Microwave Limb Sounder Temperature Research Articles

Vertical Structures of Temperature and Ozone Changes in the Stratosphere and Mesosphere during Stratospheric Sudden Warmings

We analyze the observations of temperature and ozone measured by the Microwave Limb Sounder (MLS) during the period of 2005–2016, to investigate the vertical structures of temperature and ozone in the stratosphere and mesosphere during stratospheric sudden warming (SSW). We compute the height profiles of the correlation coefficients between 55 height levels of MLS temperature anomalies and compare them with the results of Whole Atmosphere Community Climate Model simulations for three major SSWs. We also construct the temperature and ozone anomalies for the events to investigate the changes in the temperature and ozone distributions with height. There seems to always be a relatively weak but broad negative correlation between the temperature anomaly at 10 hPa and temperature anomalies over the entire mesosphere during the period before SSW events. However, this pattern gets stronger in the lower mesosphere but becomes a positive correlation in the upper mesosphere and lower thermosphere after the onset of SSW. We also found that the temperatures from the simulations show a similar trend to the observational results but with smaller variations and the transition height from negative to positive correlation in the mesosphere is much lower in the simulation than in the actual observations.

Accuracy and precision of polar lower stratospheric temperatures from reanalyses evaluated from A-Train CALIOP and MLS, COSMIC GPS RO, and the equilibrium thermodynamics of supercooled ternary solutions and ice clouds

Abstract. We investigate the accuracy and precision of polar lower stratospheric temperatures (100–10 hPa during 2008–2013) reported in several contemporary reanalysis datasets comprising two versions of the Modern-Era Retrospective analysis for Research and Applications (MERRA and MERRA-2), the Japanese 55-year Reanalysis (JRA-55), the European Centre for Medium-Range Weather Forecasts (ECMWF) interim reanalysis (ERA-I), and the National Oceanic and Atmospheric Administration (NOAA) National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (NCEP-CFSR). We also include the Goddard Earth Observing System model version 5.9.1 near-real-time analysis (GEOS-5.9.1). Comparisons of these datasets are made with respect to retrieved temperatures from the Aura Microwave Limb Sounder (MLS), Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) Global Positioning System (GPS) radio occultation (RO) temperatures, and independent absolute temperature references defined by the equilibrium thermodynamics of supercooled ternary solutions (STSs) and ice clouds. Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) observations of polar stratospheric clouds are used to determine the cloud particle types within the Aura MLS geometric field of view. The thermodynamic calculations for STS and the ice frost point use the colocated MLS gas-phase measurements of HNO3 and H2O. The estimated bias and precision for the STS temperature reference, over the 68 to 21 hPa pressure range, are 0.6–1.5 and 0.3–0.6 K, respectively; for the ice temperature reference, they are 0.4 and 0.3 K, respectively. These uncertainties are smaller than those estimated for the retrieved MLS temperatures and also comparable to GPS RO uncertainties (bias < 0.2 K, precision > 0.7 K) in the same pressure range. We examine a case study of the time-varying temperature structure associated with layered ice clouds formed by orographic gravity waves forced by flow over the Palmer Peninsula and compare how the wave amplitudes are reproduced by each reanalysis dataset. We find that the spatial and temporal distribution of temperatures below the ice frost point, and hence the potential to form ice polar stratospheric clouds (PSCs) in model studies driven by the reanalyses, varies significantly because of the underlying differences in the representation of mountain wave activity. High-accuracy COSMIC temperatures are used as a common reference to intercompare the reanalysis temperatures. Over the 68–21 hPa pressure range, the biases of the reanalyses with respect to COSMIC temperatures for both polar regions fall within the narrow range of −0.6 K to +0.5 K. GEOS-5.9.1, MERRA, MERRA-2, and JRA-55 have predominantly cold biases, whereas ERA-I has a predominantly warm bias. NCEP-CFSR has a warm bias in the Arctic but becomes substantially colder in the Antarctic. Reanalysis temperatures are also compared with the PSC reference temperatures. Over the 68–21 hPa pressure range, the reanalysis temperature biases are in the range −1.6 to −0.3 K with standard deviations ∼ 0.6 K for the CALIOP STS reference, and in the range −0.9 to +0.1 K with standard deviations ∼ 0.7 K for the CALIOP ice reference. Comparisons of MLS temperatures with the PSC reference temperatures reveal vertical oscillations in the MLS temperatures and a significant low bias in MLS temperatures of up to 3 K.

New method of estimating temperatures near the mesopause region using meteor radar observations

AbstractWe present a novel method of estimating temperatures near the mesopause region using meteor radar observations. The method utilizes the linear relationship between the full width at half maximum (FWHM) of the meteor height distribution and the temperature at the meteor peak height. Once the proportionality constant of the linear relationship is determined from independent temperature measurements performed over a specific period of time by the Microwave Limb Sounder (MLS) instrument on board the Aura satellite, the temperature can be estimated continuously according to the measurements of the FWHM alone without additional information. The temperatures estimated from the FWHM are consistent with the MLS temperatures throughout the study period within a margin of 3.0%. Although previous methods are based on temperature gradient or pressure assumptions, the new method does not require such assumptions, which allows us to estimate the temperature at approximately 90 km with better precision.

Validation of Aura MLS retrievals of temperature, water vapour and ozone in the upper troposphere and lower–middle stratosphere over the Tibetan Plateau during boreal summer

Abstract. We validate Aura Microwave Limb Sounder (MLS) version 3 (v3) and version 4 (v4) retrievals of summertime temperature, water vapour and ozone in the upper troposphere and lower–middle stratosphere (UTLS; 10–316 hPa) against balloon soundings collected during the Study of Ozone, Aerosols and Radiation over the Tibetan Plateau (SOAR-TP). Mean v3 and v4 profiles of temperature, water vapour and ozone in this region during the measurement campaigns are almost identical through most of the stratosphere (10–68 hPa), but differ in several respects in the upper troposphere and tropopause layer. Differences in v4 relative to v3 include slightly colder mean temperatures from 100 to 316 hPa, smaller mean water vapour mixing ratios in the upper troposphere (215–316 hPa) and a more vertically homogeneous profile of mean ozone mixing ratios below the climatological tropopause (100–316 hPa). These changes substantially improve agreement between ozonesondes and MLS ozone retrievals in the upper troposphere, but slightly worsen existing cold and dry biases at these levels. Aura MLS temperature profiles contain significant cold biases relative to collocated temperature measurements in several layers of the lower–middle stratosphere and in the upper troposphere. MLS retrievals of water vapour volume mixing ratio generally compare well with collocated measurements, excepting a substantial dry bias (−32 ± 11 % in v4) that extends through most of the upper troposphere (121–261 hPa). MLS retrievals of ozone volume mixing ratio are biased high relative to collocated ozonesondes in the stratosphere (18–83 hPa), but are biased low at 100 hPa. The largest relative biases in ozone retrievals (approximately +70 %) are located at 83 hPa. MLS v4 offers substantial benefits relative to v3, particularly with respect to water vapour and ozone. Key improvements include larger data yields, reduced noise in the upper troposphere and smaller fluctuations in the bias profile at pressures larger than 100 hPa. The situation for temperature is less clear, with cold biases and noise levels in the upper troposphere, both larger in v4 than in v3. Several aspects of our results differ from those of validations conducted in other locations. These differences are often amplified by monsoon onset, indicating that the Asian monsoon anticyclone poses unique challenges for remote sensing that impact the quality of MLS retrievals in this region.

Properties of the quasi 16 day wave derived from EOS MLS observations

[1] This paper describes the use of EOS Microwave Limb Sounder (MLS) data to observe the field of traveling planetary waves with quasi 16 day periods. This study utilizes MLS v2.2 temperature and geopotential data between 1 January 2005 and 31 December 2008 in the range 316 hPa to 0.001 hPa (approximately 8 to 97 km) to examine these waves. Analysis demonstrates that the quasi 16 day wavefield is made up of a number of components with westward and eastward propagating s = 1 and s = 2 waves generally dominant. In the Northern Hemisphere the westward and eastward propagating s = 1 waves have similar magnitudes and are larger than the other modes, while in the Southern Hemisphere, the eastward propagating s = 1 and s = 2 waves are larger than the westward propagating wave modes. All of the modes examined display strong seasonal patterns in the temperature amplitude, significant variability in the wave activity from year to year, and the presence of strong pulse-like patterns in the activity. All of the modes also display large median temperature amplitudes poleward of 40 degrees in both hemispheres. Our analysis also demonstrates that the variability in winter from year to year is larger in the Northern Hemisphere than the Southern Hemisphere. Detailed study also suggests that the exclusion of waves from regions of negative refractive index squared likely forms much of the seasonal pattern observed. Thus, regions of strong westward wind speeds effectively exclude vertically propagating waves as expected from theory. The reflection and absorption of waves associated with critical lines is also likely to explain the frequent occurrence of standing wave patterns in the EOS MLS temperature observations. This study highlights the potential of MLS observations for observing waves from the upper troposphere to the lower mesosphere.

Improved estimates for neutral air temperatures at 90 km and 78°N using satellite and meteor radar data

[1] A technique for using satellite-derived temperatures to calibrate initial estimates of 90 km temperatures measured by meteor wind radar is presented. Temperatures derived from the Nippon/Norway Svalbard Meteor Radar, situated on Svalbard at 78°N, 16°E, are calibrated using data from the Aura spacecraft's Microwave Limb Sounder (MLS) experiment. The calibration was performed in a two-step process: after an initial calibration of first-guess temperatures, results were used to adjust the MLS values to reflect daily means rather than the 0200–1100 UT observation period of the satellite instrument; thereafter the calibration was repeated with the revised MLS temperatures. The resulting temperature time series represents a marked improvement on earlier results calibrated using hydroxyl emission and potassium/K-Lidar observations, as the uncertainty is reduced from 17 to 7 K. These latest results represent a new step toward reliable and continual monitoring of upper mesosphere/lower thermosphere temperature.

Low latitude 2‐day planetary wave impact on austral polar mesopause temperatures: revealed by a January diminution in PMSE above Davis, Antarctica

A new characteristic of the austral summer polar mesopause as revealed by ground‐based radar and satellite temperature measurements is reported, that is linked to inter‐annual variability of the low‐latitude easterly wind jet. Four consecutive seasons of polar mesosphere summer echoes (PMSE) and mesosphere temperature observations above Davis, Antarctica (68.6°S) show a mid‐January diminution in PMSE occurrence rate that coincides with a minor mesopause warming of several degrees. Spectral analyses of PMSE, Aura Microwave Limb Sounder (MLS) temperatures and radar meridional winds show the presence of ∼4–5‐day planetary waves (PWs) throughout the austral summer in the polar upper mesosphere together with enhanced ∼2‐day PW activity from mid‐January to mid‐February. Analysis of MLS temperatures show that the ∼2‐day PWs have zonal wavenumbers (S) with both westward (S = −2, −3) and eastward (S = +2, +3) components. Although displaying some inter‐annual variation in the peak onset time, the mid‐January mesopause warming coincides with a weakening of the equatorward meridional wind above Davis and enhancement of low‐latitude 2‐day PW activity.

Assimilation of stratospheric and mesospheric temperatures from MLS and SABER into a global NWP model

Abstract. The forecast model and three-dimensional variational data assimilation components of the Navy Operational Global Atmospheric Prediction System (NOGAPS) have each been extended into the upper stratosphere and mesosphere to form an Advanced Level Physics High Altitude (ALPHA) version of NOGAPS extending to ~100 km. This NOGAPS-ALPHA NWP prototype is used to assimilate stratospheric and mesospheric temperature data from the Microwave Limb Sounder (MLS) and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instruments. A 60-day analysis period in January and February 2006, was chosen that includes a well documented stratospheric sudden warming. SABER and MLS temperatures indicate that the SSW caused the polar winter stratopause at ~40 km to disappear, then reform at ~80 km altitude and slowly descend during February. The NOGAPS-ALPHA analysis reproduces this observed stratospheric and mesospheric temperature structure, as well as realistic evolution of zonal winds, residual velocities, and Eliassen-Palm fluxes that aid interpretation of the vertically deep circulation and eddy flux anomalies that developed in response to this wave-breaking event. The observation minus forecast (O-F) standard deviations for MLS and SABER are ~2 K in the mid-stratosphere and increase monotonically to about 6 K in the upper mesosphere. Increasing O-F standard deviations in the mesosphere are expected due to increasing instrument error and increasing geophysical variance at small spatial scales in the forecast model. In the mid/high latitude winter regions, 10-day forecast skill is improved throughout the upper stratosphere and mesosphere when the model is initialized using the high-altitude analysis based on assimilation of both SABER and MLS data.

Validation of the Aura Microwave Limb Sounder temperature and geopotential height measurements

Global satellite observations of temperature and geopotential height (GPH) from the Microwave Limb Sounder (MLS) on the EOS Aura spacecraft are discussed. The precision, resolution, and accuracy of the data produced by the MLS version 2.2 processing algorithms are quantified, and recommendations for data screening are made. Temperature precision is 1 K or better from 316 hPa to 3.16 hPa, degrading to ∼3 K at 0.001 hPa. The vertical resolution is 3 km at 31.6 hPa, degrading to 6 km at 316 hPa and to ∼13 km at 0.001 hPa. Comparisons with analyses (Goddard Earth Observing System version 5.0.1 (GEOS‐5), European Centre for Medium‐range Weather Forecasts (ECMWF), Met Office (MetO)) and other observations (CHAllenging Minisatellite Payload (CHAMP), Atmospheric Infrared Sounder/Advanced Microwave Sounder Unit (AIRS/AMSU), Sounding of the Atmosphere using Broadband Radiometry (SABER), Halogen Occultation Experiment (HALOE), Atmospheric Chemistry Experiment (ACE), radiosondes) indicate that MLS temperature has persistent, pressure‐dependent biases which are between −2.5 K and +1 K between 316 hPa and 10 hPa. The 100‐hPa MLS v2.2 GPH surface has a bias of ∼150 m relative to the GEOS‐5 values. These biases are compared to modeled systematic uncertainties. GPH biases relative to correlative measurements generally increase with height owing to an overall cold bias in MLS temperature relative to correlative temperature measurements in the upper stratosphere and mesosphere.

A comparison of middle atmospheric water vapor as measured by WVMS, EOS‐MLS, and HALOE

We compare middle atmospheric water vapor measurements from the Halogen Occultation Experiment (HALOE), Water Vapor Mm‐wave Spectrometer (WVMS), and Earth Observing System (EOS) Microwave Limb Sounder (MLS) instruments from 40 to 70 km. The ground‐based WVMS measurements shown here were taken at Network for the Detection of Atmospheric Composition Change (NDACC) sites at Mauna Loa, Hawaii (19.5°N, 204.4°E), and Lauder, New Zealand (45.0°S, 169.7°E). A comparison of measurements where HALOE, MLS, and WVMS are all available shows that the average HALOE water vapor retrievals are lower than those from MLS at all altitudes from 40 to 70 km and lower than the WVMS retrievals everywhere except above 64 km at Lauder. The average difference between all coincident WVMS and MLS water vapor profiles is within 0.2 ppmv over almost the entire 40–70 km altitude range, both at Lauder and Mauna Loa. The standard deviation of the difference between weekly WVMS retrievals and coincident MLS retrievals is ∼0.2 ppmv at Mauna Loa and ∼0.3–0.4 ppmv at Lauder. The interannual correlation between water vapor observed by MLS and WVMS is slightly improved by the use of MLS temperature measurements in the WVMS retrievals. The MLS and WVMS profiles at Mauna Loa show particularly good interannual agreement, including a clear QBO signature.

EOS MLS forward model polarized radiative transfer for Zeeman-split oxygen lines

This work supplements the Earth Observing System (EOS) Microwave Limb Sounder (MLS) clear-sky unpolarized forward model with algorithms for modeling polarized emission from the Zeeman-split 118.75-GHz O/sub 2/ spectral line. The model accounts for polarization-dependent emission and for correlation between polarizations with complex, 2/spl times/2 intensity and absorption matrices. The oxygen line is split into three Zeeman components by the interaction of oxygen's electronic spin with an external magnetic field, and the splitting is of order /spl plusmn/0.5 MHz in a typical geomagnetic field. Zeeman splitting is only significant at pressures low enough that collisional broadening (/spl sim/1.6 MHz/hPa) is not very large by comparison. The polarized forward model becomes significant for MLS temperature retrievals at pressure below 1.0 hPa and is crucial at pressures below /spl sim/0.03 hPa. Interaction of the O/sub 2/ molecule with the radiation field depends upon the relative orientation of the radiation polarization mode and the geomagnetic field direction. The model provides both limb radiances and the derivatives of these radiances with respect to atmospheric temperature and composition, as required by MLS temperature retrievals. EOS MLS views the atmospheric limb at 118.75 GHz with a pair of linear-cross-polarized, 100-kHz-resolution, 10-MHz-wide spectrometers. The antennas of the associated receivers are scanned to view rays with tangent heights from the Earth's surface to 0.001 hPa. Comparisons of the modeled MLS radiances with measurements show generally good agreement in line positions and strengths, however residuals in the line centers at the highest tangent heights are larger than desired and still under investigation.

The two‐day wave in EOS MLS temperature and wind measurements during 2004–2005 winter

Two‐day wave observations during January–March 2005 are reported using the recently launched Microwave Limb Sounder (MLS) aboard NASA's Earth Observing System Aura mission. Wave‐induced disturbances in temperature, water vapor, carbon monoxide, and MLS line‐of‐sight wind appear in early January, peak near the end of January, and persist until late February. Temperature and wind amplitudes as large as 9 K and 50 m/s are observed near 90 km. The wave disturbance is initially confined in the mid to low summer latitudes where the climatological summer easterly jet exhibits strong shear. The wave then develops features akin to the third Rossby‐gravity global normal mode, with a weak temperature disturbance in the winter hemisphere (anti‐symmetric about the equator) and wind disturbance over the equator. Strong wind perturbation episode around 17–23 January 2005 in the mid‐latitude Southern hemisphere coincides with a particularly intense solar proton event.

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.

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.

Observations of the 2‐day wave in UARS MLS temperature and ozone measurements

Mesospheric ozone and temperature data obtained by the Microwave Limb Sounder (MLS) on board the Upper Atmosphere Research Satellite (UARS) have been analyzed to elucidate quasi‐two day wave activity. Data from January 1993 were analyzed to investigate the evolution and structure of the two day wave. During this period, the wave amplitude in ozone attained its maximum value in late January near 30° in the summer hemisphere. The evolution of the wave amplitude indicates a shift of the peak amplitude to higher latitudes with increasing height. Correlation of the two day wave phase observed in MLS ozone and temperature measurements indicates that the perturbations are out of phase. During the same period, the amplitudes of the perturbations in temperature and ozone were positively correlated. The results suggest that the variations in ozone are photochemically driven in the mesosphere region via changes in reaction rates that are strongly temperature dependent.

Analysis of Kelvin waves in High‐Resolution Doppler Imager and Microwave Limb Sounder stratosphere measurements using a constrained least squares method

The equatorial stratospheric wind and temperature fields have been observed by the High‐Resolution Doppler Imager (HRDI) and the Microwave Limb Sounder (MLS) instruments, both aboard the Upper Atmospheric Research Satellite (UARS). These independent data sets are analyzed for Kelvin waves through a constrained least squares projection of the data sets onto a basis consisting of Hermite and Fourier functions. This method is developed specifically for HRDI wind measurements since conventional methods do not work well for the irregular nature of HRDI sampling. The recovered eastward spectra from HRDI zonal winds and MLS temperatures are compared for zonal periods from 5 to 10 days per cycle for zonal wavenumbers 1 and 2. The vertical wavelengths of each Kelvin wave event in the two fields compare well. The temperature and the zonal wind fields are not in exact quadrature due to the first‐order effects of dissipation, density gradient, and shear. The observed values of zonal wind‐temperature correlation and the vertical flux of zonal momentum are studied using a numerical model which involves these first‐order effects.

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.

Stratospheric effects of 27‐day solar ultraviolet variations: An analysis of UARS MLS ozone and temperature data

The characteristics of upper stratospheric ozone and temperature responses at low latitudes to short‐term solar ultraviolet variations are studied by using 1000 days of UARS microwave limb sounder (MLS) and solar stellar irradiance comparison experiment data. Consistent with previous analyses of Nimbus 7 solar backscattered ultraviolet (SBUV) data, the high‐pass‐filtered solar flux in the 200‐to 205‐nm interval is most strongly correlated with MLS ozone measurements at tropical latitudes near 4 hPa with a sensitivity of about 0.4% for each 1% change in the solar flux. Reproducibility tests, power spectral, and coherency estimates support the reality of the observed ozone response at this level. The MLS solar UV/ozone response is significantly reduced at levels above ∼2 hPa as compared to earlier results based on SBUV data. This reduction appears to be a consequence of the ozone diurnal cycle at high altitudes combined with the necessary inclusion of nighttime records in calculating the MLS ozone zonal averages. Some evidence is obtained for an MLS solar UV/temperature response near the stratopause, but coherency tests are negative. Future analyses of independent data records having similar local time coverage as that of Nimbus 7 SBUV are needed to establish more definitively whether any significant change in the upper stratospheric UV response has occurred.

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.

Space‐time integrity of improved stratospheric and mesospheric sounder and microwave limb sounder temperature fields at Kelvin wave scales

Space‐time analyses, which are sensitive to details of retrieval and gridding processes not seen in zonal and time means, are used to investigate the integrity of version 8 gridded retrieved temperatures from the improved stratospheric and mesospheric sounder (ISAMS) on the upper atmosphere research satellite (UARS). This note presents results of such analyses applied to ISAMS tropical data. Comparisons are made with microwave limb sounder (MLS), also on UARS, temperatures. Prominent zonal wave number 1 features are observed with characteristics similar to those expected for Kelvin waves. Time versus longitude plots reveal quasi‐regular eastward phase progression from November 1991 to mid‐January 1992. The perturbations extend throughout the upper stratosphere and lower mesosphere (altitudes of 32–64 km), exhibiting peak‐to‐peak amplitudes of up to 2°–3° K and periods from ∼ 2 weeks in midstratosphere to ∼ 1 week at higher altitudes. Faster Kelvin waves with periods of 3–5 days are also found in the lower mesosphere. Height versus time plots reveal downward phase and upward group velocities, consistent with forcing from below. Vertical wavelengths are ∼ 20 km for the slower mode and about twice this scale for the faster 3 to 5‐day mode. The features are trapped within ±10°–15° of the equator. Kelvin wave signatures in ISAMS and MLS temperatures are compared at 10 and 1 hPa. Good agreement is found, illustrating the internal consistency and ability of both ISAMS and MLS temperature grids to capture relatively small amplitude features with space‐time scales of fast, zonally asymmetric equatorial modes.