467 research outputs found
Simulation of Evapotranspiration at a 3-Minute Time Interval Based on Remote Sensing Data and SEBAL Model
Using remote sensing to estimate evapotranspiration minute frequency is the basis for accurately calculating hourly and daily evapotranspiration from the regional scale. However, from the existing research, it is difficult to use remote sensing data to estimate evapotranspiration minute frequency. This paper uses GF-4 and moderate-resolution imaging spectroradiometer (MODIS) data in conjunction with the Surface Energy Balance Algorithm for Land (SEBAL) model to estimate ET at a 3-min time interval in part of China and South Korea, and compares those simulation results with that from field measured data. According to the spatial distribution of ET derived from GF-4 and MODIS, the texture of ET derived from GF-4 is more obvious than that of MODIS, and GF-4 is able to express the variability of the spatial distribution of ET. Meanwhile, according to the value of ET derived from both GF-4 and MODIS, results from these two satellites have significant linear correlation, and ET derived from GF-4 is higher than that from MODIS. Since the temporal resolution of GF-4 is 3 min, the land surface ET at a 3-min time interval could be obtained by utilizing all available meteorological and remote sensing data, which avoids error associated with extrapolating instantaneously from a single image
Toward a Broadband Astro-comb: Effects of Nonlinear Spectral Broadening in Optical Fibers
We propose and analyze a new approach to generate a broadband astro-comb by
spectral broadening of a narrowband astro-comb inside a highly nonlinear
optical fiber. Numerical modeling shows that cascaded four-wave-mixing
dramatically degrades the input comb's side-mode suppression and causes
side-mode amplitude asymmetry. These two detrimental effects can systematically
shift the center-of-gravity of astro-comb spectral lines as measured by an
astrophysical spectrograph with resolution \approx100,000; and thus lead to
wavelength calibration inaccuracy and instability. Our simulations indicate
that this performance penalty, as a result of nonlinear spectral broadening,
can be compensated by using a filtering cavity configured for double-pass. As
an explicit example, we present a design based on an Yb-fiber source comb (with
1 GHz repetition rate) that is filtered by double-passing through a low finesse
cavity (finesse = 208), and subsequent spectrally broadened in a 2-cm,
SF6-glass photonic crystal fiber. Spanning more than 300 nm with 16 GHz line
spacing, the resulting astro-comb is predicted to provide 1 cm/s (~10 kHz)
radial velocity calibration accuracy for an astrophysical spectrograph. Such
extreme performance will be necessary for the search for and characterization
of Earth-like extra-solar planets, and in direct measurements of the change of
the rate of cosmological expansion.Comment: 9 pages, 6 figure
Surface Versus Bulk Dirac States Tuning in a Three-Dimensional Topological Dirac Semimetal
Recently, crystalline-symmetry-protected three-dimensional (3D) bulk Dirac
semimetal phase has been experimentally identified in a stoichiometric
high-mobility compound, Cd3As2. The Dirac state observed in Cd3As2 has been
attributed to originate mostly from the bulk state while calculations show that
the bulk and surface states overlap over the entire Dirac dispersion energy
range. In this study, we unambiguously reveal doping induced evolution of the
ground state of surface and bulk electron dynamics in a 3D Dirac semimetal. We
develop a systematic technique to isolate the surface and bulk states in
Cd3As2, by simultaneously utilizing angle-resolved photoemission spectroscopy
(ARPES) and in-situ surface deposition. Our experimental results provide a
method for tuning the chemical potential as well as to observe surface states
degenerate with bulk states, which will be useful for future applications of 3D
Dirac semimetal.Comment: 5 pages, 4 figure
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