Lower Mesosphere Research Articles

Thermal tides in the middle atmosphere at mid-latitudes measured with a ground-based microwave radiometer

Abstract. In recent decades, theoretical studies and numerical models of thermal tides have gained attention. It has been recognized that tides have a significant influence on the dynamics of the middle and upper atmosphere; as they grow in amplitude and propagate upward, they transport energy and momentum from the lower to the upper atmosphere, contributing to the vertical coupling between atmospheric layers. The superposition of tides with other atmospheric waves leads to non-linear wave–wave interactions. However, direct measurements of thermal tides in the middle atmosphere are challenging and are often limited to satellite measurements in the tropics and at low latitudes. Due to orbit geometry, such observations provide only a reduced insight into the short-term variability in atmospheric tides. In this paper, we present tidal analysis from 5 years of continuous observations of middle-atmospheric temperatures. The measurements were performed with the ground-based temperature radiometer TEMPERA (TEMPErature RAdiometer), which was developed at the University of Bern in 2013 and was located in Bern (46.95° N, 7.45° E) and Payerne (46.82° N, 6.94° E). TEMPERA achieves a temporal resolution of 1–3 h and covers the altitude range between 25–50 km. Using an adaptive spectral filter with a vertical regularization (ASF2D) for the tidal analysis, we found maximum amplitudes for the diurnal tide of approximately 2.4 K, accompanied by seasonal variability. The maximum amplitude was reached on average at an altitude of 43 km, which also reflected some seasonal characteristics. We demonstrate that TEMPERA is suitable for providing continuous temperature soundings in the stratosphere and lower mesosphere with a sufficient cadence to infer tidal amplitudes and phases for the dominating tidal modes. Furthermore, our measurements exhibit a dominating diurnal tide and smaller amplitudes for the semidiurnal and terdiurnal tides in the stratosphere.

Response of the low latitude mesosphere and lower thermosphere to the recent sudden stratospheric warming events of 2017–18 and 2019

Upper mesospheric wind data acquired by the medium frequency radar at Kolhapur (16.7oN, 74.2oE) and Modern–Era Retrospective analysis for Research and Application version 2 (MERRA-2) temperature and wind reanalysis datasets are used to investigate the dynamical response of the low-latitude middle atmosphere to the sudden stratospheric warming (SSW) events that occurred during the 2017–18 and 2018–19 winters. When the amplitude of the high-latitude stratospheric planetary wave (PW) of zonal wavenumber one reduces considerably with the onset of the SSW event, the low-latitude mesospheric PW over Kolhapur also shows a considerable reduction in the PW activity. It is noteworthy that the upper mesospheric winds are eastward for approximately 3 weeks after the onset of SSW. The reduced PW activity is associated with the enhanced gravity wave activity in the meridional wind during the SSW 2018–19 event. The plane of propagation of gravity waves obtained from the perturbation ellipse method suggests that their predominant plane of propagation is in the north–south direction. The persistence of the eastward winds is suggested to be due to the interaction of the northward propagating gravity waves with the mean flow, leading to the eastward acceleration due to the Coriolis force.

The Spread of the Hunga Tonga H2O Plume in the Middle Atmosphere Over the First Two Years Since Eruption

AbstractThe eruption of Hunga in January 2022 injected a large amount of water into the stratosphere. Satellite measurements from Aura Microwave Limb Sounder (MLS) show that this water vapor (H2O) has now spread throughout the stratosphere and into the lower mesosphere, resulting in an increase of >1 ppmv throughout most of this region. Measurements from three ground‐based Water Vapor Millimeter Wave Spectrometer (WVMS) instruments and MLS are in good agreement, and show that in 2023 there was more H2O in the lower mesosphere than at any time since the WVMS measurements began in the 1990's. At Table Mountain, California all WVMS H2O measurements at 54 km since June 2023, and all of the measurements from Mauna Loa, Hawaii, since the resumption of measurements in September 2023, show larger mixing ratios than any previous measurements. At 70 km several recent WVMS retrievals since September 2023 show the largest anomalies ever measured. The MLS measurements show that maximum H2O anomalies over the 2004–2023 record have occurred throughout almost all of the stratosphere and lower mesosphere since the eruption. As of November 2023, almost all of the ∼140 Tg of water originally injected into the stratosphere by the Hunga eruption remains in the middle atmosphere at pressures below 83 hPa (altitudes above ∼17 km). The eruption occurred during a period when stratospheric H2O was already slightly elevated above the 2004–2021 MLS average, and the November 2023 anomaly of ∼160 Tg represents ∼15% of the total mass of H2O in this region.

Three‐Dimensional Modeling of the O2(1∆) Dayglow: Dependence on Ozone and Temperatures

AbstractFuture space missions dedicated to measuring CO2 on a global scale can make advantageous use of the O2 band at 1.27 μm to retrieve the air column. The 1.27 μm band is close to the CO2 absorption bands at 1.6 and 2.0 μm, which allows a better transfer of the aerosol properties than with the usual O2 band at 0.76 μm. However, the 1.27 μm band is polluted by the spontaneous dayglow of the excited state O2 (1∆), which must be removed from the observed signal. We investigate here our quantitative understanding of the O2(1∆) dayglow with a chemistry‐transport model. We show that the previously reported −13% deficit in O2(1∆) dayglow calculated with the same model is essentially due a −20% to −30% ozone deficit between 45 and 60 km. We find that this ozone deficit is due to excessively high temperatures (+15 K) of the meteorological analyses used to drive the model in the mesosphere. The use of lower analyzed temperatures (ERA5), in better agreement with the observations, slows down the hydrogen‐catalyzed and Chapman ozone loss cycles. This effect leads to an almost total elimination of the ozone and O2(1∆) deficits in the lower mesosphere. Once integrated vertically to simulate a nadir measurement, the deficit in modeled O2(1∆) brightness is reduced to −4.2 ± 2.8%. This illustrates the need for accurate mesospheric temperatures for a priori estimations of the O2(1∆) brightness in algorithms using the 1.27 μm band.

Planetary Wave Modulation of Gravity Waves Over the Andes in 2016

AbstractThe Andes account for the largest source of orographic gravity waves (GWs) in the middle atmosphere. This results from persistent, strong zonal winds at the surface encountering the north‐south mountain chain, producing strong orographic lift, and resulting GWs. Here, we consider GWs in the stratosphere and mesosphere above the Andes as observed by the Cloud Imaging and Particle Size instrument, the Solar Occultation for Ice Experiment on the AIM satellite, and the Sounding of the Atmosphere using Broadband Emission Radiometry instrument onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite. GW variability is considered in the context of the location of the stratospheric wintertime westerly jet and planetary wave (PW) amplitude and phase. The occurrence of GWs in the middle and upper atmosphere depends not only on tropospheric sources, like the Andes, but also on the background winds through which they propagate. Results suggest that the propagation of GWs into the mesosphere is well correlated with winds throughout the middle and upper stratosphere. PWs cause the westerly jet to move over the Andes, resulting in an increase in GW amplitude throughout the middle and upper stratosphere. The evolution and variability of GWs are tied closely to the phase of the PW‐1 and are linked to PW‐2 when the PW amplitudes are sufficiently large. GW amplitude, as observed by all three data sets, increases in the upper stratosphere and lower mesosphere when the trough of the PW‐1 in the stratosphere is over the Andes.

Estimating Individual Radio Occultation Uncertainties Using the Observations and Environmental Parameters

Abstract Estimation of uncertainties (random error statistics) of radio occultation (RO) observations is important for their effective assimilation in numerical weather prediction (NWP) models. Average uncertainties can be estimated for large samples of RO observations and these statistics may be used for specifying the observation errors in NWP data assimilation. However, the uncertainties of individual RO observations vary, and so using average uncertainty estimates will overestimate the uncertainties of some observations and underestimate those of others, reducing their overall effectiveness in the assimilation. Several parameters associated with RO observations or their atmospheric environments have been proposed to estimate individual RO errors. These include the standard deviation of bending angle (BA) departures from either climatology in the upper stratosphere and lower mesosphere (STDV) or the sample mean between 40 and 60 km (STD4060), the local spectral width (LSW), and the magnitude of the horizontal gradient of refractivity (|∇HN|). In this paper we show how the uncertainties of two RO datasets, COSMIC-2 and Spire BA, as well as their combination, vary with these parameters. We find that the uncertainties are highly correlated with STDV and STD4060 in the stratosphere, and with LSW and |∇HN| in the lower troposphere. These results suggest a hybrid error model for individual BA observations that uses an average statistical model of RO errors modified by STDV or STD4060 above 30 km, and LSW or |∇HN| below 8 km. Significance Statement These results contribute to the understanding of the sources of uncertainties in radio occultation observations. They could be used to improve the effectiveness of these observations in their assimilation into numerical weather prediction and reanalysis models by improving the estimation of their observational errors.

Mesospheric Water Vapor in 2022

AbstractThe eruption of the Hunga Tonga undersea volcano in January 2022 injected water vapor to altitudes as high as 53 km, but also an unprecedented and much larger amount of water vapor into the stratosphere. Several months after the eruption, measurements from the Aura Microwave Limb Sounder (MLS) and from three ground‐based Water Vapor Millimeter Wave Spectrometer instruments began to measure record‐high amounts of water vapor in the mesosphere over a wide range of latitudes. While there are indications that some of this mesospheric increase in water vapor was probably caused by the Hunga Tonga eruption, variations in water vapor mixing ratios also depend on dynamical factors. The phase of the QBO in 2015 was similar to that in 2022, and we make use of this similarity in order to better understand what role dynamics played in establishing the unusually large 2022 water vapor mixing ratios, both in the upper and lower mesosphere.

Feasibility analysis of optimal terahertz (THz) bands for passive limb sounding of middle and upper atmospheric wind

Abstract. As of now, direct measurements of middle and upper atmospheric wind are still scarce, and the observation method is limited, especially for the upper stratosphere and lower mesosphere. This paper presents a study of band selection to derive line-of-sight wind from 30 km to more than 120 km using a high-spectral-resolution terahertz (THz) radiometer, which can fill the measurement gap between lidar and interferometer data. Simulations from 0.1–5 THz for evaluating the feasibility of the spaceborne THz limb sounder are described in this study. The results show that high-precision wind (better than 5 m s−1) can be obtained from 40 to 70 km by covering a cluster of strong O3 lines. By choosing strong O2 or H2O lines, the high-quality measurement can be extended to 105 km. The O atom (OI) lines can provide wind signals in the higher atmosphere. In addition, performance of different instrument parameters, including spectral resolution, bandwidth, and measurement noise, was analyzed, and, lastly, four different band combinations are suggested.

Ozone and water vapor variability in the polar middle atmosphere observed with ground-based microwave radiometers

Abstract. Leveraging continuous ozone and water vapor measurements with the two ground-based radiometers GROMOS-C and MIAWARA-C at Ny-Ålesund, Svalbard (79∘ N, 12∘ E) that started in September 2015 and combining MERRA-2 and Aura-MLS datasets, we analyze the interannual behavior and differences in ozone and water vapor and compile climatologies of both trace gases describing the annual variation of ozone and water vapor at polar latitudes. A climatological comparison of the measurements from our ground-based radiometers with reanalysis and satellite data was performed. Overall differences between GROMOS-C and Aura-MLS ozone volume mixing ratio (VMR) climatology are mainly within ±7 % throughout the middle and upper stratosphere and exceed 10 % in the lower mesosphere (1–0.1 hPa) in March and October. For the water vapor climatology, the average 5 % agreement is between MIAWARA-C and Aura-MLS water vapor VMR values throughout the stratosphere and mesosphere (100–0.01 hPa). The comparison to MERRA-2 yields an agreement that reveals discrepancies larger than 50 % above 0.2 hPa depending on the implemented radiative transfer schemes and other model physics. Furthermore, we perform a conjugate latitude comparison by defining a virtual station in the Southern Hemisphere at the geographic coordinate (79∘ S, 12∘ E) to investigate interhemispheric differences in the atmospheric compositions. Both trace gases show much more pronounced interannual and seasonal variability in the Northern Hemisphere than in the Southern Hemisphere. We estimate the effective water vapor transport vertical velocities corresponding to upwelling and downwelling periods driven by the residual circulation. In the Northern Hemisphere, the water vapor ascent rate (5 May to 20 June in 2015, 2016, 2017, 2018, and 2021 and 15 April to 31 May in 2019 and 2020) is 3.4 ± 1.9 mm s−1 from MIAWARA-C and 4.6 ± 1.8 mm s−1 from Aura-MLS, and the descent rate (15 September to 31 October in 2015–2021) is 5.0 ± 1.1 mm s−1 from MIAWARA-C and 5.4 ± 1.5 mm s−1 from Aura-MLS at the altitude range of about 50–70 km. The water vapor ascent (15 October to 30 November in 2015–2021) and descent rates (15 March to 30 April in 2015–2021) in the Southern Hemisphere are 5.2 ± 0.8 and 2.6 ± 1.4 mm s−1 from Aura-MLS, respectively. The water vapor transport vertical velocities analysis further reveals a higher variability in the Northern Hemisphere and is suitable to monitor and characterize the evolution of the northern and southern polar dynamics linked to the polar vortex as a function of time and altitude.

Microwave radiometer observations of the ozone diurnal cycle and its short-term variability over Switzerland

Abstract. In Switzerland, two ground-based ozone microwave radiometers are operated in the vicinity of each other (ca. 40 km): the GROund-based Millimeter-wave Ozone Spectrometer (GROMOS) in Bern (Institute of Applied Physics) and the Stratospheric Ozone MOnitoring RAdiometer (SOMORA) in Payerne (MeteoSwiss). Recently, their calibration and retrieval algorithms have been fully harmonized, and updated time series are now available since 2009. Using these harmonized ozone time series, we investigate and cross-validate the strato–mesospheric ozone diurnal cycle derived from the two instruments and compare it with various model-based datasets: the dedicated GEOS-GMI Diurnal Ozone Climatology (GDOC) based on the Goddard Earth Observing System (GEOS-5) general circulation model, the Belgian Assimilation System for Chemical ObsErvations (BASCOE) – a chemical transport model driven by ERA5 dynamics, and a set of free-running simulations from the Whole Atmosphere Community Climate Model (WACCM). Overall, the two instruments show very similar ozone diurnal cycles at all seasons and pressure levels, and the models compare well with each other. There is a good agreement between the models and the measurements at most seasons and pressure levels, and the largest discrepancies can be explained by the limited vertical resolution of the microwave radiometers. However, as in a similar study over Mauna Loa, some discrepancies remain near the stratopause, at the transition region between ozone daytime accumulation and depletion. We report similar delays in the onset of the modelled ozone diurnal depletion in the lower mesosphere. Using the newly harmonized time series of GROMOS and SOMORA radiometers, we present the first observations of short-term (sub-monthly) ozone diurnal cycle variability at mid-latitudes. The short-term variability is observed in the upper stratosphere during wintertime, when the mean monthly cycle has a small amplitude and when the dynamics are more important. This is shown in the form of strong enhancements of the diurnal cycle, reaching up to 4–5 times the amplitude of the mean monthly cycle. We show that BASCOE is able to capture some of these events, and we present a case study of one such event following the minor sudden stratospheric warming of January 2015. Our analysis of this event supports the conclusions of a previous modelling study, attributing regional variability of the ozone diurnal cycle to regional anomalies in nitrogen oxide (NOx) concentrations. However, we also find periods with an enhanced diurnal cycle that do not show much change in NOx and where other processes might be dominant (e.g. atmospheric tides). Given its importance, we believe that the short-term variability of the ozone diurnal cycle should be further investigated over the globe, for instance using the BASCOE model.

Evidence for the Influence of the Quasi-Biennial Oscillation on the Semiannual Oscillation in the Tropical Middle Atmosphere

Abstract The semiannual oscillation (SAO) in zonally averaged zonal winds develops just above the quasi-biennial oscillation (QBO) and dominates the seasonal variability in the tropical upper stratosphere and lower mesosphere. The magnitude, seasonality, and latitudinal structure of the SAO vary with the phase of the QBO. There is also an annual oscillation (AO) whose magnitude at the equator is smaller than those of the SAO and QBO but not negligible. This work presents the relation between the SAO, QBO, AO, and time-mean wind in the tropical upper stratosphere and lower mesosphere using winds derived from satellite geopotential height observations. The winds are generally more westerly during the easterly phase of the QBO. The SAO extends to lower altitudes during periods where the QBO is characterized by deep easterly winds. The differences in the SAO associated with the QBO are roughly confined to the latitudes where the QBO has appreciable amplitude, suggesting that the mechanism is controlled by vertical coupling. The westerly phases of the SAO and AO show downward propagation with time. This analysis suggests that forcing by dissipation of waves with westerly momentum is responsible for the westerly acceleration of both the SAO and AO. The timing and structure of the easterly phases of the SAO and AO near the stratopause are consistent with the response to meridional advection of momentum across the equator during solstices; it is not apparent that local wave processes play important roles in the easterly phases in the region of the stratopause.

The Coexistence of Gravity Waves From Diverse Sources During a SOUTHTRAC Flight

AbstractWe use observations from one of the SOUTHTRAC (Southern Hemisphere Transport, Dynamics, and Chemistry) Campaign flights in Patagonia and the Antarctic Peninsula during September 2019 to analyze possible sources of gravity waves (GW) in this hotspot during austral late winter and early spring. Data from two of the instruments onboard the German High Altitude and Long Range Research Aircraft (HALO) are employed: the Airborne Lidar for Middle Atmosphere research (ALIMA) and the Basic HALO Measurement and Sensor System (BAHAMAS). The former provides vertical temperature profiles along the trajectory, while the latter gives the three components of velocity, pressure, and temperature at the flight position. GW‐induced perturbations are obtained from these observations. We include numerical simulations from the Weather Research and Forecast (WRF) model to place a four‐dimensional context for the GW observed during the flight and to present possible interpretations of the measurements, for example, the orientation or eventual propagation sense of the waves may not be inferred using only data obtained onboard. We first evaluate agreements and discrepancies between the model outcomes and the observations. This allowed us an assessment of the WRF performance in the generation, propagation, and eventual dissipation of diverse types of GW through the troposphere, stratosphere, and lower mesosphere. We then analyze the coexistence and interplay of mountain waves (MW) and non‐orographic (NO) GW. The MW dominate above topographic areas and in the direction of the so‐called GW belt, whereas the latter waves are mainly relevant above oceanic zones. WRF simulates NOGW as mainly upward propagating entities above the lower stratosphere. Model runs show that deep vertical propagation conditions are in general favorable during this flight but also that in the upper stratosphere and lower mesosphere and mainly above topography there is some potential for wave breaking. The numerical simulations evaluate the GW drag for the whole flight area and find that the strongest effect is located in the zonal component around the stratopause. The general behavior against height resembles that obtained with a local fixed lidar data. According to WRF results, up to 100 km horizontal wavelength MW account for about half of the force opposing the circulation of the atmosphere.

Satellite‐Borne Observations of Ozone Impact by the November 2001 Solar Proton Event

AbstractThe November 2001 Solar Proton Event (SPE) is one of the strongest events in the era of satellite observations. However, no observational case study of this exceptional event's impact on atmospheric chemistry has been reported. In this paper, we use satellite‐based observations from Optical Spectrograph and Infrared Imaging Systems (OSIRIS) to quantify the SPE impact on middle atmospheric O3 in the southern hemisphere during summertime conditions. The results show a relatively modest, yet detectable, O3 depletion in the upper stratosphere and lower mesosphere. Compared to the observations, the Whole Atmosphere Community Climate Model (WACCM‐D) simulates somewhat lower O3 levels before the event but captures well the relative ozone depletion. The largest depletion is seen on November 6th, after the Geostationary Operational Environment Satellite observed the peak proton fluxes. On this day, the O3 depletion was observed and simulated from the pole to 55°S geographic latitude. The daily polar cap (poleward of 60°S geographic latitude) averaged O3 profiles show a maximum depletion of 16.6 ± 2.2% at 1 hPa and 18.8 ± 3.3% at 1.5 hPa altitude, by OSIRIS and WACCM‐D, respectively. After the SPE, an enhancement in NOx is simulated by the results of the model within altitudes of the observation, which is well correlated with the observed and modeled O3 depletion. Challenges related to the detection of SPE impact on O3 in the summer hemisphere are discussed. We find that a careful analysis of simulation results can be essential when isolating the SPE impact from background variation.

The Mission Support System (MSS v7.0.4) and its use in planning for the SouthTRAC aircraft campaign

Abstract. The Mission Support System (MSS) is an open source software package that has been used for planning flight tracks of scientific aircraft in multiple measurement campaigns during the last decade. It consists of three major components: a web map server located close to the model data storage site that is capable of producing a variety of 2-D figures from 4-D meteorological data; a client application capable of displaying the figures in combination with the planned flight track and an assortment of additional information; and a new collaboration server component that enables real-time collaboration of multiple remote parties. During the last decade, these components were constantly improved towards being simple to set up and use and being standard compliant. Here, we describe the use of MSS during the Southern Hemisphere Transport, Dynamics, and Chemistry–Gravity Waves (SouthTRAC-GW) measurement campaign in 2019. This campaign, based in Rio Grande, Argentina, used the German research aircraft HALO to investigate several scientific objectives related to the Southern Hemisphere chemistry and dynamics. We present the diverse data products offered by the MSS web map server dedicated to the campaign, which were derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) forecast data, Chemical Lagrangian Model of the Stratosphere (CLaMS) simulations, and Atmospheric Infrared Sounder (AIRS) near-real time brightness temperature measurements. As an example for how the MSS software is used in conjunction with the different data sets, we describe the planning of a single flight, which eventually took place on 12 September 2019, probing orographic gravity waves propagating up into the lower mesosphere.

Wavenumbers 3 and 4 Quasi 2‐day Wave Activities Observed by Multiple Meteor Radars in the Two Hemispheres During Austral Summer

AbstractCombining observations from three meteor radars in the Northern Hemisphere (NH) and a meteor radar in the Southern Hemisphere (SH) and reanalysis data, we present successive wavenumbers 3 and 4 (W3 and W4) quasi 2 day wave activities in austral summer. In the mesosphere and lower thermosphere, the wave exhibits two burst activities. The W3 dominates the first burst, while the W4 is the predominate mode in the second burst. The mode transition arises in the late January and is completed around the end of January. The W3 and W4 have the nearly same dominate period of about 46 hr and vertical wavelengths of 110 km in the SH and about 55–65 km in the NH, and the similar latitude and altitude distributions in the zonal and meridional winds and temperature, except stronger magnitude and farther extension in the NH for the W3. The analysis indicates that the unstable regions of the summer jets in the lower mesosphere and stratopause region have the boundaries which toward lower latitudes are largely consistent with the critical layers, and Eliassen‐Palm (EP) fluxes of the W3 and W4 increase rapidly near the unstable regions. This demonstrates that both the W3 and W4 originate from the jet instabilities. Nevertheless, the EP flux of the W3 shows a stronger transfer across the hemispheres than that of the W4. The W3 and W4 have the same period and vertical scale, which is different from previous reports.

Harmonized retrieval of middle atmospheric ozone from two microwave radiometers in Switzerland

Abstract. We present new harmonized ozone time series from two ground-based microwave radiometers in Switzerland: GROMOS and SOMORA. Both instruments have measured hourly ozone profiles in the middle atmosphere (20–75 km) for more than 2 decades. As inconsistencies in long-term trends derived from these two instruments were detected, a harmonization project was initiated in 2019. The goal was to fully harmonize the data processing of GROMOS and SOMORA to better understand and possibly reduce the discrepancies between the two data records. The harmonization has been completed for the data from 2009 until 2022 and has been successful at reducing the differences observed between the two time series. It also explains the remaining differences between the two instruments and flags their respective anomalous measurement periods in order to adapt their consideration for future trend computations. We describe the harmonization and the resulting time series in detail. We also highlight the improvements in the ozone retrievals with respect to the previous data processing. In the stratosphere and lower mesosphere, the seasonal ozone relative differences between the two instruments are now within 10 % and show good correlation (R > 0.7) (except during summertime). We also perform a comparison of these new data series against measurements from the Microwave Limb Sounder (MLS) and Solar Backscatter Ultraviolet Radiometer (SBUV) satellite instruments over Switzerland. Seasonal mean differences with MLS and SBUV are within 10 % in the stratosphere and lower mesosphere up to 60 km and increase rapidly above that point.

The January 2022 eruption of Hunga Tonga-Hunga Ha'apai volcano reached the mesosphere.

Explosive volcanic eruptions can loft ash, gases, and water into the stratosphere, which affects both human activities and the climate. Using geostationary satellite images of the 15 January 2022 eruption of the Hunga Tonga-Hunga Ha'apai volcano, we find that the volcanic plume produced by this volcano reached an altitude of 57 kilometers at its highest extent. This places the plume in the lower mesosphere and provides observational evidence of a volcanic eruption injecting material through the stratosphere and directly into the mesosphere. We then discuss potential implications of this injection and suggest that the altitude reached by plumes from previous eruptions, such as the eruption of Mount Pinatubo in 1991, may have been underestimated because of a lack of observational data.

Analysis of four solar occultations by Titan’s atmosphere with the infrared channel of the VIMS instrument: Haze, CH4, CH3D, and CO vertical profiles

Titan, the largest satellite of Saturn, has a dense atmosphere mainly composed of nitrogen, methane at a percent level, and minor species. It is also covered by a thick and global photochemical organic haze. In the last two decades, the observations made by the Cassini orbiter and the Huygens probe have greatly improved our knowledge of Titan's system. The surface, haze, clouds, and chemical species can be studied and characterised with several instruments simultaneously. On the other hand, some compounds of its climatic cycle remain poorly known. This is clearly the case of the methane cycle, which is, however, a critical component of Titan's climate and of its evolution. We reanalysed four solar occultations by Titan's atmosphere observed with the infrared part of the Visual Infrared Mapping Spectrometer (VIMS) instrument. These observations were already analysed, but here we used significantly improved methane spectroscopic data. We retrieved the haze properties (not treated previously) and the mixing ratios of methane, deuterated methane, and CO in the stratosphere and in the low mesosphere. The methane mixing ratio in the stratosphere is much lower (about 1.1%) than expected from Huygens measurements (about 1.4 to 1.5%). This is consistent with previous results obtained with other instruments. However, features in the methane vertical profiles clearly demonstrate that there are interactions between the methane distribution and the atmosphere circulation. We also retrieved the haze extinction profiles and the haze spectral behaviour. We find that aerosols are aggregates with a fractal dimension of Df ≃ 2.3 ± 0.1, rather than Df ≃ 2 as previously thought. Our analysis also reveals noticeable changes in their size distribution and their morphology with altitude and time. These changes are also clearly connected to the atmosphere circulation and concerns the whole stratosphere and the transition between the main and the detached haze layers. We finally display the vertical profiles of CH3D and CO for the four observations. Although the latter retrievals have large error bars due to noisy data, we could derive values in agreement with other works.

Increase in the Aerosol Backscattering Ratio in the Lower Mesosphere in 2019–2021 and Its Effect on Temperature Measurements with the Rayleigh Method

Increase in the Aerosol Backscattering Ratio in the Lower Mesosphere in 2019–2021 and Its Effect on Temperature Measurements with the Rayleigh Method

The semi-annual oscillation (SAO) in the upper troposphere and lower stratosphere (UTLS)

Abstract. Both the scientific and operational communities are increasingly interested in subseasonal to seasonal variations of weather and climate. The semi-annual oscillation (SAO) has been studied extensively at the surface as well as in the middle atmosphere (upper stratosphere and the lower mesosphere). However, the SAO in the upper troposphere and lower stratosphere (UTLS) has been less discussed. Here we find evident SAO of temperature in the UTLS (250–175 hPa) from the subtropics to middle latitudes (22.5–42.5∘) using high-quality satellite measurements, reanalysis data, and model simulations. We show the mechanism of its formation by an energy budget analysis. The temperature in the Northern Hemisphere (NH) UTLS shows the first peak in February according to the dynamical heating and shows the second peak in July due to the dynamical heating and moist processes. Similar to the NH, the austral winter time maximum temperature in the Southern Hemisphere (SH) is related to dynamical heating and the austral summer time maximum is related to both moisture and dynamical heating in the UTLS. Model simulations indicate that the SAO in the UTLS is partly affected by the existence of an SAO in sea surface temperatures (SSTs) in the SH mid-latitudes and weakly affected by the SAO in SSTs in the NH mid-latitudes.