523 research outputs found
An examination of the precipitation delivery mechanisms for Dolleman Island, eastern Antarctic Peninsula
Copyright @ 2004 Wiley-BlackwellThe variability of size and source of significant precipitation events were studied at an Antarctic ice core drilling site: Dolleman Island (DI), located on the eastern coast of the Antarctic Peninsula. Significant precipitation events that occur at DI were temporally located in the European Centre for Medium-Range Weather Forecasting (ECMWF) reanalysis data set, ERA-40. The annual and summer precipitation totals from ERA-40 at DI both show significant increases over the reanalysis period. Three-dimensional backwards air parcel trajectories were then run for 5 d using the ECMWF ERA-15 wind fields. Cluster analyses were performed on two sets of these backwards trajectories: all days in the range 1979–1992 (the climatological time-scale) and a subset of days when a significant precipitation event occurred. The principal air mass sources and delivery mechanisms were found to be the Weddell Sea via lee cyclogenesis, the South Atlantic when there was a weak circumpolar trough (CPT) and the South Pacific when the CPT was deep. The occurrence of precipitation bearing air masses arriving via a strong CPT was found to have a significant correlation with the southern annular mode (SAM); however, the arrival of air masses from the same region over the climatological time-scale showed no such correlation. Despite the dominance in both groups of back trajectories of the westerly circulation around Antarctica, some other key patterns were identified. Most notably there was a higher frequency of lee cyclogenesis events in the significant precipitation trajectories compared to the climatological time-scale. There was also a tendency for precipitation trajectories to come from more northerly latitudes, mostly from 50–70°S. The El Niño Southern Oscillation (ENSO) was found to have a strong influence on the mechanism by which the precipitation was delivered; the frequency of occurrence of precipitation from the east (west) of DI increased during El Niño (La Niña) events
Foehn winds in the McMurdo Dry Valleys, Antarctic: The origin of extreme warming events
Foehn winds resulting from topographic modification of airflow in the lee of mountain barriers are frequently experienced in the McMurdo Dry Valleys (MDVs) of Antarctica. Strong foehn winds in the MDVs cause dramatic warming at onset and have significant effects on landscape forming processes; however, no detailed scientific investigation of foehn in the MDVs has been conducted. As a result, they are often misinterpreted as adiabatically warmed katabatic winds draining from the polar plateau. Herein observations from surface weather stations and numerical model output from the Antarctic Mesoscale Prediction System (AMPS) during foehn events in the MDVs are presented. Results show that foehn winds in the MDVs are caused by topographic modification of south-southwesterly airflow, which is channeled into the valleys from higher levels. Modeling of a winter foehn event identifies mountain wave activity similar to that associated with midlatitude foehn winds. These events are found to be caused by strong pressure gradients over the mountain ranges of the MDVs related to synoptic-scale cyclones positioned off the coast of Marie Byrd Land. Analysis of meteorological records for 2006 and 2007 finds an increase of 10% in the frequency of foehn events in 2007 compared to 2006, which corresponds to stronger pressure gradients in the Ross Sea region. It is postulated that the intra- and interannual frequency and intensity of foehn events in the MDVs may therefore vary in response to the position and frequency of cyclones in the Ross Sea region
Design and operation of a prototype interaction point beam collision feedback system for the International Linear Collider
A high-resolution, intratrain position feedback system has been developed to
achieve and maintain collisions at the proposed future electron-positron
International Linear Collider (ILC). A prototype has been commissioned and
tested with a beam in the extraction line of the Accelerator Test Facility at
the High Energy Accelerator Research Organization in Japan. It consists of a
stripline beam position monitor (BPM) with analogue signal-processing
electronics, a custom digital board to perform the feedback calculation, and a
stripline kicker driven by a high-current amplifier. The closed-loop feedback
latency is 148 ns. For a three-bunch train with 154 ns bunch spacing, the
feedback system has been used to stabilize the third bunch to 450 nm. The
kicker response is linear, and the feedback performance is maintained, over a
correction range of over 60 {\mu}m. The propagation of the correction has
been confirmed by using an independent stripline BPM located downstream of the
feedback system. The system has been demonstrated to meet the BPM resolution,
beam kick, and latency requirements for the ILC
Adventures of the Coupled Yang-Mills Oscillators: I. Semiclassical Expansion
We study the quantum mechanical motion in the potentials with
, which arise in the spatially homogeneous limit of the Yang-Mills (YM)
equations. These systems show strong stochasticity in the classical limit
() and exhibit a quantum mechanical confinement feature. We
calculate the partition function going beyond the Thomas-Fermi (TF)
approximation by means of the semiclassical expansion using the Wigner-Kirkwood
(WK) method. We derive a novel compact form of the differential equation for
the WK function. After separating the motion in the channels of the
equipotential surface from the motion in the central region, we show that the
leading higher-order corrections to the TF term vanish up to eighth order in
, if we treat the quantum motion in the hyperbolic channels correctly by
adiabatic separation of the degrees of freedom. Finally, we obtain an
asymptotic expansion of the partition function in terms of the parameter
The Arctic summer atmosphere: an evaluation of reanalyses using ASCOS data
The Arctic has experienced large climate changes over recent decades, the
largest for any region on Earth. To understand the underlying reasons for
this climate sensitivity, reanalysis is an invaluable tool. The Arctic
System Reanalysis (ASR) is a regional reanalysis, forced by ERA-Interim at
the lateral boundaries and incorporating model physics adapted to Arctic
conditions, developed to serve as a state-of-the-art, high-resolution
synthesis tool for assessing Arctic climate variability and monitoring
Arctic climate change.
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We use data from Arctic Summer Cloud-Ocean Study (ASCOS) to evaluate the
performance of ASR and ERA-Interim for the Arctic Ocean. The ASCOS field
experiment was deployed on the Swedish icebreaker <i>Oden</i> north of
87° N in the Atlantic sector of the Arctic during August and early
September 2008. Data were collected during the transits from and to
Longyearbyen and the 3-week ice drift with <i>Oden</i> moored to a drifting
multiyear ice floe. These data are independent and detailed enough to
evaluate process descriptions.
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The reanalyses captures basic meteorological variations coupled to the
synoptic-scale systems, but have difficulties in estimating clouds and
atmospheric moisture. While ERA-Interim has a systematic warm bias in the
lowest troposphere, ASR has a cold bias of about the same magnitude on
average. The results also indicate that more sophisticated descriptions of
cloud microphysics in ASR did not significantly improve the modeling of
cloud properties compared to ERA-Interim. This has consequences for the
radiation balance, and hence the surface temperature, and illustrate how a
modeling problem in one aspect of the atmosphere, here the clouds, feeds
back to other parameters, especially near the surface and in the boundary
layer
Scattering From a Two Dimensional Array of Flux Tubes: A Study of The Validity of Mean Field Theory
Mean Field Theory has been extensively used in the study of systems of anyons
in two spatial dimensions. In this paper we study the physical grounds for the
validity of this approximation by considering the Quantum Mechanical scattering
of a charged particle from a two dimensional array of magnetic flux tubes. The
flux tubes are arranged on a regular lattice which is infinitely long in the
``'' direction but which has a (small) finite number of columns in the
``'' direction. Their physical size is assumed to be infinitesimally small.
We develop a method for computing the scattering angle as well as the
reflection and transmission coefficients to lowest order in the Aharonov--Bohm
interaction. The results of our calculation are compared to the scattering of
the same particle from a region of constant magnetic field whose magnitude is
equal to the mean field of all the flux tubes. For an incident plane wave, the
Mean Field approximation is shown to be valid provided the flux in each tube is
much less than a single flux quantum. This is precisely the regime in which
Mean Field Theory for anyons is expected to be valid. When the flux per tube
becomes of order 1, Mean Field Theory is no longer valid.Comment: 23 pages, University of British Columbia Preprint UBCTP93-01
Implementation of the Combined--Nonlinear Condensation Transformation
We discuss several applications of the recently proposed combined
nonlinear-condensation transformation (CNCT) for the evaluation of slowly
convergent, nonalternating series. These include certain statistical
distributions which are of importance in linguistics, statistical-mechanics
theory, and biophysics (statistical analysis of DNA sequences). We also discuss
applications of the transformation in experimental mathematics, and we briefly
expand on further applications in theoretical physics. Finally, we discuss a
related Mathematica program for the computation of Lerch's transcendent.Comment: 23 pages, 1 table, 1 figure (Comput. Phys. Commun., in press
The relationship between T-lymphocyte infiltration, stage, tumour grade and survival in patients undergoing curative surgery for renal cell cancer
The present study examined the relationship between tumour stage, grade, T-lymphocyte subset infiltration and survival in patients who had undergone potentially curative surgery for renal clear-cell cancer (n=73). Intratumoural CD4+ T-lymphocyte infiltrate was associated with poor cancer-specific survival, independent of grade, in this cohort
Spherical Universe topology and the Casimir effect
The mode problem on the factored 3--sphere is applied to field theory
calculations for massless fields of spin 0, 1/2 and 1. The degeneracies on the
factors, including lens spaces, are neatly derived in a geometric fashion.
Vacuum energies are expressed in terms of the polyhedral degrees and equivalent
expressions given using the cyclic decomposition of the covering group. Scalar
functional determinants are calculated and the spectral asymmetry function
treated by the same approach with explicit forms on one-sided lens spaces.Comment: 33 pages, 1 figure. Typos corrected and one reference adde
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