Gravity waves are small-scale atmospheric waves which transport momentum and energy in the atmosphere. They exert a profound influence on the global circulation, particularly in the stratosphere, mesosphere and above. Accurately measuring and simulating gravity waves is thus vital both for understanding the dynamics of the atmosphere and for developing the next generation of weather forecasting and climate prediction models.

Recent advances in measurement techniques, especially specialist height-resolving retrievals from nadir-sensing instruments such as NASA's AIRS, allow us for the first time to study gravity waves in the real atmosphere in full 3D. This in turn lets us accurately measure and characterise many key properties of gravity waves at both global and regional scales for the first time, including their phase speeds, group velocities, and the directional fluxes of momentum they transport. These directional momentum fluxes are fundamental to how gravity waves drive the bulk circulation, and are thus of significant importance in understanding middle-atmospheric dynamics.

Here, we present an annual climatology of key 3D-resolved gravity wave properties at the global scale, derived from NASA/AIRS 3D observations. We compare these measured properties to equivalent fields of grid resolved gravity waves generated by four leading reanalysis models (specifically, the CFSR, ERA5, JRA55 and MERRA2 reanalyses). These comparisons are produced by sampling the reanalysis models at high spatial resolution to produce synthetic AIRS measurements - this ensures that the comparisons are like-for-like rather than being skewed towards the strengths of the different datasets. We then process both the AIRS and reanalysis data using a common software chain.

This allows us to cross-validate both the resolved-wave-generating capabilities of the reanalyses, and also the quality of the instrument retrievals at small spatial scales. It also provides the information necessary to properly guide the development of this component in the next generation of such models.

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The atmosphere of the Earth is full of waves, occurring at all scales and all heights from the surface to the edge of space. These internal gravity waves are key drivers of the global circulation, however their accurate representation in General Circulation Models (GCMs) used for weather and climate forecasting has proved very difficult. Large parts of the gravity wave spectrum, and many gravity wave processes, occur at scales that are near or below the resolution of even modern GCMs, so their effects must be simulated with parameterisations. But these parameterisations have been poorly constrained by observations, with far-reaching effects. At latitudes near 60S, the inadequate representation of gravity waves in GCMs has been linked to the long-standing “cold-pole problem”, where the southern stratospheric polar vortex breaks up too late in spring due to insufficient gravity-wave drag, with global implications. One reason for this is the historical lack of global 3-D measurements of wave properties, which are required to accurately constrain their drag effect on large-scale winds. Here we present results from a new analysis of global 3-D satellite gravity wave measurements from NASA AIRS/Aqua using a novel 3-D Stockwell transform method. Our approach allows us to spatially localise and accurately measure amplitudes and spectral characteristics of stratospheric gravity waves in 3-D, enabling us to compute their drag effects on an unprecedented scale. We present the first wintertime climatologies of gravity wave drag around the southern polar vortex, characterising the amplitudes, wavelengths, frequencies and fluxes of the wave field around 60S. We identify the mountains of the Southern Andes and Antarctic Peninsula as the dominant wave sources in the polar region, with other significant contributions from small mountainous islands and many non-orographic processes. We also find evidence of persistent and widespread meridional propagation of wave drag into the southern polar vortex from sources to the north and south at almost all the way around the Antarctic continent. Work is also underway to develop a 3-D retrieval for IASI/MetOp, which could potentially quadruple our available measurements. With these 3-D observations and our new analysis methodology, we can begin to lay down a new benchmark in the understanding of the nature of the wintertime gravity wave field around 60S, something that has as yet eluded researchers for many years.

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The mesosphere – lower thermosphere (MLT) is the transition region between Earth’s atmosphere and space and is influenced strongly both from the atmosphere below and from incoming solar UV, X-rays and charged particles. These drive chemical, radiative and dynamical processes which couple the MLT with the lower atmosphere and with space weather. The detailed processes that govern the MLT are poorly understood, however, because this is the least well-observed region of the atmosphere. For instance, we do not even know if the mesosphere is in global mean radiative balance on monthly or seasonal timescales. What we know about the MLT today is derived from measurement of radiative fluxes in the infrared (IR), which tell us about the thermal balance of the MLT, but give an incomplete picture of its chemical composition, both in terms of the measured geophysical products, as well as their altitude coverage.

The accompanying illustration of atmospheric temperature trends (Solomon et al., GRL, 2018) shows that by far the largest temperature response to increased levels of atmospheric greenhouse gases is expected in the MLT region above 100km. However, we can’t replicate this trend from the sparse measurements that exist today. Our lack of understanding of the processes that govern the MLT lead to huge uncertainties in the data processing. This is because the global composition and variability of major key species like atomic oxygen are quasi unknown.

Nevertheless, observed negative temperature trends strongly indicate that the MLT is a predictive indicator of climate change. But current research also indicates that a significant part of this signal could be linked to stratospheric ozone recovery. Measuring the composition of the MLT is scientifically significant in its own right, but it will also help to disentangle our interpretation of heat fluxes in the MLT and lower layers of the atmosphere, and therefore to better understand processes of climate forcing. Direct measurements at multiple THz frequencies are missing, because - until today - their technological requirements were prohibitive for space (only large and expensive gas lasers could provide enough power to pump a heterodyne detector). By exploiting newly developed quantum technology for its heterodyne receivers, the LOCUS THz limb-sounder overcomes this limitation and will provide for the first-time direct measurements of the global height-resolved atomic oxygen (O) distribution over several years, enabling examination of inter-annual variability, as well as seasonal and short-term variations. In addition to studying the thermal composition of the MLT region, other target species like nitrous oxide (NO) will provide novel information about the impact of space weather events on upper atmospheric composition and chemistry.

LOCUS was a candidate mission in the recent ESA Earth Explorer 10 round. It ranked among the 6 recommended missions, and - Whilst ultimately not selected for Phase-0 study - Its scientific relevance was highly commended by the expert review panel.

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The OSIRIS instrument on the Odin spacecraft has been measuring the vertical profile of limb scattered sunlight since shortly after launch in 2001. Here we present a new retrieval of stratospheric temperature from these observations. The measured limb radiance in the near ultraviolet is used with a spherical, multiple scattering radiative transfer model and a non-linear inversion to retrieve the density profile and then derive the temperature. The time series, seasonal, interannual cycle, and corresponding temperature anomaly from these preliminary results are compared with temperature retrievals from the Microwave Limb Sounder.

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Ground-based microwave remote sensing is extremely useful for monitoring the middle atmosphere. It is effective during both daytime and nighttime, in clear sky or mild overcast weather, and is therefore particularly suited for observing the Polar regions where darkness (or direct sunlight) lasts throughout an entire winter (summer) season. Most techniques in this frequency range observe the spectral lines emitted by the chemical species present in the atmosphere and estimate their vertical distribution by studying the shape of the emitted lines, dominated by pressure broadening. Critical parameters of the specific microwave instrument employed are therefore the overall spectral passband and its frequency resolution. A 22 GHz water vapor ground-based spectrometer, VESPA-22, was installed in July 2016 at the Thule High Arctic Atmospheric Observatory (THAAO) located at Thule Air Base (76.5° N, 68.8° W), Greenland (http://www.thuleatmos-it.it/), where a lidar, a Vaisala radiosounding system, and a FTIR provide additional information on stratospheric chemical and physical properties. VESPA-22 was designed and built at the INGV and measures the 22.235 GHz water vapor emission line with a bandwidth of 500 MHz and a resolution of 31 kHz. The collected spectra are inverted using an optimal estimation algorithm in order to retrieve water vapor vertical profiles from about 25 to 75 km with an overall uncertainty between 5 and 12%. Depending on season and weather conditions, 1 to 2 vertical profiles a day are obtained. Every 30 minutes the spectrometer also performs tipping curve measurements by observing the sky emission at various zenith angles. This procedure allows both the calibration of the spectrometer and the measurement of the sky opacity, which then provides an estimate of the water vapor total column with an uncertainty of 5%. VESPA-22 has been operating in an autonomous mode since its installation, with few periods of data gaps. This study will present a comparison between VESPA-22 and concurrent Aura/Microwave Limb Sounder (MLS) version 4.2 water vapor strato-mesospheric profiles obtained during two yearly cycles. The comparison is challenged by the water vapor rapid variations measured in winter, when the polar vortex moves towards and away from Thule, showing that measurements carried out at THAAO have unique characteristics that can prove to be particularly valuable for ground-truthing satellite-based measurements. In the sensitivity range of VESPA-22 retrievals, the intercomparison with Aura/MLS water vapor profiles reveals a correlation coefficient of about 0.8 or higher and an average difference reaching its maximum of -6% or -0.2 ppmv at the top of the sensitivity range.

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The stratospheric ozone layer is monitored constantly over a global scale by means of several techniques. Observations in limb geometry from satellite platforms are in this respect very valuable, providing ozone profiles with high spatial and temporal coverage and a good vertical resolution.
SCIAMACHY and OMPS-LP are two satellite instruments able to collect shortwave scattered radiance in limb geometry. Ozone profiles from SCIAMACHY (2002-2012) and OMPS-LP (2012-2018) were retrieved at the University of Bremen using the same radiative transfer model, spectroscopic databases and a similar retrieval algorithm. The first aim of this study is the merging of these data sets, to obtain a consistent time series of ozone global distributions. A transfer function is needed to overcome issues related to the sensors calibration, since the two missions overlap only for 3 months. To this aim, we chose measurements performed by the MLS instrument as a reference: this sensor has been collecting atmospheric emission in the microwave spectral region in limb geometry since 2004, with a comparable spatial and temporal resolution.
Monthly latitude- and longitude-resolved time series of ozone profiles were calculated for the two instruments, exploiting the high spatial resolution of the data sets. Their merging has been then performed minimizing the differences between OMPS-LP and SCIAMACHY ozone number density profiles with respect to MLS values, for each latitude, longitude and altitude independently. The seasonal cycle was found to be consistent enough among the three instruments, so that the computation of deseasonalized anomalies can also be avoided. Correlations and drifts of the single and merged timeseries with respect to MLS are discussed.
We used this merged data set to study long-term ozone changes: a multi-linear regression (MLR) model was applied over the 2003-2018 period, including fit proxies as QBO, ENSO and a solar forcing. Positive significant trends were detected between 35 and 45 km at mid-latitudes, with an increasing ozone concentration up to 2-4% per decade, related to the implementation of the Montreal Protocol and its amendments. Negative changes were found in the lower tropical stratosphere but were statistically non-significant.
Finally we analyzed a run of the TOMCAT chemistry transport model covering the 2003-2017 period in order to compare our trend results from the merged data set with the model simulations. The focus was also on the change in the trend pattern detected between SCIAMACHY and OMPS-LP periods, especially in the middle tropical stratosphere. The possible causes and the influence of single satellite drifts are discussed.

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The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) is a limb-viewing infrared Fourier transform spectrometer that operated from 2002 to 2012 onboard the ENVISAT satellite. The analysis of MIPAS measurements allows to study the temporal evolution of numerous species of interest for the study of the ozone in the stratosphere, pollutants and many green-house gases.
The objective of the MIPAS Quality Working Group is to improve the quality of the MIPAS products through a fruitful collaboration among spectroscopists, Level 1, Level 2, and validation teams. A large effort has recently led to implement significant improvements in both ESA Level 1 and Level 2 processors, as well as in spectroscopic database and in some absorption cross-sections in order to improve the quality of the products. In addition to the products already present in the V7 dataset (temperature and the VMR of H2O, O3, HNO3, CH4, N2O, NO2, CFC-11, CFC-12, N2O5, ClONO2, HCFC-22, COF2, CF4, HCN and CCl4), the VMR of six additional species (OCS, CH3Cl, HDO, C2H2, C2H6, COCl2) will be provided in V8 dataset.
In order to evaluate the impact of the changes in the products before full mission reprocessing, the analysis of the performances of the products of the modified L1 and L2 processors, as well as the auxiliary data, has been performed on a Diagnostic DataSet (DDS). The orbits of the DDS have been chosen in coincidence with correlative measurements for performing also a preliminary assessment of the accuracy of the products and to evaluate possible changes in the drift.
With respect to V7 products, main improvements consist in a reduction of the temperature bias in the first part of the mission, a reduction of the discontinuities in CH4 and N2O time series due to daily gain upgrade, a better filtering of clouds and a better handling of horizontal inhomogeneities.
The results of the assessment of the quality of MIPAS measurements will be shown, as well as the study of the temporal evolution and variability of all species. We will also investigate the spatial, seasonal, and interannual variations in the distribution of these species.

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The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) is a limb-viewing infrared Fourier transform spectrometer that operated from 2002 to 2012 on board the ENVISAT satellite. The fruitful collaboration among spectroscopists, Level 1, Level 2, and validation teams in the frame of the MIPAS Quality Working Group has recently led to the implementation of significant changes in both ESA Level 1 and Level 2 processors, as well as in the spectroscopic database and in some absorption cross-sections. In addition to the products already present in V7 dataset (temperature and the VMR of H2O, O3, HNO3, CH4, N2O, NO2, CFC-11, CFC-12, N2O5, ClONO2, HCFC-22, COF2, CF4, HCN and CCl4), the VMR of six additional species (OCS, CH3Cl, HDO, C2H2, C2H6, COCl2) will be provided in V8 dataset.
Among the new species, methyl chloride (CH3Cl) is of great interest in stratospheric ozone chemistry since it is the most abundant chlorine-containing gas in the troposphere and, in contrast to other relatively long-lived ozone-depleting gases, it has both natural and anthropogenic sources, with the known emission being mainly natural (tropical plants, biomass burning, the ocean, salt marshes and fungi). Since it is not controlled under the Montreal Protocol, its importance is expected to increase in the coming decades as emission controls alter the relative contributions from natural and anthropogenic halogen sources.
In this paper we present a preliminary assessment of the quality of the MIPAS CH3Cl data, in terms of precision, accuracy and vertical resolution, and we investigate the spatial and seasonal variations in the distribution of CH3Cl in the upper troposphere/lower stratosphere (UTLS).

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Age of Air (AoA) in stratosphere is defined as the average time elapsed since molecules of air at some location had last been in troposphere. Typical values of AoA in the upper stratosphere inferred from models and observations are 5-8 years. The spatial distribution of AoA in the stratosphere is controlled by global atmospheric circulation and had been extensively used to evaluate and compare general circulation and chemical transport models. Besides Lagrangian formulation, there are several Eulerian formulations, used to determine AoA both with numerical modeling and from observational data. Satellite and in-situ observations of various passive a and semi-passive tracers have been used to infer the AoA experimentally. One of such tracers is sulphur-hexafluoride SF6, which is extremely stable in the troposphere and stratosphere, and has no natural sources. The mixing ratio of SF6 in the troposphere grows steadily during last decades, thus allowing for AoA estimates.

We performed multi-decade simulations of SF6 and several AoA tracers in the atmosphere with Silam chemistry-transport model driven with ERA-Interim reanalysis. The AoA was simulated with both surface-emitted passive tracer and with ideal “timing” tracer. The mixing ratio of the latter is forced to zero near the surface and grows linearly with time at every point of the domain. These two tracers have different sign of sensitivity to advection artifacts, i.e. higher mixing ratio means older air for “timing” tracer and younger air for the passive one. The resulting distributions of AoA agree well with each other and with Lagrangian simulations made by other authors, however disagree with AoA derived from observed SF6 concentrations in polar areas.

The simulations of SF6 were made with and without account for mesosphere depletion and for gravity separation of SF6, i.e. four versions of SF6 were simulated. The results indicate a good agreement with observations by MIPAS satellite in case of either or both factors accounted, and have substantial discrepancies in polar areas for fully passive SF6. Thus we conclude that (1) Silam is capable of simulating both SF6 distribution and AoA, and (2) the discrepancy between SF6-derived and modeled AoA originates from the violation of the assumption of SF6 passiveness, that is often considered small.

The Figure shows a yearly-mean apparent AoA from simulated SF6 distribution in the atmosphere for 2010 (left panel). The AoA agrees well with AoA obtained form MIPAS observations of SF6. The center and right panels show AoA calculated from fully-pssive SF6 and "Ideal Age" tracers from the same simulation as left panel. The latter cases agree with each other and with numerous model studies of AoA.

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We describe the impact of recent developments of the SILAM chemistry transport model on the quality of the long-term hindcast 1980-2017. The aim of the study is to prepare the new release of this hindcast. Particular attention was given to the stratospheric transformations and new emissions of halogens. With this talk, we present the comparison with the total ozone columns recorded by satellites.

In the updated stratospheric transformation module we have sharply expanded the list of halogens. In particular, included bromines in addition to chlorines, which list was also expanded, for example, by including emission of CFC gases and methyl chloroform. Also, in order to better model the ozone chemistry, our model includes the updated mechanism of polar stratospheric clouds and heterogeneous reactions that take place in ice, NAT, and super-cooled ternary aerosols/droplets. There reactions convert halogenated species into more active ozone-destructive form. The number of heterogeneous reactions is now extended from 6 to 13, to reflect the inclusion of bromines. Additionally, we have updated our photolysis rates together with bringing a more realistic effect of clouds into the model.

In order to better model the atmospheric composition with our extended list of species, we have also generated new emissions inventories, for example, for CFCs and methyl chloroform. Comparison of the simulated CFC gas concentrations with measurements indicate the our model emission are in line with the real world emissions. Additionally, we have build a mechanistic model to release bromine species from the sea as a part of sea spray. Currently the release is in the form of Br₂, and the model takes into account the emission efficiency from different-size droplets.

Our next step is to include the new emissions inventory EDGAR 4.3.2 and the just-released CAMS global dataset extending the EDGAR data to the most-recent years. These two datasets cover standard air pollutants. Other emissions sources will also be updated to see their effect during the full hindcast period. With our current emissions inventories, the preliminary results already show a clear impact of the CFC gases on the ozone. The ozone hole area and its depth is consistent with the measurements. The dominant ozone depletion appears at altitude of about 14-23 km, the strongest destruction being at around 18 km altitude. The effect of the Montreal Protocol is also visible in these runs, showing already a slow recovery of the total ozone amount during the last decade.

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Abstract: The North Atlantic Oscillation (NAO) is an important climate pattern affecting the atmospheric transport in the middle and lower reaches of the North Atlantic to the Arctic. The rapid daily variation of the atmospheric total column ozone in Iceland is an indicator of meridional atmospheric transport, which is determined by the geographical location of Iceland. In the paper, we investigated the daily ozone variation of atmospheric ozone in Iceland during winter months caused by the NAO using AIRS daily ozone data from 2003 to 2015. The analysis shows that dramatic daily ozone variation occur frequently in Iceland during winter months, ranging from 50 to 80DU, sometimes exceeding 100DU to more than one third of the total ozone, and this harp variation is closely related to the phase and amplitude of the NAO index. Analysis shows that where NAO is in positive or negative phase, it can lead to a sharp ozone increase or decrease in Iceland. When NAO is in positive phase, the lower and middle of North Atlantic atmosphere is controlled under westerly circulation and Iceland is on the north side of westerly jet. When the westerly circulation sweeps through Iceland and North Atlantic with low latitude air from south-central North America, combining with air vertical upward movement, resulting sharp ozone daily decrease in Iceland; while when the westerly circulation sweeps through Iceland and North Atlantic with polar air from northern Canada, combining with vertical downward movement, resulting sharp ozone daily increase in Iceland. When NAO is in negative phase, the lower and middle of North Atlantic atmosphere is controlled by north-south atmospheric transport and Iceland lie on the path of the transport. When the south wind sweeps northward with warm and humid air from lower latitudes, the ozone in Iceland will decrease dramatically; while when the north wind sweeps southward with air from high latitudes, the ozone in Iceland will increase. In the positive phase of NAO, the daily ozone variation amplitude is bigger and positively correlated with that of NAO; in the negative phase of NAO, the daily ozone variation amplitude is smaller and negatively correlated with that of NAO.
Fig.1 The correlation of NAO amplitude and daily ozone variation in Iceland during winter months from 2003 to 2015. Black lines represent month mean of NAO, red lines represent the month mean ozone daily range in Iceland, the black bold line represents the annual mean NAO index, and the red bold line represents the annual mean ozone daily variation in Iceland

Comparison of ozone profiles show that the rapid daily ozone variation occurs mostly in upper troposphere to lower stratosphere.
Fig.2 Comparison of daily ozone profiles at 25, 26, and 27 December, when the rapid ozone decrease at 26 December 2015 occurred.

Keywords: the North Atlantic Oscillation (NAO), Iceland, daily ozone variation, Back-ward trajectory.

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In this presentation, we describe the inversion algorithm for retrievals of high vertical resolution temperature profiles using bi-chromatic stellar scintillation measurements in the occultation geometry. This retrieval algorithm has been improved with respect to nominal ESA processing and applied to the measurements by Global Ozone Monitoring by Occultation of Stars (GOMOS) operated on board Envisat in 2002-2012. The retrieval method exploits the chromatic refraction in the Earth’s atmosphere. The bi-chromatic scintillations allow the determination of the refractive angle, which is proportional to the time delay between the photometer signals. The paper discusses the basic principle and detailed inversion algorithm for reconstruction of high resolution density, pressure and temperature profiles (HRTP) in the stratosphere from scintillation measurements. The HRTP profiles are retrieved with very good vertical resolution of ~200 m and high accuracy of ~1-3 K for altitudes of 15-32 km and with a global coverage. The best accuracy is achieved in in-orbital-plane occultations, and the accuracy weakly depends on star brightness. The whole GOMOS dataset has been processed with the improved HRTP inversion algorithm using the FMI’s Scientific Processor; and the dataset (HRTP FSP v1) is in open access.
The validation of small-scale fluctuations in the retrieved HRTP profiles is performed via comparison of vertical wavenumber spectra of temperature fluctuations in HRTP and in collocated radiosonde data. We found that the spectral features of temperature fluctuations are very similar in HRTP and collocated radiosonde temperature profiles.
HRTP can be assimilated into atmospheric models, used in studies of stratospheric clouds and in analysis of internal gravity waves activity. As an example of geophysical applications, gravity wave potential energy has been estimated using the HRTP dataset. The obtained spatio-temporal distributions of gravity wave energy are in good agreement with the previous analyses using other measurements.

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