ALBERT

Description: Reliable ozone trends after 2000 are essential to detect early ozone recovery. However, the long-term ground-based and satellite ozone profile trends reported in the literature show a high variability. There are multiple reasons for variability in the reported long-term trends such as the measurement timing and the dataset quality. The Payerne Switzerland microwave radiometer (MWR) ozone trends are significantly positive at 2 % to 3 % per decade in the upper stratosphere (5–1 hPa, 35–48 km), with a high variation with altitude. This is in accordance with the Northern Hemisphere (NH) trends reported by other ground-based instruments in the SPARC LOTUS project. In order to determine what part of the variability between different datasets comes from measurement timing, Payerne MWR and SOCOL v3.0 chemistry–climate model (CCM) trends were estimated for each hour of the day with a multiple linear regression model. Trends were quantified as a function of local solar time (LST). In the middle and upper stratosphere, differences as a function of LST are reported for both the MWR and simulated trends for the post-2000 period. However, these differences are not significant at the 95 % confidence level. In the lower mesosphere (1–0.1 hPa, 48–65 km), the 2010–2018 day- and nighttime trends have been considered. Here again, the variation in the trend with LST is not significant at the 95 % confidence level. Based on these results we conclude that significant trend differences between instruments cannot be attributed to a systematic temporal sampling effect. The dataset quality is of primary importance in a reliable trend derivation, and multi-instrument comparison analyses can be used to assess the long-term stability of data records by estimating the drift and bias of instruments. The Payerne MWR dataset has been homogenized to ensure a stable measurement contribution to the ozone profiles and to take into account the effects of three major instrument upgrades. At each instrument upgrade, a correction offset has been calculated using parallel measurements or simultaneous measurements by an independent instrument. At pressure levels smaller than 0.59 hPa (above ∼50 km), the homogenization corrections to be applied to the Payerne MWR ozone profiles are dependent on LST. Due to the lack of reference measurements with a comparable measurement contribution at a high time resolution, a comprehensive homogenization of the sub-daily ozone profiles was possible only for pressure levels larger than 0.59 hPa. The ozone profile dataset from the Payerne MWR, Switzerland, was compared with profiles from the GROMOS MWR in Bern, Switzerland, satellite instruments (MLS, MIPAS, HALOE, SCHIAMACHY, GOMOS), and profiles simulated by the SOCOL v3.0 CCM. The long-term stability and mean biases of the time series were estimated as a function of the measurement time (day- and nighttime). The homogenized Payerne MWR ozone dataset agrees within ±5 % with the MLS dataset over the 30 to 65 km altitude range and within ±10 % of the HARMonized dataset of OZone profiles (HARMOZ, limb and occultation measurements from ENVISAT) over the 30 to 65 km altitude range. In the upper stratosphere, there is a large nighttime difference between Payerne MWR and other datasets, which is likely a result of the mesospheric signal aliasing with lower levels in the stratosphere due to a lower vertical resolution at that altitude. Hence, the induced bias at 55 km is considered an instrumental artifact and is not further analyzed.

Description: Middle atmospheric ozone, water vapour and zonal and meridional wind profiles have been measured with the two ground-based microwave radiometers GROMOS-C and MIAWARA-C. The instruments have been located at the Arctic research base AWIPEV at Ny-Ålesund, Svalbard (79∘ N, 12∘ E), since September 2015. GROMOS-C measures ozone spectra in the four cardinal directions with an elevation angle of 22∘. This means that the probed air masses at an altitude of 3 hPa (37 km) have a horizontal distance of 92 km to Ny-Ålesund. We retrieve four separate ozone profiles along the lines of sight and calculate daily mean horizontal ozone gradients which allow us to investigate the small-scale spatial variability of ozone above Ny-Ålesund. We present the evolution of the ozone gradients at Ny-Ålesund during winter 2018/2019, when a major sudden stratospheric warming (SSW) took place with the central date at 2 January, and link it to the planetary wave activity. We further analyse the SSW and discuss our ozone and water vapour measurements in a global context. At 3 hPa we find a distinct seasonal variation of the ozone gradients. The strong polar vortex during October and March results in a decreasing ozone volume mixing ratio towards the pole. In November the amplitudes of the planetary waves grow until they break in the end of December and an SSW takes place. From November until February ozone increases towards higher latitudes and the magnitude of the ozone gradients is smaller than in October and March. We attribute this to the planetary wave activity of wave numbers 1 and 2 which enabled meridional transport. The MERRA-2 reanalysis and the SD-WACCM model are able to capture the small-scale ozone variability and its seasonal changes.

Description: We analyse reprocessed electron density profiles and total electron content (TEC) profiles of the ionosphere in September 2008 (around solar minimum) and September 2013 (around solar maximum) obtained by the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC/FORMOSAT-3). The TEC profiles describe the total electron content along the ray path from the GPS satellite to the low Earth orbit as function of the tangent point of the ray. Some of the profiles in the magnetic polar regions show small-scale fluctuations on spatial scales

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

Description: Cloud fraction (CF), integrated liquid water (ILW) and integrated water vapour (IWV) were continuously measured from 2004 to 2016 by the TROpospheric WAter RAdiometer (TROWARA) in Bern, Switzerland. There are indications for interannual variations of CF and ILW. A spectral analysis shows that IWV is dominated by an annual oscillation, leading to an IWV maximum of 24 kg m−2 in July to August and a minimum of 8 kg m−2 in February. The seasonal behaviour of CF and ILW is composed by both the annual and the semiannual oscillation. However, the annual oscillation of CF has a maximum in December while the annual oscillation of ILW has a maximum in July. The semiannual oscillations of CF and ILW are strong from 2010 to 2014. The normalized power spectra of ILW and CF show statistically significant spectral components with periods of 76, 85, 97 and 150 days. We find a similarity between the power spectra of ILW and CF with those of zonal wind at 830 hPa (1.5 km) above Bern. Particularly, the occurrence of higher harmonics in the CF and ILW spectra is possibly forced by the behaviour of the lower-tropospheric wind. The mean amplitude spectra of CF, ILW and IWV show increased short-term variability on timescales less than 40 days from spring to fall. We find a weekly cycle of CF and ILW from June to September with increased values on Saturday, Sunday and Monday.

Description: A mesospheric water vapor data set obtained by the middle atmospheric water vapor radiometer (MIAWARA) close to Bern, Switzerland (46.88∘ N, 7.46∘ E) during October 2010 to September 2017 is investigated to study the long-term evolution and variability of quasi 2-day waves (Q2DWs). We present a climatological overview and an insight on the dynamical behavior of these waves with the occurring spectrum of periods as seen from a midlatitude observation site. Such a large and nearly continuous measurement data set as ours is rare and of high scientific value. The core results of our investigation indicate that the activity of the Q2DW manifests in burst-like events and is higher during winter months (November–February) than during summer months (May–August) for the altitude region of the mesosphere (up to 0.02 hPa in winter and up to 0.05 hPa in summer) accessible for the instrument. Single Q2DW events reach at most about 0.8 ppm in the H2O amplitudes. Further, monthly mean Q2DW amplitude spectra are presented and reveal a high-frequency variability between different months. A large fraction of identified Q2DW events (20 %) develop periods between 38 and 40 h. Further, we show the temporal evolution of monthly mean Q2DW oscillations continuously for all months and separated for single months over 7 years. The analysis of autobicoherence spectra gives evidence that Q2DWs are sometimes phase coupled to diurnal oscillations to a high degree and to waves with a period close to 18 h.

Description: We analyze reprocessed electron density profiles and TEC profiles of the ionosphere in September 2008 (around solar minimum) and September 2013 (around solar maximum) obtained by the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC). The TEC profiles describe the total electron content along the ray path from the GPS satellite to the low Earth orbit as function of the tangent point of the ray. Some of the profiles in the magnetic polar regions show small-scale fluctuations with spatial scales

Description: The middle atmospheric water vapor radiometer MIAWARA is located close to Bern in Zimmerwald (46.88∘ N, 7.46∘ E; 907 m) and is part of the Network for the Detection of Atmospheric Composition Change (NDACC). Initially built in the year 2002, a major upgrade of the instrument's spectrometer allowed middle atmospheric water vapor to be continuously measured since April 2007. Thenceforward to May 2018, a time series of more than 11 years has been gathered, that makes a first trend estimate possible. For the trend estimation, a robust multilinear parametric trend model has been used. The trend model encompasses a linear term, a solar activity tracker, the El Niño–Southern Oscillation (ENSO) index and the Quasi-Biennial Oscillation (QBO) as well as the annual and semi-annual oscillation. In the time period April 2007 to May 2018 we find a significant decline in water vapor by -0.6±0.2 ppm per decade between 61 and 72 km. Below the stratopause level (∼48 km) a smaller reduction of H2O of up to -0.3±0.1 ppm per decade is detected.

Description: Observing stratospheric ozone is essential to assess whether the Montreal Protocol has succeeded in saving the ozone layer by banning ozone depleting substances. Recent studies have reported positive trends, indicating that ozone is recovering in the upper stratosphere at mid-latitudes, but the trend magnitudes differ, and uncertainties are still high. Trends and their uncertainties are influenced by factors such as instrumental drifts, sampling patterns, discontinuities, biases, or short-term anomalies that may all mask a potential ozone recovery. The present study investigates how anomalies, temporal measurement sampling rates, and trend period lengths influence resulting trends. We present an approach for handling suspicious anomalies in trend estimations. For this, we analysed multiple ground-based stratospheric ozone records in central Europe to identify anomalous periods in data from the GROund-based Millimetre-wave Ozone Spectrometer (GROMOS) located in Bern, Switzerland. The detected anomalies were then used to estimate ozone trends from the GROMOS time series by considering the anomalous observations in the regression. We compare our improved GROMOS trend estimate with results derived from the other ground-based ozone records (lidars, ozonesondes, and microwave radiometers), that are all part of the Network for the Detection of Atmospheric Composition Change (NDACC). The data indicate positive trends of 1 % decade−1 to 3 % decade−1 at an altitude of about 39 km (3 hPa), providing a confirmation of ozone recovery in the upper stratosphere in agreement with satellite observations. At lower altitudes, the ground station data show inconsistent trend results, which emphasize the importance of ongoing research on ozone trends in the lower stratosphere. Our presented method of a combined analysis of ground station data provides a useful approach to recognize and to reduce uncertainties in stratospheric ozone trends by considering anomalies in the trend estimation. We conclude that stratospheric trend estimations still need improvement and that our approach provides a tool that can also be useful for other data sets.

Description: Observation and simulation of individual ozone streamers are important for the description and understanding of non-linear transport processes in the middle atmosphere. A sudden increase in mid-stratospheric ozone occurred above central Europe on 4 December 2015. The GROund-based Millimeter-wave Ozone Spectrometer (GROMOS) and the Stratospheric Ozone MOnitoring RAdiometer (SOMORA) in Switzerland measured an ozone enhancement of about 30 % at 34 km altitude (8.3 hPa) from 1 to 4 December. A similar ozone increase is simulated by the Specified Dynamics Whole Atmosphere Community Climate (SD-WACCM) model. Further, the global ozone fields at 34 km altitude (8.3 hPa) from SD-WACCM and the satellite experiment Aura/MLS show a remarkable agreement for the location and timing of an ozone streamer (large-scale tongue-like structure) extending from the subtropics in northern America over the Atlantic to central Europe. This agreement indicates that SD-WACCM can inform us about the wind inside the Atlantic ozone streamer. SD-WACCM shows an eastward wind of about 100 m s−1 inside the Atlantic streamer in the mid-stratosphere. SD-WACCM shows that the Atlantic streamer flows along the edge of the polar vortex. The Atlantic streamer turns southward at an erosion region of the polar vortex located above the Caspian Sea. The spatial distribution of stratospheric water vapour indicates a filament outgoing from this erosion region. The Atlantic streamer, the polar vortex erosion region and the water vapour filament belong to the process of planetary wave breaking in the so-called surf zone of the northern midlatitude winter stratosphere.