Northern Mid-latitude Station Research Articles

The natural oscillations in stratospheric ozone observed by the GROMOS microwave radiometer at the NDACC station Bern

Abstract. A multilinear parametric regression analysis was performed to assess the seasonal and interannual variations of stratospheric ozone profiles from the GROMOS (GROund-based Millimeter-wave Ozone Spectrometer) microwave radiometer at Bern, Switzerland (46.95° N, 7.44° E; 577 m). GROMOS takes part in the Network for the Detection of Atmospheric Composition Change (NDACC). The study covers the stratosphere from 50 to 0.5 hPa (from 21 to 53 km) and extends over the period from January 1997 to January 2015. The natural variability was fitted during the regression analysis through the annual and semi-annual oscillations (AO, SAO), the quasi-biennial oscillation (QBO), the El Niño–Southern Oscillation (ENSO) and the solar activity cycle. Seasonal ozone variations mainly appear as an annual cycle in the middle and upper stratosphere and a semi-annual cycle in the upper stratosphere. Regarding the interannual variations, they are primarily present in the lower and middle stratosphere. In the lower and middle stratosphere, ozone variations are controlled predominantly by transport processes, due to the long lifetime of ozone, whereas in the upper stratosphere its lifetime is relatively short and ozone is controlled mainly by photochemistry. The present study shows agreement in the observed naturally induced ozone signatures with other studies. Further, we present an overview of the possible causes of the effects observed in stratospheric ozone due to natural oscillations at a northern midlatitude station. For instance regarding the SAO, we find that polar winter stratopause warmings contribute to the strength of this oscillation since these temperature enhancements lead to a reduction in upper stratospheric ozone. We have detected a strong peak amplitude of about 5 % for the solar cycle in lower stratospheric ozone for our 1.5 cycles of solar activity. Though the 11-year ozone oscillation above Bern is in phase with the solar cycle, we suppose that the strong amplitude is partly due to meteorological disturbances and associated ozone anomalies in the Northern Hemisphere. Further, our observational study gave the result that ozone above Bern is anti-correlated with the ENSO phenomenon in the lower stratosphere and correlated in the middle stratosphere.

On the latitudinal changes in ionospheric electrodynamics and composition based on observations over the 76–77°E meridian from both hemispheres during a geomagnetic storm

AbstractThe relative contributions of the composition disturbances and the disturbance electric fields in the redistribution of ionospheric plasma is investigated in detail by taking the case of a long‐duration positive ionospheric storm that occurred during 18–21 February 2014. GPS total electron content (TEC) data from the Indian Antarctic station, Bharti (69.4°S, 76.2°E geographic), the northern midlatitude station Hanle (32.8°N, 78.9°E geographic), northern low‐latitude station lying in the vicinity of the anomaly crest, Ahmedabad (23.04°N, 72.54°E geographic, dip latitude 17°N), and the geomagnetic equatorial station, Trivandrum (8.5°N, 77°E geographic, dip latitude 0.01°S) are used in the study. These are the first simultaneous observations of TEC from Bharti and Hanle during a geomagnetic storm. The impact of the intense geomagnetic storm (Dst∼−130 nT) on the southern hemisphere high‐latitude station was a drastic reduction in the TEC (negative ionospheric storm) starting from around 0330 Indian standard time (IST) on 19 February which continued till 21 February, the maximum reduction in TEC at Bharti being ∼35 TEC units on 19 February. In the northern hemisphere midlatitude and equatorial stations, a positive ionospheric storm started on 19 February at around 0900 IST and lasted for 3 days. The maximum enhancement in TEC at Hanle was about ∼25 TECU on 19 February while over Trivandrum it was ∼10 TECU. This long‐duration positive ionospheric storm provided an opportunity to assess the relative contributions of disturbance electric fields and composition changes latitudinally. The results indicate that the negative ionospheric storm over Bharti and the positive ionospheric storm over Hanle are the effect of the changes in the global wind system and the storm‐induced composition changes. At the equatorial latitudes, the positive ionospheric storm was due to the interplay of prompt penetration electric field and disturbance dynamo electric field.

Trends of ozone total columns and vertical distribution from FTIR observations at eight NDACC stations around the globe

Abstract. Ground-based Fourier transform infrared (FTIR) measurements of solar absorption spectra can provide ozone total columns with a precision of 2% but also independent partial column amounts in about four vertical layers, one in the troposphere and three in the stratosphere up to about 45km, with a precision of 5–6%. We use eight of the Network for the Detection of Atmospheric Composition Change (NDACC) stations having a long-term time series of FTIR ozone measurements to study the total and vertical ozone trends and variability, namely, Ny-Ålesund (79° N), Thule (77° N), Kiruna (68° N), Harestua (60° N), Jungfraujoch (47° N), Izaña (28° N), Wollongong (34° S) and Lauder (45° S). The length of the FTIR time series varies by station but is typically from about 1995 to present. We applied to the monthly means of the ozone total and four partial columns a stepwise multiple regression model including the following proxies: solar cycle, quasi-biennial oscillation (QBO), El Niño–Southern Oscillation (ENSO), Arctic and Antarctic Oscillation (AO/AAO), tropopause pressure (TP), equivalent latitude (EL), Eliassen–Palm flux (EPF), and volume of polar stratospheric clouds (VPSC). At the Arctic stations, the trends are found mostly negative in the troposphere and lower stratosphere, very mixed in the middle stratosphere, positive in the upper stratosphere due to a large increase in the 1995–2003 period, and non-significant when considering the total columns. The trends for mid-latitude and subtropical stations are all non-significant, except at Lauder in the troposphere and upper stratosphere and at Wollongong for the total columns and the lower and middle stratospheric columns where they are found positive. At Jungfraujoch, the upper stratospheric trend is close to significance (+0.9 ± 1.0% decade−1). Therefore, some signs of the onset of ozone mid-latitude recovery are observed only in the Southern Hemisphere, while a few more years seem to be needed to observe it at the northern mid-latitude station.

Ozone trends derived from the total column and vertical profiles at a northern mid-latitude station

Abstract. The trends and variability of ozone are assessed over a northern mid-latitude station, Haute-Provence Observatory (OHP: 43.93° N, 5.71° E), using total column ozone observations from the Dobson and Système d'Analyse par Observation Zénithale spectrometers, and stratospheric ozone profile measurements from light detection and ranging (lidar), ozonesondes, Stratospheric Aerosol and Gas Experiment (SAGE) II, Halogen Occultation Experiment (HALOE) and Aura Microwave Limb Sounder (MLS). A multivariate regression model with quasi-biennial oscillation (QBO), solar flux, aerosol optical thickness, heat flux, North Atlantic Oscillation (NAO) and a piecewise linear trend (PWLT) or equivalent effective stratospheric chlorine (EESC) functions is applied to the ozone anomalies. The maximum variability of ozone in winter/spring is explained by QBO and heat flux in the ranges 15–45 km and 15–24 km, respectively. The NAO shows maximum influence in the lower stratosphere during winter, while the solar flux influence is largest in the lower and middle stratosphere in summer. The total column ozone trends estimated from the PWLT and EESC functions are of −1.47 ± 0.27 and −1.40 ± 0.25 DU yr−1, respectively, over the period 1984–1996 and about 0.55 ± 0.30 and 0.42 ± 0.08 DU yr−1, respectively, over the period 1997–2010. The ozone profiles yield similar and significant EESC-based and PWLT trends for 1984–1996, and are about −0.5 and −0.8% yr−1 in the lower and upper stratosphere, respectively. For 1997–2010, the EESC-based and PWLT estimates are of the order of 0.3 and 0.1% yr−1, respectively, in the 18–28 km range, and at 40–45 km, EESC provides significant ozone trends larger than the insignificant PWLT results. Furthermore, very similar vertical trends for the respective time periods are also deduced from another long-term satellite-based data set (GOZCARDS–Global OZone Chemistry And Related trace gas Data records for the Stratosphere) sampled at northern mid-latitudes. Therefore, this analysis unveils ozone recovery signals from total column ozone and profile measurements at OHP, and hence in the northern mid-latitudes.

Coherence of long-term stratospheric ozone vertical distribution time series used for the study of ozone recovery at a northern mid-latitude station

Abstract. The coherence of stratospheric ozone time series retrieved from various observational records is investigated at Haute-Provence Observatory (OHP–43.93° N, 5.71° E). The analysis is accomplished through the intercomparison of collocated ozone measurements of Light Detection and Ranging (lidar) with Solar Backscatter UltraViolet(/2) (SBUV(/2)), Stratospheric Aerosol and Gas Experiment II (SAGE~II), Halogen Occultation Experiment (HALOE), Microwave Limb Sounder (MLS) on Upper Atmosphere Research Satellite (UARS) and Aura and Global Ozone Monitoring by Occultation of Stars (GOMOS) satellite observations as well as with in situ ozonesondes and ground-based Umkehr measurements performed at OHP. A detailed statistical study of the relative differences of ozone observations over the whole stratosphere is performed to detect any specific drift in the data. On average, all instruments show their best agreement with lidar at 20–40 km, where deviations are within ±5 %. Discrepancies are somewhat higher below 20 and above 40 km. The agreement with SAGE II data is remarkable since average differences are within ±1 % at 17–41 km. In contrast, Umkehr data underestimate systematically the lidar measurements in the whole stratosphere with a near zero bias at 16–8 hPa (~30 km). Drifts are estimated using simple linear regression for the data sets analysed in this study, from the monthly averaged difference time series. The derived values are less than ±0.5 % yr−1 in the 20–40 km altitude range and most drifts are not significant at the 2σ level. We also discuss the possibilities of extending the SAGE II and HALOE data with the GOMOS and Aura MLS data in consideration with relative offsets and drifts since the combination of such data sets are likely to be used for the study of stratospheric ozone recovery in the future.

Seasonal variability of middle‐latitude ozone in the lowermost stratosphere derived from probability distribution functions

We present a study of the distribution of ozone in the lowermost stratosphere with the goal of characterizing the observed variability. The air in the lowermost stratosphere is divided into two population groups based on Ertel's potential vorticity at 300 hPa. High (low) potential vorticity at 300 hPa indicates that the tropopause is low (high), and the identification of these two groups is made to account for the dynamic variability. Conditional probability distribution functions are used to define the statistics of the ozone distribution from both observations and a three‐dimensional model simulation using winds from the Goddard Earth Observing System Data Assimilation System for transport. Ozone data sets include ozonesonde observations from northern midlatitude stations (1991–1996) and midlatitude observations made by the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS) (1994–1998). The conditional probability distribution functions are calculated at a series of potential temperature surfaces spanning the domain from the midlatitude tropopause to surfaces higher than the mean tropical tropopause (∼380 K). The probability distribution functions are similar for the two data sources, despite differences in horizontal and vertical resolution and spatial and temporal sampling. Comparisons with the model demonstrate that the model maintains a mix of air in the lowermost stratosphere similar to the observations. The model also simulates a realistic annual cycle. Results show that during summer, much of the observed variability is explained by the height of the tropopause. During the winter and spring, when the tropopause fluctuations are larger, less of the variability is explained by tropopause height. This suggests that more mixing occurs during these seasons. During all seasons, there is a transition zone near the tropopause that contains air characteristic of both the troposphere and the stratosphere. The relevance of the results to the assessment of the environmental impact of aircraft effluence is also discussed.

Comparison of groud-based and TOMS-EP total ozone data for antarctica and Northern Midlatitude Stations (1996–1999)

A comparison of the total ozone data by the TOMS-EP satellite and ground based Dobson spectrophotometer (Ukrainian Antarctic Station ‘Akademik Vernadsky’, 65.25S, 64.27W), and M124 UV filter radiometer (Kyiv University Observatory ‘Lisniky’, 50.30N, 30.53E) measurements was made. Comparison of the TOMS-EP and Dobson daily total ozone values for Antarctic Peninsula gives a decrease of the mean relative difference from 3.1% in 1996/1997 to 1.4% in 1998/1999. Comparison of the TOMS-EP and M124 data gives a changing relative difference of near zero (from −0.4% to 0.11% in 1997–1999). Days with cloudless sky and with totally overcast sky were selected from the data sets. The mean relative difference of TOMS — Dobson data changed during 1996–1999 for cloudless days from 6% to 11% and for cloud cover from −0.5% to −1.3%. Northern midlatitude observations give a relative difference of TOMS — M124 data for cloudless sky of 0.15% and for totally overcast sky of −3.5%. This comparison shows that differences in cloud cover influence the total ozone values obtained from ground-based and satellite measurements in the regions considered.

Stratospheric ozone depletion at northern midlatitudes after major volcanic eruptions

During winter 1982/83 and in early 1985, abnormal ozone deficiencies in the stratosphere were recorded at northern midlatitude stations. Ozone variations measured by ozonesondes at Hohenpeissenberg are analysed with respect to dynamic transport variations related to the quasibiennial oscillation (QBO). After the elimination of seasonal fluctuations, long-term trends and average QBO related variations, ozone time series are compared to particle surface area densities derived from lidar measurements of the stratospheric aerosol layer at Garmisch-Partenkirchen. The two stations are only 37 km apart. The analysis reveals that the 1985 ozone minimum is a transport effect, as well as most of the 1982/83 minimum. But at 17 to 20 km in 1982/83, 30 to 40% of the ozone deficiency is most probably related to the aerosol perturbation of the lower stratosphere following the April 1982 eruption of the Mexican volcano El Chichon.

The change in Sq(H) amplitude on Abnormal Quiet Days

Summary. Analysis of geomagnetic data has shown that the superposed northward magnetic field, which reduces the Sq(H) amplitude at northern mid-latitude stations on Abnormal Quiet Days, and increases the amplitude at stations on the equatorward side of the Sq focus, builds up in amplitude over four to five days before the AQD occurs, and subsides over a similar period after the AQD. It is inferred indirectly that the azimuthal component By of the interplanetary magnetic field varies similarly. Data for the opposite meridian show that the imposed field reverses to a southward direction at lower latitudes. The inferred currents to account for these fields are believed to flow in the ionosphere, but to arise from magnetospheric electric fields induced by the solar wind-transported IMF.

Day-to-day changes in the F-region in summer

Continuous Faraday rotation measurements of VHF signals from the synchronous satellite Early Bird were made from a Northern mid-latitude station during two months in the summer of 1965. The experimental technique and method of calculating the total electron content ( n t ) are described, and the main types of variation of n t during the daytime are classified. Using a continuous 42-day time series of measurements, cross correlation functions of several parameters with magnetic activity indices are calculated. A significant positive correlation at zero time shift is found between noon n t and K p , marginally significant positive correlations between n t at 20 hr and K p , between the time displacement from sunset of the diurnal peak in n t and ∑ 24 K p , And between noon slab thickness and K p , are also found. The apparent linear loss coefficient at midnight shows no significant correlation either with K p or with noon n t on the following day. The integrated electron production rate at dawn shows some long term correlation with both the decimetric solar flux and the sunspot number. The correlation between noon n t and K p can be explained as a natural consequence of shifts in the time of occurrence of the evening peak on moderately disturbed days. This behaviour is not observed in the maximum electron density, as it is probably masked by increases in slab thickness. Horizontal transport of plasma in the upper F-region must be occurring, this could be driven either by the neutral air wind or by electric fields.

Middle‐Latitude Ionospheric Total Electron Content: Summer 1965

The total electron content of the ionosphere’ was measured continuously from a northern midlatitude station during the summer of 1965 by monitoring the polarization twist of VHF radio signals from the Early Bird synchronous satellite. The ambiguity in the total amount of polarization twist of the Early Bird signal was resolved by measurinj!; the total polarization twist of a lunar reflected radio wave at a time when the moon coincided in dirertion with Early Bird. Large day‐to‐day differences in electron content were seen. The late afternoon maximum value of electron content was not well related to magnetic activity. Many small, irregular changes in electron content were observed. Diurnal values of electron content were much lower than those observed during 1961.Observations of polarization twist of VHF radio waves from synchronous satellites provide a simple, accurate means of measuring diurnal, seasonal, and anomalous changes in ionospheric total electron content.