Weather Radar – Part Two

Last week we discussed the first capture of a radar signature of a tornado back in 1953. The relationship of radar to weather goes back to World War II, however. Engineers and scientists understood that radar echoes from precipitation could mask aircraft flying in or behind the rain. That bit of information was helpful from both an offensive and defensive standpoint.

radar2014
National Weather Service Doppler Radar-Lincoln, Illinois –NOAA

After the war, surplus radar units were available to forecasters and electrical engineers and they began experimenting with this new forecast tool. By around 1960 the first generation of weather radar units were being deployed. One way to think of these is simply as a two dimensional operation. The radar beam went out until it hit something and was then reflected back. By comparing multiple images speed and direction could be determined and by charting the strength of the beam that returned and accounting for distance, the intensity of the precipitation could be observed. But the early radar had many limitations including not being able to reveal much about what was going on inside a storm.

In the later 1960s scientists began experimenting with a more sophisticated kind of radar called Doppler Radar. The Doppler Effect is the change in frequency of a wave as it moves relative to its source and the observer. For example, a train horn moving toward you in the distance will sound higher than the frequency of the horn if you were standing next to the locomotive and will sound lower as it moves away from you. Likewise, there are changes in the radar wave as it returns to the unit depending on motion of the target.

So what practical purpose does this serve? Doppler Radar showed that tornadic thunderstorms rotate. Half of the reflected wave moved toward the radar unit and half moved away.  That couplet in the upper right of this set of radar images of a tornado is observable as red and green colors after computer processing. Being able to show the vertical motionCouplet within a storm in addition to its horizontal motion, also allows better forecasts of hail.

Think of it this way, Doppler Radar is a vast improvement in permitting what amounts to 3-D observation of a storm. Detecting the relative motion and velocity within a storm were a huge step forward in tornado forecasting.

In recent years, dual polarization radars have recorded Tornado Debris Signatures (TDS), colloquially known as “debris balls.”

--National Severe Storms Laboratory
Tornado Debris Signature –National Severe Storms Laboratory

These radar signatures are helpful in determining when a radar-indicated possible tornado is confirmed as a damaging tornado, especially in remote locations and during night hours when ground truth verification in real time may not be possible.

There are limitations to radar of course. The radar beam is essentially line of sight. So, the further the distance between the radar site and the storm, the less the radar can see in the lower levels. That means that sometimes smaller tornadoes and spin up tornadoes on the front edge of a storm are less visible to radar. The strength of the radar beam also deteriorates over distance. Currently, many National Weather Service radar sites are networked together and this provides enhanced images on the edges of the coverage area, but there are still gaps on the lower levels of the storms.

Post-event analysis of radar data provides a significant opportunity for local offices of the National Weather Service to review the warnings they issued on a particular storm.

So what is next? At least two different systems are being examined. Phased array radars should allow more near-real time radar images. Currently the radar beam elevates slightly with each rotation of the antenna. All of these slices then are assembled by a computer to create the images with which we are familiar. That means a 7 to 10 minute delay. Phased array radar and more robust computing power coming online should decrease that time lag considerably. At the same time, a network of smaller radar systems, perhaps mounted on existing cell towers, has also been proposed to fill in the gaps covering the lower level of storms.

As is said, “Change is constant.” And so it is with the use of radar in severe storm forecasting. As technology advances and computing power increases, increased accuracy and better location specific short term forecasts of severe weather are certainly possible. As always, budgets will be a major consideration.

And, just to open new horizons, we’ve only been discussing ground-based radar. We’ve said nothing about satellite technology.