752 research outputs found
State-dependence of climate sensitivity: attractor constraints and palaeoclimate regimes
Equilibrium climate sensitivity (ECS) is a key predictor of climate change.
However, it is not very well constrained, either by climate models or by
observational data. The reasons for this include strong internal variability
and forcing on many time scales. In practise this means that the 'equilibrium'
will only be relative to fixing the slow feedback processes before comparing
palaeoclimate sensitivity estimates with estimates from model simulations. In
addition, information from the late Pleistocene ice age cycles indicates that
the climate cycles between cold and warm regimes, and the climate sensitivity
varies considerably between regime because of fast feedback processes changing
relative strength and time scales over one cycle.
In this paper we consider climate sensitivity for quite general climate
dynamics. Using a conceptual Earth system model of Gildor and Tziperman (2001)
(with Milankovich forcing and dynamical ocean biogeochemistry) we explore
various ways of quantifying the state-dependence of climate sensitivity from
unperturbed and perturbed model time series. Even without considering any
perturbations, we suggest that climate sensitivity can be usefully thought of
as a distribution that quantifies variability within the 'climate attractor'
and where there is a strong dependence on climate state and more specificially
on the 'climate regime' where fast processes are approximately in equilibrium.
We also consider perturbations by instantaneous doubling of CO and
similarly find a strong dependence on the climate state using our approach.Comment: 32 pages, 10 figure
Extreme sensitivity and climate tipping points
A climate state close to a tipping point will have a degenerate linear
response to perturbations, which can be associated with extreme values of the
equilibrium climate sensitivity (ECS). In this paper we contrast linearized
(`instantaneous') with fully nonlinear geometric (`two-point') notions of ECS,
in both presence and absence of tipping points. For a stochastic energy balance
model of the global mean surface temperature with two stable regimes, we
confirm that tipping events cause the appearance of extremes in both notions of
ECS. Moreover, multiple regimes with different mean sensitivities are visible
in the two-point ECS. We confirm some of our findings in a physics-based
multi-box model of the climate system.Comment: 11 figure
Response maxima in modulated turbulence
Isotropic and homogeneous turbulence driven by an energy input modulated in
time is studied within a variable range mean-field theory. The response of the
system, observed in the second order moment of the large-scale velocity
difference D(L,t)=>~Re(t)^2$, is calculated for varying
modulation frequencies w and weak modulation amplitudes. For low frequencies
the system follows the modulation of the driving with almost constant
amplitude, whereas for higher driving frequencies the amplitude of the response
decreases on average 1/w. In addition, at certain frequencies the amplitude of
the response either almost vanishes or is strongly enhanced. These frequencies
are connected with the frequency scale of the energy cascade and multiples
thereof.Comment: 11 pages, 6 figure
High continuous bandwidth multichannel acquisition system
Multichannel data acquisition has been a keystone of 7 ONR sponsored Arctic acoustic research programs conducted jointly by
WHOI and MIT investigators from 1978 through 1989. This report describes the status and capability of the most recent system
developed at WHOI for the purpose of acquiring digital data from up to 64 channels at sampling rates up to 20 kH per channel
with data bandwidth to 5120 Hz. ONR funded the development of and use of this system and its prototye for 2 Arctic field
experiments, PRUDEX 87 and CEAREX 89. It was most recently use during the Heard Island Feasibility Experiment in
February 1991. Of note are the auto-gain ranging capabilty offering a dynamic measurement range of greater than 120 dB, the
continuous storage capability of up to 200,000 samples per second to a Small Computer System Interface (SCSI) device, typically
optical disk, and easy expandability with additional identical chanels connected in parallel.Funding was provided by the Office of Naval Research under Contract No. N00014-91-J-1296
Ripple Effects: The Unintended Change to Jurisdictional Pleading Standards After Iqbal
This Note describes a little-observed ripple effect of the new pleading standard announced in Iqbal, the antiterrorism case whose holding swept broadly and changed the ground rules for considering allegations in so-called 12(b)(6) motions for all civil cases. This Note examines the interplay between the Twombly/Iqbal doctrine and federal courts’ practical approach to subject-matter jurisdiction. Part II describes the background jurisprudence on subject-matter jurisdiction, including the sharp line the Supreme Court has consistently re-drawn between claims lacking merit and those lacking jurisdictional basis, from Bell v. Hood through Arbaugh v. Y & H Corp. The consistent theme of this jurisprudence is that courts should not conflate merits and jurisdictional questions, and that judges should readily activate the court’s jurisdiction in response to a simple allegation in the complaint. Part III then describes the recent change to pleading standards on the merits of a claim. It explains the origins, factual context, and doctrinal bases of the altered pleading standard introduced in Iqbal and Twombly. These rationales do not, in most cases, apply to motions to dismiss for lack of subject-matter jurisdiction. In Part IV, lower-court case law reveals that since 2009, in practice, the Iqbal standard has been interfering with the agenda of Arbaugh, in a way that the Supreme Court did not intend. Federal judges, in their eagerness to apply the novel pleading standard of Iqbal, have neglected the jurisdictional teachings of the Supreme Court in the Arbaugh line of cases. As a result, since Iqbal an erroneous 12(b)(1) standard has propagated rapidly through circuit and lower courts.If the application of Iqbal to jurisdictional pleadings is an error, as this Note maintains and as recent Supreme Court reaffirmations of Arbaugh suggest, it may prove difficult to eradicate. Part V addresses the ineffectiveness of circuit courts and the rules-based system in correcting such mistakes
Elasticity of cross-linked semiflexible biopolymers under tension
Aiming at the mechanical properties of cross-linked biopolymers, we set up
and analyze a model of two weakly bending wormlike chains subjected to a
tensile force, with regularly spaced inter-chain bonds (cross-links)
represented by harmonic springs. Within this model, we compute the
force-extension curve and the differential stiffness exactly and discuss
several limiting cases. Cross-links effectively stiffen the chain pair by
reducing thermal fluctuations transverse to the force and alignment direction.
The extra alignment due to cross-links increases both with growing number and
with growing strength of the cross-links, and is most prominent for small force
f. For large f, the additional, cross-link-induced extension is subdominant
except for the case of linking the chains rigidly and continuously along their
contour. In this combined limit, we recover asymptotically the elasticity of a
weakly bending wormlike chain without constraints, stiffened by a factor four.
The increase in differential stiffness can be as large as 100% for small f or
large numbers of cross-links.Comment: 11 pages, 6 figures, submitted to PR
On the sound of snapping shrimp
Fluid dynamics video: Snapping shrimp produce a snapping sound by an
extremely rapid closure of their snapper claw. Our high speed imaging of the
claw closure has revealed that the sound is generated by the collapse of a
cavitation bubble formed in a fast flowing water jet forced out from the claws
during claw closure. The produced sound originates from the cavitation collapse
of the bubble. At collapse a short flash of light is emitted, just as in single
bubble sonoluminescence. A model based on the Rayleigh-Plesset equation can
quantitatively account for the visual and acoustical observations.Comment: Fluid dynamics vide
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