J. B. Kumer

The cryogenic limb array etalon spectrometer (CLAES) on UARS: Experiment description and performance

The cryogenic limb array etalon spectrometer (CLAES) is one of 10 experiments launched in September 1991 on the NASA Upper Atmosphere Research Satellite (UARS). CLAES measures altitude profiles of temperature, pressure, O3, H2O, CH4, N2O, NO, NO2, N2O5, HNO3, ClONO2, HCl, CFC 11, CFC 12, and aerosol absorption coefficients. These data are obtained between 10 and 60 km with 2.5-km vertical resolution and 500-km horizontal grid size and between latitudes 80° north and south. Since CLAES actually measures infrared spectral earthlimb emissions, it can operate continuously throughout the diurnal cycle. The on-orbit lifetime as dictated by stored cryogens which cool optics and detectors is estimated to be 21 months. The experiment will perform the first global mapping of stratospheric ClONO2, CFC 11, CFC 12, and N2O5, and these data, along with the simultaneous measurement of temperature and the other constituents listed above, should contribute to a significant improvement in our understanding of stratospheric and mesospheric photochemistry, radiative structure, and dynamics. CLAES began viewing the atmosphere in early October 1991, and the first several months of observations will be discussed. Examples of atmospheric spectral emission profiles for a number of constituents are presented as well as responsivity and noise parameters. These data show the instrument performance to be excellent and close to prelaunch predictions. An overview of the experiment and instrumentation is presented, various scientific observational modes are described, and the algorithms and software used to retrieve atmospheric parameters from emission spectra are discussed.

Validation of CH4 and N2O measurements by the cryogenic limb array etalon spectrometer instrument on the Upper Atmosphere Research Satellite

CH 4 and N 2 O are useful as dynamical tracers of stratospheric air transport because of their long photochemical lifetimes over a wide range of altitudes. The cryogenic limb array etalon spectrometer (CLAES) instrument on the NASA UARS provided simultaneous global measurements of the altitude profiles of CH 4 and N 2 O mixing ratios in the stratosphere between October 1, 1991, and May 5, 1993. Data between January 9, 1992, and May 5, 1993 (388 days), have been processed using version 7 data processing software, and this paper is concerned with the assessment of the quality of this data set. CLAES is a limb-viewing emission instrument, and approximately 1200 profiles were obtained each 24-hour period for each constituent over a nominal altitude range of 100 to 0.1 mbar (16 to 64 km). Each latitude was sampled 30 times per day between latitudes 34°S and 80°N, or 34°N and 80°S depending on the yaw direction of the UARS, and nearly all local times 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 estimates of experiment error, we compared CLAES profiles for both gases with a wide variety of correlative data from ground-based, rocket, aircraft, balloon, and space-borne sensors, looked at the repeatability of multiple profiles in the same location, and carried out empirical estimates of experiment error based on knowledge of instrument characteristics. These analyses indicate an average single-profile CH 4 systematic error of about 15% between 46 and 0.46 mbar, with CLAES biased high. The CH 4 random error over this range is 0.08 to 0.05 parts per million, which translates to about 7% in the midstratosphere. For N 2 O the indicated systematic error is less than 15% at all altitudes between 68 and 2 mbar, with CLAES tending to be high below 6.8 mbar and low above. The N 2 O random error is 20 to 5 ppb between 46 and 2 mbar, which also 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 (CH 4 ) and 1 mbar (N 2 O). The global fields show generally good spatial correlation and exhibit the major morphological and seasonal features seen in previous global field data. Several morphological features are pointed 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 Antarctic winter vortex, and local maxima in the tropics in 1992, probably associated with the Mount Pinatubo sulfate aerosol layer. Overall, the results of this validation exercise indicate that the version 7 CH 4 and N 2 O data sets can be used with good confidence for quantitative and qualitative studies of stratospheric and lower-mesospheric atmospheric structure and dynamics.

Simulation of Stratospheric N2O in the NCAR CCM2: Comparison with CLAES Data and Global Budget Analyses

Abstract Global variability and budgets of stratospheric nitrous oxide (N2O) are studied using output from a stratospheric version of the NCAR Community Climate Model. The model extends over 0–80 km, incorporating an N2O-like tracer with tropospheric source and upper-stratospheric photochemical sink, the latter parameterized using linear damping rates obtained from detailed two-dimensional model calculations. Results from the model over several seasonal cycles are compared with observations of N2O from the Cryogenic Limb Array Etalon Spectrometer instrument on the Upper Atmosphere Research Satellite. The model produces N2O structure and variability that is in reasonable agreement with the observations. Global budgets of stratospheric N2O are furthermore analyzed using model output, based on the transformed Eulerian-mean, zonal-mean framework. These budgets are used to quantify the importance of planetary wave constituent transport in the stratosphere, for both slow seasonal variations and fast planetary w...

Comparison of correlative data with HNO3 version 7 from the CLAES instrument deployed on the NASA Upper Atmosphere Research Satellite

The cryogenic limb array etalon spectrometer (CLAES) aboard UARS made near-global measurements of HNO3 and 388 days from January 9, 1992, to April 25, 1993, have been processed to data version 7 (V7). Results from UARS instruments, including CLAES, the improved stratospheric and mesospheric sounder, and the microwave limb sounder, provide the first near-global documentation of the evolution of denitrification in the Antarctic 1992 winter and spring vortex. We provide a description of the CLAES HNO3 V7 quality that includes comparisons with correlative measurements to assess overall quality, accuracy, and precision. Correlative profiles of volume mixing ratio (vmr) included those obtained by the space shuttle deployed ATMOS in two missions, March–April 1992 and April 1993, data from a variety of balloon-borne instruments at midlatitude (11 profiles), and in high-latitude northern winter (six profiles), and LIMS data. In general, the CLAES V7 HNO3 maximum values of vmr were of the order of 6–15% less than correlative for CLAES values ≤8 parts per billion by volume (ppbv). However, when CLAES peak vmr values were 10 to 13 ppbv, then CLAES values exceeded correlative by 0–7%. The comparisons were within the combined instrumental error estimates, or observed measurement variability, for the large majority of comparisons. As discussed, the retrieval of future versions will utilize updated spectral parameters and will also correct for a small uncompensated drift in radiometric calibration that occurred in the latter part of the mission. This is expected to improve the comparisons in the ≤8 ppbv range, perhaps at the expense of those in the ≥8 ppbv range. The data obtained January 9 to April 15, 1992, in comparison with data obtained January 9 to April 15, 1993, reveal strikingly evident 1-year period deseasonalized trends on a global basis. These trends agree quantitatively with available correlative data suitable for trend analysis. These include ATMOS in the southern midlatitudes and published long-term time series of HNO3 column obtained at 45°S and 20°N. These trends reveal a large decrease in the southern hemisphere and small increases in the northern hemisphere, such that the global average is toward a decrease. The global average decrease we attribute to the diminishing influence of heterogeneous conversion of N2O5 to HNO3 as the Pinatubo aerosol settles out during this time period, and the HNO3 recovers toward pre-Pinatubo conditions. We establish plausibility that the small increases in the north are due to hemispherically asymmetric QBO-like effects that are strong in the northern hemisphere and weak in the southern hemisphere and are phased to produce an increase in HNO3 over the 1-year time period of just the right magnitude to more than offset decrease due to settling out of the Pinatubo aerosol. Based on this study, our range of confidence in the CLAES HNO3 V7 product is from 70 to 3 mbar, in comparison with correlative data, and the precision on this range is of the order of 0.3–1.0 ppbv. This precision was derived from data repeatability and agrees within a factor of 2 or better with estimates based on instrument characterization and with error estimates embedded within the V7 data.

Chlorine deactivation in the lower stratospheric polar regions during late winter: Results from UARS

Recovery from enhanced chlorine conditions in the lower stratospheric polar regions of both hemispheres is investigated using data from the Upper Atmosphere Research Satellite (UARS). Microwave Limb Sounder (MLS) measurements of ClO within the polar vortices are used to infer ClOx (ClO + 2Cl2O2) abundances that are then correlated with simultaneous Cryogenic Limb Array Etalon Spectrometer (CLAES) measurements of ClONO2 and Halogen Occultation Experiment (HALOE) measurements of HCl obtained starting within 5 days of the end of the MLS and CLAES high-latitude observing periods in each hemisphere. Time series of vortex-averaged mixing ratios are calculated on two potential temperature surfaces (585 K and 465 K) in the lower stratosphere for approximately month-long intervals during late winter: August 17 – September 17, 1992, in the southern hemisphere and February 12 – March 16, 1993, in the northern hemisphere. The observed mixing ratios are adjusted for the effects of vertical transport using diabatic vertical velocities estimated from CLAES tracer data. In the northern hemisphere, the decrease in ClOx is balanced on both surfaces by an increase in ClONO2. In the southern hemisphere, continuing polar stratospheric cloud activity prevents ClO from undergoing sustained decline until about September 3. In contrast to the northern hemisphere, there is no significant chemical change in vortex-averaged ClONO2 at 465 K, and there is an apparent decrease in ClONO2 at 585 K, even after the enhanced ClO abundances have started to recede. Results from the SLIMCAT chemical transport model [Chipperfield et al., this issue] initialized with UARS data and run with OH + ClO → HCl + O2 as an 8% channel suggest that the primary recovery product in the south during this time period is not ClONO2, but HCl. HALOE HCl mixing ratios are extrapolated back to the time of the MLS and CLAES data. At 585 K, the chlorine budget can be made to balance by extrapolating HCl back to a value of 0.6 parts per billion by volume (ppbv) at the beginning of the study period; at 465 K, the contribution from extrapolated HCl is not sufficient to offset the loss in ClOx, and there is a slight imbalance between the decrease in reactive chlorine and the change in chlorine reservoirs. The difficulty in closing the chlorine budget in the southern hemisphere may arise from complications caused by ongoing activation, incomplete photochemical assumptions, and/or inadequate data quality.

Missing chemistry of reactive nitrogen in the upper stratospheric polar winter

Data from the CLAES on UARS indicate that a significant mechanism for production of HNO{sub 3} in the middle to upper stratosphere is missing from the chemical reaction set currently used by atmospheric models. Measured HNO{sub 3} in the polar vortex is strongly enhanced relative to the extra-vortex at 1200 K potential temperature (near 3 mbar) in January, 1992. The HNO{sub 3} vertical profile shows this enhancement forms a secondary altitude maximum from about 10 to 2 mbar (800-1500 K). A chemistry/transport model (CTM) simulation of this period produces no increase of HNO{sub 3} in the vortex near 3 mbar and no secondary maximum in the HNO{sub 3} profile. Furthermore, the CTM produces relatively high N{sub 2}O{sub 5} in the vortex, with a vertical peak near 3 mbar, while both CLAES and ISAMS show a shallow minimum there. The implication of this comparison is that some unmodeled process is acting to enhance HNO{sub 3} and reduce N{sub 2}O{sub 5} at high latitudes in the winter middle and upper stratosphere. Heterogeneous conversion of N{sub 2}O{sub 5} to HNO{sub 3} on hydrated ion clusters is proposed as a possibility for the missing mechanism. 15 refs., 5 figs.

Global evolution of the Mt. Pinatubo volcanic aerosols observed by the infrared limb-sounding instruments CLAES and ISAMS on the Upper Atmosphere Research Satellite

The cryogenic limb array etalon spectrometer (CLAES) and the improved stratospheric and mesospheric sounder (ISAMS) instruments on board the Upper Atmosphere Research Satellite (UARS) have been used to produce global information on the Mt. Pinatubo volcanic aerosol for the period from October 1991 to April 1993. The satellite infrared extinction measurements near 12 μm are converted into the aerosol-related parameters necessary for modelling the effects of the volcanic aerosol on the aeronomy of the stratosphere and are presented as zonal mean distributions for 80°S to 80°N averaged over ∼35-day periods. The aerosol composition is derived from the CLAES and ISAMS temperature measurements and the water vapour abundances are obtained from the microwave limb sounder (MLS). The aerosol volume density is obtained from the extinction measurements from which the surface area density and the effective particle radius are estimated. The maximum aerosol surface area density has a value of about 50 μm 2 cm -3 at a height of 24 km at the equator in October 1991, before decaying exponentially with a time constant of 443 ± 10 days. The surface area density remained well above preeruption values in April 1993. The effective particle radius in the tropics decays monotonically from 0.65 μm in October 1991 to 0.4 μm in April 1993. The global aerosol sulphate mass loading is 19.5 Mt in October 1991 and decays exponentially with a time constant of 342 ± 8 days to a value of 4.3 Mt by April 1993. Four months after the eruption the calculated optical thickness at 1.02 μm was ∼0.25 in the tropics. Rate constants are derived for the heterogeneous reactions of N 2 O 5 and ClONO 2 on the sulphate aerosols. The application of the aerosol parameters to the investigation of tracer transport, heterogeneous chemistry, and radiative transfer is discussed.

Accuracy and precision of cryogenic limb array etalon spectrometer (CLAES) temperature retrievals

The Cryogenic Limb Array Etalon Spectrometer (CLAES) measured emission from the 792 cm−1 Q branch of CO2, from which temperature distributions in the stratosphere and low mesosphere were derived. Here we briefly review the measurement technique, concentrating on aspects that affect the temperature determination. Comparison of many pairs of retrievals at the same location (near 32°N or 32°S) measured on sequential orbits (time separation of 96 min) shows a precision ranging from approximately 0.8 K at 68 mbar to about 3.5 K at 0.2 mbar, which agrees with simulations incorporating random noise and short-period spacecraft motions. Comparisons of globally analyzed CLAES data with National Meteorological Center (NMC) and U.K. Meteorological Office (UKMO) analyses show general agreement, with CLAES tending to be cooler by about 2 K, except in the tropics and high-latitude winter conditions. This is supported by comparisons with individual radiosondes and several lidars that indicate that the agreement is within 2 K throughout the profile (except for a narrow layer around 3 mbar). An error analysis also indicates that systematic errors should be roughly 2 K, independent of altitude. The systematic differences at low latitudes appear to be due to tropical waves, which have vertical wavelengths too short to be seen by the TIROS Operational Vertical Sounder (TOVS) instruments. There are no correlative rocketsondes or lidars to help resolve the reasons for the high-latitude differences. Comparisons with other Upper Atmosphere Research Satellite (UARS) data should shed additional light on this question.

Formation of low-ozone pockets in the middle stratospheric anticyclone during winter

Microwave limb sounder observations of midstratospheric ozone during stratospheric warmings show tongues of high ozone drawn up from low latitudes into the developing anticyclone. Several days later, an isolated pocket of low ozone mixing ratios appears, centered in the anticyclone, and extending in the vertical from ≈15 to 5 hPa, with higher mixing ratios both above and below. These low ozone mixing ratios during northern hemisphere warmings are comparable to values well inside the vortex and are ≈3 parts per million by volume lower than typical midlatitude extra-vortex mixing ratios. This type of feature is seen whenever the anticyclone is strong and persistent, including during relatively strong minor warmings in the southern hemisphere. Three-dimensional back trajectory calculations indicate that the air in the region of the low-ozone pockets originates at higher altitudes and low latitudes, where ozone mixing ratios are much higher. The air parcels studied here are typically confined together for 1 to 3 weeks before the lowest ozone mixing ratios are observed. The trajectory calculations and comparisons with passive tracer data confirm that the observed low-ozone regions in the midstratosphere could not result solely from transport processes.

Correlated observations of HCl and ClONO2 from UARS and implications for stratospheric chlorine partitioning

We present the first near-global set of correlated measurements of stratospheric HCl and ClONO2 concentrations. These data, obtained from the Upper Atmosphere Research Satellite (UARS) between August 1992 and March 1993, are analyzed using the Goddard trajectory mapping technique. These data indicate that, between 20 and 30 km altitude and 60°N to 60°S latitude, total inorganic chlorine (Cly) is fairly evenly distributed between ClONO2 and HCl, with HCl the slightly more dominant reservoir. The sum of UARS measurements of HCl and nighttime ClONO2, which approximates Cly at these altitudes, closely agrees with Cly derived from a tracer-Cly relation originally derived from aircraft data, increasing confidence in the UARS data. Comparisons between these data and two-dimensional model results suggest that models underpredict ClONO2 (overpredict HCl) near 20 km and overpredict ClONO2 (underpredict HCl) near 30 km, although the 20-km underprediction is not as large as analyses of aircraft measurements have indicated.