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Crossing new HIRDLS: The limb sounder finds a home at CU

As a postdoctoral visitor, John Gille came to the Mesa Laboratory not long after its 1966 opening. Among his projects was the first scientific paper ever written on the concept of measuring the upper atmosphere from space through limb sounding. That idea has captured John's scientific attention ever since.

Now a senior scientist at ACD, John is leading the way toward the most refined realization of his dream to date. The high-resolution dynamics limb sounder (HIRDLS), in the works since the late 1980s, is now gearing up for several years of design and construction. NASA has distributed the work--Phase C/D, in NASA terminology--to a number of contractors in the U.S. and England, with focal points in Boulder and at Oxford University. HIRDLS is scheduled for launch aboard a NASA spacecraft in 2002.

The optical bench--the heart of the HIRDLS instrument--is portrayed in this cutaway illustration. The six-bladed chopper in the center creates an alternating signal that is more readily detected. The bench will be made of beryllium, which is very lightweight but makes machining complex. (Illustration courtesy Phil Arter.)

The instrument project entered Phase C/D last month by moving to a new institutional home. The Center for Limb Atmospheric Sounding (CLAS) officially opened in the Graduate School of the University of Colorado at Boulder on 1 February. The seven former ACD staff working in the HIRDLS Project Office are now employees of CU, although their office remains in UCAR North. As the U.S. co-principal investigator, John is working half-time for CU while remaining at NCAR as senior scientist on a half-time basis. Several other co-investigators and scientists will remain at NCAR (see sidebar).

Under federal acquisition regulations, the Phase C/D work--substantially more involved than the conceptual design work of Phase B--would have been subject to cost-accounting standards different than those normally used at UCAR. The creation of CLAS lessens that major administrative burden: CU becomes the prime contractor for NASA ($118 million from February 1997 through 2008). NCAR will be a subcontractor ($19.7 million for the same period) for scientific and computing facility use and reimbursement of investigators who remain at NCAR. Accounting standards differ when the primary contractor is a university, so the overall administrative burden on the project will be lessened significantly.

Many other positives could emerge from the creation of CLAS. "It's a mutually beneficial project," says CU professor Susan Avery, the founding director of CLAS and former chair of UCAR's Board of Trustees. According to the CLAS program plan, the center "will complement the existing array of earth remote-sensing research activities at the university and provide a new link to the national and global earth science research communities." Avery adds, "There will be educational opportunities for students and collaborations across many units in the university, as well as with NCAR and Oxford."

Noting CU's close ties to NOAA and the National Institute of Standards and Technology through two CU-based institutes, the plan states that "CLAS offers a new opportunity to establish a similarly close connection to the other large, federally sponsored laboratory in Boulder, NCAR. Over the years, individual faculty have worked with NCAR scientists on a wide variety of projects, but an institutional connection has been lacking and CLAS will provide one."

From limb to limb

Much like recently developed GPS/Meteorology technology, limb sounding measures upper parts of the atmosphere from space by focusing near the earth's horizon. However, GPS/MET relies on an external signal source (satellites of the Global Positioning System), while a limb sounder uses the atmosphere itself as a source. Infrared limb sounders measure the emissions in the infrared spectrum of various atmospheric molecules and aerosols. The term 'limb' refers to the edge of the visible disk of an astronomical body--in this case, the earth's horizon as seen from a satellite.

Although HIRDLS won't be the first limb sounder to be deployed, it will far surpass earlier sounders in its three-dimensional acuity. Instruments such as those on board the Upper Atmosphere Research Satellite (UARS, launched in 1991) have sampled temperatures and trace gases from the middle stratosphere upward. However, these data were gathered along narrow strips circling the earth, with the strips separated by data-free areas about 25 degrees wide (3000 kilometers at the equator).

In contrast, HIRDLS will provide a true grid of measurements, and a dense one at that. Its data will be collected twice a day over the entire earth from grid points separated by five degrees latitude and longitude. "We'll get within 300 [horizontal] kilometers of every point on the earth twice a day," notes John.

Moreover, unlike any previous limb sounder, HIRDLS will have a programmable scanning feature enabling it to zoom in on areas of particular interest at a resolution of just one by one degree. This on-the-fly capability will make it possible, for instance, to perform regional analyses on the stratosphere immediately after and near a volcanic eruption. Data will be processed within a few hours of its collection and passed on to the scientific world at large several days later.

HIRDLS will focus its attention on the lower stratosphere and upper troposphere, at somewhat lower altitudes than its predecessors. The goal is to get a fuller picture of the interchange between these regions, where chemical, aerosol, and momentum exchange play a role in weather and climate near the surface as well as in phenomena such as ozone depletion in the stratosphere.

John and his colleagues have amassed plenty of scientific questions waiting to be solved with the help of HIRDLS data. For instance, recent satellite observations have detected streamers of ozone-poor air emerging from the seasonally depleted vortex near the South Pole. Normally, says John, temperature and circulation gradients near the polar vortex serve as barriers to the leakage of ozone-depleted air. "There were theoretical models that indicated we should see narrow streamers coming off these barriers. Present observations, with their coarse horizontal resolution, can't resolve the streamers and are showing them wider than they may be. Without something like HIRDLS we can't really tell how narrow or wide they are."

NCAR and Oxford join forces

Joanne Loh and John Gille. (Photo by Carlye Calvin.)

The branches of the HIRDLS family tree begin with the limb radiance inversion radiometer (LRIR), launched in 1975. It was conceived by John and built at Honeywell in Boston. "Paul Bailey and I specified the instrument parameters, Bill Mankin and I took part in the calibration, and we developed the first data-reduction software for this type of data. Everything was breaking new ground. LRIR proved that the concept [of limb sounding] worked and provided some very interesting results, as well as showing us how to do it right." Data from LRIR were used in the Global Atmospheric Research Program (GARP) data sets.

Following LRIR was LIMS, the limb infrared monitor of the stratosphere instrument, deployed in 1978. It provided the first high-resolution look at stratospheric temperature and ozone and the first global measurements of stratospheric water vapor, nitric acid (present only in concentrations of a few parts per billion), and nitrogen dioxide (a naturally occurring gas that catalytically destroys ozone). LIMS also clearly demonstrated the existence of the Brewer-Dobson circulation, in which air ascends from the troposphere to the stratosphere in the tropics, slowly drifts to high latitudes, and descends into the lower atmosphere there. According to John, "Calculations based on the measurements showed that the motions could be understood in terms of theoretical concepts then being developed, which then became the basis for the two-dimensional stratospheric modeling that then blossomed."

Two instruments proposed by John's group in the mid-1980s for UARS failed to make the final cut. However, another limb instrument did, and it was created by the Department of Atmospheric Physics at Oxford. Meanwhile, says John, "I was involved in the planning for EOS [NASA's Earth Observing System]. In the process of thinking about where the science was going, I came up with the original idea for HIRRLS (hi-resolution research limb sounder). At about the same time, Oxford proposed a separate dynamics limb sounder. NASA and the British national space agency said, 'Hey, these proposals are very similar. Let's combine them and develop a single instrument.' "

John met with British investigator John Barnett in 1989 "over several beers in pubs around Reading" to discuss the idea of NCAR and Oxford building an instrument together. Later that year, once the formalities had been ironed out, the HIRRLS plan entered Phase B in its new incarnation as the NCAR/Oxford collaboration on HIRDLS.

As the satellite bearing HIRDLS orbits the earth, the instrument will scan for radiative emissions in the upper troposphere and lower stratosphere along six tracks (black lines). The tracks are separated by five degrees of radial distance at the observation point (vertical white lines), where they are perpendicular to the earth. A new set of observations is taken once the satellite has completed five degrees of orbit, which takes about a minute. (Illustration courtesy HIRDLS, adapted by Liesel Brunson.)

Collaborating across the Atlantic on something this large-scale has had its moments, especially for Joanne Loh, project manager. "We've had some big fax bills," she reports. "It's like any project--it takes a while to build up a team. Part of the Phase B effort was aimed at just that. I think we're now at the point where we have a good team together."

The project leaders keep in touch via weekly international phone calls and meetings that alternate between Boulder and Oxford every few weeks. About every week, says Joanne, each site holds "a coordinating team meeting among the managers and technical leads, going over problems and action items. The report goes out by e-mail to everybody in HIRDLS."

As Phase C/D unfolds, the pace is picking up at the CLAS office in UCAR North. "As we get into the detailed design and start to build things, there are a lot of people coming on board," says John. Researchers from a number of CU departments may become involved as collaborators in instrument development and potential data application. Undergraduate and graduate students also will find opportunities. The center expects to tackle follow-on experiments and other related work in time.

First, though, there's an instrument to build--and John says the hour is later than it seems. "We did have the advantage of a relatively relaxed schedule, but even planning for a launch in 2002, it's going to be tight." •BH

More information on HIRDLS can be found at the project's Web site.

The HIRDLS crew

Most of the HIRDLS project staff at UCAR North, employees of NCAR through January, now work for CU/CLAS. They include:

John Gille, U.S. co-principal investigator
Joanne Loh, project manager
Mike Dials, technical manager
Phil Arter and Doug Woodward, project engineers
Dave Wilson, contracts manager
Russ Howard, project assurance
Linda Henderson, administrator

Those remaining at NCAR include:

Paul Bailey, co-investigator and data manager
Byron Boville, Guy Brasseur, Mike Coffey, and Bill Mankin, co-investigators
David Edwards, scientist
Chris Halvorson, Brian Johnson, and Jinxue Wang, instrument scientists
B. J. Heller, administrator

What it measures

HIRDLS is designed to measure the temperature and concentrations of the atmospheric constituents below, as well as the presence of miscellaneous aerosols (sulfur from volcanoes, for instance).

Temperature and ozone will be measured from 8 to 80 kilometers, with other species being measured over part of that range. All will be measured with a vertical resolution of about 1 km. At higher altitudes, the atmospheric components are so sparse that each species can maintain a different temperature. However, says John Gille, "for the altitudes of greatest interest to us, things bump into each other enough so that all the temperatures are the same."

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Edited by Bob Henson, bhenson@ucar.edu