4,300 research outputs found
Automated stereo retrieval of smoke plume injection heights and retrieval of smoke plume masks from AATSR and assessment with CALIPSO and MISR.
The longevity and dispersion of smoke and asso- ciated chemical constituents released from wildfire events are dependent on several factors, crucially including the height at which the smoke is injected into the atmosphere. The aim here is to provide improved emission data for the initialization of chemical transport models in order to better predict aerosol and trace gas dispersion following injection into the free atmosphere. A new stereo-matching algorithm, named M6, which can effec- tively resolve smoke plume injection heights (SPIH), is presented here. M6 is extensively validated against two alternative space- borne earth observation SPIH data sources and demonstrates good agreement. Further, due to the spectral and dual-view configuration of the Advanced Along-Track Scanning Radiometer imaging system, it is possible to automatically differentiate smoke from other atmospheric features effectively—a feat, which currently no other algorithm can achieve. Additionally, as the M6 algorithm shares a heritage with the other M-series matchers, it is here compared against one of its predecessors, M4, which, for the determination of SPIH, M6 is shown to substantially outperform
Synergy of stereo cloud top height and ORAC optimal estimation cloud retrieval: evaluation and application to AATSR
In this paper we evaluate the retrievals of cloud top height when stereo derived heights are combined with the radiometric cloud top heights retrieved from the ORAC (Optimal Retrieval of Aerosol and Cloud) algorithm. This is performed in a mathematically rigorous way using the ORAC optimal estimation retrieval framework, which includes the facility to use independent a priori information. Key to the use of a priori information is a characterisation of their associated uncertainty.
This paper demonstrates the improvements that are possible using this approach and also considers their impact on the microphysical cloud parameters retrieved. The AATSR instrument has two views and three thermal channels so is well placed to demonstrate the synergy of the two techniques. The stereo retrieval is able to improve the accuracy of the retrieved cloud top height when compared to collocated Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), particularly in the presence of boundary layer inversions and high clouds. The impact on the microphysical properties of the cloud such as optical depth and effective radius was evaluated and found to be very small with the biggest differences occurring over bright land surfaces and for high clouds. Overall the cost of the retrievals increased indicating a poorer radiative fit of radiances to the cloud model, which currently uses a single layer cloud model. Best results and improved fit to the radiances may be obtained in the future if a multi-layer model is used
Fixation of a Deleterious Allele under Mutation Pressure and Finite Selection Intensity
The mean fixation time of a deleterious mutant allele is studied beyond the
diffusion approximation. As in Kimura's classical work [M. Kimura, Proc. Natl.
Acad. Sci. U.S.A. Vol.77, 522 (1980)], that was motivated by the problem of
fixation in the presence of amorphic or hypermorphic mutations, we consider a
diallelic model at a single locus comprising a wild-type A and a mutant allele
A' produced irreversibly from A at small uniform rate v. The relative fitnesses
of the mutant homozygotes A'A', mutant heterozygotes A'A and wild-type
homozygotes AA are 1-s, 1-h and 1, respectively, where it is assumed that v<<
s. Here, we adopt an approach based on the direct treatment of the underlying
Markov chain (birth-death process) obeyed by the allele frequency (whose
dynamics is prescribed by the Moran model), which allows to accurately account
for the effects of large fluctuations. After a general description of the
theory, we focus on the case of a deleterious mutant allele (i.e. s>0) and
discuss three situations: when the mutant is (i) completely dominant (s=h);
(ii) completely recessive (h=0), and (iii) semi-dominant (h=s/2). Our
theoretical predictions for the mean fixation time and the quasi-stationary
distribution of the mutant population in the coexistence state, are shown to be
in excellent agreement with numerical simulations. Furthermore, when s is
finite, we demonstrate that our results are superior to those of the diffusion
theory that is shown to be an accurate approximation only when N_e s^2 << 1,
where N_e is the effective population size.Comment: 26 pages, 5 figures. Accepted by the Journal of Theoretical Biolog
Adaptive evolution of molecular phenotypes
Molecular phenotypes link genomic information with organismic functions,
fitness, and evolution. Quantitative traits are complex phenotypes that depend
on multiple genomic loci. In this paper, we study the adaptive evolution of a
quantitative trait under time-dependent selection, which arises from
environmental changes or through fitness interactions with other co-evolving
phenotypes. We analyze a model of trait evolution under mutations and genetic
drift in a single-peak fitness seascape. The fitness peak performs a
constrained random walk in the trait amplitude, which determines the
time-dependent trait optimum in a given population. We derive analytical
expressions for the distribution of the time-dependent trait divergence between
populations and of the trait diversity within populations. Based on this
solution, we develop a method to infer adaptive evolution of quantitative
traits. Specifically, we show that the ratio of the average trait divergence
and the diversity is a universal function of evolutionary time, which predicts
the stabilizing strength and the driving rate of the fitness seascape. From an
information-theoretic point of view, this function measures the
macro-evolutionary entropy in a population ensemble, which determines the
predictability of the evolutionary process. Our solution also quantifies two
key characteristics of adapting populations: the cumulative fitness flux, which
measures the total amount of adaptation, and the adaptive load, which is the
fitness cost due to a population's lag behind the fitness peak.Comment: Figures are not optimally displayed in Firefo
The evidence for and against astronomical impacts on climate change and mass extinctions: A review
Numerous studies over the past 30 years have suggested there is a causal
connection between the motion of the Sun through the Galaxy and terrestrial
mass extinctions or climate change. Proposed mechanisms include comet impacts
(via perturbation of the Oort cloud), cosmic rays and supernovae, the effects
of which are modulated by the passage of the Sun through the Galactic midplane
or spiral arms. Supposed periodicities in the fossil record, impact cratering
dates or climate proxies over the Phanerozoic (past 545 Myr) are frequently
cited as evidence in support of these hypotheses. This remains a controversial
subject, with many refutations and replies having been published. Here I review
both the mechanisms and the evidence for and against the relevance of
astronomical phenomena to climate change and evolution. This necessarily
includes a critical assessment of time series analysis techniques and
hypothesis testing. Some of the studies have suffered from flaws in
methodology, in particular drawing incorrect conclusions based on ruling out a
null hypothesis. I conclude that there is little evidence for intrinsic
periodicities in biodiversity, impact cratering or climate on timescales of
tens to hundreds of Myr. Furthermore, Galactic midplane and spiral arm
crossings seem to have little or no impact on biological or climate variation
above background level. (truncated)Comment: 51 pages, 7 figures, 140 references. To appear in the International
Journal of Astrobiology. For hyperref version with full resolution figures
see http://www.mpia-hd.mpg.de/homes/calj/astimpact_ija.pd
Egalitarian despots: hierarchy steepness, reciprocity and the grooming-trade model in wild chimpanzees (Pan troglodytes)
Biological-markets theory models the action of natural selection as a marketplace in which animals are viewed as traders with commodities to offer and exchange. Studies of female Old World monkeys have suggested that grooming might be employed as a commodity to be reciprocated or traded for alternative services, yet previous tests of this grooming-trade model in wild adult male chimpanzees (Pan troglodytes) have yielded mixed results. Here we provide the strongest test of the model to date for male chimpanzees: we use data drawn from two social groups (communities) of chimpanzees from different populations, and give explicit consideration to variation in dominance hierarchy steepness as such variation results in differing conditions for biological markets. First, analysis of data from published accounts of other chimpanzee communities, together with our own data, showed that hierarchy steepness varied considerably within and across communities and that the number of adult males in a community aged 20-30 years predicted hierarchy steepness. The two communities in which we tested predictions of the grooming-trade model lay at opposite extremes of this distribution. Second, in accord with the grooming-trade model, we found evidence that male chimpanzees trade grooming for agonistic support where hierarchies are steep (despotic) and consequent effective support is a rank-related commodity, but not where hierarchies are shallow (egalitarian). However, we also found that grooming was reciprocated regardless of hierarchy steepness. Our findings also hint at the possibility of agonistic competition, or at least exclusion, in relation to grooming opportunities compromising the free market envisioned by Biological Markets theory. Our results build on previous findings across chimpanzee communities to emphasise the importance of reciprocal grooming exchanges among adult male chimpanzees, which can be understood in a biological markets framework if grooming by or with particular individuals is a valuable commodity
A microscopic model of evolution of recombination
We study the evolution of recombination using a microscopic model developed
within the frame of the theory of quantitative traits. Two components of
fitness are considered: a static one that describes adaptation to environmental
factors not related to the population itself, and a dynamic one that accounts
for interactions between organisms e.g. competition. We focus on the dynamics
of colonization of an empty niche. As competition is a function of the
population, selection pressure rapidly changes in time. The simulations show
that both in the case of flat and steep static fitness landscapes,
recombination provides a high velocity of movement in the phenotypic space thus
allowing recombinants to colonize the highest fitness regions earlier than non
recombinants that are often driven to extinction. The stabilizing effects of
competition and assortativity are also discussed. Finally, the analysis of
phase diagrams shows that competition is the key factor for the evolution of
recombination, while assortativity plays a significant role only in small
populations.Comment: to appear in Physica
Yang-Lee and Fisher Zeros of Multisite Interaction Ising Models on the Cayley-type Lattices
A general analytical formula for recurrence relations of multisite
interaction Ising models in an external magnetic field on the Cayley-type
lattices is derived. Using the theory of complex analytical dynamics on the
Riemann sphere, a numerical algorithm to obtain Yang-Lee and Fisher zeros of
the models is developed. It is shown that the sets of Yang-Lee and Fisher zeros
are almost always fractals, that could be associated with Mandelbrot-like sets
on the complex magnetic field and temperature planes respectively.Comment: 9 pages, 3 figures; with minor correction
Ising Spins on Thin Graphs
The Ising model on ``thin'' graphs (standard Feynman diagrams) displays
several interesting properties. For ferromagnetic couplings there is a mean
field phase transition at the corresponding Bethe lattice transition point. For
antiferromagnetic couplings the replica trick gives some evidence for a spin
glass phase. In this paper we investigate both the ferromagnetic and
antiferromagnetic models with the aid of simulations. We confirm the Bethe
lattice values of the critical points for the ferromagnetic model on
and graphs and examine the putative spin glass phase in the
antiferromagnetic model by looking at the overlap between replicas in a
quenched ensemble of graphs. We also compare the Ising results with those for
higher state Potts models and Ising models on ``fat'' graphs, such as those
used in 2D gravity simulations.Comment: LaTeX 13 pages + 9 postscript figures, COLO-HEP-340,
LPTHE-Orsay-94-6
Kinesin's backsteps under mechanical load
Kinesins move processively toward the plus end of microtubules by hydrolyzing
ATP for each step. From an enzymatic perspective, the mechanism of mechanical
motion coupled to the nucleotide chemistry is often well explained using a
single-loop cyclic reaction. However, several difficulties arise in
interpreting kinesin's backstepping within this framework, especially when
external forces oppose the motion of kinesin. We review evidence, such as an
ATP-independent stall force and a slower cycle time for backsteps, that has
emerged to challenge the idea that kinesin backstepping is due to ATP
synthesis, i.e., the reverse cycle of kinesin's forward-stepping
chemomechanics. Supplementing the conventional single-loop chemomechanics with
routes for ATP-hydrolyzing backward steps and nucleotide-free steps, especially
under load, gives a better physical interpretation of the experimental data on
backsteps.Comment: 5 figures and 2 table
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