Upper Atmosphere Research Satellite Microwave Limb Sounder Research Articles

Diurnal variations of stratospheric ozone measured by ground-based microwave remote sensing at the Mauna Loa NDACC site: measurement validation and GEOSCCM model comparison

Abstract. There is presently renewed interest in diurnal variations of stratospheric and mesospheric ozone for the purpose of supporting homogenization of records of various ozone measurements that are limited by the technique employed to being made at certain times of day. We have made such measurements for 19 years using a passive microwave remote sensing technique at the Mauna Loa Observatory (MLO) in Hawaii, which is a primary station in the Network for Detection of Atmospheric Composition Change (NDACC). We have recently reprocessed these data with hourly time resolution to study diurnal variations. We inspected differences between pairs of the ozone spectra (e.g., day and night) from which the ozone profiles are derived to determine the extent to which they may be contaminated by diurnally varying systematic instrumental or measurement effects. These are small, and we have reduced them further by selecting data that meet certain criteria that we established. We have calculated differences between profiles measured at different times: morning–night, afternoon–night, and morning–afternoon and have intercompared these with like profiles derived from the Aura Microwave Limb Sounder (Aura-MLS), the Upper Atmosphere Research Satellite Microwave Limb Sounder (UARS-MLS), the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES), and Solar Backscatter Ultraviolet version 2 (SBUV/2) measurements. Differences between averages of coincident profiles are typically < 1.5% of typical nighttime values over most of the covered altitude range with some exceptions. We calculated averages of ozone values for each hour from the Mauna Loa microwave data, and normalized these to the average for the first hour after midnight for comparison with corresponding values calculated with the Goddard Earth Observing System Chemistry Climate Model (GEOSCCM). We found that the measurements and model output mostly agree to better than 1.5% of the midnight value, with one noteworthy exception: The measured morning–night values are significantly (2–3 %) higher than the modeled ones from 3.2 to 1.8 hPa (~39–43 km), and there is evidence that the measured values are increasing compared to the modeled values before sunrise in this region.

Nitric acid in the stratosphere based on Odin observations from 2001 to 2009 – Part 1: A global climatology

Abstract. The Sub-Millimetre Radiometer (SMR) on board the Odin satellite, launched in February 2001, observes thermal emissions of stratospheric nitric acid (HNO3) originating from the Earth limb in a band centred at 544.6 GHz. Height-resolved measurements of the global distribution of nitric acid in the stratosphere were performed approximately on two observation days per week. An HNO3 climatology based on more than 7 years of observations from August 2001 to April 2009 covering the vertical range between typically ~19 and 45 km (~1.5–60 hPa or ~500–1800 K in terms of potential temperature) was created. The study highlights the spatial and seasonal variation of nitric acid in the stratosphere, characterised by a pronounced seasonal cycle at middle and high latitudes with maxima during late fall and minima during spring, strong denitrification in the lower stratosphere of the Antarctic polar vortex during winter (the irreversible removal of NOy by the sedimentation of cloud particles containing HNO3), as well as large quantities of HNO3 formed every winter at high-latitudes in the middle and upper stratosphere. A strong inter-annual variability is observed in particular at high latitudes. A comparison with a stratospheric HNO3 climatology, based on over 7 years of UARS/MLS (Upper Atmosphere Research Satellite/Microwave Limb Sounder) measurements from the 1990s, shows good consistency and agreement of the main morphological features in the potential temperature range ~465 to ~960 K, if the different characteristics of the data sets such as the better altitude resolution of Odin/SMR as well as the slightly different altitude ranges are considered. Odin/SMR reaches higher up and UARS/MLS lower down in the stratosphere. An overview from 1991 to 2009 of stratospheric nitric acid is provided (with a short gap between 1998 and 2001), if the global measurements of both experiments are taken together.

UARS/MLS Cloud Ice Measurements: Implications for H2O Transport near the Tropopause

Abstract A technique for detecting large hydrometeors at high altitudes is described here and applied to the Upper Atmosphere Research Satellite/Microwave Limb Sounder (UARS/MLS) 203-GHz radiance measurements at tangent pressures between 200 and 46 hPa. At these tangent pressures the radiances remain optically thin and cloudy-sky radiances are brighter than normal clear-sky cases. Unlike infrared/visible cloud observations, the 203-GHz radiances can penetrate most ice clouds and are sensitive to ice crystals of convective origin. Rough ice water content (IWC) retrievals are made near the tropopause using estimated size distributions from in situ convective studies. The seasonal mean IWC observed at 100 hPa reaches vapor-equivalent 20 ppmv or more over convective centers, dominating the total water content. Convectively lofted ice, therefore, appears to be hydrologically significant at the tropical cold point. IWC is well correlated spatially with relative humidity with respect to ice (RHi) at 100 hPa during both the dry (January–March) and moist (July–September) periods.

Assimilation of stratospheric ozone in the chemical transport model STRATAQ

Abstract. We describe a sequential assimilation approach useful for assimilating tracer measurements into a three-dimensional chemical transport model (CTM) of the stratosphere. The numerical code, developed largely according to Kha00, uses parameterizations and simplifications allowing assimilation of sparse observations and the simultaneous evaluation of analysis errors, with reasonable computational requirements. Assimilation parameters are set by using χ2 and OmF (Observation minus Forecast) statistics. The CTM used here is a high resolution three-dimensional model. It includes a detailed chemical package and is driven by UKMO (United Kingdom Meteorological Office) analyses. We illustrate the method using assimilation of Upper Atmosphere Research Satellite/Microwave Limb Sounder (UARS/MLS) ozone observations for three weeks during the 1996 antarctic spring. The comparison of results from the simulations with TOMS (Total Ozone Mapping Spectrometer) measurements shows improved total ozone fields due to assimilation of MLS observations. Moreover, the assimilation gives indications on a possible model weakness in reproducing polar ozone values during springtime.

Three‐dimensional structure and evolution of stratospheric HNO3 based on UARS Microwave Limb Sounder measurements

The Upper Atmosphere Research Satellite (UARS) Microwave Limb Sounder (MLS) measured the global distribution of stratospheric HNO3 through more than seven complete annual cycles in both hemispheres. Here we present an overview of the seasonal, interhemispheric, and interannual variations in the distribution of HNO3 throughout the lower and middle stratosphere from 420 to 960 K potential temperature based on the UARS MLS version 6 HNO3 measurements. The version 6 MLS data have much better precision and a larger vertical range than previous MLS HNO3 data sets and have also been corrected to account for an oversight in the retrieval algorithms that led earlier versions to overestimate HNO3 abundances by as much as 35% at some levels in the stratosphere. HNO3 exhibits little vertical, seasonal, or interannual variability in the tropics. For the first ∼1.5 years of the mission, however, a persistent enhancement is seen at low and middle latitudes that we attribute to perturbations in reactive nitrogen chemistry under conditions of high aerosol loading from the eruption of Mount Pinatubo. The signature of Pinatubo‐induced HNO3 enhancement is considerably weaker at 420 and 465 K than it is at higher altitudes, and it is also considerably weaker at northern middle and high latitudes than it is in the Southern Hemisphere. HNO3 abundances increase toward the pole in both hemispheres at all levels and in all seasons, with the exception of the severely denitrified region inside the Antarctic vortex. A pronounced seasonal cycle is present at middle and high latitudes up to at least 960 K (∼34 km), with a winter maximum and a summer minimum. Large interannual variability in the timing, magnitude, and duration of enhanced wintertime HNO3 abundances is seen in both hemispheres. Even in the coldest Arctic winters, HNO3 depletion is modest and limited in both horizontal and vertical extent. In contrast, virtually complete removal of gas‐phase HNO3 occurs at the highest southern latitudes by July in every year throughout the lower stratosphere. Indications of denitrification are present up to at least 740 K, well above the highest altitude at which dehydration is observed, providing further evidence that denitrification can proceed in the absence of dehydration.

Dehydration in the tropical tropopause layer: Implications from the UARS Microwave Limb Sounder

Measurements of H2O from the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) are used to investigate the structure of H2O in the near tropopause region and dehydration mechanisms in the tropical tropopause layer (TTL). The new MLS data are consistent with convective input of H2O into the bottom of the TTL followed by slow ascent with a maximum relative amplitude in the seasonal cycle occurring near the tropopause nearly in phase with the tropopause temperature seasonal cycle. The relative amplitude of the seasonal cycle shows a minimum at 121 hPa in the upwelling moist phase. These features are reproduced with the “cold‐trap” dehydration hypothesis. Seasonal maps show wettest tropical 100 hPa H2O colocated with continental convection.

A new 147–56 hPa water vapor product from the UARS Microwave Limb Sounder

Measurements of H2O in the tropopause region have been obtained by production of a new data set from the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS). A modified version of the retrieval scheme used to produce upper tropospheric humidity (UTH) from the MLS 203 GHz radiometer was applied to the MLS 183 GHz radiometer measurements to produce useful H2O data at 147, 121, 100, 83, 68, and 56 hPa. These new data, for the first 18 months of the UARS mission when the MLS 183 GHz radiometer was operational, fill an important “gap” around 100 hPa where previous MLS H2O data were generally not useful. Characteristics of the new data set are discussed and compared with National Oceanic and Atmospheric Administration (NOAA), Climate Monitoring and Diagnostics Laboratory (CMDL) frost‐point hygrometer, and UARS Halogen Occultation Experiment (HALOE) measurements.

A search for mountain waves in MLS stratospheric limb radiances from the winter Northern Hemisphere: Data analysis and global mountain wave modeling

Despite evidence from ground‐based data that flow over mountains is a dominant source of gravity waves (GWs) for the Northern Hemisphere winter middle atmosphere, GW‐related signals in global limb radiances from the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) have shown little direct evidence of mountain waves. We address this issue by combining a renewed analysis of MLS limb‐track and limb‐scan radiances with global mountain wave modeling using the Naval Research Laboratory Mountain Wave Forecast Model (MWFM). MLS radiance variances show characteristics consistent with mountain waves, such as enhanced variance over specific mountain ranges and annual variations that peak strongly in winter. However, direct comparisons of MLS variance maps with MWFM‐simulated mountain wave climatologies reveal limited agreement. We further develop a detailed “MLS GW visibility function” that accurately specifies the three‐dimensional in‐orbit sensitivity of the MLS limb‐track radiance measurement to a spectrum of GWs with different wavelengths and horizontal propagation directions. On postprocessing MWFM‐generated mountain wave fields through these MLS visibility filters, we generate MWFM variance maps that agree substantially better with MLS radiance variances. This combined data analysis and MLS‐filtered MWFM modeling leads us to conclude that many MLS variance enhancements can be associated with mountain waves forced by flow over specific mountainous terrain. These include mountain ranges in Europe (e.g., Scandinavia; Alps; Scotland; Ural, Putoran, Altai, Hangay and Sayan Mountains; Yablonovyy, Stanovoy, Khingan, Verkhoyansk and Central Ranges), North America (e.g., Brooks Range, MacKenzie Mountains, Colorado Rockies), southeastern Greenland, and Iceland. Our results show that given careful consideration of the in‐orbit sensitivity of the instrument to GWs, middle atmospheric limb radiances measured from UARS MLS, as well as from the new MLS instrument on the Earth Observing System (EOS) satellite, can provide important global information on mountain waves in the extratropical Northern Hemisphere stratosphere and mesosphere.

Variations and climatology of ClO in the polar lower stratosphere from UARS Microwave Limb Sounder measurements

The Microwave Limb Sounder (MLS) on board the Upper Atmosphere Research Satellite (UARS) measured the global distribution of stratospheric ClO over annual cycles for much of the 1990s, albeit with reduced sampling frequency in the latter half of the decade. Here we present an overview of the interannual and interhemispheric variations in the distribution of ClO derived from UARS MLS measurements, with a particular emphasis on enhancements in the winter polar lower stratosphere. Although ClO enhancement within the Arctic vortex is comparable in both magnitude and spatial extent to that in the Antarctic at 465 K (∼19 km), a significant interhemispheric disparity is seen at higher altitudes, where maximum ClO abundances, and their spatial extent, are considerably larger in the Antarctic than in the Arctic. The Arctic exhibits much more interannual variability in the magnitude, timing, and distribution of ClO enhancement than does the Antarctic. Nevertheless, during the mid‐1990s, when the Arctic lower stratosphere was atypically cold, MLS observed the Arctic vortex to be almost completely filled with enhanced ClO in midwinter to late winter. The peak in the ClO profile is at a higher altitude, and the vertical extent of chlorine activation is larger, in the Antarctic than in the Arctic. The Arctic winter of 1995/1996, however, stands out as having a much more Antarctic‐like ClO distribution, with larger maximum ClO abundances, a higher altitude for the profile peak, and greater horizontal and vertical extent of activation than the other winters observed by UARS MLS. In the Southern Hemisphere, ClO becomes enhanced in the sunlit portions of the vortex by at least late May/early June every year, whereas in the Northern Hemisphere, ClO becomes enhanced in mid to late December in some years but not until January in others. Elevated levels of reactive chlorine persist for 4–5 months in the south but only 2–3 months in the north.

Variability of ozone loss during Arctic winter (1991–2000) estimated from UARS Microwave Limb Sounder measurements

A comprehensive analysis of version 5 (V5) Upper Atmosphere Research Satellite (UARS) Microwave Limb Sounder (MLS) ozone data using a Lagrangian transport (LT) model provides estimates of chemical ozone depletion for the 1991–1992 through 1997–1998 Arctic winters. These new estimates give a consistent, three‐dimensional picture of ozone loss during seven Arctic winters; previous Arctic ozone loss estimates from MLS were based on various earlier data versions and were done only for late winter and only for a subset of the years observed by MLS. We find large interannual variability in the amount, timing, and patterns of ozone depletion and in the degree to which chemical loss is masked by dynamical processes. Analyses of long‐lived trace gas data suggest that the LT model sometimes overestimates descent at levels above ∼520 K, so we have most confidence in the results at lower levels. When the vortex is shifted off the pole and the cold region is near the vortex edge (e.g., late winter 1993 and 1996), most rapid ozone depletion occurs near the vortex edge; when the vortex and cold region are pole‐centered (e.g., late winter 1994 and 1997), most ozone loss takes place in the vortex core. MLS observed the most severe ozone depletion in 1995–1996, with about 1.3 ppmv cumulative loss for the winter at 465 K by 3 March 1996; ∼1.0 ppmv cumulative loss is seen at 465 K by mid‐March 1993. Analyses of MLS data show significant ozone loss during January in most years, ranging from ∼0.3 to 0.6 ppmv at 465 K. A modified LT model used with the limited MLS data in 2000 gives rough estimates of ∼0.04 and 0.006–0.012 ppmv/day during 2–12 February and 12 February–29 March 2000, respectively, broadly consistent with other studies of the 1999–2000 winter. Estimates of depletion in MLS column ozone above 100 hPa are considerably smaller than other reported column loss estimates, primarily because many estimates include loss below 100 hPa and because MLS does not continuously observe the Arctic after early spring. Our results from analyses of MLS data confirm previous conclusions of broad overall agreement between many ozone loss estimates in the Arctic lower stratosphere near 450–480 K.

Mesospheric temperature from UARS MLS: retrieval and validation

A research algorithm is developed to retrieve temperature at 20– 90 km using 63 GHz O 2 emission measurements from Microwave Limb Sounder (MLS) on Upper Atmosphere Research Satellite (UARS). The algorithm is based on a previous MLS radiative transfer model but improved to produce more accurate radiance calculations in the cases where the geomagnetic Zeeman splitting is important. A fast version of the model is developed and implemented for practical uses of the temperature retrieval, which uses a single temperature and O 2 density profile as the linearization basis. The calculated radiances and linearization coefficients are fit to a set of explicit functions of the geomagnetic field and its direction at tangent heights of 0– 120 km , which are pre-stored in order to speed up the computation. The new algorithm has been used to process all the data available during 1991–1997 before MLS 63 GHz radiometer was powered off. The estimated precision of MLS temperature varies from 2 K at ∼20 km to 8 K at ∼80 km and increases sharply above ∼90 km . The retrieved MLS temperature are compared against CIRA’86, satellite, lidar, and rocket observations. Comparisons to CIRA’86 seasonal climatology show that the differences are latitude-and-season dependent and generally <5 K below 50 km and 10 K in the mesosphere. Comparisons with other satellite observations (ISAMS, HRDI, CRISTA1) show different patterns but a cold bias at 85– 90 km seems common in all these comparisons. Comparisons to ground-based lidar measurements suggest that MLS temperatures are warmer by 2– 4 K in the stratosphere and colder by 5– 15 K at 85– 90 km . The MLS-minus-lidar difference shows a 3– 10 K cold bias near 70 km for most of the sites selected. The comparisons with rocket measurements are similar to those with lidars at these altitudes, giving cold biases in the MLS temperatures at 85– 90 km . Most of these biases are understandable in terms of sampling and resolution differences, and some biases can be reduced with further improvements in the MLS retrieval algorithm. Despite the existing biases, the MLS temperature have been found useful in studying large-scale mesospheric phenomena such as the temperature inversion layer.

Spatial distributions of upper tropospheric water vapor measurements from the UARS Microwave Limb Sounder

We characterize measurements of upper tropospheric (∼300–150 hPa) water vapor obtained from the Microwave Limb Sounder (MLS) instrument on board the Upper Atmosphere Research Satellite (UARS) in terms of their spatial and temporal variability. We present the climatology of water vapor mixing ratio for October 1991‐June 1997, which includes the seasonal means, the root‐mean‐square deviations, and the seasonal differences. The climatology of the divergent wind field is compiled to examine the relationship of upper tropospheric moisture fields and the tropical circulations. The tropical wet and dry patterns are latitudinally distributed in accordance with the Hadley circulation, and longitudinal distributions correspond well to the Walker circulation. We quantify the frequency of dry tropical observations in seasonal maps. The most prominent dry regions are located in the subtropics. On the 316 hPa surface there is some occurrence of low values of humidity throughout the majority of the tropics, while at 215 hPa it is primarily the subsidence regions that show dry values. Subtropical dry observations are more frequent in the Southern Hemisphere winter than in the Northern Hemisphere winter. The seasonal cycle of MLS water vapor is compared with Stratospheric Aerosol and Gas Experiment II (SAGE II) measurements. Good agreement is found between these two data sets for measurements near the extratropical tropopause. Potential vorticity analysis is used to indicate whether extratropical measurements were influenced primarily by stratospheric or tropospheric air. Frequency distributions display distinct characteristics and seasonal dependence for these two regions.

Equatorial diurnal variations observed in UARS Microwave Limb Sounder temperature during 1991–1994 and simulated by the Canadian Middle Atmosphere Model

This paper presents a study of the equatorial diurnal tide at 27–55 km based on temperature data from the Upper Atmosphere Research Satellite (UARS) Microwave Limb Sounder (MLS). The MLS temperature retrieval is significantly improved in the lower stratosphere with the recent (version 4) software and able to provide useful tidal information down to 22 hPa. The diurnal amplitude and phase, derived from MLS ascending‐descending temperature differences, show a strong annual variation in the lower stratosphere and a semiannual variation in the upper stratosphere and mesosphere, which are in good agreement with other UARS measurements and simulations from the Canadian Middle Atmosphere Model (CMAM). The largest differences between the observation and the model appear at 22 and 1 hPa, reflecting some shortcomings in current tidal modeling. It is shown in MLS observations that the (1, 1) tide suffers severe reduction in amplitude at 1 hPa which could be caused by the interference between the propagating tide and the component generated by stratospheric ozone heating. The data also indicate the possible presence of a short‐lived propagating component (i.e., the (1, 2) mode) in the lower stratosphere, which can be excited in the upper troposphere but damped quickly as it propagates upward.

A UARS study of lower stratospheric polar processing in the early stages of northern and southern winters

UARS data and results from a three‐dimensional transport model have been used to compare and contrast the extent of the early stages of chemical processing by polar stratospheric clouds (PSCs) during the 1991–1993 northern and southern hemisphere winters of the Upper Atmosphere Research Satellite (UARS) mission. The modeled location and timing of regions of polar processing agree quite well with the UARS microwave limb sounder (MLS), cryogenic limb array etalon spectrometer (CLAES) and Halogen Occultation Experiment (HALOE) observations in that enhanced ClO concentrations are generally found in sunlit regions of polar‐processed air and decreases of ClONO2 and HNO3 accompany low temperatures and high values of the aerosol extinctions inside the vortex. For these winters it is found that there is a period of about 2 weeks during the northern hemisphere early winter where sporadic polar processing takes place. After this a persistent processing region is seen inside the polar vortex, and the vortex fills with polar‐processed air in a bit more than 2 weeks from the first persistent PSC occurrence. In the southern hemisphere, polar processing also fills the vortex in a bit over 2 weeks. Estimates of the spectral aerosol measure of the aerosol spectrum from CLAES observations illustrate that PSC particles are seen where cold temperatures occur inside the polar vortex and heterogeneous conversion of chlorine species on PSCs are expected.

Gravity‐wave‐scale temperature fluctuations seen by the UARS MLS

The Upper Atmosphere Research Satellite (UARS) Microwave Limb Sounder (MLS) has observed small‐ and meso‐scale temperature fluctuations during a special limb‐tracking operation. Analysis of late‐December 1994 data shows that there exist significant radiance fluctuations with horizontal scales of 10's–100's km associated with upper tropospheric convection and the stratospheric polar vortex. Phase coherence and amplitude growth with height are observed, showing the presence of vertically propagating waves. Variance maps generated for the limb‐tracking days further show that the MLS instrument can measure horizontal temperature variations at scales less than 100 km in both limb‐scanning and limb‐tracking observation modes.

Validation of UARS Microwave Limb Sounder temperature and pressure measurements

The accuracy and precision of the Upper Atmosphere Research Satellite (UARS) Microwave Limb Sounder (MLS) atmospheric temperature and tangent‐point pressure measurements are described. Temperatures and tangent‐point pressure (atmospheric pressure at the tangent height of the field of view boresight) are retrieved from a 15‐channel 63‐GHz radiometer measuring O2 microwave emissions from the stratosphere and mesosphere. The Version 3 data (first public release) contains scientifically useful temperatures from 22 to 0.46 hPa. Accuracy estimates are based on instrument performance, spectroscopic uncertainty and retrieval numerics, and range from 2.1 K at 22 hPa to 4.8 K at 0.46 hPa for temperature and from 200 m (equivalent log pressure) at 10 hPa to 300 m at 0.1 hPa. Temperature accuracy is limited mainly by uncertainty in instrument characterization, and tangent‐point pressure accuracy is limited mainly by the accuracy of spectroscopic parameters. Precisions are around 1 K and 100 m. Comparisons are presented among temperatures from MLS, the National Meteorological Center (NMC) stratospheric analysis and lidar stations at Table Mountain, California, Observatory of Haute Provence (OHP), France, and Goddard Spaceflight Center, Maryland. MLS temperatures tend to be 1–2 K lower than NMC and lidar, but MLS is often 5 – 10 K lower than NMC in the winter at high latitudes, especially within the northern hemisphere vortex. Winter MLS and OHP (44°N) lidar temperatures generally agree and tend to be lower than NMC. Problems with Version 3 MLS temperatures and tangent‐point pressures are identified, but the high precision of MLS radiances will allow improvements with better algorithms planned for the future.

UARS MLS O3 soundings compared with lidar measurements using the conservative coordinates reconstruction technique

A technique based on conservative properties of certain meteorological fields is used to compare ozone measurements from the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) with soundings from a lidar system operated at midlatitudes by the University of L'Aquila, Italy. A few typical cases are analyzed in connection with the position of the vortex relative to the observing station, and it is shown that in general lidar observations taken within the vortex compare well with the UARS data, regardless of whether they are coincident with a satellite overpass. It is shown that such analysis may be useful for comparing measurements of the same quantity taken at different sites using different measurement techniques.