L. Froidevaux

The Earth observing system microwave limb sounder (EOS MLS) on the aura Satellite

The Earth Observing System Microwave Limb Sounder measures several atmospheric chemical species (OH, HO/sub 2/, H/sub 2/O, O/sub 3/, HCl, ClO, HOCl, BrO, HNO/sub 3/, N/sub 2/O, CO, HCN, CH/sub 3/CN, volcanic SO/sub 2/), cloud ice, temperature, and geopotential height to improve our understanding of stratospheric ozone chemistry, the interaction of composition and climate, and pollution in the upper troposphere. All measurements are made simultaneously and continuously, during both day and night. The instrument uses heterodyne radiometers that observe thermal emission from the atmospheric limb in broad spectral regions centered near 118, 190, 240, and 640 GHz, and 2.5 THz. It was launched July 15, 2004 on the National Aeronautics and Space Administrations Aura satellite and started full-up science operations on August 13, 2004. An atmospheric limb scan and radiometric calibration for all bands are performed routinely every 25 s. Vertical profiles are retrieved every 165 km along the suborbital track, covering 82/spl deg/S to 82/spl deg/N latitudes on each orbit. Instrument performance to date has been excellent; data have been made publicly available; and initial science results have been obtained.

Overview of the EOS aura mission

Aura, the last of the large Earth Observing System observatories, was launched on July 15, 2004. Aura is designed to make comprehensive stratospheric and tropospheric composition measurements from its four instruments, the High Resolution Dynamics Limb Sounder (HIRDLS), the Microwave Limb Sounder (MLS), the Ozone Monitoring Instrument (OMI), and the Tropospheric Emission Spectrometer (TES). With the exception of HIRDLS, all of the instruments are performing as expected, and HIRDLS will likely be able to deliver most of their planned data products. We summarize the mission, instruments, and synergies in this paper.

Validation of the Aura Microwave Limb Sounder middle atmosphere water vapor and nitrous oxide measurements

[1] The quality of the version 2.2 (v2.2) middle atmosphere water vapor and nitrous oxide measurements from the Microwave Limb Sounder (MLS) on the Earth Observing System (EOS) Aura satellite is assessed. The impacts of the various sources of systematic error are estimated by a comprehensive set of retrieval simulations. Comparisons with correlative data sets from ground-based, balloon and satellite platforms operating in the UV/visible, infrared and microwave regions of the spectrum are performed. Precision estimates are also validated, and recommendations are given on the data usage. The v2.2 H2O data have been improved over v1.5 by providing higher vertical resolution in the lower stratosphere and better precision above the stratopause. The single-profile precision is � 0.2–0.3 ppmv (4–9%), and the vertical resolution is � 3–4 km in the stratosphere. The precision and vertical resolution become worse with increasing height above the stratopause. Over the pressure range 0.1–0.01 hPa the precision degrades from 0.4 to 1.1 ppmv (6–34%), and the vertical resolution degrades to � 12–16 km. The accuracy is estimated to be 0.2–0.5 ppmv (4–11%) for the pressure range 68–0.01 hPa. The scientifically useful range of the H2O data is from 316 to 0.002 hPa, although only the 82–0.002 hPa pressure range is validated here. Substantial improvement has been achieved in the v2.2 N2O data over v1.5 by reducing a significant low bias in the stratosphere and eliminating unrealistically high biased mixing ratios in the polar regions. The single-profile precision is � 13–25 ppbv (7–38%), the vertical resolution is � 4–6 km and the accuracy is estimated to be 3–70 ppbv (9–25%) for the pressure range 100–4.6 hPa. The scientifically useful range of the N2O data is from 100 to 1 hPa.

Validation of Aura Microwave Limb Sounder stratospheric ozone measurements

[1] The Earth Observing System (EOS) Microwave Limb Sounder (MLS) aboard the Aura satellite has provided essentially daily global measurements of ozone (O3) profiles from the upper troposphere to the upper mesosphere since August of 2004. This paper focuses on validation of the MLS stratospheric standard ozone product and its uncertainties, as obtained from the 240 GHz radiometer measurements, with a few results concerning mesospheric ozone. We compare average differences and scatter from matched MLS version 2.2 profiles and coincident ozone profiles from other satellite instruments, as well as from aircraft lidar measurements taken during Aura Validation Experiment (AVE) campaigns. Ozone comparisons are also made between MLS and balloon-borne remote and in situ sensors. We provide a detailed characterization of random and systematic uncertainties for MLS ozone. We typically find better agreement in the comparisons using MLS version 2.2 ozone than the version 1.5 data. The agreement and the MLS uncertainty estimates in the stratosphere are often of the order of 5%, with values closer to 10% (and occasionally 20%) at the lowest stratospheric altitudes, where small positive MLS biases can be found. There is very good agreement in the latitudinal distributions obtained from MLS and from coincident profiles from other satellite instruments, as well as from aircraft lidar data along the MLS track.

Microwave limb sounder measurement of stratospheric SO2 from the Mt. Pinatubo Volcano

This paper presents measurements of sulfur dioxide densities in the stratosphere made by the microwave limb sounder (MLS) on the upper atmosphere research satellite. The SO[sub 2] came from the eruption of the Mt Pinatubo volcano which injected a massive quantity of gas into the stratosphere. The MLS is able to measure the decay rate of the gas densities based on its extended time and spatial coverage, and from this decay rate infer the OH densities in the stratosphere, since OH is the major reactive species which converts the SO[sub 2] into sulfuric acid.

Validation of the Aura Microwave Limb Sounder ClO measurements

We assess the quality of the version 2.2 (v2.2) ClO measurements from the Microwave Limb Sounder (MLS) on the Earth Observing System Aura satellite. The MLS v2.2 ClO data are scientifically useful over the range 100 to 1 hPa, with a single- profile precision of similar to 0.1 ppbv throughout most of the vertical domain. Vertical resolution is similar to 3-4 km. Comparisons with climatology and correlative measurements from a variety of different platforms indicate that both the amplitude and the altitude of the peak in the ClO profile in the upper stratosphere are well determined by MLS. The latitudinal and seasonal variations in the ClO distribution in the lower stratosphere are also well determined, but a substantial negative bias is present in both daytime and nighttime mixing ratios at retrieval levels below (i. e., pressures larger than) 22 hPa. Outside of the winter polar vortices, this negative bias can be eliminated by subtracting gridded or zonal mean nighttime values from the individual daytime measurements. In studies for which knowledge of lower stratospheric ClO mixing ratios inside the winter polar vortices to better than a few tenths of a ppbv is needed, however, day - night differences are not recommended and the negative bias must be corrected for by subtracting the estimated value of the bias from the individual measurements at each affected retrieval level.

Residual circulation in the stratosphere and lower mesosphere as diagnosed from microwave limb sounder data

Results for the residual circulation in the stratosphere and lower mesosphere between September 1991 and August 1994 are reported. This circulation is diagnosed by applying an accurate radiative transfer code to temperature, ozone, and water vapor measurements acquired by the Microwave Limb Sounder (MLS) onboard the Upper Atmosphere Research Satellite (UARS), augmented by climatological distributions of methane, nitrous oxide, nitrogen dioxide, surface albedo, and cloud cover. The sensitivity of the computed heating rates to the presence of Mt. Pinatubo aerosols is explored by utilizing aerosol properties derived from the measurements obtained by the Improved Stratospheric and Mesospheric Sounder instrument, also onboard UARS. The computed vertical velocities exhibit a Semiannual oscillation (SAO) around the tropical stratopause, with the region of downward velocities reaching maximum spatial extent in February and August. This behavior reflects the semiannual oscillation in temperature and ozone and mimics that seen in past studies of the October 1978–May 1979 period based on data from the Limb Infrared Monitor of the Stratosphere onboard the Nimbus 7 satellite. The SAO vertical velocities are stronger during the northern winter phase, as expected if planetary waves from the winter hemisphere are involved in driving the SAO. A possible quasi-biennial oscillation (QBO) signal extending from the middle into the upper stratosphere is also hinted at, with the equatorial vertical velocities in the region 10–1 hPa significantly smaller (or even negative) in 1993/94 than in 1992/93. Despite the short data record, the authors believe that this pattern reflects a QBO signal rather than a coincidental interannual variability, since the time–height section of vertical velocity at the equator resembles that of the zonal wind. Wintertime high-latitude descent rates are usually greater in the Northern Hemisphere, but they also exhibit significant variability there. In the three northern winters analyzed in this study, strong downward velocities are diagnosed in the lower stratosphere during stratospheric warmings and are associated with enhanced wave forcing (computed as the momentum residual) in the mid- and upper stratosphere. The implications of the computed circulation for the distribution of tracers are illustrated by the example of the “double-peaked” structure in the water vapor distribution measured by MLS.

Analysis of UARS data in the southern polar vortex in September 1992 using a chemical transport model

We have used a new, isentropic-coordinate three-dimensional chemical transport model to investigate the decay of ClO and evolution of other species in the Antarctic polar vortex during September 1992. The model simulations cover the same southern hemisphere period studied in a companion data paper by Santee et al. [this issue]. The model is initialized using the available data from the Microwave Limb Sounder (MLS) and Cryogenic Limb Array Etalon Spectrometer (CLAES) on the Upper Atmosphere Research Satellite (UARS). During the model initialization chemical inconsistencies in the UARS data became evident. Fields of odd nitrogen (NOy) derived from CLAES N2O underestimated the sum of the direct observations of the major NOy species. Results from the model integrations at 465 K and 585 K are sampled in the same way as the various UARS instruments and compared to the observations both directly and by considering average quantities in the inner and edge vortex regions. Sampling the observed species in the same way as the UARS instruments is important in removing any spurious trends due, for example, to changing solar zenith angle. While the model can reproduce the magnitude of the MLS ClO observations at 585 K, this is not possible at 465 K. The model partitions too much ClO into Cl2O2 to reproduce the observed ClO which is around 2.0 parts per billion by volume (ppbv) averaged within the polar vortex. The model also underestimates CLAES ClONO2 in the inner vortex at 465 K due to heterogeneous processing. The observations require that effectively all of the inorganic chlorine is in the form of ClO and ClONO2 in the inner vortex at this altitude. In the basic model run, the decay of ClO produces ClONO2 which is not observed by CLAES. Our results indicate the potential importance of the speculative reaction between OH and ClO producing HCl for the recovery of HCl in the Antarctic spring. By including this reaction, the decay of model ClO into HCl is enhanced, yielding better agreement with HCl data from the Halogen Occultation Experiment (HALOE) data. Similar results can also be obtained by including the reaction between HO2 and ClO to produce HCl with a 3% channel. The model generally reproduces the observed O3 destruction during September. The most significant discrepancy for O3 is in the inner vortex at 465 K where the model underestimates the observed O3 loss rate, especially when the effects of vertical motion are included.

Validation of Aura Microwave Limb Sounder Ozone by ozonesonde and lidar measurements

We present validation studies of MLS version 2.2 upper tropospheric and stratospheric ozone profiles using ozonesonde and lidar data as well as climatological data. Ozone measurements from over 60 ozonesonde stations worldwide and three lidar stations are compared with coincident MLS data. The MLS ozone stratospheric data between 150 and 3 hPa agree well with ozonesonde measurements, within 8% for the global average. MLS values at 215 hPa are biased high compared to ozonesondes by A`20% at middle to high latitude, although there is a lot of variability in this altitude region. Comparisons between MLS and ground-based lidar measurements from Mauna Loa, Hawaii, from the Table Mountain Facility, California, and from the Observatoire de Haute-Provence, France, give very good agreement, within A`5%, for the stratospheric values. The comparisons between MLS and the Table Mountain Facility tropospheric ozone lidar show that MLS data are biased high by A`30% at 215 hPa, consistent with that indicated by the ozonesonde data. We obtain better global average agreement between MLS and ozonesonde partial column values down to 215 hPa, although the average MLS values at low to middle latitudes are higher than the ozonesonde values by up to a few percent. MLS v2.2 ozone data agree better than the MLS v1.5 data with ozonesonde and lidar measurements. MLS tropical data show the wave one longitudinal pattern in the upper troposphere, with similarities to the average distribution from ozonesondes. High upper tropospheric ozone values are also observed by MLS in the tropical Pacific from June to November.

Six years of UARS Microwave Limb Sounder HNO3 observations: Seasonal, interhemispheric, and interannual variations in the lower stratosphere

We present an overview of the seasonal, interhemispheric, and interannual variations in the distribution of HNO 3 in the lower stratosphere based on measurements of gas-phase HNO 3 made by the UARS Microwave Limb Sounder (MLS) through six complete annual cycles in both hemispheres. Outside of the winter polar regions, zonal-mean HNO 3 mixing ratios on the 465-K potential temperature surface are comparable in the two hemispheres in all latitude bands and in all years examined. Except at high latitudes, interannual variability is minimal, and there is no significant hemispheric asymmetry in the overall HNO 3 distribution or its seasonal cycle. Although the Antarctic experiences widespread severe denitrification, the MLS data indicate that the denitrification is not complete; that is, not all polar stratospheric cloud (PSC) particles sediment out of the lower stratosphere. Replenishment of HNO 3 at 465 K during the mid- to late-winter period (when temperatures, though still low, are generally rising) is most likely achieved through a combination of PSC evaporation and continuing weak diabatic descent. Despite large interhemispheric and interannual differences in the extent and duration of PSC activity and denitrification, HNO 3 recovers to similar values at the end of every winter in both the Arctic and the Antarctic. Zonal-mean HNO 3 values for the two hemispheres are virtually indistinguishable for the latitudes equatorward of 65°, even during the winter months. Thus the effects of severe denitrification are confined in both space and time to the regions poleward of 65°S during the winter and early spring.