High Resolution Dynamics Limb Sounder
HIRDLS HISTORY
Space-based experiments have contributed much to our knowledge of the stratosphere in recent years. These observations have been characterized by large horizontal or vertical scales, leaving a range of unobserved phenomena at smaller scales. This is especially true at the tropopause, the boundary between the troposphere and stratosphere, where rapid vertical changes in temperature and composition have been unobserved on a global basis.
The High Resolution Dynamics Limb Sounder (HIRDLS) experiment was designed to address problems for which these scales are important. HIRDLS is a 21 channel limb-scanning infrared radiometer designed to make global measurements at smaller vertical and horizontal scales than have been previously observed, from pole to pole, at altitudes of 8-80 km.
It is the result of the collaborative efforts of scientists and engineers in 3 groups; the Atmospheric, Oceanic and Planetary Physics group from Oxford University in the United Kingdom, the Center for Limb Atmospheric Sounding of the University of Colorado at Boulder, and the Atmospheric Chemistry Division of the National Center for Atmospheric Research, also in Boulder. Work on HIRDLS began with separate proposals to NASA in 1988. While some of the instrumental details were different, their scientific objectives were similar, and the groups agreed to work together on a single experiment. The experiment was selected for flight as part of the Earth Observing System (EOS) program, and later assigned to the Aura spacecraft.
During launch an unfortunate accident occurred, resulting in a blockage of most of the optical aperture, significantly reducing the coverage and longitudinal resolution. Only a portion of the optical beam width at a large azimuth from the orbital plane on the side away from the sun can see the atmosphere. Irrecoverable loss of capabilities include limitation of coverage to the region 65°S-82°N, and inability to obtain longitudinal resolution finer than an orbital spacing. While this optical blockage also impacted radiometric performance, extensive effort has gone into developing corrections for the several effects of the obstruction so that at this stage radiances from some of the channels can be put into retrievals for temperature, ozone, and nitric acid. Changes were also necessary for the retrieval algorithm. While further improvements in the data are expected, these data will allow HIRDLS to provide important support toward reaching the Aura objectives.