First AMOS (Automated Meteorological Observation System — one of the first automated weather observation systems), then AWOSs I, II, and III, now ASOS. The groundswell to automate the United States' weather sensing equipment continues apace. What next?
Wind profilers, that's what. Wind profilers are ground-based installations that remotely sense winds and temperatures aloft. To do this, profilers use a combination of Doppler radar and acoustic sounding.
Here's how the wind-sensing part of the profilers work: A sequence of three Doppler radar beams (at radio frequencies of 404.37 MHz) is transmitted from the site. The first beam is transmitted vertically. The next beam is sent to the north of the first beam, at an angle 16 degrees from the vertical. The third beam also goes up at a 16-degree angle from the vertical but to the east. Each beam dwells for two minutes at each position, and after one round of sampling, or six minutes, the wind profiler's computer software computes the directions and speeds of winds aloft — at 72 different altitudes, all the way up to 53,000 feet.
The amazing thing is that the profilers can perform this feat — unattended, of course — every six minutes of every day and regardless of cloud cover or precipitation. The accuracy of the wind speed measurements is within about 2 miles per hour.
The reason for this kind of hair-splitting accuracy is the extreme sensitivity of the profilers' Doppler radars. We're definitely not talking about your plain-vanilla Doppler weather radar type of signal, the kind being used in the new "Nexrad" WSR-88Ds.
"These are the most sensitive production radars ever built," says Russ Chadwick, director of the National Oceanic and Atmospheric Administration's Forecast Systems Laboratory and head of the Wind Profiler Demonstration Network (WPDN). "They can measure signals as weak as 168 decibels below a milliwatt. That's about 10[-20] Watts of signal measurement."
It's this kind of sensitivity that allows the profilers to derive wind data from even the clearest of skies. They don't need cloud water droplets, precipitation, or even passing insects to detect air movements. Instead, they sense the minute frequency shifts and changes in atmospheric impedance that occur when air molecules overturn and create small eddies, which is a constant atmospheric occurrence. Once sensed, the profilers then track the speed and movement of these eddies, thereby producing a huge — and hugely precise — look at atmospheric motions above the profiler site.
Temperatures aloft are sensed by a different means. What amounts to four huge loudspeakers — one at each corner of the profiler site — send bursts of acoustic energy skyward. This sound is at a frequency of 850 hertz, and it's audible. "We had to do some fine-tuning of the frequency we used," Chadwick said. "The first frequencies sounded like police sirens, so the site sounded like 'whoop-whoop-whoop' all the time. That was annoying, so the new, 850-hertz signal is quite different. Some say it sounds like jungle sounds — monkey and parrot calls — but others think it sounds like an air conditioner bearing about to go out."
Whatever the sound, its speed is measured by the Doppler radar. There is a relationship between temperature and the speed of sound, and after more manipulation by more software, the computer spits out a temperature profile of the atmosphere. Temperature information is given for altitudes up to 18,000 feet. Above this altitude, the sounds become too weak to use or are dissipated by wind. Above 18,000 feet, researchers have been relying on the temperature information provided by the trusty old network of radiosonde balloons, plus the data fed them automatically by aircraft tied into the Airborne Communications and Reporting System (an automated air-to-ground transmission of position, altitude, wind, and temperature).
Since 1988, a network of some 32 wind profilers has been operating in the central United States. Operated and funded by NOAA, these profilers are technically part of a prototype, demonstration program. But in reality, information from these profilers is used daily by the National Aviation Weather Advisory Unit (NAWAU) in Kansas City, Missouri. NAWAU is part of the National Severe Storms Forecast Center and is responsible for issuing sigmets, convective sigmets, airmets, and area forecasts.
Meteorologists at NAWAU appreciate the speed and accuracy of the profiler data and have used it successfully in analyzing and predicting the strength and movement of a wide range of phenomena. Generally, the meteorologists use hourly averages of the six-minute observations.
The Area Forecast, Airmet, and Sigmet units of NAWAU have found profiler data useful in detecting turbulence (by identifying fluctuations in air parcel velocities) at all levels of the atmosphere; upslope flows over the high plains; strong surface winds; the structure and strength of fronts; and identification of small-scale weather systems and features.
The Convective Sigmet unit uses the demonstration network to identify the onset, cessation, and location of low-level jet streams; areas of upward vertical motion; areas of differential temperature that enhance air mass destabilization; the presence and location of upper-level diffluence (divergence aloft that helps create and sustain convection); and areas and layers of wind shear that boost convection.
Other entities also make use of wind profiler observations. Information from the demonstration sites are fed to each Weather Service Forecast Office and are used there to help prepare certain terminal forecasts. The National Meteorological Center's Cray supercomputers are also fed wind profiler data in support of their mission to create forecast and analysis models. Finally, profiler observations are sent to 95 universities and several foreign meteorological organizations.
As it stands now, the Wind Profiler Demonstration Network has a lot going for it. Its accuracy, coupled with its near-real-time data stream, sure beats the twice-daily launches of radiosonde balloons. The balloons leave huge gaps in the information flow because they're launched at 12-hour intervals. On top of this is the additional "pirep gap" of the late evening and early morning hours, when there's less flying. In addition, with a mere 75 balloons for the entire nation, the United States' winds and temperatures aloft data grid isn't very dense at all.
WPDN's 32 sites are all within a 950-square-mile area and thus are able to give quick warning of such volatile events as fast-breaking convective action — even in the middle of the night. (That's the major reason why WPDN was located smack dab in the middle of Tornado Alley.) Plans call for the fully implemented, nationwide wind profiler network to have approximately 200 sites. This would give research and forecast meteorologists a quantum leap in atmospheric data density, a concomitant increase in short-term forecast accuracy, vastly superior, more frequent winds and temperatures aloft forecasts, and many more avenues to explore in the world of long-range forecast algorithms.
There's more. Researchers at the Forecast Systems Laboratory and Unisys (the prime contractor for the profilers) are working on a new method of measuring humidity at each profiler site. Using very high precision, survey-quality GPS signals, they've learned that the apparent motion of water vapor slows down signal transmission. Meteorologists believe they can use the alteration of these signals to develop formulas for figuring out the total humidity ("precipitable water" is the more correct, meteorological term) in the air above a site.
The hope here is to somehow correlate temperatures aloft readings with the humidity readings to come up with more precise locations of suspected icing conditions. Right now, the problem is that this system only shows the total precipitable water (i.e., the water depth should all water vapor suddenly condense) and doesn't assign an altitude to water vapor concentrations. With more tweaking, researchers may just reach an elusive goal: accurate plots of altitudes and areas conducive to icing.
Now for the sad part of this business. Due to budgetary constraints, the humidity sensing program is on hold. Even worse, WPDN may remain in demonstration status for an indefinite period. Full implementation isn't in next year's NOAA budget, a victim, it's said, of the ongoing battle between the "wet" (National Ocean Survey) and the "dry" (National Weather Service) sides of the NOAA bureaucracy.
Too bad for us if the wind profiler program doesn't pan out. Unlike ASOS, there seems to be little debate over the quality of wind profiler observations.
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