Decoding weather and climate by using global positioning systems technology

Decoding weather and climate by using global positioning systems technology

Knox, John

The U.S. military created it decades ago, but now it is exploding onto the civilian scene with seemingly limitless applications. The Internet? No, it is global positioning system technology, better known simply as GPS. If you have heard of GPS, it is probably because you have seen a newspaper item on how it will guide cars of the future to the nearest gas station, or point recalcitrant husbands to the laundry room (see comic strip below). In this column, we will examine a more exotic, but potentially even more revolutionary application of GPS: the improvement of weather forecasts and climate observations.

WHAT IS GPS?

PS works like this: the U.S. military has placed about two dozen satellites in orbit around the earth, roughly halfway between the surface and where weather satellites typically reside. This network of satellites routinely checks each others positions and then transmits this information to receivers via radio waves. If a receiver intercepts signals from four or more satellites, then it can calculate its own location very accurately. How accurately? Amazingly, to within about a foot and a half, or just the size of the little saucer-shaped antenna! Little wonder, then, that everyone from aviators to seismologists to foresters is snapping up this technology; wherever precise locations matter, GPS is the wave of the future.

GPS AND WEATHER

The application of GPS to weather forecasting, like weather radar, transforms a fly-in-theointment into a scientific bonanza.

Radar was invented to detect enemy aircraft during World War II. However, the military soon discovered that weather conditions interfered with aircraft detection. But this wartime drawback could pay peacetime dividends, because severe weather is a mortal foe, too. Weather radar was born, and with it came the improved detection of thunderstorms and tornadoes.

In the case of GPS, the accuracy of an estimate of location depends just as with radar – on the transmission of radio waves through the earth’s atmosphere. Thunderstorms and tornadoes are invisible to GPS, but the continuous changes in atmospheric temperature, pressure, and especially moisture delay GPS signals on their route to Earth by tiny fractions of a second – just enough to reduce the accuracy of navigation.

This navigator’s headache is the meteorologist’s “Eureka.” The delay in GPS signals can be “inverted” to infer the conditions of the atmosphere around the globe. This information is the Holy Grail of weather forecasting, especially when it comes to moisture, which is the energy source for storms. Many (if not most) of the glitches in weather forecasting today result from our imperfect knowledge of what the atmosphere is doing overhead, over the entire globe. Currently our best sources of such information are weather balloons; but few of them are launched over the oceans, and even in the United States only one balloon is launched every twelve hours at about one site per state. GPS technology promises much more continuous and global information.

GPS AND CLIMATE

The debate over “global warming” pivots on a few key issues: Is the atmosphere really warming globally? If so, will atmospheric moisture increase and wreak havoc via more severe storms? Can we trust our data sources, which often are based on a variety of incompatible methods? And do weather and climate phenomena get in the way of the reliability of the data-gathering methods?

GPS can make a solid contribution to this debate through a technique called “radio occultation,” in which the radio signals are received by another satellite in a low orbit over Earth. As a global data source, GPS can avoid the bias of observing temperature only in populated regions over land. GPS is especially sensitive to water-vapor concentrations in the atmosphere, so changes can be detected rapidly and reliably. Meanwhile, GPS is not hampered by clouds, volcanic ash, or other atmospheric features that impair other methods of climate monitoring. GPS space-based technology contains virtually no parts that require maintenance or re-calibration, unlike some other satellite-based methods of measuring global temperatures.

Finally, because meteorological GPS applications piggy-back onto the military (and now commercial) networks of satellites, there is relatively little overhead involved in using it for scientific purposes. In short, GPS may offer a cheap, reliable way out of the endless squabbles over the veracity of global warming and help keep tabs on meaningful changes in global climate.

DOES GPS WORK FOR WEATHER AND CLIMATE?

Applications of GPS technology to weather and climate are still in their infancy, but the trial runs are very promising. A field experiment in Oklahoma in the early 1990s demonstrated that GPS provided accurate and exceptionally high-time-resolution data for water vapor. Significant improvements were noted in the location of atmospheric patterns that trigger thunderstorm activity, raising hopes that GPS might eventually lead to better forecasts for the western High Plains.

More recent spot-comparisons of temperature observations using GPS versus conventional weather balloons have revealed the advantage of GPS in determining temperatures near jetstream level – a region that is important for aviation and forecasting and is also very difficult to observe. These tests, performed in 1995 and 1996 in places ranging from West Virginia to Western Samoa, highlight the versatility and universality of this approach. A project currently underway in Japan will employ the extensive GPS network there (created originally for earthquake prediction) to improve the forecasting of the rainy season. In short, meteorological GPS applications are moving from theory toward reality.

THE SKEPTIC’S CORNER

Because you should now think GPS is the best invention since e-mail, here are a few cautionary appraisals to appease skeptical readers: Heard It Before: We heard the same predictions about weather satellites in the 1960s and 1970s, but weather forecasting has yet to benefit fully from satellites. The main reason seems to be that satellite data and weather balloon data are apples and oranges, and com puter forecast models must be told how to digest each one in the most nourishing way. This turns out to be a daunting task; the same might well apply to GPS-derived data.

Theory, Schmeory: Proponents of new technologies often compare the best theoretical performance of their wares to the day-to-day, error-ridden performance of existing technology. However, in practice the new technologies inevitably suffer from those same mundane real-life errors. For example, weather balloons are a much-maligned method of atmospheric information-gathering but the experienced weather forecasters I know wish that we had more of them and fewer of the much-ballyhooed, but often less useful, higher-tech alternatives.

These doubts notwithstanding, I find GPS to be a fascinating innovation in meteorology. Given its advantages of low cost, high reliability, and impressive accuracy, I predict that GPS will become one (not the) important component in the observational network that leads to better weather forecasts in the 21st century – and might tell us whether or not our globe really is warming.

John Knox is an assistant professor of geography and meteorology at Valparaiso University, the same school emblazoned on Sally Forth’s husband’s t-shirt. He can be contacted at John. Knox@valpo.edu.

Copyright National Forum: Phi Kappa Phi Journal Spring 1999

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