Cryogenic Limb Array Etalon Spectrometer Research Articles

The Global Space-based Stratospheric Aerosol Climatology (version 2.0): 1979–2018

Abstract. A robust stratospheric aerosol climate data record enables the depiction of the radiative forcing of this highly variable component of climate. In addition to the radiative forcing, stratospheric aerosol also plays a key role in the chemical processes leading to ozone depletion. Therefore, stratospheric aerosol is one of the crucial parameters in understanding climate change in the past and potential changes in the future. As a part of Stratospheric-tropospheric Processes and their Role in Climate (SPARC) Stratospheric Sulfur and its Role in Climate (SSiRC) activity, the Global Space-based Stratospheric Aerosol Climatology (GloSSAC) was created (Thomason et al., 2018) to support the World Climate Research Programme's (WCRP) Coupled Model Intercomparison Project Phase 6 (CMIP6) (Eyring et al., 2016). This data set is a follow-on to one created as a part of SPARC's Assessment of Stratospheric Aerosol Properties (ASAP) activity (SPARC, 2006) and a data created for the Chemistry-Climate Model Initiative (CCMI) in 2012 (Eyring and Lamarque, 2012). Herein, we discuss changes to the original release version including those as a part of v1.1 that was released in September 2018 that primarily corrects an error in the conversion of Cryogenic Limb Array Etalon Spectrometer (CLAES) data to Stratospheric Aerosol and Gas Experiment (SAGE) II wavelengths, as well as the new release, v2.0. Version 2.0 is focused on improving the post-SAGE II era (after 2005) with the goal of mitigating elevated aerosol extinction in the lower stratosphere at mid- and high latitudes noted in v1.0 as noted in Thomason et al. (2018). Changes include the use of version 7.0 of the Optical Spectrograph and InfraRed Imaging System (OSIRIS), the recently released Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) lidar Level 3 stratospheric aerosol profile monthly product and the new addition of SAGE III/ISS. Here, we use an observed relationship between (i) OSIRIS extinction at 750 nm and (ii) SAGE II and SAGE III/ISS extinction at 525 nm to derive an altitude–latitude-based monthly climatology of Ångström exponent to compute OSIRIS extinction at 525 nm, resulting in a better agreement between OSIRIS and SAGE measurements. We employ a similar approach to convert OSIRIS 750 nm extinction to 1020 nm extinction for the post-SAGE II period. Additionally, we incorporate the recently released standard CALIPSO stratospheric aerosol profile monthly product into GloSSAC with an improved conversion technique of the 532 nm backscatter coefficient to extinction using an observed relationship between OSIRIS 525 nm extinction and CALIPSO 532 nm backscatter. SAGE III/ISS data are also incorporated in GloSSAC to extend the climatology to the present and to test the approach used to correct OSIRIS/CALIPSO data. The GloSSAC v2.0 netCDF file is accessible at https://doi.org/10.5067/glossac-l3-v2.0 (Thomason, 2020).

Impacts of Mt Pinatubo volcanic aerosol on the tropical stratosphere in chemistry–climate model simulations using CCMI and CMIP6 stratospheric aerosol data

Abstract. To simulate the impacts of volcanic eruptions on the stratosphere, chemistry–climate models that do not include an online aerosol module require temporally and spatially resolved aerosol size parameters for heterogeneous chemistry and aerosol radiative properties as a function of wavelength. For phase 1 of the Chemistry-Climate Model Initiative (CCMI-1) and, later, for phase 6 of the Coupled Model Intercomparison Project (CMIP6) two such stratospheric aerosol data sets were compiled, whose functional capability and representativeness are compared here. For CCMI-1, the SAGE-4λ data set was compiled, which hinges on the measurements at four wavelengths of the SAGE (Stratospheric Aerosol and Gas Experiment) II satellite instrument and uses ground-based lidar measurements for gap-filling immediately after the 1991 Mt Pinatubo eruption, when the stratosphere was too optically opaque for SAGE II. For CMIP6, the new SAGE-3λ data set was compiled, which excludes the least reliable SAGE II wavelength and uses measurements from CLAES (Cryogenic Limb Array Etalon Spectrometer) on UARS, the Upper Atmosphere Research Satellite, for gap-filling following the Mt Pinatubo eruption instead of ground-based lidars. Here, we performed SOCOLv3 (Solar Climate Ozone Links version 3) chemistry–climate model simulations of the recent past (1986–2005) to investigate the impact of the Mt Pinatubo eruption in 1991 on stratospheric temperature and ozone and how this response differs depending on which aerosol data set is applied. The use of SAGE-4λ results in heating and ozone loss being overestimated in the tropical lower stratosphere compared to observations in the post-eruption period by approximately 3 K and 0.2 ppmv, respectively. However, less heating occurs in the model simulations based on SAGE-3λ, because the improved gap-filling procedures after the eruption lead to less aerosol loading in the tropical lower stratosphere. As a result, simulated tropical temperature anomalies in the model simulations based on SAGE-3λ for CMIP6 are in excellent agreement with MERRA and ERA-Interim reanalyses in the post-eruption period. Less heating in the simulations with SAGE-3λ means that the rate of tropical upwelling does not strengthen as much as it does in the simulations with SAGE-4λ, which limits dynamical uplift of ozone and therefore provides more time for ozone to accumulate in tropical mid-stratospheric air. Ozone loss following the Mt Pinatubo eruption is overestimated by up to 0.1 ppmv in the model simulations based on SAGE-3λ, which is a better agreement with observations than in the simulations based on SAGE-4λ. Overall, the CMIP6 stratospheric aerosol data set, SAGE-3λ, allows SOCOLv3 to more accurately simulate the post-Pinatubo eruption period.

A multidecadal study of cirrus in the tropical tropopause layer

AbstractChanges in the distributions of cirrus in the upper troposphere centered at 100 hPa in the tropical tropopause layer (TTL) during the last 27 years (1985–2012) are quantified based upon an analysis of five independent data sets. Stratospheric Aerosol and Gas Experiment II, Halogen Occultation Experiment, Cryogenic Limb Array Etalon Spectrometer, High Resolution Dynamics Limb Sounder, and Cloud‐Aerosol Lidar with Orthogonal Polarization data are used to determine cirrus frequency‐of‐occurrence geospatial distributions in the TTL in multiple year segments during 1985–2012. Full width at half maximum (FWHM) tropical widths of cirrus zonal averages of occurrence frequency centered at 100 hPa are calculated for each experiment. The FWHMs have a 2σ trend of 0.28 ± 1.5° decade−1. This statistically insignificant trend is similar to those calculated by Davis and Rosenlof (2012) based upon analyses of reanalysis tropopause height and tropopause latitudinal gradients near 100 hPa.

Longitudinal variability of water vapor and cirrus in the tropical tropopause layer

The Microwave Limb Sounder (MLS) and the Cryogenic Limb Array Etalon Spectrometer (CLAES) are both instruments on the Upper Atmosphere Research Satellite and are sensitive to water vapor and subvisible cirrus in the tropical tropopause layer. We analyze the longitudinal and vertical variations of water vapor in the tropical tropopause layer using a new version of MLS water vapor that has improved vertical resolution. The water vapor fields show similarity with subvisible cirrus and sea surface temperature (SST) on seasonal and intraseasonal timescales. The formation of cirrus is more prevalent over continental landmasses and over warm oceanic areas where the upper troposphere is also moist. The intraseasonal variability in cirrus is correlated with upper tropospheric water vapor and linked to a variability in SST of about 1°C. The incidence of cirrus increases with SST until temperatures exceed about 29°C. A further rise in sea surface temperature does not see a corresponding increase in cirrus. Similarly, the incidence of cirrus increases as 100 hPa temperatures fall, with clouds being more likely when temperatures are about 190K. At temperatures colder than this, when in situ formation of cirrus should be more likely, there is a decrease in the likelihood of finding cirrus. Thus evidence points strongly to a dominant role for the convective formation of these cirrus.

Indication of convectively generated gravity waves observed by CLAES

Cryogenic Limb Array Etalon Spectrometer temperature data are analyzed for gravity waves (GWs) using the same methods previously employed for CRISTA data. We obtain nearly 1.5 years of continuous GW data between 34°S and 34°N and good coverage at higher latitudes depending on UARS yaw maneuvers. Correlations of GW variances with sea surface temperature indicate strong convective forcing above the Gulf of Mexico and the Kuro-Shio stream as well as above source regions in the southern subtropics during the summer months of the respective hemisphere. However, it appears difficult to clearly separate between convective forcing and the effects of the background winds, for which similar correlations are found. Further progress in accessing GW sources therefore can be expected mainly from improved measurements, which could provide also information about the GW horizontal wavelength as well as from quantitative comparisons to model predictions.

Vertical profiles of aerosol volume from high spectral resolution infrared transmission measurements: Results

The high‐resolution infrared absorption spectra of the Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment are utilized to derive vertical profiles of sulfate aerosol volume density and extinction coefficient. Following the eruption of Mt. Pinatubo in June 1991, the ATMOS spectra obtained on three Space Shuttle missions (1992, 1993, and 1994) provide a unique opportunity to study the global stratospheric sulfate aerosol layer shortly after a major volcanic eruption and periodically during the decay phase. Synthetic sulfate aerosol spectra are fit to the observed spectra, and a global fitting inversion routine is used to derive vertical profiles of sulfate aerosol volume density. Vertical profiles of sulfate aerosol volume density for the three missions over portions of the globe are presented, with the peak in aerosol volume density occurring from as low as 10 km (polar latitudes) to as high as 20 km (subtropical latitudes). Derived aerosol volume density is as high as 2–3.5 μm3 cm−3 ±10% in 1992, decreasing to 0.2–0.5 μm3 cm−3 ±20% in 1994, in agreement with other experiments. Vertical extinction profiles derived from ATMOS are compared with profiles from Improved Stratospheric And Mesospheric Sounder (ISAMS) and Cryogenic Limb Array Etalon Spectrometer (CLAES) that coincide in space and time and show good general agreement. The uncertainty of the ATMOS vertical profiles is similar to CLAES and consistently smaller than ISAMS at similar altitudes.

Stratospheric N2O5 in the austral spring 2002 as retrieved from limb emission spectra recorded by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS)

N2O5 was retrieved from infrared limb emission spectral radiances made by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on board the European environmental satellite (ENVISAT). The measurements were taken during the period covering the Antarctic polar vortex split between 20 September and 13 October 2002. The retrieval of N2O5 is aggravated by its continuum‐like emission feature covering a wide spectral region, which is hardly distinguishable from background continuum emission. The method of constraining the background continuum in the N2O5 analysis spectral region to its value in a N2O5‐free spectral region was found to be appropriate to solve this problem. Retrieved volume mixing ratios (VMR) of N2O5 exhibit features consistent with the dynamics prevalent at the time in the Antarctic and known N2O5 chemistry governing diurnal variability. The observations of low N2O5 inside vortex air mass and rich N2O5 exvortex air mass are strongly in support of the chemistry that governs its partitioning within the NOy family. The enhanced nighttime high geographic latitude N2O5 VMR with a peak of 4.4 ppbv in the altitude range of 32–37 km during the last week of September 2002 is consistent with air mass transport from lower to high latitudes and temperature‐sensitive N2O5 formation chemistry. N2O5 enhancement up to 6 ppbv was also observed by the Cryogenic Limb Array Etalon Spectrometer(CLAES) and Improved Stratospheric and Mesospheric Sounder (ISAMS) experiments on Upper Atmospheric Research Satellite (UARS) during Northern Hemisphere January 1992 stratospheric warming which was a factor of 3 larger than any measurements of N2O5 prior to UARS. In contrast, a maximum of 4.4 ppbv N2O5 VMR observed by MIPAS at 32–37 km is only a factor of 2 larger than its prewarming values.

Intraseasonal variations of water vapor and cirrus clouds in the tropical upper troposphere

Space‐time variations of tropical upper tropospheric water vapor and cirrus clouds associated with the intraseasonal oscillation (ISO) are investigated using data from the Microwave Limb Sounder (MLS) and the Cryogenic Limb Array Etalon Spectrometer (CLAES) on board the Upper Atmosphere Research Satellite (UARS). Composite moisture and meteorological fields based on five ISO events selected in two boreal winters (1991–1993) are analyzed using 20–80 day band‐pass‐filtered data. At 215 and 146 hPa, wet anomalies with frequent appearance of cirrus clouds exist over the convective system and move eastward from the Indian Ocean to the central Pacific, suggesting a direct effect of convective activity up to this level. At 100 hPa, however, the moisture field seems to be indirectly affected by convective activity through the dynamical response to the convective heating. Dry anomalies are observed over the Indian Ocean around the developing stage and over the eastern Pacific around the mature‐to‐decaying stage of the ISO. Cirrus clouds are frequently found over the cold region located to the east of the convective system. These structures around the tropopause level are closely related to the eastward moving Kelvin and Rossby wave responses to the convective heating with the equatorial cold anomaly and with the subtropical anticyclonic gyres. Between the two gyres the easterly wind blowing through the equatorial cold region may cause dehydration through cirrus formation when the convective system develops over the Indian Ocean and the western Pacific. As the northern gyre intensifies, tropical dry air is transported to the subtropical Pacific and eventually to the equatorial eastern Pacific. It is suggested that the temperature and flow variations due to the coupled Kelvin‐Rossby wave structure play an important role in dehydrating air in the tropical and subtropical tropopause region.

Kelvin wave signatures in stratospheric trace constituents

Connections, sometimes tenuous, have previously been noted between stratospheric Kelvin waves and several stratospheric trace constituents. The present study finds evidence of Kelvin wave signatures in ozone from the Microwave Limb Sounder (MLS) and Cryogenic Limb Array Etalon Spectrometer (CLAES) instruments aboard the Upper Atmosphere Research Satellite. Predominant variations near 10 days are associated with a Kelvin wave mode previously identified in MLS temperature. Variations in CLAES nitrous oxide also show some evidence of influence by this Kelvin mode. The results presented here show that the observing characteristics of the instrument can influence the derived structure and properties of Kelvin waves.

Cirrus and water vapor in the tropical tropopause layer observed by Upper Atmosphere Research Satellite (UARS)

The Microwave Limb Sounder (MLS) on Upper Atmosphere Research Satellite (UARS) is sensitive to water vapor and ozone in the lower stratosphere. The Cryogenic Limb Array Etalon Spectrometer (CLAES), another of the instruments on UARS, has a spatial and temporal coverage similar to that of MLS and can be used to indicate the presence of cirrus. We examine the relationships among water vapor, ozone, and cirrus in the tropical region at 68 and 83 hPa during December–February 1991/1992, an El Niño period. For most longitudes at 68 hPa, three‐dimensional trajectories show that the cirrus form predominately in air which ascends slowly through the tropical tropopause layer. Cirrus form in air which has travelled a long way in the horizontal, in the westerly jet, before heading equatorward in subtropical anticyclones. In the Indonesian region, however, a greater proportion is found in air which is sinking than in air which is ascending. The western Pacific region sees the highest proportion of cirrus which may have been influenced by convection, and the possible contribution from convection is greater at 83 hPa than at 68 hPa. In cloudy regions, the air is likely to have a high relative humidity. Mixing ratios of ozone from MLS have a tendency to be lower, and this may indicate that ozone is destroyed on the ice particles that comprise the clouds.

A stratospheric aerosol climatology from SAGE II and CLAES measurements: 2. Results and comparisons, 1984–1999

This paper presents a climatology of the stratospheric aerosol produced with our look‐up table (LUT) technique using data from the Stratospheric Aerosol and Gas Experiment (SAGE II) and the Cryogenic Limb Array Etalon Spectrometer (CLAES) instruments. The LUT climatology spans the period from December 1984 to August 1999. It includes values and uncertainties of measured extinction and optical depth at four SAGE II wavelengths (0.385–1.02 μm) and supplements these with results from the CLAES 12.82 μm wavelength during the key period January 1992 through May 1993, when the large particle sizes from the Pinatubo volcanic injection made many SAGE II extinction and optical depth spectra wavelength independent. Also included are retrieved values of effective radius Reff, distribution width σg, surface area S, and volume V. Aerosol retrievals show notable increases in all these parameters after major volcanic eruptions, with increases in Reff lagging increases in the others. Postvolcanic increases in σg, indicative of broader size distributions, are consistent with sudden increases in numbers of both small and large particles that exceed increases in intermediate‐size particles. After Pinatubo, retrieved Reff and σg took nearly 5 years to return to pre‐eruption values, while slightly shorter recovery times were obtained for S and V. During low‐aerosol‐loading periods, size distributions narrow in going from the tropical core to higher latitudes at altitudes between 20 and 22 km. Seasonal variations in S and V are observed at high latitudes for several altitude bands, but are less obvious in Reff. With some exceptions, LUT retrievals agree well with most previous climatologies. For example, agreement is good between LUT retrieved surface area and results from balloon‐borne measurements, where available. However, there are a few noteworthy discrepancies. For example, values of surface area from principal component analysis of SAGE II data are less than LUT retrievals during near‐background periods (e.g., 1989 to mid‐1991, and after 1996) and greater than LUT retrievals in the peak of the Pinatubo plume. The smaller LUT‐derived surface areas during the Pinatubo peak result from the constraint provided by the CLAES 12.82 μm extinction measurements.

A stratospheric aerosol climatology from SAGE II and CLAES measurements: 1. Methodology

This paper presents the methods used to produce a global climatology of the stratospheric aerosol using data from two satellite instruments: the Stratospheric Aerosol and Gas Experiment (SAGE II) and the Cryogenic Limb Array Etalon Spectrometer (CLAES). The climatology, which spans from December 1984 to August 1999, includes values and uncertainties of measured extinction and optical depth and of retrieved particle effective radius Reff, distribution width σg, surface area S, and volume V. As a basis for aerosol retrievals, a multiwavelength look‐up table (LUT) algorithm was developed that matches the satellite‐measured extinction ratios to precomputed ratios that are based on a range of unimodal lognormal size distributions. For cases in which the LUT does not find an acceptable match between measured and precomputed extinction spectra, a different technique called the parameter search technique is utilized. The combination of these two techniques and data from both satellites allows us to retrieve values of Reff, σg, S, and V over a wider range of conditions and from a wider range of wavelengths than used by other methods. This greater wavelength range helps constrain retrieved results, especially in postvolcanic conditions when particle sizes are greatly increased and SAGE II extinction spectra become essentially independent of wavelength. Our method includes an altitude‐ and time‐dependent procedure that uses bimodal size distributions from in situ measurements to estimate bias and uncertainty introduced by assuming a unimodal functional form. Correcting for this bias reduces uncertainty in retrievals of Reff, S, and V by about 7%, 5%, and 1% (averaged over all altitude bands), leaving remaining uncertainties from the unimodal assumption of about ±18%, ±20%, and ±21%, respectively. Additional uncertainties, resulting from measurement error and spatiotemporal variability, are evaluated by propagating input uncertainties through the retrieval algorithm. In an accompanying paper we report on a climatology of Reff, S, and V and consider uncertainties in our retrieved values of these parameters. In this paper we examine the sensitivity of our retrievals to refractive index and measurement wavelength. We find, for example, that changing refractive index from a value for the stratospheric temperature of 215 K to that for 300 K can increase retrieved Reff by ∼7.5%, owing largely to effects at the CLAES 12.82 μm wavelength. When only SAGE II wavelengths are used, corresponding changes in Reff are much smaller.

MLS observations of atmospheric gravity waves over Antarctica

Stratospheric gravity waves over Antarctica are studied with the radiance variances observed by Upper Atmosphere Research Satellite Microwave Limb Sounder (UARS MLS). Strong wave activities are found during August and September in 1992–1994 over the Drake Passage region (290°–315° longitude and 50°S–70°S latitude), where the enhancement seems associated with the topography of both South America and Antarctic Peninsula tips. During the same period, significant gravity wave activities are also evident along the Antarctic coastal region. The vertical growth of the wave variance shows a scale height of ∼14 km at low altitudes for both Drake Passage and the coastal regions but reduces substantially at high altitudes. The growth rate transition occurs at a lower altitude in the Drake Passage region where the variance is larger. The gravity wave activities observed near the Antarctic may contribute significantly to the formation of polar stratospheric clouds (PSC) because of the overlapping with the vortex edge and with the PSC activities as observed by CLAES (Cryogenic Limb Array Etalon Spectrometer).

UARS Microwave Limb Sounder upper tropospheric humidity measurement: Method and validation

The methodology and validation of upper tropospheric humidity (UTH) measurements by the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) are described. The MLS instrument was not specifically designed to measure UTH, but its measurement is achieved by modeling the radiance signal from a window channel as the instrument field of view is scanned through the troposphere. UTH is retrieved as a piecewise‐linear function in relative humidity with respect to ice (% RHi) with breakpoints at 147, 215, 316, and 464 hPa. MLS version 4.9 is believed to be the most accurate version to date; its estimated global‐averaged accuracy (precision) is 23(21), 22(10), 22(8), and 50(19) %RHi at 147, 215, 316, and 464, hPa respectively. The validation of UTH is established by its consistency with (1) climatological fields, (2) coincident comparisons with in situ Vaisala RS80 balloon sondes, frostpoint hygrometers and Measurement of Ozone and Water Vapor by Airbus In‐Service Aircraft measurements, (3) cirrus observations, and (4) meteorological dynamics and assimilation fields. MLS climatological UTH is morphologically consistent with Stratospheric Aerosol and Gas Experiment II and Vaisala zonal climatologies. Comparisons with coincident in situ data show a wide range of agreement. Relative to the other in situ instruments, V4.9 MLS humidities agree on average within 11% at 147 hPa, 3–36% dry at 215 hPa, and 14–39% dry at 316 and 464 hPa. The standard deviation of the individual comparisons about the mean between MLS UTH and the other techniques is approximately 25%RHi at all heights. Humidity measurements in cirrus are compared with UARS Cryogenic Limb Array Etalon Spectrometer (CLAES) extinction measurements and an MLS cirrus detection algorithm. CLAES extinction measurements from 316 to 147 hPa which indicate the presence of cirrus correspond well with MLS UTH measurements greater than ∼100 %RHi. Global MLS UTH distributions appear consistent with those expected from upper tropospheric dynamics and with the Goddard Data Assimilation Office humidity fields.

On the formation of HNO3 in the Antarctic mid to upper stratosphere in winter

We address the previously unresolved puzzle of nitric acid formation in the polar winter mid to upper stratosphere, first indicated by Limb Infrared Monitor of the Stratosphere observations in the Arctic winter of 1978–1979. Several theoretical studies over the past 2 decades have tried to reproduce these observations with varying success. More recently, the onset, altitude range, and duration of the formation process have been clarified by the Cryogenic Limb Array Etalon Spectrometer onboard UARS and by a series of ground‐based observations taken at the South Pole during the Antarctic winters of 1993, 1995, and 1999. Using the Stony Brook‐SSt. Petersburg two‐dimensional photochemical model, we have reexplored HNO3 formation via both the ion cluster chemistry and heterogeneous chemistry on sulfate aerosols considered by earlier investigators. By including what we believe to be a realistic flux of NOy from the mesosphere, we find that the model can generate observed mixing ratios through a combination of ion‐clusterenhanced chemistry in the upper to mid stratosphere, augmented by heterogeneous chemistry on sulfate aerosol below ∼40 km. Results are presented which clarify the relative role of various processes and assumed NOy fluxes. These also point up the need to incorporate more accurate downward NOy fluxes in models being used to simulate the polar stratosphere. Finally, we emphasize the need to consider the influence of repartitioning of NOy or NOx into HNO3 before observed variations in amounts of the former reaching the mid to lower stratosphere in winter and early spring can properly be used as tracers to reflect variations in thermospheric‐mesospheric NOy production or transport.

Onset, extent, and duration of dehydration in the southern hemisphere polar vortex

Satellite observations of water vapor and aerosol extinction along with temperature trajectory calculations are analyzed for the Southern Hemisphere winter of 1992 in order to determine the onset, extent, and duration of dehydration within the polar vortex. Our investigation uses measurements of water vapor from the Microwave Limb Sounder (MLS) and aerosol extinction from the Cryogenic Limb Array Etalon Spectrometer (CLAES), both on board the Upper Atmosphere Research Satellite (UARS). Evidence of persistent ice cloud formation, supported by temperature statistics obtained from air parcel trajectories, suggests that the onset of the dehydration process occurs between late June and early July. By late August‐early September water vapor depleted areas within the vortex no longer coincide with high aerosol extinctions, indicating that severe dehydration has occurred with the irreversible removal of water vapor over vast areas. Areas with depleted levels of water vapor, below the prewinter values, persist well into November. Evidence for dehydration is found on potential temperature surfaces from 420 K (the lower limit of the MLS measurements) to 520 K (approximately 16 to 22 km). The horizontal extent of the dehydrated area at 465 K encompasses up to 35% of the total vortex area equatorward of 80°S. A comparison of CLAES aerosol extinction measurements and model calculations of aerosol extinction suggests an average ice particle number concentration and size of 10−2–10−3 cm−3 and 10–30 μm, respectively. We show that the difference between the timing of the onset of dehydration found here and that in a recent analysis of Polar Ozone and Aerosol Measurement III (POAM) observations can be explained by the latitudinal sampling pattern of the POAM instrument.

Upper tropospheric humidity and thin cirrus

Upper tropospheric water vapor data from the UARS Microwave Limb Sounder (MLS) are compared with coincident optically thin (often subvisible) cirrus cloud measurements from the UARS Cryogenic Limb Array Etalon Spectrometer (CLAES) at 147 hPa in the tropics. A strong correlation is found between mean relative humidity with respect to ice (RHI) and percent occurrence of cirrus clouds. The cloudiness‐RHI relationship is established from the coincident CLAES and MLS observations (Oct. 1991–May 1993), and used with 1991–97 MLS data to find by proxy the percent occurrence of cirrus for 1993–97. Interannual variability of cloudiness is investigated with the extended (1991–97) cirrus data, and a clear El Nino signature is seen. The observed relationship of RHI and optically thin cirrus provides a potential diagnostic of climatological behavior of atmospheric models, in which cirrus is important for radiative balance, heterogeneous chemistry, and the near‐tropopause water budget.

Temporal evolution of chlorine monoxide in the middle stratosphere

The temporal evolution of stratospheric chlorine monoxide (ClO) at northern midlatitudes is studied over different scales: diurnal and seasonal variations. Data sets include temperature profiles, and ClO, HCl, ClONO2, NO2, O3, H2O, and CH4 mixing ratios as measured from 10 to 0.46 hPa (i.e., from ∼30 to ∼55 km) by the Microwave Limb Sounder (MLS), by the Halogen Occultation Experiment, and by the Cryogenic Limb Array Etalon Spectrometer instruments aboard the UARS satellite from at most September 1991 to June 1997 within the band 40°–50°N. The time evolution of these species is interpreted by comparison with results from a “zero‐dimensional” (0‐D) purely photochemical model and from the SLIMCAT three‐dimensional (3‐D) chemical transport model over the Plateau de Bure station (45°N, 10°E) from September 1991 to March 1998. A maximum confidence value for the yield in the reaction ClO + OH → HCl + O2 is deduced from the analysis to be in the range 0.05–0.10±0.03, which is consistent with recent laboratory data, suggesting a value of ∼0.05–0.06±0.02. Comparisons between the ClO diurnal variations measured by MLS and calculated by the 0‐D model agree to within 10% around the maximum of ClO (∼40 km) and within 5% elsewhere. The ClO diurnal variation is induced by variations in the partitioning with HOCl and ClONO2 below 50 km and by variations in the partitioning within the ClOx (=Cl+ClO) family above. The agreement between measured and modeled seasonal variations of ClO implies an evolution above 30 km essentially dictated by the variation in the partitioning with HCl, together with the partitioning within the ClOx family above 40 km. The differences between measured and modeled ClO seasonal variations above 50 km are attributed to uncertainties in the relative rates of the sources of HCl (reactions of Cl with CH4 and HO2).

Trajectory hunting: A case study of rapid chlorine activation in December 1992 as seen by UARS

Trajectory hunting (i.e., a technique to find air parcels sampled at least twice over the course of a few days) is applied to analyze Upper Atmosphere Research Satellite (UARS) measurements in conjunction with the Atmospheric and Environmental Research, Inc. (AER) photochemical box model. As a case study, we investigate rapid chlorine activation in the Arctic lower stratosphere on December 29, 1992 associated with a polar stratospheric cloud (PSC) event. Eleven air parcels that have been sampled several times along 5‐day trajectories at the 465 K (∼46 hPa), 520 K (∼31 hPa), and 585 K (∼22 hPa) levels were investigated. For the first time, the latest versions of the Cryogenic Limb Array Etalon Spectrometer (CLAES, version 9) and Microwave Limb Sounder (MLS, version 5) data sets are analyzed, and their consistency is assessed. A detailed sensitivity study with the AER photochemical box model along these trajectories leads to the conclusion that for the December 24–29, 1992 episode (1) the individual CLAES version 9 ClONO2 and MLS version 5 ClO measurements are self‐consistent within their uncertainties; and (2) most of the time, UARS measurements of ClO, ClONO2, HNO3, and aerosol extinction at 780 cm−1 agree within the range of their uncertainties with the model calculations. It appears that the HNO3 and aerosol extinction measurements for four parcels at 520 K look more supportive for the nitric acid trihydrate (NAT) scheme. However, the uncertainties in the individual UARS measurements and U.K. Meteorological Office temperature do not allow a definite discrimination between the NAT and supercooled ternary solution (STS) PSC schemes for this chlorine activation episode in December 1992.

A polar stratospheric cloud parameterization for the global modeling initiative three‐dimensional model and its response to stratospheric aircraft

We describe a new parameterization of polar stratospheric clouds (PSCs) which was written for and incorporated into the three‐dimensional (3‐D) chemistry and transport model (CTM) developed for NASA's Atmospheric Effects of Aviation Project (AEAP) by the Global Modeling Initiative (GMI). The parameterization was designed to respond to changes in NOy and H2O produced by high‐speed civilian transport (HSCT) emissions. The parameterization predicts surface area densities (SADs) of both Type 1 and Type 2 PSCs for use in heterogeneous chemistry calculations. Type 1 PSCs are assumed to have a supercooled ternary sulfate (STS) composition, and Type 2 PSCs are treated as water ice with a coexisting nitric acid trihydrate (NAT) phase. Sedimentation is treated by assuming that the PSC particles obey lognormal size distributions, resulting in a realistic mass flux of condensed phase H2O and HNO3. We examine a simulation of the Southern Hemisphere high‐latitude lower stratosphere winter and spring seasons driven by temperature and wind fields from a modified version of the National Center for Atmospheric Research (NCAR) Middle Atmosphere Community Climate Model Version 2 (MACCM2). Predicted PSC SADs and median radii for both Type 1 and Type 2 PSCs are consistent with observations. Gas phase HNO3 and H2O concentrations in the high‐latitude lower stratosphere qualitatively agree with Cryogenic Limb Array Etalon Spectrometer (CLAES) HNO3 and Microwave Limb Sounder (MLS) H2O observations. The residual denitrification and dehydration of the model polar vortex after polar winter compares well with atmospheric trace molecule spectroscopy (ATMOS) observations taken during November 1994. When the NOx and H2O emissions of a standard 500‐aircraft HSCT fleet with a NOx emission index of 5 are added, NOx and H2O concentrations in the Southern Hemisphere polar vortex before winter increase by up to 3%. This results in earlier onset of PSC formation, denitrification, and dehydration. Active Cly increases and produces small (∼1%) decreases in lower stratospheric vortex O3 concentrations during the spring relative to the HSCT‐free run.