798 research outputs found
Bose-glass to Superfluid transition in the three-dimensional Bose-Hubbard Model
We present a Monte Carlo study of the Bose-glass to superfluid transition in
the three-dimensional Bose-Hubbard model. Simulations are performed on the
classical (3 + 1) dimensional link-current representation using the geometrical
worm algorithm. Finite-size scaling analysis (on lattices as large as
16x16x16x512 sites) of the superfluid stiffness and the compressibility is
consistent with a value of the dynamical critical exponent z = 3, in agreement
with existing scaling and renormalization group arguments that z = d. We find
also a value of for the correlation length exponent,
satisfying the relation . However, a detailed study of the
correlation functions, C(r, tau), at the quantum critical point are not
consistent with this value of z. We speculate that this discrepancy could be
due to the fact that the correlation functions have not reached their true
asymptotic behavior because of the relatively small spatial extent of the
lattices used in the present study.Comment: 9 pages, 8 figures, submitted to PR
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Zonal-mean dynamics of extended recoveries from stratospheric sudden warmings
The recovery of the Arctic polar vortex following stratospheric sudden warmings is found to take upward of 3 months in a particular subset of cases, termed here polar-night jet oscillation (PJO) events. The anomalous zonal-mean circulation above the pole during this recovery is characterized by a persistently warm lower stratosphere, and above this a cold midstratosphere and anomalously high stratopause, which descends as the event unfolds. Composites of these events in the Canadian Middle Atmosphere Model show the persistence of the lower-stratospheric anomaly is a result of strongly suppressed wave driving and weak radiative cooling at these heights. The upper-stratospheric and lower-mesospheric anomalies are driven immediately following the warming by anomalous planetary-scale eddies, following which, anomalous parameterized nonorographic and orographic gravity waves play an important role. These details are found to be robust for PJO events (as opposed to sudden warmings in general) in that many details of individual PJO events match the composite mean. Azonal-mean quasigeostrophic model on the sphere is shown to reproduce the response to the thermal and mechanical forcings produced during a PJO event. The former is well approximated by Newtonian cooling. The response can thus be considered as a transient approach to the steady-state, downward control limit. In this context, the time scale of the lower-stratospheric anomaly is determined by the transient, radiative response to the extended absence of wave driving. The extent to which the dynamics of the wave-driven descent of the stratopause can be considered analogous to the descending phases of the quasi-biennial oscillation (QBO) is also discussed
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Quantifying Eddy Feedbacks and Forcings in the Tropospheric Response to Stratospheric Sudden Warmings
This is the final version of the article. It first appeared from American Meteorological Society via https://doi.org/10.1175/JAS-D-16-0056.1Abstract
The equatorward shift of the zonal-mean midlatitude tropospheric jet following a stratospheric sudden warming in a comprehensive stratosphere-resolving model is found to be well quantified by the simple model of tropospheric eddy feedbacks proposed by Lorenz and Hartmann. This permits a decomposition of the shift into a component driven by the stratospheric anomalies and a component driven by tropospheric feedbacks.
This is done by extending the simple model to include three effective forcing mechanisms by which the stratosphere may influence the tropospheric jet. These include 1) the zonally symmetric adjustments associated with the mean meridional circulation and the direct influence of the stratospheric anomalies on 2) the tropospheric synoptic-scale or 3) the tropospheric planetary-scale eddies. Although the anomalous tropospheric winds are primarily maintained against surface friction by the synoptic-scale eddies, this response can be entirely attributed to the eddy feedback term. The response of the planetary-scale eddies, in contrast, can be directly attributed to the stratosphere. The zonally symmetric tropospheric circulation associated with downward control is found to play little role in driving the tropospheric response.
The prospects of applying this methodology to reanalysis data are also considered, but statistical limitations and the relatively weak projection of the vertically integrated composite wind anomalies onto the leading EOF preclude any conclusions from being drawn.PH acknowledges funding support from the European Research Council through the ACCI project (grant number 267760) lead by John Pyle and from an NSERC post-doctoral fellowship. IRS acknowledges support from National Science Foundation funding to the National Center for Atmospheric Research and NSF award AGS-1317469
Progranulin regulates neurogenesis in the developing vertebrate retina
We evaluated the expression and function of the microgliaāspecific growth factor, Progranulināa (Pgrnāa) during developmental neurogenesis in the embryonic retina of zebrafish. At 24 hpf pgrnāa is expressed throughout the forebrain, but by 48 hpf pgrnāa is exclusively expressed by microglia and/or microglial precursors within the brain and retina. Knockdown of Pgrnāa does not alter the onset of neurogenic programs or increase cell death, however, in its absence, neurogenesis is significantly delayedāretinal progenitors fail to exit the cell cycle at the appropriate developmental time and postmitotic cells do not acquire markers of terminal differentiation, and microglial precursors do not colonize the retina. Given the link between Progranulin and cell cycle regulation in peripheral tissues and transformed cells, we analyzed cell cycle kinetics among retinal progenitors following Pgrnāa knockdown. Depleting Pgrnāa results in a significant lengthening of the cell cycle. These data suggest that Pgrnāa plays a dual role during nervous system development by governing the rate at which progenitors progress through the cell cycle and attracting microglial progenitors into the embryonic brain and retina. Collectively, these data show that Pgrnāa governs neurogenesis by regulating cell cycle kinetics and the transition from proliferation to cell cycle exit and differentiation. Ā© 2017 The Authors. Developmental Neurobiology Published by Wiley Periodicals, Inc. Develop Neurobiol 77: 1114ā1129, 2017Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138360/1/dneu22499.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/138360/2/dneu22499_am.pd
Dynamic expression of the basic helix-loop-helix transcription factor neuroD in the rod and cone photoreceptor lineages in the retina of the embryonic and larval zebrafish
NeuroD is a basic helix-loop-helix (bHLH) transcription factor critical for determining neuronal cell fate and regulating withdrawal from the cell cycle. We showed previously that, in goldfish, neuroD is expressed in the rod photoreceptor lineage, and we inferred that neuroD is also expressed in a subset of amacrine cells and nascent cone photoreceptors. Here we extended that study by examining the temporal and spatial expression pattern of neuroD in the embryonic and larval zebrafish and by identifying the cell types that express this gene. NeuroD expression in the developing zebrafish retina is dynamic, spanning early retinogenesis and the maturation of cone photoreceptors. In early retinogenesis neuroD expression expands from a small patch in the ventronasal retina, through the remaining retinal neuroepithelium. As retinogenesis progresses, neuroD expression becomes restricted to amacrine cells, immature cones, and cells of rod and cone lineages. This expression achieves an adult pattern by 96 hours postfertilization (hpf), whereupon the temporal pattern of neuroD expression in central retina is spatially recapitulated at the germinative margin. The cellular pattern of expression suggests that neuroD regulates aspects of rod and cone genesis, but through separate cellular lineages. Furthermore, neuroD is coexpressed with the cone-rod-homeobox transcription factor (Crx) in putative cone progenitors and nascent cone photoreceptors, suggesting that, in the zebrafish retina, as in other vertebrate retinas, similar genetic cascades regulate photoreceptor genesis and maturation. J. Comp. Neurol. 501:1ā12, 2007. Ā© 2007 Wiley-Liss, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/55874/1/21150_ftp.pd
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