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101.
Howard and Harvey (1970) analyzed Mt. Wilson Doppler shifts to obtain a daily measure of the Sun's differential rotation. The data were fitted to give an angular velocity of the form = a + b sin2
B + c sin4
B (B = heliographic latitude). Changes in a, b, c were found to be correlated (Howard and Harvey, 1970). Yoshimura (1972) used the anticorrelation of the b and c parameters to infer the existence of large-scale convection. Wolff (1975) used the b-c anticorrelation and a weak correlation between a and b to infer that variations of the Sun's polar and equatorial rotation rates are anticorrelated. In this paper, the anticorrelation of b and c is shown to be due to numerical coupling. 相似文献
102.
Radiative transfer corrections for accurate spectroscopic measurements of volcanic gas emissions 总被引:2,自引:1,他引:1
Christoph Kern Tim Deutschmann Leif Vogel Markus Wöhrbach Thomas Wagner Ulrich Platt 《Bulletin of Volcanology》2010,72(2):233-247
There is widespread use of passive remote sensing techniques to quantify trace gas column densities in volcanic plumes utilizing
scattered sunlight as a light source. Examples include passive DOAS, COSPEC, and the SO2 camera. In order to calculate trace gas concentrations or volcanic emission fluxes, knowledge about the optical path through
the plume is necessary. In the past, a straight photon path through the plume has always been assumed although it was known
that this is not always true. Here we present the results of model studies conducted specifically to quantify the effects
of realistic radiative transfer in and around volcanic plumes on ground-based remote sensing measurements of SO2. The results show that measurements conducted without additional information on average photon paths can be inaccurate under
certain conditions, with possible errors spanning more than an order of magnitude. Both over and underestimation of the true
column density can occur. Actual errors depend on parameters such as distance between instrument and plume, plume SO2 concentration, plume aerosol load, as well as aerosol conditions in the ambient atmosphere. As an example, a measurement
conducted with an SO2 camera is discussed, the results of which can only be correctly interpreted if realistic radiative transfer is considered.
Finally, a method is presented which for the first time allows the retrieval of actual average photon paths in spectroscopic
(i.e. DOAS) measurements of adequate resolution. By allowing for a wavelength dependent column density during the evaluation
of DOAS measurements, we show how radiative transfer effects can be corrected using information inherently available in the
measured spectra, thus greatly enhancing the accuracy of DOAS measurements of volcanic emissions. 相似文献
103.
Christoph Kern Holger Sihler Leif Vogel Claudia Rivera Martha Herrera Ulrich Platt 《Bulletin of Volcanology》2009,71(6):659-670
Active Long Path Differential Optical Absorption Spectroscopy (LP-DOAS) measurements of halogen oxides were conducted at Masaya
Volcano, in Nicaragua from April 14 to 26, 2007. The active LP-DOAS system allowed night-time halogen measurements and reduced
the ClO detection limit by an order of magnitude when compared to previous passive DOAS measurements, as wavelengths below
300 nm could be used for the DOAS retrievals. BrO was detected with an average BrO/SO2 molecular ratio of approximately 3 × 10−5 during the day. However, BrO values were below the detection limit of the instrument for all night-time measurements, a strong
indication that BrO is not directly emitted, but rather the result of photochemical formation in the plume itself according
to the autocatalytic “bromine explosion” mechanism. Despite the increased sensitivity, both ClO and OClO could not be detected.
The achieved upper limits for the X/SO2 ratios were 5 × 10−3 and 7 × 10−6, respectively. A rough calculation suggests that ClO and OClO should be present at similar abundances in volcanic plumes.
Since the DOAS technique is orders of magnitude more sensitive for OClO than for ClO, this indicates that OClO should always
be detectable in plumes in which ClO is found. However, further LP-DOAS studies are needed to conclusively clarify the role
of chlorine oxides in volcanic plumes. 相似文献
104.
Michael S. Zhdanov Masashi Endo Leif H. Cox Martin Čuma Johnathan Linfoot Chris Anderson Noel Black Alexander V. Gribenko 《Geophysical Prospecting》2014,62(3):552-572
A towed streamer electromagnetic system capable of simultaneous seismic and electromagnetic data acquisition has recently been developed and tested in the North Sea. We introduce a 3D inversion methodology for towed streamer electromagnetic data that includes a moving sensitivity domain. Our implementation is based on the 3D integral equation method for computing responses and Fréchet derivatives and uses the re‐weighted regularized conjugate gradient method for minimizing the objective functional with focusing regularization. We present two model studies relevant to hydrocarbon exploration in the North Sea. First, we demonstrate the ability of a towed electromagnetic system to detect and characterize the Harding field, a medium‐sized North Sea hydrocarbon target. We compare our 3D inversion of towed streamer electromagnetic data with 3D inversion of conventional marine controlled‐source electromagnetic data and observe few differences between the recovered models. Second, we demonstrate the ability of a towed streamer electromagnetic system to detect and characterize the Peon discovery, which is representative of an infrastructure‐led shallow gas play in the North Sea. We also present an actual case study for the 3D inversion of towed streamer electromagnetic data from the Troll field in the North Sea and demonstrate our ability to image all the Troll West Oil and Gas Provinces and the Troll East Gas Province. We conclude that 3D inversion of data from the current generation of towed streamer electromagnetic systems can adequately recover hydrocarbon‐bearing formations to depths of approximately 2 km. We note that by obviating the need for ocean‐bottom receivers, the towed streamer electromagnetic system enables electromagnetic data to be acquired over very large areas in frontier and mature basins for higher acquisition rates and relatively lower cost than conventional marine controlled‐source electromagnetic methods. 相似文献
105.
On the modifications of near-inertial waves at fronts: implications for energy transfer across scales 总被引:1,自引:1,他引:0
Leif N. Thomas 《Ocean Dynamics》2017,67(10):1335-1350
In the ocean, wind-generated kinetic energy (KE) manifests itself primarily in balanced currents and near-inertial waves. The dynamics of these flows is strongly constrained by the Earth’s rotation, causing the KE in balanced currents to follow an inverse cascade but also preventing wave-wave interactions from fluxing energy in the near-inertial band to lower frequencies and higher vertical wavenumbers. How wind-generated KE is transferred to small-scale turbulence and dissipated is thus a non-trivial problem. This article presents a review of recent theoretical calculations and numerical simulations that demonstrate how some surprising modifications to internal wave physics by the lateral density gradients present at ocean fronts allow for strong interactions between balanced currents and near-inertial waves that ultimately result in energy loss for both types of motion. 相似文献
106.
107.
Based on seismic profiles, multibeam bathymetry and sediment cores, an improved understanding of the deglaciation/postglacial history of the southern part of the Norwegian Channel has been obtained. The Norwegian Channel Ice Stream started to recede from the shelf edge ca. 15.5 ka BP (14C ages are used throughout). Approximately 500–1000 years later the ice margin was located east of the deep Skagerrak trough. At that time, the Norwegian Channel off southern Norway had become a large fjord-like embayment, surrounded by the grounded ice sheet along the northern slope and possibly stagnant ice remnants at the southern flank. The Norwegian Channel off southern Norway has been the main sediment trap of the North Sea, and south of Egersund more than 200 m of sediments have been deposited since the start of the deglaciation. Five seismic units are mapped. The oldest unit E occurs in some of the deepest troughs, and was deposited immediately after the ice became buoyant. Unit D is acoustically massive and comprises mass-movement deposits in eastern Skagerrak and south of Egersund. Unit C (in the channel southwest of Lista/Egersund) is interpreted to comprise mainly bottom current deposits derived from palaeo-rivers, e.g. Elben. During deposition of unit C (ca. 14.5–13 ka BP), there was limited inflow of Atlantic water. A change in depositional environment at ca. 13 ka BP is related to an increased inflow of saline water and more open hydrographic circulation. Widely distributed, acoustically stratified clays of unit B were deposited ca. 13–10 ka BP. The Holocene Unit A shows a depositional pattern broadly similar to that of unit B. 相似文献
108.
Patterns in community structure and functioning of motile epibenthic fauna were investigated in shallow (0–1 m) sediment habitats along the Skagerrak–Baltic estuarine gradient (salinity range from 4 to 34). The study area was divided into five regions, reflecting different sea-basins along the 1260 km coastline, and fauna was collected at six sites within each region. Ten replicate samples of motile epibenthic fauna were taken randomly at each site with a portable drop trap (bottom area 1 m2) in June and September in 2004. 相似文献
109.
Leif Kahl Kristensen 《Celestial Mechanics and Dynamical Astronomy》2009,105(4):275-287
Initial orbit determination by least squares of N observations is essentially a linear problem if the coordinates x
0 and x
1 at two standard epochs are used as elements. The orbit of a main belt object is approximated within the observational errors
by a third degree polynomial during a month. A 4-observation orbit is useful for the initial linking between two nights. Parallax
is treated rigorously and future simultaneous space and Earth based observations determine the critical distance directly.
The N-observation method is a great simplification of the classical 3-observation orbit followed by a differential correction by
N observations. 相似文献
110.