By D.A. Barber

The monsoon that ambles up from Mexico into Arizona and New Mexico every summer is considered the least understood meteorological phenomenon in North America. When it will arrive is one good question, but the most pressing issue each year is how much rain it will bring. UA’s Dr. E. Philip Krider, professor of atmospheric sciences, believes he may have the answer: lightning.

With lightning you can trace the movement of the storms and then what we’re finding is that the more lightning that you have, the more rain,” says Krider.

Krider received his doctorate in physics from the UA in 1969 and did post-doctoral work at the NASA Manned Spacecraft Center in Houston. He returned to UA as faculty in the spring of 1971. During the early 1990s he served as the head of the UA’s department of atmospheric sciences and director of the Institute of Atmospheric Physics.

"We began this kind of work back in the early ’80s at the NASA Kennedy Space Center where they have a lot of instrumentation to detect lightning and they also have a network of rain gauges. I had a master’s student examine relationships between lightning and rainfall and we’ve recently resurrected the topic here.”

The monsoon (Arabic for “season”) begins in the Southwest when cumulus clouds travel northeastward from Mexico with water from the Gulf of California. As the clouds move toward North America, they continue to pick-up moisture building in to cumulonimbus clouds with their distinct anvil-shaped tops towering some 30,000 feet, where temperatures can be as low as 60 degrees below zero.

The atmosphere becomes unstable since it’s reversed itself with the heavier cold air on top and the lighter warm air below. The violent uplifting movement of air results in convection as the atmosphere tries to correct the normal stability as warm, humid air rises producing water droplets that fall as rain.

The sky darkens and sudden gusty winds, heavy rain, hail and possible flooding becomes inevitable. Finally bolts of lightning light up the sky followed by claps of thunder. And it’s the link between how often lightning strikes and how much rain will occur that Krider is searching for.

“In these monsoon thunderstorms, characterized by intense convection, we find that lightning comes 5 to 20 minutes before the rain hits the ground,” says Krider. “And the larger the storm, the more intense the updraft and the longer the lag time.”

Determining how much rain is coming in the short term can save lives and property, even if the lag time is short. While the average monsoon in Tucson normally brings 6.06 inches of rain ­ less than half the area’s average annual rainfall. In 1964 the area received 14 inches in one summer. Then there was the flood of 1993 from heavy rains.

Lightning alone is an interesting phenomenon. Inside an unstable thundercloud air, water and ice are churned relentlessly to the point they become charged with static electricity. Positive charged particles accumulate at the top while the heavier, negative-charged particles gather at the bottom. Meanwhile on the ground, positive charges build-up on the normally negatively charged earth, following the passing clouds like a shadow. The negative electrons begin zigzagging within the cloud and form the invisible “stepped leader” to the ground, which attracts a stream of positive electrons and forms an electrical current. When the positive electrons travel up the stepped leader from the ground, the return stroke creates a bright flash: the lightning observed on the ground. The lightning channel is heated to as much as 15,000 Celsius ­ 2.5 times the surface temperature of the sun. The air around that channel is heated and expands faster than the speed of sound creating a sonic boom heard as “thunder.”

Actually only one-third of lightning strokes within a storm ever strike the ground, with the rest of the bolts (averaging two to six inches in diameter) traveling from cloud to cloud, commonly referred to as “sheet lightning.” But it’s those ground strikes Krider is studying with the help of the U.S. National Lightning Detection Network (NLDN).

“The processes that generate electricity in clouds invariably involve interactions with precipitation, so the processes that generate electricity are the same ultimately,” says Krider.

What the UA researchers found was that a “smart” sensor unit ­ a “gated, wideband lightning detection finder” ­ could be developed to respond to only electromagnetic signatures characteristic to cloud-to-ground lightning strokes while filtering out all other sources of electrical interference, such as cloud-to-cloud lightning, and radio and TV signals using “pulse shape discriminators.”

“The pulses have to have the rise time within a certain window and the characteristics of the shape of the pulse must be that of cloud to ground lightning. The way they detect the lightning and locate where it’s striking is by measuring both the magnetic direction with radio direction finding antennas, and also by timing very precisely the peak of the signal coming from the lightning. And from very accurate arrival time measurements at three or four or five stations, you can work back and figure out exactly where the source originated,” says Krider who notes the sensors also rely on GPS units in the sensors.

Krider says the equipment isn’t necessarily “better” than tools like Doppler radar, but is different in a very critical way.

“The one thing about the lightning detection sensors is that you have the answer within a few tens of seconds, where with radar you have to wait five or six minutes for it to complete a scan and then you’ve got to process that information and display it,” says Krider.

In Arizona, NLDN has six sensors, about four in southern Arizona and all with a 400-mile range. The equipment stems from a Tucson-based company originally known as Lightning Location and Protection (LLP). It’s a company ­ now known as Vaisala GAI Inc. ­ that Krider helped to create. Krider and A. Richard Kassander, former UA vice president for research, formed LLP and in 1976 signed an agreement with UA to pay royalties in exchange for the exclusive right to manufacture the detection equipment Krider and his colleagues had developed.

The first commercial LLP system was produced in 1978, and it turned out to be a tech transfer boom for the UA.

“For more than 20 years, it was by far the largest total revenue stream driven by the University licensing program,” says Krider.

Krider sold LLP in 1983, though he remains a consultant. The company name was changed to Vaisala which operates the National Lightning Detection Network from their offices near Tucson International Airport, supplying data to federal agencies, utility companies and others. The detection equipment Krider helped develop now operates in more than 40 countries. But Krider still favors the Tucson area’s lightning storms.

Despite being known as the “Lightning Capital of the World,” Tucson and the state actually rank behind Florida, Georgia, Colorado and other states.

“The kind of lightning we have here is really good for research,” says Krider.

“In Florida, you’re looking at cloud bases of 1 or 2 kilometers, whereas here in Arizona it’s typically 3 or 4 kilometers so you can see an awful lot more of the discharge below the cloud and you can also see much further because the dry air and visibility is so clean.”

While creating a sensor that could also detect cloud to cloud lightning could vastly improve predicting precipitation levels even further, coming up with a technique for actually predicting when and where lightning will strike is probably not in the forecast beyond statistical guesses.

“We cannot predict today where clouds will form tomorrow. When you really get into the details of where lightning will strike, you need to start with predicting where the cloud will be,” says Krider.

And once you do have the cloud, there are more uncertainties.

“The leader developing down is a highly random process and they change direction in a highly random fashion. So you really can’t predict ahead of time where any given discharge will hit the ground.”

Still, Krider and his colleagues’ lightning research at the department of atmospheric sciences has already changed the way meteorologists observe violent storms. It’s a far cry from that summer day in 1752 when Benjamin Franklin stood outside Philadelphia with a key on a kite string.