Why the <Emphasis Type="Italic">Euler</Emphasis> scheme in particle tracking is not enough: the shallow-sea pycnocline test case

During the last decades, the Euler scheme was the common “workhorse” in particle tracking, although it is the lowest-order approximation of the underlying stochastic differential equation. To convince the modelling community of the need for better methods, we have constructed a new test case that will show the shortcomings of the Euler scheme. We use an idealised shallow-water diffusivity profile that mimics the presence of a sharp pycnocline and thus a quasi-impermeable barrier to vertical diffusion. In this context, we study the transport of passive particles with or without negative buoyancy. A semi-analytic solutions is used to assess the performance of various numerical particle-tracking schemes (first- and second-order accuracy), to treat the variations in the diffusivity profile properly. We show that the commonly used Euler scheme exhibits a poor performance and that widely used particle-tracking codes shall be updated to either the Milstein scheme or second-order schemes. It is further seen that the order of convergence is not the only relevant factor, the absolute value of the error also is.     

Evolution and degradation of flat‐top mesas in the hyper‐arid Negev,Israel revealed from 10Be cosmogenic nuclides

Mesas are ubiquitous landforms in arid and semiarid regions and are often characterized by horizontal stratified erodible rocks capped by more resistant strata. The accepted conceptual model for mesa evolution and degradation considers reduction in the width of the mesa flat‐top plateau due to cliff retreat but ignores possible denudation of the mesa flat‐top and the rates and mechanism of erosion. In this study we examine mesas in the northeastern hyperarid Negev Desert where they appear in various sizes and morphologies and represent different stages of mesa evolution. The variety of mesas within a single climatic zone allows examination of the process of mesa evolution through time. Two of the four sites examined are characterized by a relatively wide (200–230 m) flat‐top and a thick caprock whereas the other two are characterized by a much narrower remnant flat‐top (several meters) and thinner caprock. We use the concentration of the cosmogenic nuclide ¹⁰Be for: (a) determining the chronology of the various geomorphic features associated with the mesa; and (b) understanding geomorphic processes forming the mesa. The ¹⁰Be data, combined with field observations, suggest a correlation between the width of flat‐top mesa and the denudation and cliff retreat rates. Our results demonstrate that: (a) cliff retreat rates decrease with decreasing width of the flat‐top mesa; (b) vertical denudation rates increase with decreasing width of the flat‐top mesa below a critical value (~60 m, for the Negev Desert); (c) the reduction in the width of the flat‐top mesa is driven mainly by cliff retreat accompanied by extremely slow vertical denudation rate which can persist for a very long time (>10⁶ Ma); and (d) when the width of the mesa decreases below a certain threshold, its rate of denudation increases dramatically and mesa degradation is completed in a short time. Copyright © 2014 John Wiley & Sons, Ltd.     

Seasonal surface ocean circulation and dynamics in the Philippine Archipelago region during 2004–2008

The dynamics of the seasonal surface circulation in the Philippine Archipelago (117°E–128°E, 0°N–14°N) are investigated using a high-resolution configuration of the Regional Ocean Modeling System (ROMS) for the period of January 2004–March 2008. Three experiments were performed to estimate the relative importance of local, remote and tidal forcing. On the annual mean, the circulation in the Sulu Sea shows inflow from the South China Sea at the Mindoro and Balabac Straits, outflow into the Sulawesi Sea at the Sibutu Passage, and cyclonic circulation in the southern basin. A strong jet with a maximum speed exceeding 100 cm s⁻¹ forms in the northeast Sulu Sea where currents from the Mindoro and Tablas Straits converge. Within the Archipelago, strong westward currents in the Bohol Sea carry the surface water of the western Pacific (WP) from the Surigao Strait into the Sulu Sea via the Dipolog Strait. In the Sibuyan Sea, currents flow westward, which carry the surface water from the WP near the San Bernardino Strait into the Sulu Sea via the Tablas Strait.These surface currents exhibit strong variations or reversals from winter to summer. The cyclonic (anticyclonic) circulation during winter (summer) in the Sulu Sea and seasonally reversing currents within the Archipelago region during the peak of the winter (summer) monsoon result mainly from local wind forcing, while remote forcing dominates the current variations at the Mindoro Strait, western Sulu Sea and Sibutu passage before the monsoons reach their peaks. The temporal variations (with the mean removed), also referred to as anomalies, of volume transports in the upper 40 m at eight major Straits are caused predominantly by remote forcing, although local forcing can be large during sometime of a year. For example, at the Mindoro Strait, the correlation between the time series of transport anomalies due to total forcing (local, remote and tides) and that due only to the remote forcing is 0.81 above 95% significance, comparing to the correlation of 0.64 between the total and local forcing. Similarly, at the Sibutu Passage, the correlation is 0.96 for total versus remote effects, comparing to 0.53 for total versus local forcing. The standard deviations of transports from the total, remote and local effects are 0.59 Sv, 0.50 Sv, and 0.36 Sv, respectively, at the Mindoro Strait; and 1.21 Sv, 1.13 Sv, and 0.59 Sv at the Sibutu Passage. Nonlinear rectification of tides reduces the mean westward transports at the Surigao, San Bernardino and Dipolog Straits, and it also has non-negligible influence on the seasonal circulation in the Sulu Sea.     

Climate change: a profile of US climate scientists’ perspectives

Climate scientists have played a significant role in investigating global climate change. In the USA, a debate has swirled about whether a consensus on climate change exists among reputable scientists and this has entered the policy process. In order to better understand the views of US climate scientists, we conducted an empirical survey of US climate scientists (N?=?468) in 2005, and compared the results with the 2007 Intergovernmental Panel on Climate Change (IPCC) physical science report and policy summaries. Our results reveal that survey respondents generally agree about the nature, causes, and consequences of climate change, and are in agreement with IPCC findings. We also found that there is strong support for a variety of policy initiatives to reduce greenhouse gas emissions.     

The Keurusselkä impact structure,Finland—Impact origin confirmed by characterization of planar deformation features in quartz grains

Abstract– Although the meteorite impact origin of the Keurusselkä impact structure (central Finland) has been established on the basis of the occurrence of shatter cones, no detailed microscopic examination of the impactites from this structure has so far been made. Previous microscope investigations of in situ rocks did not yield any firm evidence of shock features (Raiskila et al. 2008; Kinnunen and Hietala 2009). We have carried out microscopic observations on petrographic thin sections from seven in situ shatter cone samples and report here the discovery of planar fractures (PFs) and planar deformation features (PDFs) in quartz and feldspar grains. The detection and characterization of microscopic shock metamorphic features in the investigated samples substantiates a meteorite impact origin for the Keurusselkä structure. The crystallographic orientations of 372 PDF sets in 276 quartz grains were measured, using a universal stage (U‐stage) microscope, for five of the seven distinct shatter cone samples. Based on our U‐stage results, we estimate that investigated shatter cone samples from the Keurusselkä structure have experienced peak shock pressures from approximately 2 GPa to slightly less than 20 GPa for the more heavily shocked samples. The decoration of most of the PDFs with fluid inclusions also indicates that these originally amorphous shock features were altered by postimpact processes. Finally, our field observations indicate that the exposed surface corresponds to the crater floor; it is, however, difficult to estimate the exact diameter of the structure and the precise amount of material that has been eroded since its formation.     

Jason-1 global statistical evaluation and performance assessment: Calibration and cross-calibration results

Since Jason-1launch, extensive validation of Jason-1 data and cross-calibration relative to TOPEX/Poseidon (T/P) have been performed by the CLS validation team within the CNES Jason-1 project. These validation activities are routinely operated as part of the Jason-1 ground segment, and often lead to in-depth studies to understand all validation conclusions. This paper presents the main results in terms of Jason-1 data quality: verification of data availability and validity, monitoring of the most relevant altimeter and radiometer parameters, assessment of the Jason-1 altimeter system performances. From global statistical analysis of more than 2 years of Jason-1 GDR data, results for all components of the altimeter measurement are derived in terms of bias, trend and precision. This work also represents a contribution to the estimation of the Jason-1 error budget. Thorough studies have been more focused on specific issues in relation to data quality: this is the case for the analysis of the high frequency content of the Jason-1 data and its impact on the T/P to Jason-1 comparison. From the results presented in this paper, it is demonstrated that the Jason-1 mission fulfils the requirements of high precision altimetry. In particular, it allows continuing the observation of the Mean Sea Level (MSL) variations at the same accuracy as T/P, which was one of the challenges of the Jason-1 mission. Potential improvements and open issues are also identified, with the objective of still making progress in terms of altimeter data quality.     

A SOLAR α - ω DYNAMO STUDY AND ITS TRANSITION TO CHAOS

In this contribution we present a nonlinear dynamo model, described by an infinite dimensional system of differential equations, whose solutions depend on the essential parameter D, the dynamo number. The solutions and the bifurcation points of the system are determined with the help of a new developed computer code. We show that, depending on D, stationary, oscillatory and chaotic solutions, which are characterized by Lyapunov exponents, result. We find that the solar dynamo may operate either in the chaotic or in the stable limit cycle domain, depending on the characteristic value of the dynamo number or the motion of the convection zone.     

Mechanized Algebraic Operations (MAO)

A software package for Mechanized Algebraic Operations (MAO) is described. With MAO one is able to manipulate on the computer Poisson series in literal form. The system is operational; it has application in the fields of celestial mechanics, astrodynamics, and nonlinear mechanics. Besides describing the system, the present paper suggests various techniques to prepare problems such that they lend themselves for an automated treatment with MAO. Optimized implementation of the general subroutines is discussed.     

General time elements for the two body problem

When integrating a perturbed two-body problem, very often the propagation of the numerical error is reduced by using a new time variables defined by dt/ds=|q| ⁿ , (|q| is the radial distance,t the time). This paper introduces a time element for such transformations, i.e., a new variablet _n is defined so that dt _n/ds=1+ (perturbing terms) andt=F _n(t_n), whereF _n is a known function. The time element equation should be useful in reducing the error in the determination of the timet.F _n is given explicitly forn=1, 3/2, 2, 5/2 and 3, and a general expression is given for other values.The work was performed while the author was an NRC Senior Research Associate, Goddard Space Flight Center, Greenbelt, Md., U.S.A.