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Summer 1998

New instrumentation: The 1998 lineup

This year sees the debut of a number of powerful new observing tools that will help scientists across the entire range of UCAR fields of research--and one instrument that will improve the lives of the air-traveling public as well. Each instrument described in the next few pages has an NCAR connection, either by invention, design, operation, or some combination of all these. Without further ado, here are this season's rookies.

Balloon-borne continuous cloud particle replicator

by Zhenya Gallon

Larry Miloshevich (left) and Andrew Heymsfield with the replicator. (Photo by Carlye Calvin.)

How do you capture a tiny ice crystal at the top of a cirrus cloud when that cloud's top is higher than most research aircraft can fly? Scientists Larry Miloshevich and Andrew Heymsfield of NCAR's Mesoscale and Microscale Meteorology (MMM) division catch cloud particles like flies in honey, then preserve their crystalline shapes as instant fossils. They've developed a balloon-launched instrument that brings back detailed replicas of the ice crystals that make up the clouds at the top of the troposphere. Their collaborators on the project are Steven Aulenbach (MMM), Harold Cole (Atmospheric Technology Division), and Walter Grotewold (ATD, retired).

The balloon-borne continuous cloud particle replicator is a corrugated-cardboard box (51 x 17 x 5 cm; 20 x 6.5 x 2 in) containing a continuous loop of clear 35-mm film mounted on an acrylic frame. The film is coated with Formvar, a polyvinyl plastic. A small motor winds the film past the collection opening at a constant speed, while a tube dispenses solvent to soften the Formvar just before it moves past the opening. Water droplets or ice crystals land in the solution, leaving behind a three-dimensional cast of the particle when the solvent evaporates and the Formvar hardens again.

Mounted on the outside of the box are a radiosonde and transmitter beacon. The radiosonde provides temperature, pressure, position coordinates, and relative humidity records for the flight. Holes at specific intervals on the film trigger an interruption in the radiosonde's data stream, creating a time stamp to permit correlation between the data from the radiosonde and the replicator.

At a predetermined height, the replicator is cut free from the balloon and descends on a parachute. The radiosonde's position coordinates and the transmitter beacon help the researchers locate and retrieve the reusable instrument package on the ground. Total weight of the replicator, including the parachute, is 1.6 kilograms (3.5 pounds, which is within FAA guidelines for free balloon launches). Now that a prototype has been built, Miloshevich estimates the cost of producing the replicator at $500 per unit.

The shapes of cloud particles play a major role in determining how clouds transmit, absorb, and reflect solar and infrared radiation. Atmospheric scientists would like to have a better grasp of the radiative properties of the high, thin cirrus clouds that hover in the upper troposphere between about 8 and 13 kilometers (5 and 8 miles) above sea level. The balloon-borne replicator fills in the gaps in cirrus cloud data from aircraft flights.

Choji Magono and Seiichi Tazawa (Hokkaido University, Japan) developed the basic technique for continuous replication on a balloon-borne instrument in 1966. On their team for that National Science Foundation-funded project was Heymsfield, then an undergraduate. In 1989 Heymsfield made some improvements to the instrument and began using replicator data to characterize cirrus-cloud microphysics. Further improvements in the quality and capabilities of the replicator have come since 1991, when Miloshevich began working with Heymsfield.

Results are available to the research community, but Miloshevich is the only one launching the replicator. "There are a hundred things to know to get it right," he explains. "It's one of the lowest-tech scientific instruments you'll find, but the apparent simplicity is deceptive."

For more information, contact Miloshevich (303-497-8963 or milo@ucar.edu) or Heymsfield (303-497-8943 or heyms1@ucar.edu).

Precision solar photometric telescope

by Carol Rasmussen

The PSPT (front dome) in action. (Photo courtesy of MLSO.)

A new dome has sprung from the lava at Mauna Loa Solar Observatory, housing a new telescope pointed at the disk of the sun. This instrument, called the precision solar photometric telescope (PSPT), records images of the full solar disk with its facial features--sunspots and their surrounding regions, called faculae and plages--at least once a day. These features mark places where magnetic flux tubes emerge on the visible surface of the sun and change the amount of radiation emitted toward the earth.

The PSPT will help answer the question of how changes in the solar features modify the star's total radiative output. Over the course of the sun's 11-year cycle, the percentage of the sun that's covered by spots changes from zero at solar minimum to as much as 30% at the peak of the cycle. Thus "It was extremely important to bring this telescope on line early enough so we could catch the solar minimum," says Michael Knölker, director of HAO. The telescope began taking observations in September 1997--close to the solar minimum, which occurred in the previous year. Last fall, virtually no features were visible, but already the sun's activity has increased considerably, with more sunspots and increasing solar activity such as a massive ejection that occurred in May.

The telescope is the second of its kind; the first is in Rome, operated by the Rome Observatory, and a third is planned for the National Solar Observatory (NSO) at Sacramento Peak, New Mexico. They are spaced around the globe to allow the longest possible uninterrupted observations of the solar disk each day. The instruments were designed by Jeff Kuhn (NSO) and built by him and his colleagues Hao-Sheng Lin and Roy Coulter. These new instruments are the centerpiece of NSF's Radiative Input from Sun to Earth (SunRISE) program, an initiative to measure and understand solar radiative variability. The HAO SunRISE group studying and interpreting the new data is Peter Fox, Mark Rast, Randle Meisner, and Oran (Dick) White.

The instrument includes a telescope and a filter wheel that rotates to allow pictures in three wavelengths. Behind the telescope is a charge-coupled-device (CCD) camera of 2,048 x 2,048 pixels. The three wavelengths are red (605-610 nanometers), blue (408-412 nm), and another blue wavelength of about 393 nm, called the calcium channel because it's the frequency at which the calcium IIK ion emits light. White explains that the wavelengths were chosen to show sunspots, plages, and faculae in two regions of the solar atmosphere: the photosphere, the lowest layer observable in visible light; and the chromosphere, a thin layer lying above the photosphere. (In fact, plages and faculae are names describing the same magnetic phenomenon as it appears at the two different heights.) "The calcium channel shows the bright regions in the chromosphere associated with sunspots," White explains. "Just a little bit to the red of the calcium band, we chose the so-called [blue] continuum channel to show sunspots and faculae lower in the photosphere. With the red channel near 600 nm, we're going to longer wavelengths where the amount of radiation from sunspots and plages will be different. Understanding such differences over the solar spectrum is a key component in the SunRISE program."

The precision of the instrument is certainly worth noting in its name: it's an extraordinary 0.1% per pixel. The PSPT is still being tested, with data being analyzed "over and over again," says Knölker. Fox and Meisner are making the first calculations with the new images. These first studies include surveys of data quality and "flat fielding" to remove artifacts due to sensitivity changes from pixel to pixel in the CCD camera. Since each image contains 4 million pixels, preparation of the final images requires a lot of computer resources to create a reliable and accurate data base. The data will be posted on the Web when the processing is complete, but certain special images are available now to colleagues who request them. The HAO PSPT group sees a steady stream of precision solar images being available soon. For further information, contact Fox (303-497-1511 or pfox@ucar.edu) or Meisner (303-497-1533 or meisner@ucar.edu).

Clear-air turbulence sensor

by Anatta

In April, during test flights aboard the NCAR/NSF Electra aircraft, a lidar sensor seeking clear-air turbulence over the Rocky Mountains signaled choppy air ahead, which aircraft occupants experienced seconds later as the plane flew through the turbulent patches. The lidar could eventually be part of a detection and warning system used on commercial planes to prevent injuries to passengers and crew.

The experiment is a collaboration among NASA, NSF, and Coherent Technologies, Inc. (CTI), of Lafayette, Colorado. The NASA Dryden Flight Research Center is leading the experiment, called ACLAIM (the Airborne Coherent Lidar Advanced In-flight Measurement). CTI built the Doppler lidar sensor, which uses laser beams to track the velocities of aerosol particles, some as small as a millionth of a meter, in air several kilometers ahead of the plane. As long as the velocities remain uniform, no turbulence exists. But if the laser beam detects changes in the velocities, it's a clear indication of turbulence ahead.

The technology used in the laser sensor can be envisioned as a kind of infrared radar. "The infrared radar concept uses a light pulse transmitted from the laser, and some of the light is reflected off the particles back to a sensor at the source," said Rod Bogue, NASA Dryden project manager. "The reflected light has a slight Doppler shift, due to the aircraft's motion relative to the particles. By analyzing the frequency of the shift, the changes in wind velocity along the laser beam's path can be determined."

On the first shakedown flight, "the system clearly established the viability of this technique by detecting even mild turbulence ahead," said NCAR project manager Allen Schanot (Atmospheric Technology Division), who was aboard the plane. On the second flight the lidar successfully detected moderate turbulence eight to ten seconds in advance of the plane as it flew several times into disturbed air along the mountain ridges near Pueblo, Colorado. According to Bogue, "If an alarm were sounded when the turbulence was first detected, passengers would have been able to return a short distance to their seats, seat themselves, and fasten seat belts prior to the encounter."

NCAR atmospheric scientist Larry Cornman will work with CTI scientists to analyze the data and to assess the ability of the lidar to measure turbulence quantitatively. Says Cornman, "Now that we know the device can 'see' clear-air turbulence just ahead, there's incentive to make it powerful enough to look farther and extend warning times."

For more information, contact Cornman (303-497-8439 or cornman@ucar.edu).

Glidersonde Demonstrator Program

by Zhenya Gallon

Ken Howard tests an early version of the glidersonde last year near Tucson, Arizona. (Photo by Mike Douglas, NSSL.)

In the late 1980s the escalating costs of radiosondes threatened to reduce or eliminate routine soundings at some sites in the worldwide observing network--particularly in developing countries. Dean Lauritsen of NCAR's Atmospheric Technology Division (ATD) had an idea. "It occurred to me that if a cheaper radiosonde was unlikely, perhaps a more costly but recoverable and reusable radiosonde would lower sounding costs," Lauritsen recalls. In early 1990, he sketched out his idea in an informal paper, suggesting that recent advances in Global Positioning System (GPS) technology, including miniaturization of receivers, made it possible to develop a miniature aircraft capable of flying a radiosonde along a predetermined route and returning it to a designated recovery site. Lauritsen was awarded a patent in 1991, but other commitments pushed the idea to the back burner.

Meanwhile, unaware of Lauritsen's work, Ken Howard at the National Severe Storms Laboratory (NSSL) in Norman, Oklahoma, and some colleagues were also investigating miniature aircraft possibilities. With consulting by Lauritsen and ATD colleagues Terrence Hock and Harold Cole, NSSL launched the Glidersonde Demonstrator Program, a collaboration among Davis Egle; Dudley Smith; Frank Gallagher, III (all of the University of Oklahoma); Nilton Renno (University of Arizona); and Sherman Fredrickson, Michael Douglas, and Howard (all at NSSL). The National Weather Service Office of International Activities has provided program seed money.

The glidersonde is an unpowered aircraft with a one-meter wingspan, carrying a radiosonde (or any other small atmospheric sampling device). Also on board are an autopilot, a GPS receiver, and a sonde microcomputer. The glidersonde is carried aloft by a standard 300-gram (10-ounce) balloon. An operator selects launch and landing coordinates and the altitude at which the glidersonde should release from the balloon. During balloon flight, the sonde microcomputer records temperature, pressure, and humidity, and derives wind speed and direction from the GPS data. Data can be transmitted or stored for downloading after landing.

Once at release altitude, the autopilot computes the return flight path. The autopilot continually adjusts the flight track in response to glidersonde heading, altitude, and position throughout the flight. When the glidersonde is about 30 meters (100 feet) above its landing target, a small parachute deploys, and the sonde floats down for a landing. Test flights this spring were designed to improve flight path control and landing target accuracy. The goal is to land repeatedly within 40 m (130 ft) of target.

To minimize costs, the glidersonde uses off-the-shelf components. The estimated cost of a production model will be under $1,500, with a usable life cycle of at least 30 flights. Since a standard operational radiosonde costs about $150, the savings are considerable.

Renno has also built four motorized, remotely piloted vehicles. Based on hobbyists' model airplanes, the current generation of research RPVs has the potential to carry instruments to places larger aircraft cannot go. In contrast to the long-range flights of the Australian aerosonde (see p. 15), the U.S. RPV project is focusing on microscale data from the boundary layer, including the fine detail of convective plumes and the updrafts of thunderstorms. "RPVs can fly into a convective system as long as it doesn't hail," Howard explains. "But we don't send them into the rain shaft or right into a tornado." At least not yet. Additional projects are under way to fly RPVs into dust devils in Arizona, with the ultimate goal of flying them into tornadoes.

See more about the Glidersonde Research Program on the Web .

Chromospheric helium imaging photometer

by Bob Henson

A solar disk image collected by CHIP on 24 May 1998. (Photo courtesy of Alice Lecinski.)

When the sun approaches solar maximum, great bubbles of atomic particles and magnetic fields are hurled out of the corona into space. If these mass ejections are pointed in the right direction, they can reach the earth in only a few hours and shower our atmosphere for one to four days, knocking out power grids and severing communication links. In the past, there was no way to spot these coronal mass ejections in advance, because only the edge of the sun could be monitored, not the disk, where the ejections coming our way originate. "It's the [mass ejections] on the disk that are dangerous," says Oran White (HAO), who is anticipating the fourth solar peak of his career.

An instrument at the NCAR High Altitude Observatory's Mauna Loa Solar Observatory is one of a new breed that's changing all that. These sensors can survey the entire solar disk, not just the sun's edge, for solar storms in the making. The chromospheric helium imaging photometer, nicknamed CHIP, takes a highly detailed picture of the solar disk every three minutes. Unlike a typical camera, it only sees a narrow band of light produced by helium. CHIP's frequent photos are the only ones on earth that can profile an hour-long mass ejection from start to finish. HAO associate scientist Alice Lecinski, who maintains a Web site with ejections captured in detail by CHIP, calls it "a unique and wonderful instrument."

The CHIP measurements are complemented by observations from two new satellites: the Solar Heliospheric Observatory (SOHO, deployed by NASA and the European Space Agency) and Yohkoh (deployed by the Japanese Space Agency). Scientists at NCAR are finding both satellites a boon to long-term research, but their data may also serve as warning tools in the next solar cycle. If a storm big enough to knock out regional power occurs, says White, "between CHIP and SOHO and Yohkoh, we'll see it." With the help of these sentries, alerts could be sent to storm-vulnerable utilities and satellite controllers.

For a look at CHIP and other solar images collected at Mauna Loa, see the observatory's Web site.

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Edited by Carol Rasmussen, carolr@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Tue Apr 4 14:55:01 MDT 2000