12,375 research outputs found
The spectral signature of recent climate change
Spectrally resolved measurements of the Earth’s reflected shortwave (RSW) and outgoing longwave radiation (OLR) at the top of the atmosphere intrinsically contain the imprints of a multitude of climate relevant parameters. Here, we review the progress made in directly using such observations to diagnose and attribute change within the Earth system over the past four decades. We show how changes associated with perturbations such as increasing greenhouse gases are expected to be manifested across the spectrum and illustrate the enhanced discriminatory power that spectral resolution provides over broadband radiation measurements. Advances in formal detection and attribution techniques and in the design of climate model evaluation exercises employing spectrally resolved data are highlighted. We illustrate how spectral observations have been used to provide insight into key climate feedback processes and quantify multi-year variability but also indicate potential barriers to further progress. Suggestions for future research priorities in this area are provided
Spectral signature of short attosecond pulse trains
We report experimental measurements of high-order harmonic spectra generated
in Ar using a carrier-envelope-offset (CEO) stabilized 12 fs, 800nm laser field
and a fraction (less than 10%) of its second harmonic. Additional spectral
peaks are observed between the harmonic peaks, which are due to interferences
between multiple pulses in the train. The position of these peaks varies with
the CEO and their number is directly related to the number of pulses in the
train. An analytical model, as well as numerical simulations, support our
interpretation
Determining the Spectral Signature of Spatial Coherent Structures
We applied to an open flow a proper orthogonal decomposition (pod) technique,
on 2D snapshots of the instantaneous velocity field, to reveal the spatial
coherent structures responsible of the self-sustained oscillations observed in
the spectral distribution of time series. We applied the technique to 2D planes
out of 3D direct numerical simulations on an open cavity flow. The process can
easily be implemented on usual personal computers, and might bring deep
insights on the relation between spatial events and temporal signature in (both
numerical or experimental) open flows.Comment: 4 page
An efficient methodology to simulate mixed spectral signatures of land covers through Field Radiometry data
An efficient methodology to simulate mixed spectral signatures of land covers, from endmember data, using linear statistical modelling based on the least squares estimation approach, is proposed. The optimal set of endmember has been obtained by measurements in situ with a field spectroradiometer GER 1500. Also, it is proposed the use of new sub-pixel methods based on statistics and certain “units of sampling” to apply to the landscapes. The resultant point estimations for these new units will be the “observations” and all of them will carry out an special role to simulate the final spectral signature. This methodology is used to simulate spectral signatures of a Mediterranean forest landscape near to Madrid (Spain). Furthermore the spectral signature model obtained through Field Radiometry data will be correlated with the image data of the same zone provided by the Landsat 7 Enhaced Thematic Mapper Plus (ETM+) sensor once corrected. The results obtained in correlation studies seem to conclude its efficiency. At the same time, the results open new research guidelines
The 10 micron amorphous silicate feature of fractal aggregates and compact particles with complex shapes
We model the 10 micron absorption spectra of nonspherical particles composed
of amorphous silicate. We consider two classes of particles, compact ones and
fractal aggregates composed of homogeneous spheres. For the compact particles
we consider Gaussian random spheres with various degrees of non-sphericity. For
the fractal aggregates we compute the absorption spectra for various fractal
dimensions. The 10 micron spectra are computed for ensembles of these particles
in random orientation using the well-known Discrete Dipole Approximation. We
compare our results to spectra obtained when using volume equivalent
homogeneous spheres and to those computed using a porous sphere approximation.
We conclude that, in general, nonspherical particles show a spectral signature
that is similar to that of homogeneous spheres with a smaller material volume.
This effect is overestimated when approximating the particles by porous spheres
with the same volume filling fraction. For aggregates with fractal dimensions
typically predicted for cosmic dust, we show that the spectral signature
characteristic of very small homogeneous spheres (with a volume equivalent
radius r_V<0.5 micron) can be detected even in very large particles. We
conclude that particle sizes are underestimated when using homogeneous spheres
to model the emission spectra of astronomical sources. In contrast, the
particle sizes are severely overestimated when using equivalent porous spheres
to fit observations of 10 micron silicate emission.Comment: Accepted for publication in A&
Visible and near infrared spectroscopy of Hayabusa re-entry using semi-autonomous tracking
A ground-based tracking camera and co-aligned slit-less spectrograph were used to measure the spectral signature of visible radiation emitted from the Hayabusa capsule as it entered into the Earth's atmosphere in June 2010. Good quality spectra were obtained that showed the presence of radiation from the heat shield of the vehicle and the shock-heated air in front of the vehicle. An analysis of the black body nature of the radiation concluded that the peak average temperature of the surface was about (3100±100) K
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