471 research outputs found
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
Retrieval of tropospheric water vapour by using spectra of a 22 GHz radiometer
In this paper, we present an approach to retrieve tropospheric water vapour profiles from pressure broadened emission spectra at 22 GHz, measured by a ground based microwave radiometer installed in the south of Bern at 905 m. <br><br> Classical microwave instruments concentrating on the troposphere observe several channels in the center and the wings of the water vapour line (20–30 Ghz), whereas our retrieval approach uses spectra with a bandwidth of 1 GHz and a high resolution around the center of the 22 GHz water vapour line. <br><br> The retrieval is sensitive up to 7 km with a vertical resolution of 3–5 km. Comparisons with profiles from operational balloon soundings, performed at Payerne, 40 km away from the radiometer location, showed a good agreement up to 7 km with a correlation of above 0.8. The retrievals shows a wet bias of 10–20% compared to the sounding
An integrated approach toward the incorporation of clouds in the temperature retrievals from microwave measurements
In this paper, we address the characterization of clouds and its inclusion in
microwave retrievals in order to study its effect on tropospheric temperature
profiles measured by TEMPERA radiometer. TEMPERA is the first ground-based
microwave radiometer that makes it possible to obtain temperature profiles in the
troposphere and stratosphere at the same time. In order to characterize the
clouds a multi-instrumental approach has been adopted. Cloud base altitudes
were detected using ceilometer measurements while the integrated liquid water
was measured by TROWARA radiometer. Both instruments are co-located with
TEMPERA in Bern (Switzerland). Using this information and a constant Liquid
Water Content value inside the cloud a liquid profile is provided to
characterize the clouds in the inversion algorithm. Microwave temperature
profiles have been obtained incorporating this water liquid profile in the
inversion algorithm and also without considering the clouds, in order to
assess its effect on the retrievals. The results have been compared with the
temperature profiles from radiosondes which are launched twice a day at the
aerological station of MeteoSwiss in Payerne (40 km W of Bern). Almost 1
year of data have been analysed and 60 non-precipitating cloud cases were
studied. The statistical analysis carried out over all the cases evidenced
that temperature retrievals improved in most of the cases when clouds were
incorporated in the inversion algorithm
Statistical-Thermal Model Calculations using THERMUS
Selected results obtained using THERMUS, a newly-developed
statistical-thermal model analysis package, are presented.Comment: Contributed to 8th International Conference on Strangeness in Quark
Matter, Cape Town, South Africa, 15-20 September 200
Contribution of the massive photon decay channel to the neutrino cooling of neutron stars
The role of the massive photon decay via intermediate states of electron-electron-holes and proton-proton-holes into neutrino-anti-neutrino pairs in the course of neutron star cooling is investigated. These reactions may ba operative in hot neutron stars in the region of proton pairing. The corresponding contribution to the neutrino emissivity is calculated. It varies with the temperature as T3/2e−mγ/T for T < m γ, where mγ is an effective photon mass in superconducting matter. Estimates show that this process appears as strong cooling channel of neutron stars at temperatures T ≈ ( 10^9 - 10^10) K
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° 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 7Â 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.</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 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 at 0.2 hPa (59 km)
Non-Linear Compton Scattering of Ultrashort and Ultraintense Laser Pulses
The scattering of temporally shaped intense laser pulses off electrons is
discussed by means of manifestly covariant quantum electrodynamics. We employ a
framework based on Volkov states with a time dependent laser envelope in
light-cone coordinates within the Furry picture. An expression for the cross
section is constructed, which is independent of the considered pulse shape and
pulse length. A broad distribution of scatted photons with a rich pattern of
subpeaks like that obtained in Thomson scattering is found. These broad peaks
may overlap at sufficiently high laser intensity, rendering inappropriate the
notion of individual harmonics. The limit of monochromatic plane waves as well
as the classical limit of Thomson scattering are discussed. As a main result, a
scaling law is presented connecting the Thomson limit with the general result
for arbitrary kinematics. In the overlapping regions of the spectral density,
the classical and quantum calculations give different results, even in the
Thomson limit. Thus, a phase space region is identified where the differential
photon distribution is strongly modified by quantum effects.Comment: 31 pages, 10 figure
Centrality Dependence of Thermal Parameters Deduced from Hadron Multiplicities in Au + Au Collisions at sqrt{s_{NN}} = 130 GeV
We analyse the centrality dependence of thermal parameters deduced from
hadron m ultiplicities in Au + Au collisions at .
While the chemical freeze-out temperature and chemical potentials are found to
be roughly centrality-independent, the strangeness saturation factor
increases with participant number towards unity, supporting the assumption of
equilibrium freeze-out conditions in central collisions
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