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The pixel size of the Infrared Atmospheric Sounding Interferometer (IASI) remote sensor is much smaller than the horizontal grid size of current Chemical Transport Models (CTMs). In order to assimilate the maximum of information from the IASI retrievals, we have increased the horizontal resolution of our model MOCAGE to be consistent with the IASI pixel size. Experiments are carried out with the Valentina data assimilation system using the standard and the high resolution versions of the model. Two resolutions of the horizontal Gaussian grid have been used for the model: with a T42 and a T170 triangular truncations. Our study is based on the combination of data from the IASI instrument and from the Microwave Limb Sounder (MLS), since this latter dataset allows the information to be spread through the whole atmospheric columns at a low computational cost. Two datasets of ozone super-observations have been constructed by averaging the IASI data on the two model grids. Direct model simulations without data assimilation first show that the increase of the horizontal resolution modifies the ozone smallest scale structures as well as the ozone meridional distribution. This modification results from a better representation of the vertical velocity with the T170 configuration. When the ozone assimilation is performed there is less influence of the horizontal resolution of the model. Nevertheless, in a general way, comparisons with independent data show large reductions of the ozone standard deviations when the resolution is increased. When the ozone assimilation is performed with the high resolution dataset, the high resolution model does not improve the ozone analysis compared to the one obtained with the same model resolution but with the low resolution IASI dataset. This result is due to the difficulty to combine IASI data and MLS data. For assimilating IASI data at high resolution the horizontal correlation length-scale has to be decreased to catch the small scale structures present in the dataset. By doing so the influence of the coarser resolution MLS data is decreased and part of the information brought on the vertical shape of the ozone profile is lost. It is concluded that it is essential to add information on the vertical distribution of ozone column when the IASI data is assimilated at a resolution close to the pixel size. Using IASI averaging kernels would likely improve the simulations, but the computational cost would be much higher. Alternatively, better results might be obtained by a careful tuning of the horizontal correlation length-scale.

Forest fires in British Columbia in August 2017 caused a pyrocumulonimbus event that injected a polluted air mass into the lower stratosphere. The Microwave Limb Sounder (MLS) on the Aura satellite first observed the polluted air mass on 14 August 2017 and continued to observe it for 60 d (100 d in water vapour). We estimate the mass of CO injected into the stratosphere to be 2400 Gg. Events in which a fire injects its burning products directly into the stratosphere are rare: this is the third of four such events in the 16 years since the launch of Aura, the second largest of the four events, and the only one in the Northern Hemisphere. The other three events occurred in Australia in December 2006, February 2009 and from December 2019 to January 2020. Unlike the 2006 and 2009 events, but like the 2019–2020 event, the polluted air mass described here had a clearly elevated water vapour content: between 2.5 and 5 times greater than that in the surrounding atmosphere. We describe the evolution of the polluted air mass, showing that it rose to an altitude of about 24 km (31 hPa) and divided into several identifiable parts. In addition to CO and H2O, we observe enhanced amounts of HCN, CH3CN, CH3Cl and CH3OH with mixing ratios in the range to be expected from a variety of measurements in other biomass burning plumes. We use back trajectories and plume-dispersion modelling to demonstrate that the pollutants observed by MLS originated in the British Columbia fires, the likeliest source being at 53.2∘ N, 121.8∘ W at 05:20 UTC on 13 August 2017.

Total column ozone (TCO) over the Tibetan Plateau (TP) is lower than that over other regions at the same latitude, particularly in summer. This feature is known as the “TP ozone valley”. This study evaluates long-term changes in TCO and the ozone valley over the TP from 1984 to 2100 using Coupled Model Intercomparison Project Phase 6 (CMIP6). The TP ozone valley consists of two low centers, one is located in the upper troposphere and lower stratosphere (UTLS), and the other is in the middle and upper stratosphere. Overall, the CMIP6 models simulate the low ozone center in the UTLS well and capture the spatial characteristics and seasonal cycle of the TP ozone valley, with spatial correlation coefficients between the modeled TCO and the Multi Sensor Reanalysis version 2 (MSR2) TCO observations greater than 0.8 for all CMIP6 models. Further analysis reveals that models which use fully coupled and online stratospheric chemistry schemes simulate the anticorrelation between the 150 hPa geopotential height and zonal anomaly of TCO over the TP better than models without interactive chemistry schemes. This suggests that coupled chemical-radiative-dynamical processes play a key role in the simulation of the TP ozone valley. Most CMIP6 models underestimate the low center in the middle and upper stratosphere when compared with the Microwave Limb Sounder (MLS) observations. However, the bias in the middle and upper stratospheric ozone simulations has a marginal effect on the simulation of the TP ozone valley. Most CMIP6 models predict the TP ozone valley in summer will deepen in the future.

The Microwave Limb Sounder (MLS), launched on NASA's Aura spacecraft in 2004, measures vertical profiles of the abundances of key atmospheric species from the upper troposphere to the mesosphere with daily near-global coverage. We review the first 15 years of the record of H2O and N2O measurements from the MLS 190 GHz subsystem (along with other 190 GHz information), with a focus on their long-term stability, largely based on comparisons with measurements from other sensors. These comparisons generally show signs of an increasing drift in the MLS “version 4” (v4) H2O record starting around 2010. Specifically, comparisons with v4.1 measurements from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) indicate a ∼ 2 %–3 % per decade drift over much of the stratosphere, increasing to as much as ∼ 7 % per decade around 46 hPa. Larger drifts, of around 7 %–11 % per decade, are seen in comparisons to balloon-borne frost point hygrometer measurements in the lower stratosphere. Microphysical calculations considering the formation of polar stratospheric clouds in the Antarctic winter stratosphere corroborate a drift in MLS v4 water vapor measurements in that region and season. In contrast, comparisons with the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission, and with ground-based Water Vapor Millimeter-wave Spectrometer (WVMS) instruments, do not show statistically significant drifts. However, the uncertainty in these comparisons is large enough to encompass most of the drifts identified in other comparisons. In parallel, the MLS v4 N2O product is shown to be generally decreasing over the same period (when an increase in stratospheric N2O is expected, reflecting a secular growth in emissions), with a more pronounced drift in the lower stratosphere than that found for H2O. Comparisons to ACE-FTS and to MLS N2O observations in a different spectral region, with the latter available from 2004 to 2013, indicate an altitude-dependent drift, growing from 5 % per decade or less in the mid-stratosphere to as much as 15 % per decade in the lower stratosphere. Detailed investigations of the behavior of the MLS 190 GHz subsystem reveal a drift in its “sideband fraction” (the relative sensitivity of the 190 GHz receiver to the two different parts of the microwave spectrum that it observes). Our studies indicate that sideband fraction drift accounts for much of the observed changes in the MLS H2O product and some portion of the changes seen in N2O. The 190 GHz sideband fraction drift has been corrected in the new “version 5” (v5) MLS algorithms, which have now been used to reprocess the entire MLS record. As a result of this correction, the MLS v5 H2O record shows no statistically significant drifts compared to ACE-FTS. However, statistically significant drifts remain between MLS v5 and frost point measurements, although they are reduced. Drifts in v5 N2O are about half the size of those in v4 but remain statistically significant. Scientists are advised to use MLS v5 data in all future studies. Quantification of interregional and seasonal to annual changes in MLS H2O and N2O will not be affected by the drift. However, caution is advised in studies using the MLS record to examine long-term (multiyear) variability and trends in either of these species, especially N2O; such studies should only be undertaken in consultation with the MLS team. Importantly, this drift does not affect any of the MLS observations made in other spectral regions such as O3, HCl, CO, ClO, or temperature.

Starting in 1969 and comprising three launches a week, the Uccle (Brussels, Belgium) ozonesonde dataset is one of longest and densest in the world. Moreover, as the only major change was the switch from Brewer-Mast (BM) to electrochemical concentration cell (ECC) ozonesonde types in 1997 (when the emissions of ozone-depleting substances peaked), the Uccle time series is very homogenous. In this paper, we briefly describe the efforts that were undertaken during the first 3 decades of the 50 years of ozonesonde observations to guarantee the homogeneity between ascent and descent profiles, under changing environmental conditions (e.g. SO2), and between the different ozonesonde types. This paper focuses on the 50-year-long Uccle ozonesonde dataset and aims to demonstrate its past, present, and future relevance to ozone research in two application areas: (i) the assessment of the temporal evolution of ozone from the surface to the (middle) stratosphere, and (ii) as the backbone for validation and stability analysis of both stratospheric and tropospheric satellite ozone retrievals. Using the Long-term Ozone Trends and Uncertainties in the Stratosphere (LOTUS) multiple linear regression model (SPARC/IO3C/GAW, 2019), we found that the stratospheric ozone concentrations at Uccle have declined at a significant rate of around 2 % per decade since 1969, which is also rather consistent over the different stratospheric levels. This overall decrease can mainly be assigned to the 1969–1996 period with a rather consistent rate of decrease of around −4 % per decade. Since 2000, a recovery of between +1 % per decade and +3 % per decade of the stratospheric ozone levels above Uccle has been observed, although it is not significant and is not seen for the upper stratospheric levels measured by ozonesondes. Throughout the entire free troposphere, a very consistent increase in the ozone concentrations of 2 % per decade to 3 % per decade has been measured since both 1969 and 1995, with the trend since 1995 being in almost perfect agreement with the trends derived from the In-service Aircraft for a Global Observing System (IAGOS) ascent/descent profiles at Frankfurt. As the number of tropopause folding events in the Uccle time series has increased significantly over time, increased stratosphere-to-troposphere transport of recovering stratospheric ozone might partly explain these increasing tropospheric ozone concentrations, despite the levelling-off of (tropospheric) ozone precursor emissions and notwithstanding the continued increase in mean surface ozone concentrations. Furthermore, we illustrate the crucial role of ozonesonde measurements for the validation of satellite ozone profile retrievals. With the operational validation of the Global Ozone Monitoring Experiment-2 (GOME-2), we show how the Uccle dataset can be used to evaluate the performance of a degradation correction for the MetOp-A/GOME-2 UV (ultraviolet) sensors. In another example, we illustrate that the Microwave Limb Sounder (MLS) overpass ozone profiles in the stratosphere agree within ±5 % with the Uccle ozone profiles between 10 and 70 hPa. Another instrument on the same Aura satellite platform, the Tropospheric Emission Spectrometer (TES), is generally positively biased with respect to the Uccle ozonesondes in the troposphere by up to ∼ 10 ppbv, corresponding to relative differences of up to ∼ 15 %. Using the Uccle ozonesonde time series as a reference, we also demonstrate that the temporal stability of those last two satellite retrievals is excellent.

Stratospheric water vapor variations, which may play an important role in surface climate, have drawn extensive studies. Here, the variation in stratospheric water vapor is investigated by using data from observations of the Microwave Limb Sounder (MLS) on the Aura satellite, from the ECMWF Interim Reanalysis (ERAI), and simulations by the Whole Atmosphere Community Climate Model (WACCM). We find that the differences of annual mean stratospheric water vapor among these datasets may be partly caused by the differences in vertical transports. Using budget analysis, we find that the upward transport of water vapor at 100 hPa is mainly located over the Pacific warm pool region and South America in the equatorial tropics in boreal winter and over the southeast of the South Asian high and south of North America in boreal summer. It is found that temperature averaged over regions with upward transport is a better indicator of interannual variability of tropical mean stratospheric water vapor than the tropical mean temperature. It seems that the distributions of the seasonal cycle amplitude of lower stratospheric water vapor in the tropics can also be impacted by the vertical transport. The radiative effects of the interannual changes in water vapor in the lowermost stratosphere are underestimated by approximately 29% in both ERAI and WACCM compared to MLS, although the interannual variations of water vapor in the lowermost stratosphere are dramatically overestimated in ERAI and WACCM. The results here indicate that the radiative effect of long-term changes in water vapor in the lowermost stratosphere may be underestimated in both ERAI and WACCM simulations.

We investigate the influence of different chemical and physical processes on the water vapour distribution in the lower stratosphere (LS), in particular in the Asian and North American monsoon anticyclones (AMA and NAMA, respectively). Specifically, we use the chemistry transport model CLaMS to analyse the effects of large-scale temperatures, methane oxidation, ice microphysics, and small-scale atmospheric mixing processes in different model experiments. All these processes hydrate the LS and, particularly, the AMA. While ice microphysics has the largest global moistening impact, it is small-scale mixing which dominates the specific signature in the AMA in the model experiments. In particular, the small-scale mixing parameterization strongly contributes to the water vapour transport to this region and improves the simulation of the intra-seasonal variability, resulting in a better agreement with the Aura Microwave Limb Sounder (MLS) observations. Although none of our experiments reproduces the spatial pattern of the NAMA as seen in MLS observations, they all exhibit a realistic annual cycle and intra-seasonal variability, which are mainly controlled by large-scale temperatures. We further analyse the sensitivity of these results to the domain-filling trajectory set-up, here-called Lagrangian trajectory filling (LTF). Compared with MLS observations and with a multiyear reference simulation using the full-blown chemistry transport model version of CLaMS, we find that the LTF schemes result in a drier global LS and in a weaker water vapour signal over the monsoon regions, which is likely related to the specification of the lower boundary condition. Overall, our results emphasize the importance of subgrid-scale mixing and multiple transport pathways from the troposphere in representing water vapour in the AMA.

A newly upgraded German Weather Service (DWD) ozone and temperature lidar (HOH) located at the Hohenpeißenberg Meteorological Observatory (47.8∘ N, 11.0∘ E) has been evaluated through comparison with the travelling standard lidar operated by NASA's Goddard Space Flight Center (NASA GSFC Stratospheric Ozone (STROZ) lidar), satellite overpasses from the Microwave Limb Sounder (MLS), the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER), the Ozone Mapping and Profiler Suite (OMPS), meteorological radiosondes launched from Munich (65 km northeast), and locally launched ozonesondes. The “blind” evaluation was conducted under the framework of the Network for the Detection of Atmospheric Composition Change (NDACC) using 10 clear nights of measurements in 2018 and 2019. The campaign, referred to as the Hohenpeißenberg Ozone Profiling Study (HOPS), was conducted within the larger context of NDACC validation activities for European lidar stations. There was good agreement between all ozone lidar measurements in the range of 15 to 41 km with relative differences between co-located ozone profiles of less than ±10 %. Differences in the measured ozone number densities between the lidars and the locally launched ozone sondes were also generally less than 5 % below 30 km. The satellite ozone profiles demonstrated some differences with respect to the ground-based lidars which are due to sampling differences and geophysical variation. Both the original and new DWD lidars continue to meet the NDACC standard for lidar ozone profiles by exceeding 3 % accuracy between 16.5 and 43 km. Temperature differences for all instruments were less than ±5 K below 60 km, with larger differences present in the lidar–satellite comparisons above this region. Temperature differences between the DWD lidars met the NDACC accuracy requirements of ±1 K between 17 and 78 km. A unique cross-comparison between the HOPS campaign and a similar, recent campaign at Observatoire de Haute-Provence (Lidar Validation NDACC Experiment; LAVANDE) allowed for an investigation into potential biases in the NASA-STROZ reference lidar. The reference lidar may slightly underestimate ozone number densities above 43 km with respect to the French and German NDACC lidars. Below 20 km, the reference lidar temperatures profiles are 5 to 10 K cooler than the temperatures which are reported by the other instruments.

In atmospheric chemistry retrievals and data assimilation systems, observation errors associated with satellite radiances are chosen empirically and generally treated as uncorrelated. In this work, we estimate inter-channel error covariances for the Infrared Atmospheric Sounding Interferometer (IASI) and evaluate their impact on ozone assimilation with the chemistry transport model MOCAGE (Modèle de Chimie Atmosphérique à Grande Echelle). The method used to calculate observation errors is a diagnostic based on the observation and analysis residual statistics already adopted in many numerical weather prediction centres. We used a subset of 280 channels covering the spectral range between 980 and 1100 cm−1 to estimate the observation-error covariance matrix. This spectral range includes ozone-sensitive and atmospheric window channels. We computed hourly 3D-Var analyses and compared the resulting O3 fields against ozonesondes and the measurements provided by the Microwave Limb Sounder (MLS) and by the Ozone Monitoring Instrument (OMI). The results show significant differences between using the estimated error covariance matrix with respect to the empirical diagonal matrix employed in previous studies. The validation of the analyses against independent data reports a significant improvement, especially in the tropical stratosphere. The computational cost has also been reduced when the estimated covariance matrix is employed in the assimilation system, by reducing the number of iterations needed for the minimizer to converge.

A weak El Niño during 2014–2015 boreal winter developed as a strong boreal summer event in 2015 which continued and even enhanced during the following winter. In this work, the detailed changes in the structure, dynamics, and trace gases within the Asian summer monsoon anticyclone (ASMA) during the extreme El Niño of 2015–2016 is delineated by using Aura Microwave Limb Sounder (MLS) measurements, COSMIC radio occultation (RO) temperature, and National Centers for Environmental Prediction (NCEP) reanalysis products. Our analysis concentrates only on the summer months of July and August 2015 when the Niño 3.4 index started to exceed values of 1.5. The results show that the ASMA structure was quite different in summer 2015 as compared to the long-term (2005–2014) mean. In July, the spatial extension of the ASMA is greater than the long-term mean in all the regions except over northeastern Asia, where it exhibits a strong southward shift in its position. The ASMA splits into two, and the western Pacific mode is evident in August. Interestingly, the subtropical westerly jet (STJ) shifted southward from its normal position over northeastern Asia, and as a result midlatitude air moved southward in 2015. Intense Rossby wave breaking events along with STJ are also found in July 2015. Due to these dynamical changes in the ASMA, pronounced changes in the ASMA tracers are noticed in 2015 compared to the long-term mean. A 30 % (20 %) decrease in carbon monoxide (water vapor) at 100 hPa is observed in July over most of the ASMA region, whereas in August the drop is strongly concentrated at the edges of the ASMA. A prominent increase in O3 (> 40 %) at 100 hPa is clearly evident within the ASMA in July, whereas in August the increase is strongly located (even at 121 hPa) over the western edges of the ASMA. Further, the temperature around the tropopause shows significant positive anomalies (∼ 5 K) within the ASMA in 2015. The present results clearly reveal the El-Niño-induced dynamical changes caused significant changes in the trace gases within the ASMA in summer 2015.

Ozone (O3) plays a significant role in weather and climate on regional to global spatial scales. Most studies on the variability in the total column of O3 (TCO) are typically carried out using daytime data. Based on knowledge of the chemistry and transport of O3, significant deviations between daytime and night-time O3 are only expected either in the planetary boundary layer (PBL) or high in the stratosphere or mesosphere, with little effect on the TCO. Hence, we expect the daytime and night-time TCO to be very similar. However, a detailed evaluation of satellite measurements of daytime and night-time TCO is still lacking, despite the existence of long-term records of both. Thus, comparing daytime and night-time TCOs provides a novel approach to verifying the retrieval algorithms of instruments such as the Atmospheric Infrared Sounder (AIRS) and the Microwave Limb Sounder (MLS). In addition, such a comparison also helps to assess the value of night-time TCO for scientific research. Applying this verification on the AIRS and the MLS data, we identified inconsistencies in observations of O3 from both satellite instruments. For AIRS, daytime–night-time differences were found over oceans resembling cloud cover patterns and over land, mostly over dry land areas, which is likely related to infrared surface emissivity. These differences point to issues with the representation of both processes in the AIRS retrieval algorithm. For MLS, a major issue was identified with the “ascending–descending” orbit flag, used to discriminate night-time and daytime MLS measurements. Disregarding this issue, MLS day–night differences were significantly smaller than AIRS day–night differences, providing additional support for the retrieval method origin of AIRS in stratospheric column ozone (SCO) day–night differences. MLS day–night differences are dominated by the upper-stratospheric and mesospheric diurnal O3 cycle. These results provide useful information for improving infrared O3 products.

We investigate the impact of the so-called energetic particle precipitation (EPP) indirect effect on lower stratospheric ozone, ClO, and ClONO2 in the Antarctic springtime. We use observations from the Microwave Limb Sounder (MLS) and Ozone Monitoring Instrument (OMI) on Aura, the Atmospheric Chemistry Experiment – Fourier Transform Spectrometer (ACE-FTS) on SCISAT, and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat, covering the period from 2005 to 2017. Using the geomagnetic activity index Ap as a proxy for EPP, we find consistent ozone increases with elevated EPP during years with an easterly phase of the quasi-biennial oscillation (QBO) in both OMI and MLS observations. While these increases are the opposite of what has previously been reported at higher altitudes, the pattern in the MLS O3 follows the typical descent patterns of EPP-NOx. The ozone enhancements are also present in the OMI total O3 column observations. Analogous to the descent patterns found in O3, we also found consistent decreases in springtime MLS ClO following winters with elevated EPP. To verify if this is due to a previously proposed mechanism involving the conversion of ClO to the reservoir species ClONO2 in reaction with NO2, we used ClONO2 observations from ACE-FTS and MIPAS. As ClO and NO2 are both catalysts in ozone destruction, the conversion to ClONO2 would result in an ozone increase. We find a positive correlation between EPP and ClONO2 in the upper stratosphere in the early spring and in the lower stratosphere in late spring, providing the first observational evidence supporting the previously proposed mechanism relating to EPP-NOx modulating Clx-driven ozone loss. Our findings suggest that EPP has played an important role in modulating ozone depletion in the last 15 years. As chlorine loading in the polar stratosphere continues to decrease in the future, this buffering mechanism will become less effective, and catalytic ozone destruction by EPP-NOx will likely become a major contributor to Antarctic ozone loss.

The amount of ice injected into the tropical tropopause layer has a strong radiative impact on climate. A companion paper (Part 1) used the amplitude of the diurnal cycle of ice water content (IWC) as an estimate of ice injection by deep convection, showed that the Maritime Continent (MariCont) region provides the largest injection to the upper troposphere (UT; 146 hPa) and to the tropopause level (TL; 100 hPa). This study focuses on the MariCont region and extends that approach to assess the processes, the areas and the diurnal amount and duration of ice injected over islands and over seas during the austral convective season. The model presented in the companion paper is again used to estimate the amount of ice injected (ΔIWC) by combining ice water content (IWC) measured twice a day by the Microwave Limb Sounder (MLS; Version 4.2) from 2004 to 2017 and precipitation (Prec) measurements from the Tropical Rainfall Measurement Mission (TRMM; Version 007) binned at high temporal resolution (1 h). The horizontal distribution of ΔIWC estimated from Prec (ΔIWCPrec) is presented at 2∘×2∘ horizontal resolution over the MariCont. ΔIWC is also evaluated by using the number of lightning events (Flash) from the TRMM-LIS instrument (Lightning Imaging Sensor, from 2004 to 2015 at 1 h and 0.25∘ × 0.25∘ resolution). ΔIWCPrec and ΔIWC estimated from Flash (ΔIWCFlash) are compared to ΔIWC estimated from the ERA5 reanalyses (ΔIWCERA5) with the vertical resolution degraded to that of MLS observations (ΔIWCERA5). Our study shows that the diurnal cycles of Prec and Flash are consistent with each other in phase over land but different over offshore and coastal areas of the MariCont. The observational ΔIWC range between ΔIWCPrec and ΔIWCFlash, interpreted as the uncertainty of our model in estimating the amount of ice injected, is smaller over land (where ΔIWCPrec and ΔIWCFlash agree to within 22 %) than over ocean (where differences are up to 71 %) in the UT and TL. The impact of the MLS vertical resolution on the estimation of ΔIWC is greater in the TL (difference between ΔIWCERA5 and 〈ΔIWCERA5〉 of 32 % to 139 %, depending on the study zone) than in the UT (difference of 9 % to 33 %). Considering all the methods, in the UT, estimates of ΔIWC span 4.2 to 10.0 mg m−3 over land and 0.4 to 4.4 mg m−3 over sea, and in the TL estimates of ΔIWC span 0.5 to 3.9 mg m−3 over land and 0.1 to 0.7 mg m−3 over sea. Finally, based on IWC from MLS and ERA5, Prec and Flash, this study highlights that (1) at both levels, ΔIWC estimated over land can be more than twice that estimated over sea and (2) small islands with high topography present the largest ΔIWC (e.g., island of Java).

Tropospheric ozone plays an important role in atmospheric processes. However, the acquisition of tropospheric ozone content from satellite observations is a crucial challenge for atmospheric pollut...

This paper reports on a project to compare the representation of the semiannual oscillation (SAO) in the equatorial stratosphere and lower mesosphere within six major global atmospheric reanalysis datasets and with recent satellite Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER) and Microwave Limb Sounder (MLS) observations. All reanalyses have a good representation of the quasi-biennial oscillation (QBO) in the equatorial lower and middle stratosphere and each displays a clear SAO centered near the stratopause. However, the differences among reanalyses are much more substantial in the SAO region than in the QBO-dominated region. The degree of disagreement among the reanalyses is characterized by the standard deviation (SD) of the monthly mean zonal wind and temperature; this depends on latitude, longitude, height, and time. The zonal wind SD displays a prominent equatorial maximum that increases with height, while the temperature SD reaches a minimum near the Equator and is largest in the polar regions. Along the Equator, the zonal wind SD is smallest around the longitude of Singapore, where consistently high-quality near-equatorial radiosonde observations are available. Interestingly, the near-Singapore minimum in SD is evident to at least ∼3 hPa, i.e., considerably higher than the usual ∼10 hPa ceiling for in situ radiosonde observations. Our measurement of the agreement among the reanalyses shows systematic improvement over the period considered (1980–2016), up to near the stratopause. Characteristics of the SAO at 1 hPa, such as its detailed time variation and the displacement off the Equator of the zonal wind SAO amplitude maximum, differ significantly among the reanalyses. Disagreement among the reanalyses becomes still greater above 1 hPa. One of the reanalyses in our study also has a version produced without assimilating satellite observations, and a comparison of the SAO in these two versions demonstrates the very great importance of satellite-derived temperatures in the realistic analysis of the tropical upper stratospheric circulation.

Observations from the Ozone Monitoring Instrument (OMI) on the Aura satellite are used to study the effect of energetic particle precipitation (EPP, as proxied by the geomagnetic activity index, Ap ) on the Antarctic stratospheric NO2 column in late winter–spring (August–December) during the period from 2005 to 2017. We show that the polar (60–90 ∘ S) stratospheric NO2 column is significantly correlated with EPP throughout the Antarctic spring, until the breakdown of the polar vortex in November. The strongest correlation takes place during years with the easterly phase of the quasi-biennial oscillation (QBO). The QBO modulation may be a combination of different effects: the QBO is known to influence the amount of the primary NOx source ( N2O ) via transport from the Equator to the polar region; and the QBO phase also affects polar temperatures, which may provide a link to the amount of denitrification occurring in the polar vortex. We find some support for the latter in an analysis of temperature and HNO3 observations from the Microwave Limb Sounder (MLS, on Aura). Our results suggest that once the background effect of the QBO is accounted for, the NOx produced by EPP significantly contributes to the stratospheric NO2 column at the time and altitudes when the ozone hole is present in the Antarctic stratosphere. Based on our findings, and the known role of NOx as a catalyst for ozone loss, we propose that as chlorine activation continues to decrease in the Antarctic stratosphere, the total EPP- NOx needs be accounted for in predictions of Antarctic ozone recovery.

Seven years of high-resolution measurements of upper tropospheric humidity (UTH) from the Indian Geostationary satellite, Kalpana-1, are used to understand the hydration and dehydration processes during the active and break phases of the Indian summer monsoon (ISM). The active and break phases are identified using the TRMM-3B42 daily rainfall data over the Central Indian region. The most significant and new observation is that the upper troposphere is dry during the active and wet during the break phases of the ISM. Satellite measurements of humidity from Megha-Tropiques SAPHIR and water vapor mixing ratio from Aura microwave limb sounder also show dryness in the upper troposphere during the active and wetness during the break phases of ISM. The analysis illustrates that dehydration dominates during the active and hydration during the break phases. Cloud-aerosol lidar with orthogonal polarization data shows that cirrus (sub-categorized as sub-visible, thin, and thick) occurrence is more during the active phase as compared with the break phase. The dehydration mechanism during the active phase of ISM is attributed to the presence of cirrus. This study provides evidence that the upper troposphere shows dehydration during the active and hydration during the break phase of the ISM, thereby exhibiting intra-seasonal variability in the UTH.

Acetonitrile (CH3CN) is a volatile organic compound (VOC) and a potential tracer of biomass burning. We evaluated the capability of using observations derived from the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) on the International Space Station (ISS) to measure CH3CN profiles. The error in a CH3CN vertical profile from the Level-2 research (L2r) product version 3.0.0 was estimated by theoretical error analysis and also compared with other instrumental measurements. We estimated the systematic and random errors to be ∼5.8 ppt (7.8 %) and 25 ppt (60 %), respectively, for a single observation at 15.7 hPa in the tropics, where the CH3CN measurements are enhanced. The major source of systematic error was the pressure-broadening coefficient, and its contribution to the total systematic error was approximately 60 % in the middle stratosphere (15.7–4.8 hPa). The random error decreased to less than 40 % after averaging 10 profiles in the pressure range of 28.8–1.6 hPa. The total error due to uncertainties in other molecular spectroscopic parameters (2.8 ppt) was comparable to that of CH3CN spectroscopic parameters. We compared the SMILES CH3CN profiles with those of the microwave limb sounder (MLS) on the Aura satellite (version 4.2). The SMILES CH3CN values were consistent with those from MLS within the standard deviation (1σ) of the MLS observations. The difference between the SMILES and MLS CH3CN profiles increased with altitude and was within 20–35 ppt (20 %–260 %) at 15.7–1.6 hPa. We observed discrepancies of 5–10 ppt (10 %–30 %) between the SMILES CH3CN profiles observed by different spectrometers, and hence, we do not recommend merging SMILES CH3CN profiles derived from different spectrometers. We found that the SMILES CH3CN volume mixing ratio (VMR) in the upper stratosphere has a seasonal maximum in February.

Using ERA-Interim and ERA-5 reanalysis datasets combined with measurements from the Microwave Limb Sounder (MLS), we divided the Tibetan Plateau (TP) into three regions to analyse the long-term trends of water vapor in the upper troposphere and its possible mechanisms. The conclusions are as follows. The water vapor at 200 hPa in the summers from 1979 to 2016 over Region 1 (34°N-40°N, 80°E-100°E) shows a positive trend, which is caused by the rising surface temperature and the northward shift of the South Asian High (SAH). The positive trend of water vapor over Region 2 (28°N-33°N, 80°E-90°E) is attributed to the enhanced South Asian summer monsoon, which leads to increases in horizontal transport of water vapor and vertical water vapor flux. The increase in the vertical transport of water vapor is responsible for the increase in water vapor over the Region 2. The negative trend of water vapor over Region 3 (27°N-30°N, 93°E-98°E) is mainly due to the weakening of water vapor transport caused by the weakening of convective activities.

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.

This work focuses on studying the presence and characteristics of 27 d solar signatures in middle atmospheric temperature observed by the microwave limb sounder (MLS) on NASA's Aura spacecraft. The 27 d signatures in temperature are extracted using the superposed epoch analysis (SEA) technique. We use time-lagged linear regression (sensitivity analysis) and a Monte Carlo test method (significance test) to explore the dependence of the results on latitude and altitude, solar activity, and season, as well as on different parameters (e.g., smoothing filter, window width and epoch centers). Using different parameters does impact the results to a certain degree, but it does not affect the overall results. Analyzing the 13-year data set shows that highly significant 27 d solar signatures in middle atmospheric temperature are present at many altitudes and latitudes. A tendency to higher temperature sensitivity to solar forcing in the winter hemisphere compared to the summer hemisphere is found. In addition, the sensitivity of temperature to 27 d solar forcing tends to be larger at high latitudes than at low latitudes. For 11-year solar minimum conditions no statistically significant identification of a 27 d solar signature is possible at most altitudes and latitudes. Several results we obtained suggest that processes other than solar variability drive atmospheric temperature variability at periods around 27 d. Comparisons of the obtained sensitivity values with earlier experimental and model studies show good overall agreement.

A 3 d episode of anomalously low ozone concentrations in the stratosphere over northern Europe occurred on 3–5 November 2018. A reduction of the total ozone column down to ∼ 200–210 Dobson units was predicted by the global forecasts of the System for Integrated modeLling of Atmospheric coMposition (SILAM) driven by the weather forecast of the Integrated Forecasting System (IFS) of the European Centre for Medium-Range Weather Forecasts (ECMWF). The reduction down to 210–215 DU was subsequently observed by satellite instruments, such as the Ozone Monitoring Instrument (OMI) and Ozone Mapping Profile Suite (OMPS). The episode was caused by an intrusion of tropospheric air, which was initially uplifted by a storm in the northern Atlantic, south-east of Greenland. Subsequent transport towards the east and further uplift over the Scandinavian ridge of this humid and low-ozone air brought it to ∼25 km altitude, causing ∼30 % reduction of the ozone layer thickness over northern Europe. The low-ozone air was further transported eastwards and diluted over Siberia, so that the ozone concentrations were restored a few days later. Comparison of the model predictions with OMI, OMPS, and MLS (Microwave Limb Sounder) satellites demonstrated the high accuracy of the 5 d forecast of the IFS–SILAM system: the ozone anomaly was predicted within ∼10 DU accuracy and positioned within a couple of hundreds of kilometres. This episode showed the importance of the stratospheric composition dynamics and the possibility of its short-term forecasting, including such rare events.

We retrieved lower stratospheric vertical profiles of O3, HNO3, and HCl from solar spectra taken with a ground-based Fourier transform infrared spectrometer (FTIR) installed at Syowa Station, Antarctica (69.0∘ S, 39.6∘ E), from March to December 2007 and September to November 2011. This was the first continuous measurement of chlorine species throughout the ozone hole period from the ground in Antarctica. We analyzed temporal variation of these species combined with ClO, HCl, and HNO3 data taken with the Aura MLS (Microwave Limb Sounder) satellite sensor and ClONO2 data taken with the Envisat MIPAS (the Michelson Interferometer for Passive Atmospheric Sounding) satellite sensor at 18 and 22 km over Syowa Station. An HCl and ClONO2 decrease occurred from the end of May at both 18 and 22 km, and eventually, in early winter, both HCl and ClONO2 were almost depleted. When the sun returned to Antarctica in spring, enhancement of ClO and gradual O3 destruction were observed. During the ClO-enhanced period, a negative correlation between ClO and ClONO2 was observed in the time series of the data at Syowa Station. This negative correlation was associated with the relative distance between Syowa Station and the edge of the polar vortex. We used MIROC3.2 chemistry–climate model (CCM) results to investigate the behavior of whole chlorine and related species inside the polar vortex and the boundary region in more detail. From CCM model results, the rapid conversion of chlorine reservoir species (HCl and ClONO2) into Cl2, gradual conversion of Cl2 into Cl2O2, increase in HOCl in the winter period, increase in ClO when sunlight became available, and conversion of ClO into HCl were successfully reproduced. The HCl decrease in the winter polar vortex core continued to occur due to both transport of ClONO2 from the subpolar region to higher latitudes, providing a flux of ClONO2 from more sunlit latitudes into the polar vortex, and the heterogeneous reaction of HCl with HOCl. The temporal variation of chlorine species over Syowa Station was affected by both heterogeneous chemistries related to polar stratospheric cloud (PSC) occurrence inside the polar vortex and transport of a NOx-rich air mass from the polar vortex boundary region, which can produce additional ClONO2 by reaction of ClO with NO2. The deactivation pathways from active chlorine into reservoir species (HCl and/or ClONO2) were confirmed to be highly dependent on the availability of ambient O3. At 18 km, where most ozone was depleted, most ClO was converted to HCl. At 22 km where some O3 was available, an additional increase in ClONO2 from the prewinter value occurred, similar to the Arctic.

This study presents an analysis of the quasi-16-day wave (Q16DW) at three stations in the middle latitudes by using a meteor radar chain in conjunction with Aura Microwave Limb Sounder temperature data and MERRA2 (Modern-Era Retrospective Analysis for Research and Applications, Version 2) reanalysis data from 2008 to 2017. The radar chain consists of three meteor radar stations located at Mohe (MH, 53.5°N, 122.3°E), Beijing (BJ, 40.3°N, 116.2°E), and Wuhan (WH, 30.5°N, 114.6°E). The Q16DW wave exhibits similar seasonal variation in the neutral wind and temperature, and the Q16DW amplitude is generally strong during winter and weak around summer. The Q16DW at BJ was found to have secondary enhancement around September in the zonal wind, which is rarely reported at similar latitudes. The latitudinal variations of the Q16DW in the neutral wind and temperature are quite different. The Q16DW at BJ is the most prominent in both neutral wind components among the three stations and the Q16DW amplitudes at MH and WH are comparable, whereas the wave amplitude in temperature decreases with decreasing latitude. The quasi-geostrophic refractive index squared at the three stations in the period from 2008 to 2017 was revealed. The results indicate that the Q16DW in the mesosphere and lower thermosphere (MLT) at MH has a limited contribution from the lower atmosphere. Around March and October, the Q16DW in the troposphere at BJ can propagate upward into the MLT region, whereas at WH, the contribution to the Q16DW in the MLT region is largely from the mesosphere.

The long-term record of Umkehr measurements from four NOAA Dobson spectrophotometers was reprocessed after updates to the instrument calibration procedures. In addition, a new data quality-control tool was developed for the Dobson automation software (WinDobson). This paper presents a comparison of Dobson Umkehr ozone profiles from NOAA ozone network stations – Boulder, the Haute-Provence Observatory (OHP), the Mauna Loa Observatory (MLO), Lauder – against several satellite records, including Aura Microwave Limb Sounder (MLS; ver. 4.2), and combined solar backscatter ultraviolet (SBUV) and Ozone Mapping and Profiler Suite (OMPS) records (NASA aggregated and NOAA cohesive datasets). A subset of satellite data is selected to match Dobson Umkehr observations at each station spatially (distance less than 200 km) and temporally (within 24 h). Umkehr Averaging kernels (AKs) are applied to vertically smooth all overpass satellite profiles prior to comparisons. The station Umkehr record consists of several instrumental records, which have different optical characterizations, and thus instrument-specific stray light contributes to the data processing errors and creates step changes in the record. This work evaluates the overall quality of Umkehr long-term measurements at NOAA ground-based stations and assesses the impact of the instrumental changes on the stability of the Umkehr ozone profile record. This paper describes a method designed to correct biases and discontinuities in the retrieved Umkehr profile that originate from the Dobson calibration process, repair, or optical realignment of the instrument. The Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2) Global Modeling Initiative (M2GMI) and NASA Global Modeling Initiative chemistry transport model (GMI CTM) ozone profile model output matched to station location and date of observation is used to evaluate instrumental step changes in the Umkehr record. Homogenization of the Umkehr record and discussion of the apparent stray light error in retrieved ozone profiles are the focus of this paper. Homogenization of ground-based records is of great importance for studies of long-term ozone trends and climate change.

Sudden stratospheric warmings (SSW) are large-scale disruptions of the wintertime state of the stratosphere that can affect the circulation at synoptic and global scales, including altitudes up to the mesopause in both winter and summer hemispheres. In this study, the response of the summer mesosphere is analyzed during the SSW in the winter stratosphere. In particular, we focus on major SSW events where the climatological stratopause disappears and subsequently reforms at higher altitude, which we refer to as “extreme SSW” in this article. The summer mesosphere response to such extreme SSW events is analyzed in three different phases: (a) stratosphere warming phase, (b) stratopause discontinuity phase, and (c) stratopause reformation phase. Composites of anomalies with respect to climatology derived from the Microwave Limb Sounder and the extended version of the Whole Atmosphere Community Climate Model with specified dynamics are analyzed. The polar summer mesosphere cools during the stratospheric warming phase and warms in subsequent phases. A detailed lag-correlation analysis shows strong negative correlation of −0.6 to −0.8 between the summer mesosphere and the winter stratosphere during the stratosphere warming phase, and a positive correlation of 0.4–0.6 in the phases thereafter. An attempt is made to explain the apparent drivers and dynamics responsible for these couplings, supported with evidence from observations and model output. Ministry of Education (MOE) Published version This research is supported by the Ministry of Education, Singapore, under grant AcRF Tier 1-2018-T1-002-166 (RG 196/17).

We use a forward Lagrangian trajectory model to diagnose mechanisms that produce the water vapor seasonal cycle observed by the Microwave Limb Sounder (MLS) and reproduced by the Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM) in the tropical tropopause layer (TTL). We confirm in both the MLS and GEOSCCM that the seasonal cycle of water vapor entering the stratosphere is primarily determined by the seasonal cycle of TTL temperatures. However, we find that the seasonal cycle of temperature predicts a smaller seasonal cycle of TTL water vapor between 10 and 40∘ N than observed by MLS or simulated by the GEOSCCM. Our analysis of the GEOSCCM shows that including evaporation of convective ice in the trajectory model increases both the simulated maximum value of the 100 hPa 10–40∘ N water vapor seasonal cycle and the seasonal-cycle amplitude. We conclude that the moistening effect from convective ice evaporation in the TTL plays a key role in regulating and maintaining the seasonal cycle of water vapor in the TTL. Most of the convective moistening in the 10–40∘ N range comes from convective ice evaporation occurring at the same latitudes. A small contribution to the moistening comes from convective ice evaporation occurring between 10∘ S and 10∘ N. Within the 10–40∘ N band, the Asian monsoon region is the most important region for convective moistening by ice evaporation during boreal summer and autumn.

Changes in stratospheric ozone have to be assessed continuously to evaluate the effectiveness of the Montreal Protocol. In the southern hemisphere, few ground-based observational datasets exist, making measurements at the Network for the Detection of Atmospheric Composition Change (NDACC) station at Lauder, New Zealand invaluable. Investigating these datasets in detail is essential to derive realistic ozone trends. We compared lidar data and microwave radiometer data with collocated Aura Microwave Limb sounder (MLS) satellite data and ERA5 reanalysis data. The detailed comparison makes it possible to assess inhomogeneities in the data. We find good agreement between the datasets but also some possible biases, especially in the ERA5 data. The data uncertainties and the inhomogeneities were then considered when deriving trends. Using two regression models from the Long-term Ozone Trends and Uncertainties in the Stratosphere (LOTUS) project and from the Karlsruhe Institute of Technology (KIT), we estimated resulting ozone trends. Further, we assessed how trends are affected by data uncertainties and inhomogeneities. We find positive ozone trends throughout the stratosphere between 0% and 5% per decade and show that considering data uncertainties and inhomogeneities in the regression affects the resulting trends.

Free tropospheric ozone (O3) trends in the Central East China (CEC) and export regions are investigated for 2008–2017 using the IASI (Infrared Atmospheric Sounding Interferometer) O3 observations and the LMDZ-OR-INCA model simulations, including the most recent Chinese emission inventory. The observed and modelled trends in the CEC region are −0.07 ± 0.02 and −0.08 ± 0.02 DU yr−1, respectively, for the lower free troposphere (3–6 km column) and −0.05 ± 0.02 and −0.06 ± 0.02 DU yr−1, respectively, for the upper free troposphere (6–9 km column). The statistical p value is smaller to 0.01 for all the derived trends. A good agreement between the observations and the model is also observed in the region, including the Korean Peninsula and Japan and corresponding to the region of pollution export from China. Based on sensitivity studies conducted with the model, we evaluate, at 60 % and 52 %, the contribution of the Chinese anthropogenic emissions to the trend in the lower and upper free troposphere, respectively. The second main contribution to the trend is the meteorological variability (34 % and 50 %, respectively). These results suggest that the reduction in NOx anthropogenic emissions that has occurred since 2013 in China led to a decrease in ozone in the Chinese free troposphere, contrary to the increase in ozone at the surface. We designed some tests to compare the trends derived by the IASI observations and the model to independent measurements, such as the In-service Aircraft for a Global Observing System (IAGOS) or other satellite measurements (Ozone Monitoring Instrument (OMI)/Microwave Limb Sounder (MLS)). These comparisons do not confirm the O3 decrease and stress the difficulty in analysing short-term trends using multiple data sets with various sampling and the risk of overinterpreting the results.

Specular meteor radars (SMRs) and partial reflection radars (PRRs) have been observing mesospheric winds for more than a solar cycle over Germany (~54 °N) and northern Norway (~69 °N). This work investigates the mesospheric mean zonal wind and the zonal mean geostrophic zonal wind from the Microwave Limb Sounder (MLS) over these two regions between 2004 and 2020. Our study focuses on the summer when strong planetary waves are absent and the stratospheric and tropospheric conditions are relatively stable. We establish two definitions of the summer length according to the zonal wind reversals: (1) the mesosphere and lower thermosphere summer length (MLT-SL) using SMR and PRR winds, and (2) the mesosphere summer length (M-SL) using PRR and MLS. Under both definitions, the summer begins around April and ends around mid-September. The largest year to year variability is found in the summer beginning in both definitions, particularly at high-latitudes, possibly due to the influence of the polar vortex. At high-latitudes, the year 2004 has a longer summer length compared to the mean value for MLT-SL, as well as 2012 for both definitions. The M-SL exhibits an increasing trend over the years, while MLT-SL does not have a well-defined trend. We explore a possible influence of solar activity, as well as large-scale atmospheric influences (e.g. quasi-biennial oscillations (QBO), El Niño-southern oscillation (ENSO), major sudden stratospheric warming events). We complement our work with an extended time series of 31 years at mid-latitudes using only PRR winds. In this case, the summer length shows a breakpoint, suggesting a non-uniform trend, and periods similar to those known for ENSO and QBO.

Planetary waves in the mesosphere are studied using observational data and models to establish their origin, as there are indications of their generation independently of waves in the stratosphere. The quantitative relationships between zonal wave 1 and wave 2 were studied with a focus on the mid-latitude mesosphere at 50°N latitude. Aura Microwave Limb Sounder measurements were used to estimate wave amplitudes in geopotential height during sudden stratospheric warmings in recent boreal winters. The moving correlation between the wave amplitudes shows that, in comparison with the anticorrelation in the stratosphere, wave 2 positively correlates with wave 1 and propagates ahead of it in the mesosphere. A positive correlation r = 0.5–0.6, statistically significant at the 95% confidence level, is observed at 1–5-day time lag and in the 75–91 km altitude range, which is the upper mesosphere–mesopause region. Wavelet analysis shows a clear 8-day period in waves 1 and 2 in the mesosphere at 0.01 hPa (80 km), while in the stratosphere–lower mesosphere, the period is twice as long at 16 days; this is statistically significant only in wave 2. Possible sources of mesospheric planetary waves associated with zonal flow instabilities and breaking or dissipation of gravity waves are discussed.

Until now our understanding of the 11-year solar cycle signal (SCS) in stratospheric ozone has been largely based on high-quality but sparse ozone profiles from the Stratospheric Aerosol and Gas Experiment (SAGE) II or coarsely resolved ozone profiles from the nadir-viewing Solar Backscatter Ultraviolet Radiometer (SBUV) satellite instruments. Here, we analyse 16 years (2005–2020) of ozone profile measurements from the Microwave Limb Sounder (MLS) instrument on the Aura satellite to estimate the 11-year SCS in stratospheric ozone. Our analysis of Aura-MLS data suggests a single-peak-structured SCS profile (about 3 % near 4 hPa or 40 km) in tropical stratospheric ozone, which is significantly different to the SAGE II and SBUV-based double-peak-structured SCS. We also find that MLS-observed ozone variations are more consistent with ozone from our control model simulation that uses Naval Research Laboratory (NRL) v2 solar fluxes. However, in the lowermost stratosphere modelled ozone shows a negligible SCS compared to about 1 % in Aura-MLS data. An ensemble of ordinary least squares (OLS) and three regularised (lasso, ridge and elastic net) linear regression models confirms the robustness of the estimated SCS. In addition, our analysis of MLS and model simulations shows a large SCS in the Antarctic lower stratosphere that was not seen in earlier studies. We also analyse chemical transport model simulations with alternative solar flux data. We find that in the upper (and middle) stratosphere the model simulation with Solar Radiation and Climate Experiment (SORCE) satellite solar fluxes is also consistent with the MLS-derived SCS and agrees well with the control simulation and one which uses Spectral and Total Irradiance Reconstructions (SATIRE) solar fluxes. Hence, our model simulation suggests that with recent adjustments and corrections, SORCE data can be used to analyse effects of solar flux variations. Furthermore, analysis of a simulation with fixed solar fluxes and one with fixed (annually repeating) meteorology confirms that the implicit dynamical SCS in the (re)analysis data used to force the model is not enough to simulate the observed SCS in the middle and upper stratospheric ozone. Finally, we argue that the overall significantly different SCS compared to previous estimates might be due to a combination of different factors such as much denser MLS measurements, almost linear stratospheric chlorine loading changes over the analysis period, variations in the stratospheric dynamics as well as relatively unperturbed stratospheric aerosol layer that might have influenced earlier analyses.

High-quality stratospheric ozone profile data sets are a key requirement for accurate quantification and attribution of long-term ozone changes. Satellite instruments provide stratospheric ozone profile measurements over typical mission durations of 5–15 years. Various methodologies have then been applied to merge and homogenise the different satellite data in order to create long-term observation-based ozone profile data sets with minimal data gaps. However, individual satellite instruments use different measurement methods, sampling patterns and retrieval algorithms which complicate the merging of these different data sets. In contrast, atmospheric chemical models can produce chemically consistent long-term ozone simulations based on specified changes in external forcings, but they are subject to the deficiencies associated with incomplete understanding of complex atmospheric processes and uncertain photochemical parameters. Here, we use chemically self-consistent output from the TOMCAT 3-D chemical transport model (CTM) and a random-forest (RF) ensemble learning method to create a merged 42-year (1979–2020) stratospheric ozone profile data set (ML-TOMCAT V1.0). The underlying CTM simulation was forced by meteorological reanalyses, specified trends in long-lived source gases, solar flux and aerosol variations. The RF is trained using the Stratospheric Water and OzOne Satellite Homogenized (SWOOSH) data set over the time periods of the Microwave Limb Sounder (MLS) from the Upper Atmosphere Research Satellite (UARS) (1991–1998) and Aura (2005–2016) missions. We find that ML-TOMCAT shows excellent agreement with available independent satellite-based data sets which use pressure as a vertical coordinate (e.g. GOZCARDS, SWOOSH for non-MLS periods) but weaker agreement with the data sets which are altitude-based (e.g. SAGE-CCI-OMPS, SCIAMACHY-OMPS). We find that at almost all stratospheric levels ML-TOMCAT ozone concentrations are well within uncertainties of the observational data sets. The ML-TOMCAT (V1.0) data set is ideally suited for the evaluation of chemical model ozone profiles from the tropopause to 0.1 hPa and is freely available via https://doi.org/10.5281/zenodo.5651194 (Dhomse et al., 2021).

A new atmospheric ozone profile climatology has been constructed by combining ozone profiles from the Aura Microwave Limb Sounder (MLS) and Modern‐Era Retrospective Analysis for Research Applications version 2 (MERRA2) Global Modeling Initiative (GMI) model simulation (M2GMI). The MLS and M2GMI ozone profiles are merged between 13 and 17 km (~159 and 88 hPa) with MLS used for stratospheric and GMI for primarily tropospheric levels. The time record for profiles from MLS and GMI is August 2004–December 2016. The derived seasonal climatology consists of monthly zonal-mean ozone profiles in 5-degree latitude bands from 90° S–90° N covering altitudes (in Z* log-pressure altitude) from zero to 80 km in 1 km increments. This climatology can be used as a priori information in satellite ozone retrievals, in atmospheric radiative transfer studies, and as a baseline to compare with other measured or model-simulated ozone. The MLS/GMI seasonal climatology shows a number of improvements compared to previous ozone profile climatologies based on MLS and ozonesonde measurements. These improvements are attributed mostly to continuous daily global coverage of GMI tropospheric ozone compared to sparse regional measurements from sondes. Only daytime measurements for MLS are used in the MLS/GMI climatology compared to the previous MLS/sonde climatology that averaged MLS day and night measurements together; the daytime-only measurements are important for applications involving the upper stratosphere and lower mesosphere where the ozone diurnal cycle is large. In addition to the seasonal climatology, we also derive an additive climatology to account for inter-annual variability in stratospheric zonal-mean ozone profiles which is based on a rotated empirical orthogonal function (REOF) analysis of Aura MLS ozone profiles. This REOF climatology starts in 1970 and captures most of the inter-annual variability in global stratospheric ozone including Quasi-Biennial Oscillation (QBO) signatures.

Decreases in stratospheric NOx associated with enhanced aerosol have been observed after large volcanic eruptions, for example, after the eruption of Mount Pinatubo in 1991. While the 1991 Mount Pinatubo eruption was the last large explosive eruption, recent studies have shed light on the impacts of moderate-sized eruptions since the year 2000 on the global stratospheric aerosol budget. We use an ensemble of simulations from a coupled climate-chemistry model to quantify and analyze changes in NO and NO2 (NOx), N2O5, HNO3, ClO, and ClONO2 during periods of increased stratospheric volcanic aerosol concentrations since 2000. By using an ensemble approach, we are able to distinguish forced responses from internal variability. We also compare the model ensemble results to satellite measurements of these changes in atmospheric composition, including measurements from the Optical Spectrograph and Infrared Imaging Spectrometer on the Odin satellite and the Aura Microwave Limb Sounder. We find decreases in stratospheric NOx concentrations up to 20 hPa, consistent with increases in stratospheric HNO3 concentrations. The HNO3 perturbations also extend higher, up to 5 hPa, associated with periods of increased volcanic aerosol concentrations in both model simulations and observations, though correlations with volcanic aerosol are considerably higher in the model simulations. The model simulates increases in ClO at altitudes and magnitudes similar to the NOx reductions, but this response is below the detectable limit in the available observations (100 pptv). We also demonstrate the value of accounting for transport-related anomalies of atmospheric trace gases by regression onto N2O anomalies.

Deep convection systems (DCSs) can rapidly lift water vapor and other pollutants from the lower troposphere to the upper troposphere and lower stratosphere. The main detrainment height determines the level to which the air parcel is lifted. We analyzed the main detrainment height over the Tibetan Plateau and its southern slope based on the CloudSat Cloud Profiling Radar 2B_GEOPROF dataset and the Aura Microwave Limb Sounder Level 2 cloud ice product onboard the A-train constellation of Earth-observing satellites. It was found that the DCSs over the Tibetan Plateau and its southern slope have a higher main detrainment height (about 10–16 km) than other regions in the same latitude. The mean main detrainment heights are 12.9 and 13.3 km over the Tibetan Plateau and its southern slope, respectively. The cloud ice water path decreases by 16.8% after excluding the influences of DCSs, and the height with the maximum increase in cloud ice water content is located at 178 hPa (about 13 km). The main detrainment height and outflow horizontal range are higher and larger over the central and eastern Tibetan Plateau, the west of the southern slope, and the southeastern edge of the Tibetan Plateau than that over the northwestern Tibetan Plateau. The main detrainment height and outflow horizontal range are lower and broader at nighttime than during daytime.

The impact of a major sudden stratospheric warming (SSW) in the Arctic in February 2018 on the midlatitude mesosphere is investigated by performing the microwave radiometer measurements of carbon monoxide (CO) and zonal wind above Kharkiv, Ukraine (50.0∘ N, 36.3∘ E). The mesospheric peculiarities of this SSW event were observed using a recently designed and installed microwave radiometer in eastern Europe for the first time. Data from the ERA-Interim and MERRA-2 reanalyses, as well as the Aura microwave limb sounder measurements, are also used. Microwave observations of the daily CO profiles in January–March 2018 allowed for the retrieval of mesospheric zonal wind at 70–85 km (below the winter mesopause) over the Kharkiv site. Reversal of the mesospheric westerly from about 10 m s−1 to an easterly wind of about −10 m s−1 around 10 February was observed. The local microwave observations at our Northern Hemisphere (NH) midlatitude site combined with reanalysis data show wide-ranging daily variability in CO, zonal wind, and temperature in the mesosphere and stratosphere during the SSW of 2018. The observed local CO variability can be explained mainly by horizontal air mass redistribution due to planetary wave activity. Replacement of the CO-rich polar vortex air by CO-poor air of the surrounding area led to a significant mesospheric CO decrease over the station during the SSW and fragmentation of the vortex over the station at the SSW start caused enhanced stratospheric CO at about 30 km. The results of microwave measurements of CO and zonal wind in the midlatitude mesosphere at 70–85 km altitudes, which still are not adequately covered by ground-based observations, are useful for improving our understanding of the SSW impacts in this region.

In this study, we analyze the Aura Microwave Limb Sounder water vapor data in the tropical upper troposphere and the lower and middle stratosphere (UTLMS) (from 215 to 6 hPa) for the period from August 2004 to September 2017 using time-lag regression analysis and composite analysis to explore the interannual variations of tropical UTLMS water vapor and their connections to El Niño–Southern Oscillation (ENSO) and quasi-biennial oscillation (QBO). Our analysis shows that the interannual tropical UTLMS water vapor anomalies are strongly related to ENSO and QBO which together can explain more than half (∼ 50 %–60 %) but not all variance of the interannual tropical water vapor anomalies. We find that ENSO's impact is strong in the upper troposphere (∼ 215–∼ 120 hPa) and near the tropopause (∼ 110–∼ 90 hPa), with a ∼ 3-month lag but weak in the lower and middle stratosphere (∼ 80 to ∼ 6 hPa). In contrast, QBO's role is large in the lower and middle stratosphere, with an upward-propagating signal starting at the tropopause (100 hPa) with a ∼ 2-month lag, peaking in the middle stratosphere near 15 hPa with a ∼ 21-month lag. The phase lag is based on the 50 hPa QBO index used by many previous studies. This observational evidence supports that the QBO's impact on the tropical stratospheric water vapor is from its modulation on the tropical tropopause temperature and then transported upward with the tape recorder as suggested by many previous studies. In the upper troposphere, ENSO is more important than QBO for the interannual tropical water vapor anomalies that are positive during the warm ENSO phases but negative during the cold ENSO phases. Near the tropopause, both ENSO and QBO are important for the interannual tropical water vapor anomalies. Warm ENSO phase and westerly QBO phase tend to cause positive water vapor anomalies, while cold ENSO phase and easterly QBO phase tend to cause negative water vapor anomalies. As a result, the interannual tropical water vapor anomalies near the tropopause are different depending on different ENSO and QBO phase combinations. In the lower and middle stratosphere, QBO is more important than ENSO for the interannual tropical water vapor anomalies. For the westerly QBO phases, interannual tropical water vapor anomalies are positive near the tropopause and in the lower stratosphere but negative in the middle stratosphere and positive again above. Vice versa for the easterly QBO phases.

Improving measurements of water vapour in the upper troposphere and lower stratosphere (UTLS) is a priority for the atmospheric science community. In this work, UTLS water vapour profiles derived from Atmospheric Chemistry Experiment (ACE) satellite measurements are assessed with coincident ground-based measurements taken at a high Arctic observatory at Eureka, Nunavut, Canada. Additional comparisons to satellite measurements taken by the Atmospheric Infrared Sounder (AIRS), Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), Microwave Limb Sounder (MLS), Scanning Imaging Absorption Spectrometer for Atmospheric CHartography (SCIAMACHY), and Tropospheric Emission Spectrometer (TES) are included to put the ACE Fourier transform spectrometer (ACE-FTS) and ACE Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation (ACE-MAESTRO) results in context. Measurements of water vapour profiles at Eureka are made using a Bruker 125HR solar absorption Fourier transform infrared spectrometer at the Polar Environment Atmospheric Research Laboratory (PEARL) and radiosondes launched from the Eureka Weather Station. Radiosonde measurements used in this study were processed with software developed by the Global Climate Observing System (GCOS) Reference Upper-Air Network (GRUAN) to account for known biases and calculate uncertainties in a well-documented and consistent manner. ACE-FTS measurements were within 11 ppmv (parts per million by volume; 13 %) of 125HR measurements between 6 and 14 km. Between 8 and 14 km ACE-FTS profiles showed a small wet bias of approximately 8 % relative to the 125HR. ACE-FTS water vapour profiles had mean differences of 13 ppmv (32 %) or better when compared to coincident radiosonde profiles at altitudes between 6 and 14 km; mean differences were within 6 ppmv (12 %) between 7 and 11 km. ACE-MAESTRO profiles showed a small dry bias relative to the 125HR of approximately 7 % between 6 and 9 km and 10 % between 10 and 14 km. ACE-MAESTRO profiles agreed within 30 ppmv (36 %) of the radiosondes between 7 and 14 km. ACE-FTS and ACE-MAESTRO comparison results show closer agreement with the radiosondes and PEARL 125HR overall than other satellite datasets – except for AIRS. Close agreement was observed between AIRS and the 125HR and radiosonde measurements, with mean differences within 5 % and correlation coefficients above 0.83 in the troposphere between 1 and 7 km. Comparisons to MLS at altitudes around 10 km showed a dry bias, e.g. mean differences between MLS and radiosondes were −25.6 %. SCIAMACHY comparisons were very limited due to minimal overlap between the vertical extent of the measurements. TES had no temporal overlap with the radiosonde dataset used in this study. Comparisons between TES and the 125HR showed a wet bias of approximately 25 % in the UTLS and mean differences within 14 % below 5 km.

Using Microwave Limb Sounder (MLS) satellite observations, ERA‐Interim reanalysis data, and a chemistry transport model simulation, we analyze and investigate the causes of the asymmetric hemispheric trends of N2O, CH4, and HCl in the stratosphere during the period 2004–2012. We find significant hemispheric asymmetries in the trends of these trace gases in the midlatitude middle and lower stratosphere. With regard to N2O and CH4, the enhanced downwelling branch of the residual circulation in the Northern Hemisphere (NH) middle and upper stratosphere transports more N2O/CH4‐poor air from the upper stratosphere to the lower stratosphere. The enhanced poleward meridional branch of the residual circulation in the Southern Hemisphere (SH) stratosphere brings more N2O/CH4‐rich air from lower to middle latitudes. These processes therefore contribute to the negative trends of N2O and CH4 in the NH lower stratosphere and the positive trends in the SH middle stratosphere. A corresponding positive trend is found for HCl in the NH, where the deep branch of the residual circulation located in the middle and upper stratosphere strengthens, bringing more HCl‐rich air downward to the lower stratosphere, while the shallow branch of the residual circulation in the lower stratosphere weakens and leads to enhanced conversion of chlorine‐containing source gases of different lifetimes to HCl. A reversed picture emerges in the SH, where the deep branch of the residual circulation in the middle and upper stratosphere weakens, while the shallow branch in the lower stratosphere strengthens, resulting in less HCl there. In addition, the southward shift of the upwelling branch of the residual circulation in recent decades can partly explain trace gas trends above 20 hPa, while the eddy mixing has a small effect on the trends. Understanding these contributions from different processes to the hemispheric asymmetries in trends of these trace gases can help us to evaluate more accurately future changes in stratospheric composition.