A severe thunderstorm southeast of Boulder, Colo., appeared in brilliant rainbow colors on the computerized radar display, as though painted by an electronic Van Gogh. There, outlined in contrasting hues, was the distinctive swirl of a rotating updraft that might soon spawn a tornado. Interpreting radar data faster than could a human operator, the computer flagged the dangerous whirl, known as a mesocyclone, with a boldly stylized ``M.''
``You're providing the meteorologist with a view of the wind which he's never had before,'' said Raytheon Company engineer James G. Wieler as he proudly showed off the prototype system. Although he had used a scene drawn from an archival data bank, the demonstration typifies the electronic revolution that is giving severe-weather forecasters a new life-saving edge -- especially in their ability to predict tornado hazards.
The annual death rate resulting from tornadoes has been cut in half in recent decades as the forecast and warning system has improved and public alertness has increased. (See accompanying story.) Now severe-weather experts anticipate further gains in public safety and forecast accuracy as better radar, ``real time'' computer-data processing, and faster communications add to forecasting capabilities.
In this electronic revolution, the type of radar Mr. Wieler demonstrated makes use of an effect first described in 1842 by Austrian physicist Johann Christian Doppler. Moving objects shift the frequency of sound, light, or radar waves that they emit or reflect.
Radar signals returned by an object moving toward the radar are shifted toward higher frequencies. Signals returned by retreating objects are lowered in frequency.
Conventional weather radar shows the precipitation areas in storms. Doppler radar, by sensing the frequency shifts, also shows the motion of wind-borne precipitation, dust, and debris toward or away from the radar site. Hence, as Wieler notes, it gives meteorologists an unprecedented view of the winds.
The United States National Weather Service (NWS) wants to replace its 230 aging radars with a new Doppler network. It is developing this system as part of a joint program with the Department of Defense and the Federal Aviation Agency (FAA). The goal is to install 160 of the new radars by early in the next decade, with 139 units covering the continental United States and the rest located overseas mainly at military sites. The FAA plans to install another 100 units at air terminals.
Called NEXRAD (NEXt RADar), the program is now in the final development phase, though it still may be difficult to gain installation approval for the billion-dollar network.
Be that as it may, severe-weather forecasters haven't had to wait for the NEXRAD system to reap major benefits from electronic aids. A complimentary data processing and display system already is boosting the accuracy of tornado watch predictions. Nearly instant data collection
It's called the Centralized Storm Information System (CSIS) and four interconnected computers form the system's core. The system collects, manipulates, and displays mountains of weather data in forms that aid quick decisionmaking by forecasters. This information is gathered from conventional weather data lines and from satellites, using a 15-foot dish antenna on the center's roof.
Satellite information is only 5 to 6 minutes old when it arrives by this direct route, as opposed to 35 minutes with the previous system. ``When you're dealing on a time and space scale [such] as we're dealing with, minutes are critical and getting timely information is important,'' says Fredrick P. Ostby, director of the National Severe Storms Forecast Center (NSSFC) at Kansas City, Mo.
Dr. Ostby's center prepares severe-weather watches for all parts of the continental US. A watch alerts residents of a specific region as small as a county that there is a potential for severe weather during the next 2 to 6 hours. This is in contrast to warnings that indicate a severe-weather hazard is imminent. Local weather stations issue such warnings. But if your area is under a severe weather watch, it has been declared by the NSSFC in Kansas City.
The CSIS puts reliable and timely information literally at the forecaster's fingertips. Satellites provide both cloud pictures, which reveal development and movement of major storm systems, and the distribution of temperature and moisture through the depth of the atmosphere. The latter probing, which reaches to the ground in cloud-free areas, is important for assessing how prone air in a region is to thunderstorm development. Balloon-carried instruments called radiosondes sent up from about 100 stations supply this kind of information twice daily. The satellite soundings are a useful supplement occurring at intervals of 40 miles every hour.
With CSIS, forecasters can overlay this and other information on top of satellite cloud pictures displayed by the computers. They can call in radar pictures from anywhere in the country. Maps of local areas can be laid right on top of satellite images of storms. Then the computer can display the names and locations of the 20 closest communities and the local weather station.
With a few key strokes, a forecaster can prepare and issue a weather watch for the area displayed. This takes about 10 minutes compared with as much as an hour the old way, using paper maps, paper forms and manual typing of the messages.
``It has really been the one factor we can hang our hats on in improving forecasting skill,'' enabling forecasters to concentrate on using their experience in `short-fuse situations,'' says NSSFC deputy director Edward W. Ferguson.
The severe-weather watch forecasts are not only more timely, they also are gaining in reliability. Dr. Ferguson notes that, when CSIS began being used in February 1982, tornado watch verification ran in the low 40 percent range. That's a false alarm rate of 55 to 60 percent.
Then the verification rate began to climb. It jumped to 52 or 53 percent correct forecasts in 1982, 54 percent in 1983, and around 59 percent last year. Since CSIS began operating three years ago, false alarms have dropped 15 to 20 percentage points with no indication of leveling off.
``I think public awareness [of tornado alerts] has gone up, and I think our greater accuracy has contributed to this,'' Ferguson says. That awareness contributes directly to the continuing decline in the annual number of deaths caused by tornadoes. A need for better warning capability
While the CSIS has sharpened tornado watches, it can do little to improve the warnings issued by local weather stations when a tornado is about to pounce. Right now, there is essentially no lead time. Usually, a warning can't be issued until a tornado has actually touched down. It is here that forecasters expect Doppler radar and NEXRAD to give them an edge.
Doppler radar can pick up the swirl of an incipient tornado vortex while it is still aloft. It tracks the formation and development of the tornado parent mesocyclone, a swirling mass of air several miles wide within a severe thunderstorm. Not all mesocyclones produce tornadoes. But when tornadoes occur, they are born of these larger circulations. Doppler radar can pick up a mesocyclone when it forms, generally at midlevels within the thunderstorm updraft.
This capability is a major reason why the NWS wants to replace existing weather radars, which are not good tools for tornado warning, with the NEXRAD system. Tests by the National Severe Storms Laboratory at Norman, Okla., show that Doppler radars can give local forecasters 20 minutes or more of lead time in issuing tornado warnings before a funnel reaches the ground. And it reduces false alarms substantially.
``Up to now, you could do nothing about lead time with warnings,'' says Ferguson. With Doppler radar, he adds, ``You can almost finish your cup of coffee before taking cover.''
A NEXRAD station would tie its radar directly to a computerized data-processing and communications system giving forecasters a variety of aids, such as severe weather probability or flood potential, as well as traditional radar displays.
As with CSIS, operators can superimpose maps of local areas on the radar image. They can put an ``alert box'' over a local map area, such as an airport, and have the system show the probability of hazardous weather and type of hazardous weather within it.
Not only will color-coded images show the whirling winds of probable tornadoes, but the system will be able to detect those automatically, flag them, and alert an operator.
NEXRAD data also will be kept in archives and provide an unprecedented resource for training and research, says Raytheon's NEXRAD Program Manager Thomas McDonagh. He says the data bank will become increasingly useful as it builds up year by year.
Despite its apparent advantages, NEXRAD's future has come under a financial cloud. It's an expensive system and the administration is exploring the possibility of a cheaper alternative. As a result, NOAA has put NEXRAD on hold.
A high-ranking NWS official, who asked that his name be withheld, says that, while some off-the-shelf equipment has been suggested in place of NEXRAD, it won't do the job the weather service thinks should be done.
Meanwhile, tornado forecasters such as Edward Ferguson remain enthusiastic about the progress already made in improving tornado watches and the prospect for gaining lead time and reliability for warnings.
``It's the most exciting and most challenging job in trying to forecast severe weather I've seen in 32 years in the business,'' Ferguson says, ``We're in a life-saving business. We take it seriously. But it's challenging and it's fun.''