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High Resolution Dynamic Limb Sounder (HIRDLS) Group
HIRDLS Group Members:
Summary of Activities:
The High Resolution Dynamics Limb Sounder (HIRDLS) is a 21 channel infrared limb scanning radiometer, jointly developed by ACD, the University of Colorado , and the Physics Department of Oxford University. It is designed to make observations of temperature, O 3 , H 2 O, and 8 other trace species, as well as PSC's, aerosols and cirrus clouds, from the upper troposphere to the mesosphere, with higher vertical resolution than has previously been available from space observations. NASA funded the U.S. share of the HIRDLS development.
Previous reports have described how, when HIRDLS was launched on the Aura spacecraft in July 2004, a plastic lamina from inside HIRDLS came loose and obstructed most of the instrument's aperture, limiting the view to the atmosphere to a small fraction of the width of the optical beam. Attempts to shake it off were unsuccessful.
However, enough had been learned for the team, led by John Gille, the U.S. PI, and John Barnett ( Oxford ), the U.K. PI, to propose that they be allowed several months to show they could make use of the signals that could be seen through the partial aperture. These efforts demonstrated that there was recoverable atmospheric information in the signals, and initial results for temperature, ozone, PSC's and cirrus were very encouraging. Based on these results, NASA approved the continuation of the HIRDLS effort.
These data were presented and discussed at the Aura Science Team meeting in the Hague , the Netherlands in November. Immediately thereafter the HIRDLS Science Team met in Oxford for more extensive review of the data, with the goal of identifying problems and planning the following steps.
The top-level approach is to correct the measured signals to make them as close as possible to the expected radiances. The major efforts this year were to improve and refine the correction algorithms, make them more robust, and if possible develop a physical basis for them that would assure that they would work under all conditions.
The 3 required corrections are the removal of spurious oscillations (due to mechanical oscillation of the plastic) from the signal, correction for the reduced viewing area, and subtraction of the infrared signal from the plastic itself. A key method for determining these corrections is to have the spacecraft pitch by ~5°, so that HIRDLS looks above the atmosphere and measures signals only from the plastic sheet, thus allowing their characteristics to be determined. James Craft (CU, NCAR Visitor) coordinated 4 of these pitch maneuvers this year. Hyunah Lee and Chris Halvorson developed the algorithm to remove the effects of the oscillations, which turned out to be quite regular in frequency and phase, although differing between up and down scans. The method involves determining sets of 16 empirical orthogonal functions (EOF's) to describe the oscillations in one channel, and scaling the results to the other channels. Sets of EOF's have been developed for all 4 scan patterns, and incorporated recently in the operational code. Tom Eden showed that it was possible to model the variation of the radiometric signals from the obstruction during the night side of the orbit as a function of instrument temperatures. By doing this for vertical scans at different azimuth angles, it is possible to estimate the open area fraction (OAF) through which the different channels view the atmosphere. To correct for the infrared signal reflected and emitted from the plastic, Gene Francis developed a precise parameterization of the pitch-up signal in terms of a mean and 3 EOF's, while Bruno Nardi developed a physically based model. Rashid Khosravi and Cheryl Craig modified the level 2 retrieval algorithm to accommodate the new conditions. The requirements to develop these algorithms has meant that the scientists have had to concentrate on these activities, and not spend their time on use of the data for scientific studies.
The NCAR part of the team hosted the next HIRDLS science team meeting, held in June. Again, the team reviewed the correction algorithms and approaches, presented comparisons of HIRDLS results with other data, and discussed future activities. The comparisons showed that the nitric acid data had also reached an acceptable quality level. The team agreed to make the temperature, ozone, nitric acid and cloud top pressure data publicly available to those who wanted to assist in its validation and evaluation.
The HIRDLS team hosted the Aura Science Team meeting, attended by over 220 people, at NCAR on September 11-15. In association with this, several working groups met to discuss data validation as well as science applications of the data. HIRDLS team members presented results based on improved processing codes.
Among a number of noteworthy results, Steve Massie showed that HIRDLS could map sub-visible cirrus in the tropical upper troposphere. These showed a movement of the thickest layers from SE Asia to India to Indonesia , and from Central to South America , over the May to October time period. These cirrus layers play an important role in the earth's radiative balance as well as in dehydrating the UT/LS, and are a unique measurement by HIRDLS on Aura.
Further improvements continue to be incorporated in the operational data processing code. Initially they are used to process of days for which there are validation data. The goal is to improve the results as rapidly as possible, with the desire to recover all the originally expected species.
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