VALIDATION OF CH4 AND N2O MEASUREMENTS BY THE CRYOGENIC LIMB ARRAY ETALON SPECTROMETER INSTRUMENT ON THE UPPER-ATMOSPHERE RESEARCH SATELLITE

CH4 and N2O are useful as dynamical tracers of stratospheric air trans port because of their long photochemical lifetimes over a wide range o f altitudes. The cryogenic limb array etalon spectrometer (CLAES) inst rument on the NASA UARS provided simultaneous global measurements of t he altitude profiles of CH4 and N2O mixing ratios in the stratosphere between October 1, 1991, and May 5, 1993. Data between January 9, 1995 and May 5, 1993 (388 days), have been processed using version 7 data processing software, and this paper is concerned with the assessment o f the quality of this data set. CLAES is a limb-viewing emission instr ument, and approximately 1200 profiles were obtained each 24-hour peri od for each constituent over a nominal altitude range of 100 to 0.1 mb ar (16 to 64 km). Each latitude was sampled 30 times per day between l atitudes 34 degrees S and 80 degrees N, or 34 degrees N and 80 degrees S depending on the yaw direction of the UARS, and nearly all local ti mes were sampled in about 36 days. This data set extends the altitude, latitude, and seasonal coverage of previous experiments, particularly in relation to measurements at high winter latitudes. To arrive at es timates of experiment error, we compared CLAES profiles for both gases with a wide variety of correlative data from ground-based, rocket, ai rcraft, balloon, and space-borne sensors, looked at the repeatability of multiple profiles in the same location, and carried out empirical e stimates of experiment error based on knowledge of instrument characte ristics. These analyses indicate an average single-profile CH4 systema tic error of about 15% between 46 and 0.46 mbar, with CLAES biased hig h. The CH4 random error over this range is 0.08 to 0.05 parts per mill ion, which translates to about 7% in the midstratosphere. For N2O the indicated systematic error is less than 15% at all altitudes between 6 8 and 2 mbar, with CLAES tending to be high below 6.8 mbar and low abo ve. The N2O random error is 20 to 5 ppb between 46 and 2 mbar, which a lso translates to 7% in the low to midstratosphere. Both tracers have useful profile information to as low as 68 mbar, excluding the tropics , and as high as 0.2 mbar (CH4) and 1 mbar (N2O). The global fields sh ow generally good spatial correlation and exhibit the major morphologi cal and seasonal features seen in previous global field data. Several morphological features are painted out for regions and conditions for which there have been essentially no previous data. These include the differential behavior of the tracer isopleths near and inside the Anta rctic winter vortex, and local maxima in the tropics in 1992, probably associated with the Mount Pinatubo sulfate aerosol layer. Overall, th e results of this validation exercise indicate that the version 7 CH4 and N2O data sets can be used with good confidence for quantitative an d qualitative studies of stratospheric and lower-mesospheric atmospher ic structure and dynamics.