Gap filling and noise reduction of unevenly sampled data by means of the Lomb-Scargle periodogram

International audienceThe Lomb-Scargle periodogram is widely used for the estimation of the power spectral density of unevenly sampled data. A small extension of the algorithm of the Lomb-Scargle periodogram permits the estimation of the phases of the spectral components. The amplitude and phase information is sufficient for the construction of a complex Fourier spectrum. The inverse Fourier transform can be applied to this Fourier spectrum and provides an evenly sampled series (Scargle, 1989). We are testing the proposed reconstruction method by means of artificial time series and real observations of mesospheric ozone, having data gaps and noise. For data gap filling and noise reduction, it is necessary to modify the Fourier spectrum before the inverse Fourier transform is done. The modification can be easily performed by selection of the relevant spectral components which are above a given confidence limit or within a certain frequency range. Examples with time series of lower mesospheric ozone show that the reconstruction method can reproduce steep ozone gradients around sunrise and sunset and superposed planetary wave-like oscillations observed by a ground-based microwave radiometer at Payerne

Influence of AFP Process Parameters on the Temperature Distribution Used for Thermal in-process Monitoring

Automated Fiber Placement is an important manufacturing process when it comes to aerospace part generation. To minimize the machine downtime, an online monitoring system is necessary to prevent time consuming visual inspection. Therefore, a new thermal in-process monitoring system is integrated. Herewith spotted temperature difference enable the localization of placed tows and detection of defects. The thermal contrast between subsurface and tows depends on different process parameters such as lay-up speed, tooling temperature and compaction pressure. This influence is analyzed and discussed within lay-up studies to gain further knowledge about the different effects to increase the reliability of in-process monitoring as well as AFP processes.Therm-O-PlanNiedersächsisches Vorab, Volkswagen Foundatio

Long-term observation of midlatitude quasi 2-day waves by a water vapor radiometer

A mesospheric water vapor data set obtained by the middle atmospheric water vapor radiometer (MIAWARA) close to Bern, Switzerland (46.88°&thinsp;N, 7.46°&thinsp;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&thinsp;hPa in winter and up to 0.05&thinsp;hPa in summer) accessible for the instrument. Single Q2DW events reach at most about 0.8&thinsp;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&thinsp;%) develop periods between 38 and 40&thinsp;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&thinsp;h.</p

Global sounding of F region irregularities by COSMIC during a geomagnetic storm

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 &lt;50&thinsp;km. Possibly the trajectory of the tangent point intersects spatial electron density irregularities in the magnetic polar region. For derivation of the morphology of the electron density and TEC fluctuations, a 50&thinsp;km high-pass filter is applied in the s domain, where s is the distance between a reference point (bottom tangent point) and the tangent point. For each profile, the mean of the fluctuations is calculated for tangent point altitudes between 400 and 500&thinsp;km. At first glance, the global maps of ΔNe and ΔTEC are quite similar. However, ΔTEC might be more reliable since it is based on fewer retrieval assumptions. We find a significant difference if the arithmetic mean or the median is applied to the global map of September 2013. In agreement with literature, ΔTEC is enhanced during the post-sunset rise of the equatorial ionosphere in September 2013, which is associated with spread F and equatorial plasma bubbles. The global map of ΔTEC at solar maximum (September 2013) has stronger fluctuations than those at solar minimum (September 2008). We obtained new results when we compare the global maps of the quiet phase and the storm phase of the geomagnetic storm of 15 July 2012. It is evident that the TEC fluctuations are increased and extended over the southern magnetic polar region at the day of the geomagnetic storm. The north–south asymmetry of the storm response is more pronounced in the upper ionosphere (ray tangent points h&thinsp;=&thinsp;400–500&thinsp;km) than in the lower ionosphere (ray tangent points h&thinsp;=&thinsp;200–300&thinsp;km).</p

Trend analysis of the 20-year time series of stratospheric ozone profiles observed by the GROMOS microwave radiometer at Bern

The ozone radiometer GROMOS (GROund-based Millimeter-wave Ozone Spectrometer) has been performing continuous observations of stratospheric ozone profiles since 1994 above Bern, Switzerland (46.95° N, 7.44° E, 577 m). GROMOS is part of the Network for the Detection of Atmospheric Composition Change (NDACC). From November 1994 to October 2011, the ozone line spectra were measured by a filter bench (FB). In July 2009, a fast Fourier transform spectrometer (FFTS) was added as a back end to GROMOS. The new FFTS and the original FB measured in parallel for over 2 years. The ozone profiles retrieved separately from the ozone line spectra of FB and FFTS agree within 5% at pressure levels from 30 to 0.5 hPa, from October 2009 to August 2011. A careful harmonisation of both time series has been carried out by taking the FFTS as the reference instrument for the FB. This enables us to assess the long-term trend derived from stratospheric ozone observations at Bern. The trend analysis was performed by using a robust multilinear parametric trend model which includes a linear term, the solar variability, the El Niño–Southern Oscillation (ENSO) index, the quasi-biennial oscillation (QBO), the annual and semi-annual oscillation and several harmonics with period lengths between 3 and 24 months. Over the last years, some experimental and modelling trend studies have shown that the stratospheric ozone trend is levelling off or even turning positive. With our observed ozone profiles, we are able to support this statement by reporting a statistically significant trend of +3.14% decade$^{-1}$ at 4.36 hPa (37.76 km), covering the period from January 1997 to January 2015, above Bern. Additionally, we have estimated a negative trend over this period of -3.94%decade$^{-1}$ at 0.2 hPa (59 km)

Significant decline of mesospheric water vapor at the NDACC site near Bern in the period 2007 to 2018

The middle atmospheric water vapor radiometer MIAWARA is located close to Bern in Zimmerwald (46.88∘&thinsp;N, 7.46∘&thinsp;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&thinsp;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&thinsp;ppm per decade is detected.</p