The Earth Observer

--Lelia B. Vann (l.b.vann@larc.nasa.gov), SAGE III Science Manager, Aerosol Research Branch, NASA Langley Research Center

On May 7-9, a Stratospheric Aerosol and Gas Experiment (SAGE) III Science Team meeting was conducted at the Langley Research Center (LaRC) to begin formal discussion of the data validation approach and to finalize discussion of the algorithm theoretical basis documents (ATBDs). M. Patrick McCormick, SAGE III Principal Investigator (PI), welcomed everyone and reviewed the agenda for the next three days. The first day was devoted to validation, and the remaining two days were devoted to ATBD discussions. McCormick introduced Dave Woods and Charles Trepte as the leads for coordinating the SAGE III Validation Planning.

Validation Discussions

Trepte discussed the preliminary plans for validation campaigns, and Woods discussed aircraft availability and possible airborne and balloon sensors. The focus was primarily on the 1998 Meteor-3M/SAGE III launch with the consideration that similar approaches would be used for the International Space Station mission in 2001 and the flight of opportunity (FOO) mission in 2005, with appropriate adjustments for the differences in orbits and launch times.

The SAGE III Standard Data Products were used as a guide for determining validation requirements. The proposed approaches included a combination of airborne campaigns, balloon flights, measurements at cooperative network ground sites, and comparisons with other satellite sensors. Candidate sensors for validating each SAGE III data product were discussed as well.

The following two University of Wyoming balloon packages were discussed:

1. The dustsonde, used for previous SAGE validations, measures vertical profiles of aerosol size distribution. The package also provides ozone and water vapor profiles.

2. The backscattersonde measures aerosol backscatter at two wavelengths and also provides ozone and water vapor profiles.

Either of these balloon systems can be deployed from one of several high-latitude sites (Northern Finland, Sweden, or Alaska).

The proposed aircraft flight options included: a late fall or early spring deployment from Poker Flat to observe a quiescent region; a late winter deployment from Thule in search of polar stratospheric clouds (PSCs); or flights to Canada or Alaska anytime.

The following aircraft are being considered for service as validation platforms:

• The ER-2 or WB-57, primarily for high-altitude in situ measurements, but with lidar on board if possible [either the Lidar Atmospheric Sensing Experiment (LASE) or Goddard's cloud lidar].

• The DC-8 for mid-altitude long-range flights with multiple lidars and other remote sensors as well as in situ measurements.

• The P-3 and C-130 are limited to altitudes less than 30,000 ft, where weather systems may interfere with measurements. However, they do have plenty of cargo capacity (can support multiple lidars) and very good range capability. Because of their altitude limitations, they are viewed as possible backups in case the DC-8 cannot be made available.

• The University of Washington's Convair-580 with a large suite of instruments equipped for studying clouds and tropospheric aerosols and gases.

McCormick strongly emphasized that the initial planning of validation campaigns should be driven by the science requirements.

Volker Mohnen stated that the validation should focus on aerosols and ozone in the upper troposphere and lower stratosphere (8 - 12 km) because this is the region of greatest need and greatest scientific challenge. Phil Russell agreed with Mohnen, but pointed out that this altitude region is much more difficult to study than the stratosphere because of spatial inhomogeneities. Mohnen advised considering the use of Mike Profit's UV-Absorption method for ozone on all aircraft flights because it is the best in situ method available for ozone.

Peter Hobbs pointed out the importance of knowing the lower detectable limits of each SAGE III product because this determines the range over which SAGE III can measure that species. He also mentioned that FIRE III will be conducted in Alaska in 1998. Since the ER-2 will be involved, this could potentially be an opportunity for us to work in some SAGE III validation flights while the aircraft are in Alaska.

Mohnen stated that we should use ozone sondes at the southern tip of Argentina because of their low cost and low risk. He did not think it was important to do an airborne campaign in the southern hemisphere. Jack Kaye mentioned a workshop in November in which sondes will be one of the main topics of discussion.

Derek Cunnold pointed out that a southern hemisphere campaign might be necessary to resolve differences in SAGE response between sunrises and sunsets.

Dave Rind made the point that we should always fly two water vapor sensors on aircraft for intercomparisons.

Albert Chernikov and Nikolai Elansky presented discussions on a proposed Russian network that will include several high-latitude ground stations with measurements of aerosols, nitrogen dioxide, ozone, and water vapor.

Phil Russell presented some data on the Altus remotely piloted vehicle (RPV), which is capable of reaching 65,000 ft., but is very limited in payload capacity.

Validation Conclusions

The validation portion of the Science Team meeting closed with the Science Team in general agreement on the approach of planning three aircraft campaigns within the first year after launch. Assuming an August 1998 launch, the first campaign will involve deploying the DC-8 from Fairbanks in the November/December time frame. This will be followed in January/February with the DC-8 and ER-2 or WB-57 deployed out of Thule in search of PSCs. A third campaign in the summer of 1999 will be deployed over either Canada or Alaska and will involve the DC-8, ER-2, or WB-57, and perhaps the Convair-580.

ATBD Discussions

The next two days (May 8-9), were devoted to the ATBDs and to spectroscopy requirements. The SAGE III team members are to complete and submit the ATBDs to the Mission to Planet Earth (MTPE) EOS Project Science Office in July of this year. The ATBDs will be externally peer reviewed, with written comments, followed by two weeks of oral presentation to a visiting committee.

To support this effort and reach group consensus on the content and quality of the documents, the Team focused on the review and revision of the documents with particular attention to the science justification, algorithm description, and quality (error budget) of the SAGE III products. Four subgroups were formed and focused on specific topics (aerosol, cloud, gases, and temperature). Plenary sessions were held to provide time for criticism, group discussion, and consensus.

Each subgroup focused on written revisions of the ATBDs and was requested to review the following areas:

Summary of issues:

• Science rationale:
  1. Justification
     
  2. Historical context and why SAGE III was
     selected for this measurement

• Algorithm:
  1. A clear and concise connection between the instrument
     measurements and the output products
     
  2. Justify the choice of algorithm (building on the SAM/SAGE
     heritage, and expected to evolve with experience with SAGE III)

• Quality of the Products:
  1. Error discussion
     
  2. Systematic error sources and discussion
     
  3. Implications for trend studies and validation

During the last plenary session of the day, it was suggested that since our products are closely related it made sense to combine all nine ATBDs into one document. McCormick discussed this approach with Michael King, the EOS Senior Project Scientist, and the decision was to keep all ATBDs separate so as to ease the review process.

Spectroscopy Discussions

Following the discussions at the January 1996 SAGE III Science Team meeting, the Team has reached consensus on the necessity and priority of laboratory spectroscopy measurements of key gaseous species. Eric Shettle prepared a detailed assessment of the current databases and provided recommendations.

The SAGE III Science Team examined the systematic uncertainties that will occur in the SAGE III measurements resulting from the quality of laboratory spectroscopic measurements and recommended that priority and funding be given to the following molecular-constituent determinations:

1. improved oxygen spectroscopy in the A band region (with a strong desire for simultaneous measurements of the B and C bands);

2. improved water vapor spectroscopy in the 940 nm band (870 to 1000 nm);

3. improved ozone spectroscopy at 940 nm and in all `aerosol-only' channels (1550, 1020, 872, 757, 675, 525, and 385 nm), especially at stratospheric temperatures, (down to 180 K);

4. improved nitrogen trioxide spectroscopy, 580 to 680 nm, including the temperature dependence over the range of stratospheric temperatures.

Some further considerations regarding the laboratory determinations are as follows:

1. Given the wide range of the various measurements of the intensities and line widths for the oxygen A band, it was agreed that additional measurements are needed to have confidence that the oxygen A band absorption cross sections are known to within the 2% needed to meet the SAGE III accuracy requirements.

2. Given the 50% uncertainties applied to the weakest water vapor lines when integrated over the 2 nm resolution of the SAGE III detector, the water vapor absorption is known to about 10-15%. To achieve greater accuracy for water vapor retrievals will require new measurements of the water vapor spectroscopy in the near infrared.

3. Since ozone absorbs at most of the wavelengths used for the retrieval of the other species, its contribution must be removed from the transmission measurements at those wavelengths. Particularly in the IR, the uncertainties in the ozone cross sections (especially at stratospheric temperatures) can impact the retrieval of water vapor, aerosols, and to a lesser extent oxygen.

4. The uncertainties in the NO2 absorption cross sections are 10-15% at room temperature and 20-40% at stratospheric temperatures. Improved measurements are needed, given these uncertainties.

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