bluuebunny/crossref_metadata_2025 huggingface dataset:
Frictional constitutive law at intermediate slip rates accounting for flash heating and thermally activated slip process
Authors: Noda Hiroyuki
Published: September 2008

A constitutive law in a rate‐ and state‐dependent framework accounting for flash heating at microscopic contacts is proposed on the basis of a simple asperity model and a thermally activated slip process thought to cause logarithmic dependency of the friction coefficient on slip rate. This law is probably applicable in an intermediate slip rate regime (about 0.001–0.1 m/s), where contact time of asperities is shorter than a cutoff time for time‐dependent healing, and a phase transformation such as melting does not take place at the microscopic contacts. The steady state friction coefficient, which is constructed numerically and derived analytically, depends on slip rate and background temperature, gives a good coverage of experimental data of gabbro friction at slip rate on the order of 0.1 m/s, and explains linear dependency of the friction coefficient of Al2O3 ceramics on temperature at around 0.1 m/s. Similar to the usual rate‐ and state‐dependent friction law, the transition behavior on a sudden step in slip rate includes a positive direct effect and a following evolution effect, which are essential in considering the problems dealing with coupling of a frictional surface and surrounding elastic medium. This constitutive law illuminates the importance of the change in not only microscopic, but also macroscopic temperature; the latter, as well as the slip rate, probably changes dynamically during the nucleation of rupture and the coseismic phase.
Effect of thermal diffusion on the stability of strongly tilted mantle plume tails
Authors: Kerr R. C. , Mériaux C. , Lister J. R.
Published: September 2008

The effect of thermal diffusion on the stability of strongly tilted mantle plume tails is explored by investigating experimentally and numerically the gravitational instability of a rising horizontal cylindrical region of buoyant viscous fluid. At large viscosity ratios, we find that the instability is unaffected by diffusion when the Rayleigh number Ra is greater than about 300. When Ra is less than 300, diffusion significantly increases the time for instability, as the rising fluid region needs to grow substantially by entrainment before it becomes unstable. When Ra is less than about 140 and the rise height available H is less than about 40 times the cylinder radius, the rising region of fluid is unable to grow sufficiently and instability is prevented. When our results are applied to the Earth, we predict that thermal diffusion will stabilize plume tails in both the upper and lower mantle. We also predict that some of the buoyancy flux in mantle plumes is lost during ascent to form downstream thermal wakes in any larger‐scale mantle flow.
The assimilation of satellite‐derived sea surface temperatures into a diurnal cycle model
Authors: Pimentel S. , Haines K. , Nichols N. K.
Published: September 2008

Satellite sea surface temperature (SST) observations available from infrared and microwave radiometers derive a skin and sub‐skin temperature measurement from very near the ocean surface. These measurements, particularly those taken during the day under clear calm conditions, are often seen to have a diurnal warming signal. These diurnal SST signals can result in errors and aliasing when observations collected at different times of the day and from a variety of observation sources are merged together to obtain “foundation” or bulk SST observation products. A similar problem occurs when SST observations are assimilated into ocean models, which typically do not resolve a diurnal cycle. In this article, a novel data assimilation method is developed and implemented that explicitly utilizes diurnal signal information in SST observations. The technique assimilates SST observations taken over the day into a diurnal cycle model by making corrections, within uncertainty bounds, to the surface boundary forcing of the model. In particular, the surface wind speeds and the fractional cloud cover parameter, which are typically poorly known over the oceans, are tuned in the process. This method is shown to improve the estimate of diurnal SST variability, and it has the potential to reduce uncertainties in estimates of foundation or bulk SST. As such, the procedure can be viewed as a dynamic observation operator.
Variability in the teleconnection between the El Niño–Southern Oscillation and West Antarctic climate deduced from West Antarctic ice core isotope records
Authors: Gregory Scott , Noone David
Published: September 2008

The U.S. contribution to the International Trans‐Antarctic Scientific Expedition (ITASE) program obtained several ice cores from the West Antarctic Ice Sheet. Because of proximity to the Pacific Ocean, the West Antarctic ice cores are expected to have an El Niño–Southern Oscillation (ENSO) signature. The ITASE 2001‐5 core δ18O isotope was selected for detailed analysis here because its location, high annual accumulation, and record length make it an ideal candidate for capturing the effects of regional circulation anomalies in the isotopic composition. The 2001‐5 core is compared to two other cores, the 2001‐2 and 2001‐3 cores, which are further west and therefore capture some spatial variability of the regional circulation on various time scales. Analysis shows that several phenomena, including ENSO, leave a signature in the ice cores. Evidence suggests that ENSO signals in the ice cores are significantly modulated by low‐frequency variability. Correlation with the Southern Annular Mode (SAM), global temperature, Pacific Decadal Oscillation, and ENSO shows that temperature and ENSO generally appear to have the strongest influence on the 2001‐5 isotope signal while there is no clearly dominant single influence in the other cores. Results suggest that the teleconnection between ENSO and the 2001‐5 core is quite dependent on the state of the SAM. Specifically, when the Southern Oscillation Index (SOI) and SAM are in phase, there is an ENSO related pressure anomaly west of the Antarctic Peninsula, in the vicinity of the ice cores studied. This extends previous findings to span the entire 20th century.
Numerical simulations of the three‐dimensional distribution of meteoric dust in the mesosphere and upper stratosphere
Authors: Bardeen C. G. , Toon O. B. , Jensen E. J. , Marsh D. R. , Harvey V. L.
Published: September 2008

Micrometeorites that ablate in the lower thermosphere and upper mesosphere are thought to recondense into nanometer‐sized smoke particles and then coagulate into larger dust particles. Previous studies with one‐dimensional models have determined that the meteoric dust size distribution is sensitive to the background vertical velocity and have speculated on the importance of the mesospheric meridional circulation to the dust spatial distribution. We conduct the first three‐dimensional simulations of meteoric dust using a general circulation model with sectional microphysics to study the distribution and characteristics of meteoric dust in the mesosphere and upper stratosphere. We find that the mesospheric meridional circulation causes a strong seasonal pattern in meteoric dust concentration in which the summer pole is depleted and the winter pole is enhanced. This summer pole depletion of dust particles results in fewer dust condensation nuclei (CN) than has traditionally been assumed in numerical simulations of polar mesospheric clouds (PMCs). However, the total number of dust particles present is still sufficient to account for PMCs if smaller particles can nucleate to form ice than is conventionally assumed. During winter, dust is quickly transported down to the stratosphere in the polar vortex where it may participate in the nucleation of sulfate aerosols, the formation of the polar CN layer, and the formation of polar stratospheric clouds (PSCs). These predictions of the seasonal variation and resulting large gradients in dust concentration should assist the planning of future campaigns to measure meteoric dust.
Charge separation in thunderstorm conditions
Authors: Pereyra Rodolfo G. , Bürgesser Rodrigo E. , Ávila Eldo E.
Published: September 2008

A laboratory investigation of the electric charge transfer in collisions between vapor‐grown ice crystals and a riming target is presented in this work. A series of experiments were conducted for ambient temperatures between −8°C and −29°C, air velocity of 8 m s−1, and effective liquid water content from 0.5 to 10 g m−3, with the goal of studying the performance of the noninductive mechanism under a wide range of temperature and liquid water content. At low temperatures (below −19°C), the results revealed no dependence of the charge separated per collision upon variations of the liquid water content. While at temperatures above −19°C, the efficiency of the graupel charging could decrease as the liquid water content increases, as a consequence of the decrease of the probability that the ice crystals impact and rebound from the graupel surface in the dry growth regime. We found that the dominant sign of the graupel charging was negative for temperatures below −15°C and positive at higher temperatures. A simple functional representation of our laboratory results is given so that they can be incorporated in cloud electrification models.
Temporally transitional mantle convection: Implications for Mars
Authors: Loddoch Alexander , Hansen Ulrich
Published: September 2008

The thermal evolution of terrestrial planets such as Earth, Mars, and Venus is strongly dominated by the convective processes in the planet's silicate mantle. The actual style of convection controls the efficiency of heat transport and thus the cooling behavior of the whole planet. In the present study we investigate the heat transport properties of variable viscosity convection, focusing on the temporally transitional behavior discovered recently. While the difference of the newly found convective regime to the already known stagnant lid and episodic behavior has been elaborated in our previous study, the present work investigates the applicability of the observed intermittent behavior to the thermal evolution of terrestrial planets. A 3‐D numerical mantle convection code is applied and calculations are carried out in the parameter range for which the temporally transitional behavior has been found. Using the described approach, it is possible to investigate the transition from a (temporarily) mobilized to a stagnant surface in a fluid dynamically consistent manner. While such a scenario has been suggested for Mars' early history, it has so far been investigated only by means of parameterized convection models. We show that the sporadic surface mobilization events may indeed occur on time scales relevant for Mars. In order to assess their influence on the subsequent thermal evolution of planetary bodies, an internal heating of the mantle and a secular cooling of the core are additionally taken into account. The obtained results are compared to the findings of thermal evolution studies employing parameterized convection models. We show that the thermal consequences of a temporal transition from a mobile to a stagnant surface are indeed correctly described by parameterized models as done in previous studies.
Iceberg calving flux and mass balance of the Austfonna ice cap on Nordaustlandet, Svalbard
Authors: Dowdeswell J. A. , Benham T. J. , Strozzi T. , Hagen J. O.
Published: September 2008

Satellite radar interferometry, 60 MHz airborne ice‐penetrating radar data, and visible band satellite imagery were used to calculate the velocity structure, ice thickness, and the changing ice‐marginal extent of Austfonna (8000 km2 and 2500 km3), the largest ice cap in the Eurasian Arctic. Ice cap motion is generally less than about 10 m a−1, except where faster flowing curvilinear features with velocities of several tens to over 200 m a−1 are present. Most drainage basins of Austfonna have undergone ice‐marginal retreat over the past few decades at an average of a few tens of meters per year. Integrating margin change around the whole ice cap gives a total area loss of about 10 km2 a−1. Iceberg flux from the marine margins of Austfonna is about 2.5 ± 0.5 km3 a−1 (water equivalent), about 45% of the total calving flux from the whole Svalbard archipelago. When mass loss by iceberg production is taken into account, the total mass balance of Austfonna is negative by between about 2.5 and 4.5 km3 a−1. This iceberg flux represents about 33 ± 5% of total annual mass loss from Austfonna, with the remainder lost through surface ablation. Iceberg flux should be included in calculations of the total mass balance of the many large Arctic ice caps, including those located in the Russian and Canadian Arctic that end in tidewater. The neglect of this term has led to underestimates of mass loss from these ice caps and, thus, to underestimates of the contribution of Arctic ice caps to global sea level rise.
Effects of soil skeleton deformations on hysteretic soil water characteristic curves: Experiments and simulations
Authors: Miller Gerald A. , Khoury Charbel N. , Muraleetharan Kanthasamy K. , Liu Chunyang , Kibbey Tohren C. G.
Published: September 2008

Soil water characteristic curves (SWCCs) represent the relationship between suction and water content in unsaturated soils. The SWCCs exhibit hysteresis during wetting‐drying cycles; however, the empirical expressions used to describe SWCCs have typically ignored the hysteresis. Additionally, the shape of the SWCC will vary depending on the void ratio of the soil and changes resulting from soil skeleton deformations, which may also show hysteretic behavior under various loading conditions. Therefore, it is important to investigate, both experimentally and theoretically, the relationship between soil skeleton deformations and the SWCC for different soils. There is limited information in the literature that examines, both experimentally and theoretically, the complex coupling between the soil skeleton deformation and SWCC behavior, and generally, this behavior is not well understood. This paper presents laboratory test results of SWCCs determined under different confining stresses on similarly prepared samples of a silty soil; drying, wetting, second drying, and scanning curves were obtained. The influence of soil skeleton deformations on SWCCs is inferred from the curves measured in an oedometer under different stress conditions. An elastoplastic phenomenological constitutive model based on the bounding surface plasticity theory was utilized to simulate the coupled mechanical‐hydraulic behavior of measured results. This research demonstrates that the model is capable of predicting hysteresis in SWCCs and soil skeleton deformation and the coupling between the hydraulic and mechanical behavior of unsaturated soils.
Real‐time groundwater flow modeling with the Ensemble Kalman Filter: Joint estimation of states and parameters and the filter inbreeding problem
Authors: Hendricks Franssen H. J. , Kinzelbach W.
Published: September 2008

Real‐time groundwater flow modeling with filter methods is interesting for dynamical groundwater flow systems, for which measurement data in real‐time are available. The Ensemble Kalman Filter (EnKF) approach is used here to update states together with parameters by adopting an augmented state vector approach. The performance of EnKF is investigated in a synthetic study with a two‐dimensional transient groundwater flow model where (1) only the recharge rate is spatiotemporally variable, (2) only transmissivity is spatially variable with σlnT2 = 1.0 or (3) with σlnT2 = 2.7, and (4) both recharge rate and transmissivity are uncertain (a combination of (1) and (3)). The performance of EnKF for simultaneous state and parameter estimation in saturated groundwater flow problems is investigated in dependence of the number of stochastic realizations, the updating frequency and updating intensity of log‐transmissivity, the amount of measurements in space and time, and the method (iterative versus noniterative EnKF), among others. Satisfactory results were also obtained if both transmissivity and recharge rate were uncertain. However, it was found that filter inbreeding is much more severe if hydraulic heads and transmissivities are jointly updated than if only hydraulic heads are updated. The filter inbreeding problem was investigated in more detail and could be strongly reduced with help of a damping parameter, which limits the intensity of the perturbation of the log‐transmissivity field. An additional reduction of filter inbreeding could be achieved by combining two measures: (1) inflating the elements of the predicted state covariance matrix on the basis of a comparison between the model uncertainty and the observed errors at the measurement points and (2) starting the flow simulations with a very large number of realizations and then sampling the desired number of realizations after one simulation time step by minimizing the differences between the local cpdfs (and bivariate cpdfs) of hydraulic head for the large ensemble and the corresponding cpdfs for the reduced ensemble. The two measures, which cause very limited CPU costs, allowed making 100 stochastic realizations for the reproduction of the states as efficient as 200–500 untreated stochastic realizations.
Assessment of damage caused by high groundwater inundation
Authors: Kreibich Heidi , Thieken Annegret H.
Published: September 2008

In several cities around the world, problems with rising groundwater levels were reported, but up to now, losses caused by high groundwater levels have been neglected in loss assessment studies. However reliable loss assessments are necessary to evaluate the cost‐effectiveness of mitigating measures like groundwater withdrawal and drainage. To improve the knowledge about flood losses and the loss‐influencing factors, 1697 households affected by the Elbe and Danube floods in 2002 in Germany were interviewed. Cases affected by a high groundwater level were identified and analyzed in comparison with cases affected by riverine flooding. Inundations due to high groundwater show significantly different impact and damage characteristics in comparison with riverine floods. Most common loss models tested were as such not suitable to estimate losses due to high groundwater. Therefore, new loss models have to be developed. One step in this direction is the development of loss models just on basis of loss data from groundwater flooding. From all such models tested, the multifactorial model FLEMOps+ performed best, confirming that the uncertainty in loss estimation can be reduced if more predictive variables, besides the water level, are taken into consideration. However further research is necessary to investigate the main factors influencing the losses due to groundwater flooding and to develop specific models for their assessment.
Influence of environmental moisture on TRMM‐derived tropical cyclone precipitation over land and ocean
Authors: Jiang Haiyan , Halverson Jeffrey B. , Zipser Edward J.
Published: September 2008

The rainfall climatology and persistence model (R‐CLIPER) used operationally in the Atlantic Ocean basin mainly utilizes tropical cyclone (TC) intensity to predict TC rainfall. However, the rain production by TCs is also influenced by environmental parameters such as total moisture availability, horizontal moisture convergence, vertical wind shear, and sea surface temperature (SST). Previous TC case studies have used environmental moisture parameters to diagnose TC rainfall. In this study, we composite over 3000 snapshots of 3‐hourly TRMM 3B42 rainfall fields for Atlantic landfalling tropical cyclones between 1998–2006 to analyze the rainfall distribution and storm total volumetric rain as a function of total precipitable water (TPW), horizontal moisture convergence (HMC), and ocean surface flux (OSF) over land and over ocean. For over ocean conditions, higher TPW, HMC, or OSF values are associated with higher azimuthally averaged rain rates. Over land, this is still the case but less obvious. Computing the linear correlation coefficients between total volumetric rain and moisture parameters shows this fact much more clearly. These coefficients are generally higher for over ocean conditions than those for over land conditions. To test if moisture parameters can provide additional information other than TC intensity to help TC rainfall forecasts, a multiple linear regression is performed between TC volumetric rain and several variables including TC maximum wind speed, TPW, HMC, and OSF. By adding moisture parameters as additional variables, TC volumetric rain will be better predicted than using TC intensity (maximum wind speed) only. The correlation coefficient between volumetric rain and maximum wind speed can increase from 0.52 (0.51) to 0.67 (0.65) for over ocean (land) conditions by adding TPW, HMC and OSF. By adding TPW only, the correlation coefficient increases to 0.59 and 0.64 for over ocean and land, respectively.
Seasonality in the “grasshopping” and atmospheric residence times of persistent organic pollutants over the oceans
Authors: Jurado Elena , Dachs Jordi
Published: September 2008

Persistent Organic Pollutants (POPs) are toxic, bioaccumulable and semi‐volatile compounds that have been detected in all environmental compartments, even in remote pristine regions where they have never been produced or used. Long range atmospheric transport (LRAT) and subsequent deposition is the major route of introduction of POPs to the oceans, including remote regions, and thus to the marine food‐webs. It has been suggested that when POPs travel through the atmosphere, they tend to move in stages. This is known as the grasshopper effect, a repeated process of volatilization and deposition, even though the measure of grasshopping potential has been elusive. Here we show the spatial and seasonal variability of the grasshopper and LRAT potentials for polychlorinated biphenyls (PCB), a significant class of POPs. The number of hops that a chemical undergoes during LRAT over the oceans presents an important spatial and seasonal variability driven by biogeochemical processes occurring in the water column. These results have important implications for the understanding of the widespread distribution and fractionation processes of POPs at the global scale, showing that transport events to remote regions, such the Arctic, are maximized during seasons of low primary productivity.
High sensitivity of subglacial hydrological pathways in Antarctica to small ice‐sheet changes
Authors: Wright A. P. , Siegert M. J. , Le Brocq A. M. , Gore D. B.
Published: September 2008

The flow of water beneath the Antarctic Ice Sheet has recently been shown to involve drainage from and exchange between subglacial lakes, with flow routing over distances of >100km. As subglacial water is known to be important to ice‐sheet dynamics, changes to the flow routing of basal water may yield an ice‐sheet response. Here we show that the directions of subglacial hydrological flowpaths are highly sensitive to the ice‐sheet surface topography at certain sites in Antarctica. Surface elevation changes, of between 5 and 15 m, are found to be sufficient to produce significant changes to the subglacial water flowpaths, which affect the potential drainage routes of large subglacial lakes. We identify the reasons for this high sensitivity and recognise several locations in East Antarctica where changes to the drainage regime could have important effects on large‐scale basal water distribution.
Activating and reactivating pairs of nested collapses during caldera‐forming eruptions: Campi Flegrei (Italy)
Authors: Acocella Valerio
Published: September 2008

A structural model for Campi Flegrei Caldera (CFC; Italy), one of Earth's most hazardous calderas, is provided. Data from the north (outer) rim show that this formed by downsag soon after the Neapolitan Yellow Tuff emplacement (∼15 ka). Campanian Ignimbrite's (CFC's other major eruption; ∼39 ka) displacement at depth also suggests the activity of a buried caldera fault. Therefore, during the Campanian Ignimbrite eruption the north rim was activated as caldera‐bounding fault and during the Neapolitan Yellow Tuff eruption as downsag. Previously published data reveal an inner collapsed structure to the south, formed during the Campanian Ignimbrite eruption and reactivated during the Neapolitan Yellow Tuff eruption. So, CFC consists of 2 nested depressions, both active during the Campanian Ignimbrite and Neapolitan Yellow Tuff eruptions, but with different amounts of collapse. CFC is a major example of how 2 nested calderas may: (1) form during the same collapse episode; (2) be reactivated together during caldera‐forming eruptions.
On blanketing sporadic E and polarization effects near the equatorial electrojet
Authors: Tsunoda Roland T.
Published: September 2008

A number of puzzling features have been observed that occur during the day in the vicinity of the equatorial electrojet (EEJ). They include (1) an equatorial belt, essentially collocated with the EEJ, that is virtually devoid of blanketing sporadic E (Esb); (2) appearances of Esb within that “forbidden” belt but only when the EEJ is weak or reversed (i.e., a counterelectrojet); (3) occasional appearances of a secondary current layer above the EEJ; and (4) observations at 4.3°N dip latitude of a latitudinal tilt in the bottomside E layer on one occasion and an unexpectedly large electron drift velocity on another. These features are shown to be accountable if the usual framework for electrodynamics in this region is modified to include the following: (1) transport of Esb that is controlled by a shear in the neutral wind with a crucial contribution from the polarization electric field of the EEJ; (2) the formation of an equatorward boundary for Esb by a dynamical balance between the neutral wind and the polarization electric field; (3) a buildup in conductivity within a band‐like region, just poleward of the Esb boundary, from an accumulation of metallic ions; and (4) the appearance of a secondary polarization electric field across the band of enhanced conductivity and current flow throughout the region pervaded by that secondary electric field.
Origin of Saturn's aurora: Simultaneous observations by Cassini and the Hubble Space Telescope
Authors: Bunce E. J. , Arridge C. S. , Clarke J. T. , Coates A. J. , Cowley S. W. H. , Dougherty M. K. , Gérard J.‐C. , Grodent D. , Hansen K. C. , Nichols J. D. , Southwood D. J. , Talboys D. L.
Published: September 2008

Outer planet auroras have been imaged for more than a decade, yet understanding their physical origin requires simultaneous remote and in situ observations. The first such measurements at Saturn were obtained in January 2007, when the Hubble Space Telescope imaged the ultraviolet aurora, while the Cassini spacecraft crossed field lines connected to the auroral oval in the high‐latitude magnetosphere near noon. The Cassini data indicate that the noon aurora lies in the boundary between open‐ and closed‐field lines, where a layer of upward‐directed field‐aligned current flows whose density requires downward acceleration of magnetospheric electrons sufficient to produce the aurora. These observations indicate that the quasi‐continuous main oval is produced by the magnetosphere‐solar wind interaction through the shear in rotational flow across the open‐closed‐field line boundary.
Variation of the Martian ionospheric electron density from Mars Express radar soundings
Authors: Morgan D. D. , Gurnett D. A. , Kirchner D. L. , Fox J. L. , Nielsen E. , Plaut J. J.
Published: September 2008

The Mars Advanced Radar for Subsurface and Ionospheric Sounding aboard Mars Express has been in operation for over 2 years. Between 14 August 2005 and 31 July 2007, we obtain 34,492 ionospheric traces, of which 14,060 yield electron density profiles and 12,291 yield acceptable fits to the Chapman ionospheric model. These results are used to study the Martian ionosphere under changing conditions: the presence or absence of solar energetic particles, solar EUV flux, season, solar zenith angle, and latitude. The 2‐year average subsolar maximum electron density n0 is 1.62 × 105 cm−3, the average subsolar electron density altitude h0 is 128.2 km, and the average neutral scale height H is 12.9 km. Solar energetic particle events are associated with a 6% increase in n0, a 3 km decrease in h0, and a 0–7 km decrease in H. The value of n0 varies smoothly between 1.4 × 105 and 1.8 × 105 cm−3, yielding d ln n0/d ln F10.7 = 0.30 ± 0.4; h0 varies between 115 and 135 km, while H remains relatively constant with EUV flux and season, in contrast with previous work. The value of h0 decreases toward the terminator at low latitude but increases poleward during summer; H varies from 11 km, for solar zenith angle less than 40°, to between 14 and 17 km near the terminator, depending on season. Near‐peak temperatures vary between 220 K and 300 K, less variation than indicated by modeling, probably due to sampling near solar minimum.
Investigation of the deposition and emission of mercury in arctic snow during an atmospheric mercury depletion event
Authors: Johnson Kelsey P. , Blum Joel D. , Keeler Gerald J. , Douglas Thomas A.
Published: September 2008

Mechanisms of air‐snow exchange of mercury (Hg) during and after atmospheric mercury depletion events (AMDEs) remain poorly constrained and this has limited our understanding of the arctic Hg cycle. We measured the Hg concentrations of surface snow through time and carried out flux chamber experiments during AMDE and non‐AMDE conditions in the spring of 2006 near Barrow, Alaska. Clear skies, low‐velocity onshore winds, and a stable boundary layer characterized the meteorology during this AMDE. Surface snow Hg concentrations (upper 1 cm) increased throughout a 9‐day AMDE from background levels (4.1–15.5 ng/L) to elevated levels (147 and 237 ng/L) at two sampling sites and returned to near‐baseline values within 2 days of AMDE cessation. The Hg concentrations of core samples from the full snowpack did not increase significantly during the AMDE and demonstrate that the Hg enhancement of surface snow resulted from deposition of atmospheric Hg to surface snow. We estimate that complete deposition of background Hg to a height of 200–450 m in the near‐surface troposphere could account for the Hg gains to surface snow during this event. Snow incubated in field‐based flux chambers emitted 4 to 7% of its total Hg content within 1 day and may represent an upper limit for the photo‐reduction rate of “easily” reducible Hg in snow under post‐AMDE conditions. Full‐column snow core samples collected in the late springtime have comparable Hg loads to those observed during the AMDE season and imply that a significant fraction of the Hg deposited during the 3‐month AMDE season was retained until snowmelt at this location.
Radiative properties of aerosol mixture observed during the dry season 2006 over M'Bour, Senegal (African Monsoon Multidisciplinary Analysis campaign)
Authors: Derimian Y. , Léon J.‐F. , Dubovik O. , Chiapello I. , Tanré D. , Sinyuk A. , Auriol F. , Podvin T. , Brogniez G. , Holben B. N.
Published: September 2008

During the dry season in the Western Africa region the outbreaks of biomass burning aerosols occur on a background of frequent dust storms. In the frame of the African Monsoon Multidisciplinary Analysis (AMMA) campaign we studied optical properties and radiative effects of aerosol mixture and dust observed from January to March 2006 over M'Bour, Senegal. The aerosol burden during the observing period was notably below the average of previous years. However, variability of aerosol sizes and spectral absorption were found as representative. The lidar obtained extinction profiles suggested aerosol transport in two layers (or one elevated layer) for events with two different aerosol types, called aerosol mixture in the paper. Low‐altitude transport was observed for dust. The broadband solar radiative fluxes and aerosol radiative effects were analyzed by combining measurements and simulations. The simulations relied on retrieved column aerosol properties and are products of Aerosol Robotic Network data processing. The 24‐h average aerosol radiative effect yielded cooling at the bottom and at the top of the atmosphere for all the analyzed events of aerosol mixture and dust. The radiative efficiency (radiative effect scaled by optical thickness) at the bottom of the atmosphere and within the aerosol layer appeared to be stronger in case of mixture than in case of pure dust due to enhanced absorption ability of former. We also evaluated the importance of accounting for dust particles nonsphericity for simulating radiative effects. Our test showed that neglecting of aerosol particle shape can result in pronounced bias, up to 10% overestimation of daily average aerosol radiative effect.
Electric and magnetic fields and field derivatives from lightning stepped leaders and first return strokes measured at distances from 100 to 1000 m
Authors: Jerauld J. , Uman M. A. , Rakov V. A. , Rambo K. J. , Jordan D. M. , Schnetzer G. H.
Published: September 2008

Using electric and magnetic field and field derivative sensors arrayed over an area of about 1 km2, we measured the close fields of stepped leaders and first return strokes in 18 negative cloud‐to‐ground lightning flashes at distances to individual sensors ranging from about 100 m to about 1 km. We present examples of the close field waveforms along with their statistical characterization as a function of the distance to the lightning. Statistical data are presented for the half‐peak width of the stepped‐leader/return‐stroke electric field waveform; the stepped‐leader electric field change; the return‐stroke electric field change at 20, 100, and 1000 μs after return‐stroke initiation; the peak electric field derivative; the risetime of the electric field derivative waveform; and the magnetic field initial peak, largest peak, risetime, and half‐peak width. For example, in the 100–200 m range, the average half‐peak width of the leader/return‐stroke electric field change was about 0.8 ms; the average observed leader electric field change was about 40 kV m−1; the average return‐stroke electric field change at 20 μs was about 35 kV m−1; and the average peak electric field derivative was about 15 kV m−1μs−1, the largest unsaturated measurement being about 20 kV m−1μs−1. Peak derivative values observed at close range are consistent with derivative measurements made for return strokes over salt water at distances of some tens of kilometers if an inverse‐distance dependence is assumed for the field amplitude.
The Generational Cycle of State Spaces and Adequate Genetical Representation
Authors: Lloyd Elisabeth A. , Lewontin Richard C. , Feldman Marcus W.
Published: September 2008

Most models of generational succession in sexually reproducing populations necessarily move back and forth between genic and genotypic spaces. We show that transitions between and within these spaces are usually hidden by unstated assumptions about processes in these spaces. We also examine a widely endorsed claim regarding the mathematical equivalence of kin-, group-, individual-, and allelic-selection models made by Lee Dugatkin and Kern Reeve. We show that the claimed mathematical equivalence of the models does not hold.
The Evolution of Cooperation in the Centipede Game with Finite Populations
Authors: Smead Rory
Published: September 2008

The partial cooperation displayed by subjects in the Centipede Game deviates radically from the predictions of traditional game theory. Even standard, infinite population, evolutionary settings have failed to provide an explanation for this behavior. However, recent work in finite population evolutionary models has shown that such settings can produce radically different results from the standard models. This paper examines the evolution of partial cooperation in finite populations. The results reveal a new possible explanation that is not open to the standard models and gives us reason to be cautious when employing these otherwise helpful idealizations.
Two Ways of Analogy: Extending the Study of Analogies to Mathematical Domains
Authors: Schlimm Dirk
Published: September 2008

The structure-mapping theory has become the de facto standard account of analogies in cognitive science and philosophy of science. In this paper I propose a distinction between two kinds of domains and I show how the account of analogies based on structure-preserving mappings fails in certain (object-rich) domains, which are very common in mathematics, and how the axiomatic approach to analogies, which is based on a common linguistic description of the analogs in terms of laws or axioms, can be used successfully to explicate analogies of this kind. Thus, the two accounts of analogies should be regarded as complementary, since each of them is adequate for explicating analogies that are drawn between different kinds of domains. In addition, I illustrate how the account of analogies based on axioms has also considerable practical advantages, for example, for the discovery of new analogies.
Two Views on Time Reversal
Authors: North Jill
Published: September 2008

In a recent paper, Malament (2004) employs a time reversal transformation that differs from the standard one, without explicitly arguing for it. This is a new and important understanding of time reversal that deserves arguing for in its own right. I argue that it improves upon the standard one. Recent discussion has focused on whether velocities should undergo a time reversal operation. I address a prior question: What is the proper notion of time reversal? This is important, for it will affect our conclusion as to whether our best theories are time-reversal symmetric, and hence whether our spacetime is temporally oriented.
INTREPID—INformation-theoretic TREe traversal for Protein functional site IDentification
Authors: Sankararaman Sriram , Sjölander Kimmen
Published: September 2008

Motivation: Identification of functionally important residues in proteins plays a significant role in biological discovery. Here, we present INTREPID—an information–theoretic approach for functional site identification that exploits the information in large diverse multiple sequence alignments (MSAs). INTREPID uses a traversal of the phylogeny in combination with a positional conservation score, based on Jensen–Shannon divergence, to rank positions in an MSA. While knowledge of protein 3D structure can significantly improve the accuracy of functional site identification, since structural information is not available for a majority of proteins, INTREPID relies solely on sequence information. We evaluated INTREPID on two tasks: predicting catalytic residues and predicting specificity determinants.

Results: In catalytic residue prediction, INTREPID provides significant improvements over Evolutionary Trace, ConSurf as well as over a baseline global conservation method on a set of 100 manually curated enzymes from the Catalytic Site Atlas. In particular, INTREPID is able to better predict catalytic positions that are not globally conserved and hence, attains improved sensitivity at high values of specificity. We also investigated the performance of INTREPID as a function of the evolutionary divergence of the protein family. We found that INTREPID is better able to exploit the diversity in such families and that accuracy improves when homologs with very low sequence identity are included in an alignment. In specificity determinant prediction, when subtype information is known, INTREPID-SPEC, a variant of INTREPID, attains accuracies that are competitive with other approaches for this task.

Availability: INTREPID is available for 16919 families in the PhyloFacts resource (http://phylogenomics.berkeley.edu/phylofacts).

Contact:  sriram_s@cs.berkeley.edu

Supplementary information: Relevant online supplementary material is available at http://phylogenomics.berkeley.edu/INTREPID.
varDB: a pathogen-specific sequence database of protein families involved in antigenic variation
Authors: Hayes C. Nelson , Diez Diego , Joannin Nicolas , Honda Wataru , Kanehisa Minoru , Wahlgren Mats , Wheelock Craig E. , Goto Susumu
Published: September 2008

Summary: Infectious diseases are a major threat to global public health and prosperity. The causative agents consist of a suite of pathogens, ranging from bacteria to viruses, including fungi, helminthes and protozoa. Although these organisms are extremely varied in their biological structure and interactions with the host, they share similar methods of evading the host immune system. Antigenic variation and drift are mechanisms by which pathogens change their exposed epitopes while maintaining protein function. Accordingly, these traits enable pathogens to establish chronic infections in the host. The varDB database was developed to serve as a central repository of protein and nucleotide sequences as well as associated features (e.g. field isolate data, clinical parameters, etc.) involved in antigenic variation. The data currently contained in varDB were mined from GenBank as well as multiple specialized data repositories (e.g. PlasmoDB, GiardiaDB). Family members and ortholog groups were identified using a hierarchical search strategy, including literature/author-based searches and HMM profiles. Included in the current release are>29 000 sequences from 39 gene families from 25 different pathogens. This resource will enable researchers to compare antigenic variation within and across taxa with the goal of identifying common mechanisms of pathogenicity to assist in the fight against a range of devastating diseases.

Availability: varDB is freely accessible at http://www.vardb.org/

Contact:  nelson@kuicr.kyoto-u.ac.jp; goto@kuicr.kyoto-u.ac.jp

Supplementary information:  Supplementary data are available at Bioinformatics online.
Genetic and environmental contributions to the diversity of substances used in adolescent twins: a longitudinal study of age and sex effects
Authors: Derringer Jaime , Krueger Robert F. , McGue Matt , Iacono William G.
Published: September 2008

Aims  To determine how genetic and environmental contributions affecting the number of psychoactive substances used varies with age and gender over the course of adolescence.

Design  Estimates of genetic, shared environmental and non‐shared environmental contributions to total variance in diversity of substances used at ages 11, 14 and 17 years were obtained by fitting a multivariate behavior genetic (Cholesky) model.

Participants  A total of 711 male and 675 female twins.

Measurements  Participants reported whether they had used each of 11 substances.

Findings  The average diversity of substances used increased over time for both males and females, and males generally reported a wider diversity of substances used than females. Influences of genetic factors increased with age and were greater for males than for females at ages 14 and 17 years. Genetic factors remained consistent (i.e. highly correlated) across ages for both males and females, as did shared environmental influences for males. Non‐shared environmental factors for both sexes and females' shared environmental factors were age‐specific.

Conclusions  Regardless of sex, the proportion of variance in substances used attributable to genetic factors increases during adolescence, although it is greater for males than females at later ages. These findings indicate that prevention interventions may be most effective if they target early adolescence when environmental factors account for the majority of variance in substance use. The high correlation of genetic factors across ages suggests that early use may sometimes signal an early expression of a developmentally stable genetic predisposition.
Overdose training and take‐home naloxone for opiate users: prospective cohort study of impact on knowledge and attitudes and subsequent management of overdoses
Authors: Strang John , Manning Victoria , Mayet Soraya , Best David , Titherington Emily , Santana Laura , Offor Elizabeth , Semmler Claudia
Published: September 2008

Aim  To examine the impact of training in overdose management and naloxone provision on the knowledge and confidence of current opiate users; and to record subsequent management of overdoses that occur during a 3‐month follow‐up period.

Design  Repeated‐measures design to examine changes in knowledge and confidence immediately after overdose management training; retention of knowledge and confidence at 3 months; and prospective cohort study design to document actual interventions applied at post‐training overdose situations.

Method  A total of 239 opiate users in treatment completed a pre‐training questionnaire on overdose management and naloxone administration and were re‐assessed immediately post‐training, at which point they were provided with the take‐home emergency supply of naloxone. Three months later they were re‐interviewed.

Results  Significant improvements were seen in knowledge of risks of overdose, characteristics of overdose and appropriate actions to be taken; and in confidence in the administration of naloxone. A 78% follow‐up rate was achieved (186 of 239) among whom knowledge of both the risks and physical/behavioural characteristics of overdose and also of recommended management actions was well retained. Eighteen overdoses (either experienced or witnessed) had occurred during the 3 months between the training and the follow‐up. Naloxone was used on 12 occasions (a trained client's own supply on 10 occasions). One death occurred in one of the six overdoses where naloxone was not used. Where naloxone was used, all 12 resulted in successful reversal.

Conclusions  With overdose management training, opiate users can be trained to execute appropriate actions to assist the successful reversal of potentially fatal overdose. Wider provision may reduce drug‐related deaths further. Future studies should examine whether public policy of wider overdose management training and naloxone provision could reduce the extent of opiate overdose fatalities, particularly at times of recognized increased risk.
No reason for optimism: the expected impact of commitments in the European Commission's Alcohol and Health Forum
Authors: De Bruijn Avalon
Published: September 2008

Background Europe is the heaviest‐drinking region in the world, more than 2.5 times the rest of the world's average.

The Commission's conclusion The cornerstone for the European Commission's action to decrease the alcohol‐related harm of this consumption and the main emphasis for its work is the Alcohol and Health Forum, with its Task Forces on Marketing Communication and Youth‐Specific Aspects of Alcohol. The Forum, which was launched in June 2007, aims to provide a common platform for all interested stakeholders. Forum members are invited to make commitments to reduce alcohol‐related harm, in the form of a monitored and evaluated action plan. By 29 February 2008, 79 commitments have been provided by the members of the Forum.

Appraisal of the initiative Taking into account the limited information available, the proposed commitments indicate few evidence‐based approaches. A large majority of the summaries do not, or only slightly, address the relevance of their commitment and give no evidence of why their proposed action is important in reducing alcohol‐related harm. Even fewer commitments mention indicators of effectiveness of the proposed actions or propose to evaluate the effectiveness of the proposed actions.

Conclusion While most economic operators in the Forum have resources to undertake actions which could be highly effective, they commit principally to educational programmes which have been found to be mainly ineffective. This, and the neglect of existing legislation, do not give reason for optimism on the impact of the proposed commitments.