The Newest Technology

The TV station I work for was the first in the world to install a Dual Polarimetric Radar. The NWS is upgrading most of their Doppler radars to this technology as well. The reason: The ability to determine the shape of objects in the beam.

Image from our ARMOR radar. Click image for live view.

Image from our ARMOR radar. Click image for live view.

This has fabulous possibilities. We have even detected debris from a large tornado on our radar. We call it ARMOR for Advanced Radar for Meteorological Operations and Research. The University of Alabama at Huntsville and WHNT developed ARMOR jointly. I use it for forecasting and on air, and they are doing some incredible research science with it.

ONE LAST THING

A line of storms or an area of rain will change as it moves along. Do not assume that a storm 100 km away will be as strong, or even still exist when it reaches you. Remember also that the further away it is, the higher in the storm you are looking. The rainfall pattern may look much different near the ground. Radar is a remote sensing tool. It does great things, but it’s only detecting radio waves. No more, and no less!

Later,

Dan