NASA - National Aeronautics and Space Administration

The High Resolution Dynamics Limb Sounder (HIRDLS) experiment was designed to provide global temperature and composition data on the region from the upper troposphere to the mesopause with vertical and horizontal resolution not previously available. The science objectives are the study of small-scale dynamics and transports, including stratosphere-troposphere exchange, upper troposphere/lower stratosphere chemistry, aerosol, cirrus and PSC distributions, and gravity waves. The instrument features 21 channels, low noise levels, high vertical resolution, and a mechanical cooler for long life. During launch most of the optical aperture became obscured, so that only a potion of an 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 radiances from some of the channels can be put into retrievals for temperature. Changes were also necessary for the retrieval algorithm. The validation of the resulting temperature retrievals is presented to demonstrate the effectiveness of these corrections. The random errors range from ~0.5 K at 20 km to ~1.0 at 60 km, close to those predicted. Comparisons with high-resolution radiosondes, lidars, ACE-FTS, and ECMWF analyses give a consistent picture of HIRDLS temperatures being 1–2 K warm from 200 to 10 hPa and within ±2 K of standards from 200 to 2 hPa (but warmer in the region of the tropical tropopause), above which HIRDLS appears to be cold. Comparisons show that both COSMIC and HIRDLS can see small vertical features down to about 2 km