scholarly journals Comparison of ozone profiles and influences from the tertiary ozone maximum in the night-to-day ratio above Switzerland

2017 ◽  
Vol 17 (17) ◽  
pp. 10259-10268 ◽  
Author(s):  
Lorena Moreira ◽  
Klemens Hocke ◽  
Niklaus Kämpfer
Keyword(s):  
Annual Variation ◽  
Lower Stratosphere ◽  
Microwave Radiometer ◽  
Relative Difference ◽  
Atmospheric Composition ◽  
Composition Change ◽  
Ozone Maximum ◽  
Ozone Profile ◽  
Mesospheric Ozone ◽  
The Mean

Abstract. Stratospheric and middle-mesospheric ozone profiles above Bern, Switzerland (46.95° N, 7.44° E; 577 m) have been continually measured by the GROMOS (GROund-based Millimeter-wave Ozone Spectrometer) microwave radiometer since 1994. GROMOS is part of the Network for the Detection of Atmospheric Composition Change (NDACC). A new version of the ozone profile retrievals has been developed with the aim of improving the altitude range of retrieval profiles. GROMOS profiles from this new retrieval version have been compared to coincident ozone profiles obtained by the satellite limb sounder Aura Microwave Limb Sounder (MLS). The study covers the stratosphere and middle mesosphere from 50 to 0.05 hPa (from 21 to 70 km) and extends over the period from July 2009 to November 2016, which results in more than 2800 coincident profiles available for the comparison. On average, GROMOS and MLS comparisons show agreement generally over 20 % in the lower stratosphere and within 2 % in the middle and upper stratosphere for both daytime and nighttime, whereas in the mesosphere the mean relative difference is below 40 % during the daytime and below 15 % during the nighttime. In addition, we have observed the annual variation in nighttime ozone in the middle mesosphere, at 0.05 hPa (70 km), characterized by the enhancement of ozone during wintertime for both ground-based and space-based measurements. This behaviour is related to the middle-mesospheric maximum in ozone (MMM).

scholarly journals Comparison of ozone profiles and influences from the tertiary ozone maximum in the night-to-day ratio above Switzerland

2017 ◽  
Author(s):  
Lorena Moreira ◽  
Klemens Hocke ◽  
Niklaus Kämpfer
Keyword(s):  
Annual Variation ◽  
Microwave Radiometer ◽  
Atmospheric Composition ◽  
Altitude Range ◽  
Ozone Maximum ◽  
Winter Anomaly ◽  
Mesospheric Ozone ◽  
The Mean ◽  
Standard Deviations ◽  
Relative Differences

Abstract. Stratospheric and middle mesospheric ozone profiles have been continually measured by the GROMOS (GROund-based Millimeter-wave Ozone Spectrometer) microwave radiometer since 1994 above Bern, Switzerland (46.95° N, 7.44° E, 577 m). GROMOS is part of the Network for the Detection of Atmospheric Composition Change (NDACC). A new version for the retrieval of ozone profiles has been developed with the aim to improve the altitude range of retrieval profiles. GROMOS profiles from this new retrieval version have been compared to coincident ozone profiles obtained by the satellite limb sounder Aura/MLS. The study covers the stratosphere and middle mesosphere from 50 to 0.05 hPa (from 21 to 70 km) and extends over the period from July 2009 to November 2016, which results in more than 3500 coincident profiles available for the comparison. GROMOS and Aura/MLS profiles agree within 3 % for the altitude range from 25 to 55 km, with standard deviations of the mean relative differences around 5 % from 30 to 40 km and tending to 10 % towards the lower and upper stratosphere. Above the stratosphere, the mean relative differences and its standard deviations are increasing with altitude up to 50 % at 70 km. In addition, we have observed the annual variation of nighttime ozone in the middle mesosphere, at 0.05 hPa (70 km), characterised by the enhancement of ozone during wintertime for both ground-based and space-based measurements. This behaviour is explained by the middle mesospheric maximum of ozone (MMM). On the other hand, the amplitude of the diurnal variation, night-to-day ratio (NDR), is not as strong as the observed one at higher latitudes, nevertheless we observe the winter anomaly of the night-to-day ratio.

scholarly journals Construction of merged satellite total O<sub>3</sub> and NO<sub>2</sub> time series in the tropics for trend studies and evaluation by comparison to NDACC SAOZ measurements

2014 ◽  
Vol 7 (10) ◽  
pp. 3337-3354 ◽  
Author(s):  
M. Pastel ◽  
J.-P. Pommereau ◽  
F. Goutail ◽  
A. Richter ◽  
A. Pazmiño ◽  
...  
Keyword(s):  
Time Series ◽  
Lower Stratosphere ◽  
Atmospheric Composition ◽  
Data Sets ◽  
Composition Change ◽  
Long Time ◽  
Uv Visible ◽  
The Tropics ◽  
Lower Noise ◽  
Total Column

Abstract. Long time series of ozone and NO2 total column measurements in the southern tropics are available from two ground-based SAOZ (Système d'Analyse par Observation Zénithale) UV-visible spectrometers operated within the Network for the Detection of Atmospheric Composition Change (NDACC) in Bauru (22° S, 49° W) in S-E Brazil since 1995 and Reunion Island (21° S, 55° E) in the S-W Indian Ocean since 1993. Although the stations are located at the same latitude, significant differences are observed in the columns of both species, attributed to differences in tropospheric content and equivalent latitude in the lower stratosphere. These data are used to identify which satellites operating during the same period, are capturing the same features and are thus best suited for building reliable merged time series for trend studies. For ozone, the satellites series best matching SAOZ observations are EP-TOMS (1995–2004) and OMI-TOMS (2005–2011), whereas for NO2, best results are obtained by combining GOME version GDP5 (1996–2003) and SCIAMACHY – IUP (2003–2011), displaying lower noise and seasonality in reference to SAOZ. Both merged data sets are fully consistent with the larger columns of the two species above South America and the seasonality of the differences between the two stations, reported by SAOZ, providing reliable time series for further trend analyses and identification of sources of interannual variability in the future analysis.

scholarly journals Intercomparison of stratospheric ozone profiles for the assessment of the upgraded GROMOS radiometer at Bern

2013 ◽  
Vol 6 (4) ◽  
pp. 6097-6146 ◽  
Author(s):  
S. Studer ◽  
K. Hocke ◽  
M. Pastel ◽  
S. Godin-Beekmann ◽  
N. Kämpfer
Keyword(s):  
Time Series ◽  
Stratospheric Ozone ◽  
Microwave Radiometer ◽  
Mean Difference ◽  
Atmospheric Composition ◽  
Time Period ◽  
Lidar Measurements ◽  
Mesospheric Ozone ◽  
Relative Differences

Abstract. Since November 1994, the GROund-based Millimeter-wave Ozone Spectrometer (GROMOS) measures stratospheric and lower mesospheric ozone in Bern, Switzerland (47.95° N, 7.44° E). GROMOS is part of the Network for the Detection of Atmospheric Composition Change (NDACC). In July 2009, a Fast-Fourier-Transform spectrometer (FFTS) has been added as backend to GROMOS. The new FFTS and the original filter bench (FB) measured parallel for over two years. In October 2011, the FB has been turned off and the FFTS is now used to continue the ozone time series. For a consolidated ozone time series in the frame of NDACC, the quality of the stratospheric ozone profiles obtained with the FFTS has to be assessed. The FFTS results from July 2009 to December 2011 are compared to ozone profiles retrieved by the FB. FFTS and FB of the GROMOS microwave radiometer agree within 5% above 20 hPa. A later harmonization of both time series will be realized by taking the FFTS as benchmark for the FB. Ozone profiles from the FFTS are also compared to coinciding lidar measurements from the Observatoire Haute Provence (OHP), France. For the time period studied a maximum mean difference (lidar – GROMOS FFTS) of +3.8% at 3.1 hPa and a minimum mean difference of +1.4% at 8 hPa is found. Further, intercomparisons with ozone profiles from other independent instruments are performed: satellite measurements include MIPAS onboard ENVISAT, SABER onboard TIMED, MLS onboard EOS Aura and ACE-FTS onboard SCISAT-1. Additionally, ozonesondes launched from Payerne, Switzerland, are used in the lower stratosphere. Mean relative differences of GROMOS FFTS and these independent instruments are less than 10% between 50 and 0.1 hPa.

scholarly journals Intercomparison of stratospheric nitrogen dioxide columns retrieved from ground-based DOAS and FTIR and satellite DOAS instruments over the subtropical Izana station

2016 ◽  
Vol 9 (9) ◽  
pp. 4471-4485 ◽  
Author(s):  
Cristina Robles-Gonzalez ◽  
Mónica Navarro-Comas ◽  
Olga Puentedura ◽  
Matthias Schneider ◽  
Frank Hase ◽  
...  
Keyword(s):  
Relative Difference ◽  
Field Of View ◽  
Atmospheric Composition ◽  
Optical Absorption Spectroscopy ◽  
Data Series ◽  
Composition Change ◽  
Ozone Monitoring Instrument ◽  
Vertical Column ◽  
Ozone Monitoring ◽  
Scanning Imaging

Abstract. A 13-year analysis (2000–2012) of the NO2 vertical column densities derived from ground-based (GB) instruments and satellites has been carried out over the Izaña NDACC (Network for the Detection of the Atmospheric Composition Change) subtropical site. Ground-based DOAS (differential optical absorption spectroscopy) and FTIR (Fourier transform infrared spectroscopy) instruments are intercompared to test mutual consistency and then used for validation of stratospheric NO2 from OMI (Ozone Monitoring Instrument) and SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY). The intercomparison has been carried out taking into account the various differences existing in instruments, namely temporal coincidence, collocation, sensitivity, field of view, etc. The paper highlights the importance of considering an “effective solar zenith angle” instead of the actual one when comparing direct-sun instruments with zenith sky ones for a proper photochemical correction. Results show that NO2 vertical column densities mean relative difference between FTIR and DOAS instruments is 2.8 ± 10.7 % for a.m. data. Both instruments properly reproduce the NO2 seasonal and the interannual variation. Mean relative difference of the stratospheric NO2 derived from OMI and DOAS is −0.2 ± 8.7 % and from OMI and FTIR is −1.6 ± 6.7 %. SCIAMACHY mean relative difference is of 3.7 ± 11.7 and −5.7 ± 11.0 % for DOAS and FTIR, respectively. Note that the days used for the intercomparison are not the same for all the pairs of instruments since it depends on the availability of data. The discrepancies are found to be seasonally dependent with largest differences in winter and excellent agreement in the spring months (AMJ). A preliminary analysis of NO2 trends has been carried out with the available data series. Results show increases in stratospheric NO2 columns in all instruments but larger values in those that are GB than that expected by nitrous oxide oxidation. The possible reasons for the discrepancy between instruments and the positive trends are discussed in the text.

scholarly journals Validation of the CrIS fast physical NH<sub>3</sub> retrieval with ground-based FTIR

2017 ◽  
Vol 10 (7) ◽  
pp. 2645-2667 ◽  
Author(s):  
Enrico Dammers ◽  
Mark W. Shephard ◽  
Mathias Palm ◽  
Karen Cady-Pereira ◽  
Shannon Capps ◽  
...  
Keyword(s):  
Relative Difference ◽  
Atmospheric Composition ◽  
Absolute Difference ◽  
Lower Sensitivity ◽  
Composition Change ◽  
Atmospheric Concentration ◽  
The Absolute ◽  
Cross Track ◽  
Median Absolute Difference ◽  
Total Column

Abstract. Presented here is the validation of the CrIS (Cross-track Infrared Sounder) fast physical NH3 retrieval (CFPR) column and profile measurements using ground-based Fourier transform infrared (FTIR) observations. We use the total columns and profiles from seven FTIR sites in the Network for the Detection of Atmospheric Composition Change (NDACC) to validate the satellite data products. The overall FTIR and CrIS total columns have a positive correlation of r  =  0.77 (N  =  218) with very little bias (a slope of 1.02). Binning the comparisons by total column amounts, for concentrations larger than 1.0  ×  1016 molecules cm−2, i.e. ranging from moderate to polluted conditions, the relative difference is on average ∼ 0–5 % with a standard deviation of 25–50 %, which is comparable to the estimated retrieval uncertainties in both CrIS and the FTIR. For the smallest total column range (< 1.0  × 1016 molecules cm−2) where there are a large number of observations at or near the CrIS noise level (detection limit) the absolute differences between CrIS and the FTIR total columns show a slight positive column bias. The CrIS and FTIR profile comparison differences are mostly within the range of the single-level retrieved profile values from estimated retrieval uncertainties, showing average differences in the range of  ∼ 20 to 40 %. The CrIS retrievals typically show good vertical sensitivity down into the boundary layer which typically peaks at  ∼ 850 hPa (∼ 1.5 km). At this level the median absolute difference is 0.87 (std  =  ±0.08) ppb, corresponding to a median relative difference of 39 % (std  =  ±2 %). Most of the absolute and relative profile comparison differences are in the range of the estimated retrieval uncertainties. At the surface, where CrIS typically has lower sensitivity, it tends to overestimate in low-concentration conditions and underestimate in higher atmospheric concentration conditions.

scholarly journals Improvement of OMI ozone profile retrievals in the upper troposphere and lower stratosphere by the use of a tropopause-based ozone profile climatology

2013 ◽  
Vol 6 (9) ◽  
pp. 2239-2254 ◽  
Author(s):  
J. Bak ◽  
X. Liu ◽  
J. C. Wei ◽  
L. L. Pan ◽  
K. Chance ◽  
...  
Keyword(s):  
Lower Stratosphere ◽  
Meteorological Data ◽  
A Priori ◽  
Tropopause Height ◽  
Ozone Monitoring Instrument ◽  
Upper Troposphere ◽  
Variable Position ◽  
Ozone Profile ◽  
The Mean ◽  
Standard Deviations

Abstract. Motivated by the need of obtaining a more accurate global ozone distribution in the upper troposphere and lower stratosphere (UTLS), we have investigated the use of a tropopause-based (TB) ozone climatology in ozone profile retrieval from the Ozone Monitoring Instrument (OMI). Due to the limited vertical ozone information in the UTLS region from OMI backscattered ultraviolet radiances, better climatological a priori information is important for improving ozone profile retrievals. We present the new TB climatology and evaluate the result of retrievals against previous work. The TB climatology is created using ozonesonde profiles from 1983 through 2008 extended with climatological ozone data above sonde burst altitude (~35 km) with the corresponding temperature profiles used to identify the thermal tropopause. The TB climatology consists of the mean states and 1σ standard deviations for every month for each 10° latitude band. Compared to the previous TB climatology by Wei et al. (2010), three additional processes are applied in deriving our climatology: (1) using a variable shifting offset to define the TB coordinate, (2) separating ozonesonde profiles into tropical and extratropical regimes based on a threshold of 14 km in the thermal tropopause height, and (3) merging with an existing climatology from 5–10 km above the tropopause. The first process changes the reference of profiles to a variable position between local and mean tropopause heights within ±5 km of the tropopause and to the mean tropopause elsewhere. The second helps to preserve characteristics of either tropical or extratropical ozone structures depending on tropopause height, especially in the subtropical region. The third improves the climatology above ozonesonde burst altitudes and in the stratosphere by using climatology derived from many more satellite observations of ozone profiles. With aid from the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) tropopause height, the new climatology and retrieval can better represent the dynamical variability of ozone in the tropopause region. The new retrieval result demonstrates significant improvement of UTLS ozone, especially in the extratropical UTLS, when evaluated using ozonesonde measurements and the meteorological data. The use of TB climatology significantly enhances the spatial consistency and the statistical relationship between ozone and potential vorticity/tropopause height in the extratropical UTLS region. Comparisons with ozonesonde measurements show substantial improvements in both mean biases and their standard deviations over the extratropical lowermost stratosphere and upper troposphere. Overall, OMI retrievals with the TB climatology show improved ability in capturing ozone gradients across the tropopause found in tropical/extratropical ozonesonde measurements.

scholarly journals The SPARC water vapor assessment II: intercomparison of satellite and ground-based microwave measurements

2017 ◽  
Vol 17 (23) ◽  
pp. 14543-14558 ◽  
Author(s):  
Gerald E. Nedoluha ◽  
Michael Kiefer ◽  
Stefan Lossow ◽  
R. Michael Gomez ◽  
Niklaus Kämpfer ◽  
...  
Keyword(s):  
Water Vapor ◽  
Middle Atmosphere ◽  
Atmospheric Composition ◽  
Microwave Measurements ◽  
Interannual Variations ◽  
Composition Change ◽  
Mauna Loa ◽  
Trend Assessment ◽  
The Mean ◽  
Almost All

Abstract. As part of the second SPARC (Stratosphere–troposphere Processes And their Role in Climate) water vapor assessment (WAVAS-II), we present measurements taken from or coincident with seven sites from which ground-based microwave instruments measure water vapor in the middle atmosphere. Six of the ground-based instruments are part of the Network for the Detection of Atmospheric Composition Change (NDACC) and provide datasets that can be used for drift and trend assessment. We compare measurements from these ground-based instruments with satellite datasets that have provided retrievals of water vapor in the lower mesosphere over extended periods since 1996. We first compare biases between the satellite and ground-based instruments from the upper stratosphere to the upper mesosphere. We then show a number of time series comparisons at 0.46 hPa, a level that is sensitive to changes in H2O and CH4 entering the stratosphere but, because almost all CH4 has been oxidized, is relatively insensitive to dynamical variations. Interannual variations and drifts are investigated with respect to both the Aura Microwave Limb Sounder (MLS; from 2004 onwards) and each instrument's climatological mean. We find that the variation in the interannual difference in the mean H2O measured by any two instruments is typically  ∼  1%. Most of the datasets start in or after 2004 and show annual increases in H2O of 0–1 % yr−1. In particular, MLS shows a trend of between 0.5 % yr−1 and 0.7 % yr−1 at the comparison sites. However, the two longest measurement datasets used here, with measurements back to 1996, show much smaller trends of +0.1 % yr−1 (at Mauna Loa, Hawaii) and −0.1 % yr−1 (at Lauder, New Zealand).

scholarly journals Nitrogen dioxide stratospheric column at the subtropical NDACC station of Izaña from DOAS, FTIR and satellite instrumentation

2016 ◽  
Author(s):  
Cristina Robles-Gonzalez ◽  
Mónica Navarro-Comas ◽  
Olga Puentedura ◽  
Matthias Schneider ◽  
Frank Hase ◽  
...  
Keyword(s):  
Relative Difference ◽  
Field Of View ◽  
Atmospheric Composition ◽  
Optical Absorption Spectroscopy ◽  
Data Series ◽  
Composition Change ◽  
Ozone Monitoring Instrument ◽  
Vertical Column ◽  
Ozone Monitoring ◽  
Scanning Imaging

Abstract. A 13-years analysis (2000–2012) of the NO2 vertical column densities (VCD) derived from ground-based (GB) instruments and satellites has been carried out over the Izaña NDACC (Network for the Detection of the Atmospheric Composition Change) subtropical site. Ground-based DOAS (Differential Optical Absorption Spectroscopy) and FTIR (Fourier Transform InfraRed spectroscopy) instruments are intercompared to test mutual consistency and then use for validation of stratospheric NO2 from OMI (Ozone Monitoring Instrument) and SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY). The intercomparison has been carried out taking into account the various differences existing in instruments, namely temporal coincidence, collocation, sensitivity, field of view, etc. The paper highlights the importance of considering an "effective solar zenith angle" instead of the actual one when comparing direct sun instruments with zenith sky ones for a proper photochemical correction. Results show that NO2 vertical column densities mean relative difference between FTIR and DOAS instruments is 2.8 ± 10.7 % for AM data. Both instruments properly reproduce the NO2 seasonal and the interannual variation. Mean relative difference of the stratospheric NO2 derived from OMI and DOAS is −0.2 ± 8.7 % and from OMI and FTIR is −1.6 ± 6.7. SCIAMACHY mean relative difference is of 3.7 ± 11.7 % and −5.7 ± 11.0 % for DOAS and FTIR, respectively. Note that the days used for the intercomparison are not the same for all the pairs of instruments since it depends on the availability of data. The discrepancies are found to be seasonally dependent with largest differences in winter and excellent agreement in the spring months (AMJ). A preliminary analysis of NO2 trends has been carried out with the available data series. Results show positive values in all instruments but larger values on the ground-based than that expected by nitrous oxide oxidation. The possible reasons of the discrepancy between instruments and the positive trends are discussed in the text.

scholarly journals Improvement of OMI ozone profile retrievals in the upper troposphere and lower stratosphere by the use of a tropopause-based ozone profile climatology

2013 ◽  
Vol 6 (3) ◽  
pp. 4333-4369 ◽  
Author(s):  
J. Bak ◽  
X. Liu ◽  
J. C. Wei ◽  
L. L. Pan ◽  
K. Chance ◽  
...  
Keyword(s):  
Lower Stratosphere ◽  
Meteorological Data ◽  
A Priori ◽  
Tropopause Height ◽  
Ozone Monitoring Instrument ◽  
Upper Troposphere ◽  
Variable Position ◽  
Ozone Profile ◽  
The Mean ◽  
Standard Deviations

Abstract. Motivated by the need of obtaining a more accurate global ozone distribution in the upper troposphere and lower stratosphere (UTLS), we have investigated the use of a tropopause-based (TB) ozone climatology in ozone profile retrieval from the Ozone Monitoring Instrument (OMI). Due to the limited vertical ozone information in the UTLS region from OMI backscattered ultraviolet radiances, better climatological a priori information is important for improving ozone profile retrievals. We present the new TB climatology and evaluate the result of retrievals against previous work. The improved TB climatology is created using ozonesonde profiles from 1983 through 2008 extended with climatological ozone data above sonde burst altitude (~35 km) with the corresponding temperature profiles used to identify the thermal tropopause. The TB climatology consists of the mean states and 1σ standard deviations every month for each 10° latitude band. Three additional processes are applied in deriving our climatology: (1) using a variable shifting offset to define the TB coordinate, (2) separating ozonesonde profiles into tropical and extratropical regimes based on a threshold of 14 km in the thermal tropopause height, and (3) merging with an existing climatology from 5–10 km above the tropopause. The first process changes the reference of profiles to variable position between local and mean tropopause heights within ±5 km at the tropopause and to the mean tropopause elsewhere. The second helps to preserve characteristics of either tropical or extratropical ozone structures depending on tropopause height, especially in the subtropical region. The third improves the climatology above ozonesonde burst altitudes and in the stratosphere by using climatology derived from many more satellite observations of ozone profiles. With aid from the National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) tropopause height, the new climatology and retrieval can better represent the dynamical variability of ozone in the tropopause region. The new retrieval result demonstrates significant improvement of UTLS ozone, especially in the extratropical UTLS, when evaluated using ozonesonde measurements and the meteorological data. The use of TB climatology significantly enhances the spatial consistency and the statistically relationship between ozone and potential vorticity/tropopause height in the extratropical UTLS region. Comparisons with ozonesonde measurements show substantial improvements in both mean biases and their standard deviations over the extratropical lowermost stratosphere and UT. Overall, OMI retrievals with the TB climatology show improved ability in capturing ozone gradients across the tropopause demonstrated by tropical/extratropical ozonesonde measurements.

scholarly journals The SPARC water vapor assessment II: intercomparison of satellite and ground-based microwave measurements

2017 ◽  
Author(s):  
Gerald E. Nedoluha ◽  
Michael Kiefer ◽  
Stefan Lossow ◽  
R. Michael Gomez ◽  
Niklaus Kämpfer ◽  
...  
Keyword(s):  
Time Series ◽  
Water Vapor ◽  
Middle Atmosphere ◽  
Atmospheric Composition ◽  
Microwave Measurements ◽  
Interannual Variations ◽  
Composition Change ◽  
Trend Assessment ◽  
The Mean ◽  
Almost All

Abstract. As part of the second SPARC (Stratosphere-troposphere Processes And their Role in Climate) water vapour assessment (WAVAS-II), we present measurements taken from, or coincident with, seven sites from which ground-based microwave instruments measure water vapor in the middle atmosphere. Six of the ground-based instruments are part of the Network for the Detection of Atmospheric Composition Change (NDACC) and provide datasets which can be used for drift and trend assessment. We compare measurements from these ground-based instruments with satellite datasets that have provided retrievals of water vapor in the lower mesosphere over extended periods since 1996. We first compare biases between the satellite and ground-based instruments from the upper stratosphere to the upper mesosphere. We then show a number of time series comparisons at 0.46 hPa, a level that is sensitive to changes in H2O and CH4 entering the stratosphere, but, because almost all CH4 has been oxidized, is relatively insensitive to dynamical variations. Interannual variations and drifts are investigated both with respect to the Aura Microwave Limb Sounder (MLS) (from 2004 onwards), and with respect to each instrument's climatological mean. We find that the variation in the interannual difference in the mean H2O measured by any two instruments is typically ~ 1 %. Most of the datasets start in, or after, 2004, and show annual increases in H2O of 0–1 %/year. In particular, MLS shows a trend of between 0.5 %/year and 0.7 %/year at the comparison sites. However the two longest measurement datasets used here, with measurements back to 1996, show a much smaller trend of between +0.1 %/year and −0.1 %/year.

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