Electromagnetic 2.5-dimensional forward modelling with a boundary integral formulation

An effective and accurate technique for the numerical solution of 2-D electromagnetic scattering problems with 3-D sources is presented. This solution introduces a set of the usual boundary integral equations and uses a scalar Green's function. In this scalar version, the unknowns of the problem are the boundary values of the longitudinal fields and their normal derivatives in the Fourier domain. A generalization of the usual boundary integral formulation enables us to handle a large class of models composed of piecewise homogeneous domains, including contiguous domains, multiply-connected domains and unbounded domains. This formulation involves the solution of a system of linear equations, and results in a significant saving in computation time in comparison with other rigorous methods.
  The requirements for the numerical implementation of this solution are described in detail. Numerical tests were carried out using the important example of electromagnetic tomography. The specific symmetry properties of the response function in this case are illustrated. Numerical accuracy is verified over a large frequency range, up to 1  MHz.     

Tropical cyclones, 6–25 day oscillations, and tropical-extratropical interaction over the northwestern Pacific

Summary This paper investigates tropical-extratropical interactions over the northwestern Pacific Ocean that involve tropical cyclones and subtropical jet streaks. Another aspect of this study is to examine the relation between 6–25 day convective variability and tropical cyclones. This investigation is conducted for the fall and early winter season, with a focus on the months, October through December (OND). In addition to outgoing longwave radiation (OLR) data, we use 10 years (1985–1994) of WCRP/TOGA archive II analyses produced by ECMWF to compute equivalent temperature, _e , precipitable water, W, and kinematic and kinetic energy transfer variables. These variables are composited for two classes of tropical cyclones, recurving cyclones (RCs) and non-recurving cyclones (NCRs), in order to examine the influence of tropical cyclones and baroclinic processes on changes in the jet streak intensity.We found that RCs interacted with extratropical regions during all composite days. A strong baroclinic zone developed throughout the troposphere on the north side of the composite cyclone as it propagated poleward. Between the day of recurvature, DR, and the day after recurvature, DR+1, the main band of convection shifted from the RC to a frontal band within the baroclinic zone indicating a transformation of the tropical cyclone into an extratropical one. An eastward propagating jet streak at 200 hPa, located north of the RC and in the vicinity of the baroclinic zone, increased its speed from 57 ms^–1 to 79 ms^–1 on DR+1. Although we could not measure the role of baroclinic processes in this regard, we were able to infer that upper-level outflow from the RC did supply momentum and energy to the jet streak.Whereas we expected tropical-extratropical interactions for the RCs, we also found evidence that NRCs that stay south of 20° N throughout their lifetime and that dissipate over Indo-China have an influence on the subtropical jet by their upper-level outflow, especially in the late OND season. The tropical (i.e., momentum) forcing did appear to cause increases in the speed of the jet after the composited storm crossed the Phillippines on the fourth day of its life cycle, D4. Concurrently, a baroclinic zone developed along the coast of southern China by about D4, but it was confined to the lower troposphere.Finally, our spectral analysis investigations for the northwestern Pacific showed significant peaks at 6–10 days and 15–20 days from late September to early December. The first peak is well known and is associated with typhoon activity. In several of the investigated autumn seasons (1987, 1989, 1992, and 1993), the second peak was clearly related to the recurrence interval of northwestern Pacific tropical cyclones. This result is in accordance with the findings of Hartmann et al. (1992). For some years of the investigation period (1985, 1986, and 1988), however, our results showed that westward propagating convective disturbances that fail to reach tropical depression strength also contribute to the power in the 15–25 day band, whereas in a few years (1990 and 1991), no OLR peak between 15 and 20 days could be found at all. Therefore, it appears that further work needs to be done with regard to the relationship between convective systems and their accompanying relationships on time scales ranging between 10 and 25 days.With 15 Figures     

Investigation and sensitivity analysis of a mechanistic phytoplankton model implemented in a new modular numerical tool (Eco3M) dedicated to biogeochemical modelling

A new class of phytoplankton models with a mechanistic basis has been presented in a companion paper (Baklouti, M., Diaz, F., Pinazo, C., Faure, V., Queguiner, B., 2006. Investigation of mechanistic formulations depicting phytoplankton dynamics for models of marine pelagic ecosystems. Progress in Oceanography). It is the default class of models implemented in our new numerical tool Eco3M, which is dedicated to Ecological, Mechanistic and Modular Modelling. A brief overview of its main features is given in Section 2 of the present paper. In the next sections, a particular phytoplankton model among the aforementioned class has been tested with special emphasis on the mechanistic photosynthesis component relating the photosynthetic rate to the proportion of open photosystems II. The present study encompasses several essential steps that are inherent to any modelling, including model reduction, model sensitivity analysis and comparison of model outputs with experiments. The global sensitivity analysis of the plankton model for one-at-a-time parameter perturbations revealed a restricted set of parameters having major influence on the model outputs. Sensitivity tests involving simultaneous parameter perturbations within the range actually encountered in the literature provided a confidence interval for the outputs. Chemostat experiments performed on nitrate-limited diatoms grown under low (LL) and high-light (HL) conditions have been used for comparison with model outputs. The good fit between measured data and model outputs using the same parameter values in both the LL and HL cases demonstrates the ability of our model to represent the main features of phytoplankton dynamics including photoacclimation. Finally, Eco3M is ultimately intended to include explicit bacterial and zooplankton compartments, as well as to be coupled with ocean circulation models, but the intrinsic behavior of the phytoplankton model has been investigated first, independently of physical forcing.     

Coupling mechanisms in double sandbar systems. Part 2: impact on alongshore variability of inner‐bar rip channels

Double sandbar systems are common morphological features along sandy, wave‐dominated, micro‐ to meso‐tidal coastlines. In the companion paper, we demonstrated how various alongshore inner‐bar rip‐channel patterns can develop through morphological coupling to an alongshore‐variable outer bar. The simulated coupling patterns are, however, scarcely observed in the field. Instead, inner‐bar rip channels more often possess remarkably smaller and more variable alongshore length scales, suggesting that coupling mechanisms do not play a substantial role in the overall double‐sandbar dynamics. Here we use a numerical model to show that the relative importance of self‐organization and morphological coupling changes in favour of the latter with an increase in waterdepth variability along the outer‐bar crest. Furthermore, we find that the typical alongshore variability in inner‐bar rip‐channel scale is indicative of a mixture of self‐organization and morphological coupling rather than self‐organization alone. Morphological coupling may thus be more important to understanding and predicting the evolution of inner‐bar rip channels than previously envisaged. Copyright © 2010 John Wiley and Sons, Ltd.     

Data Assimilation (4D-VAR) for Shallow-Water Flow: The Case of the Chicoutimi River

This is a web presentation of the work presented at the 10th Annual Conference of the CFD Society of Canada, “CFD 2002”, at the University of Windsor on June 9-11, 2002. This discussion paper presents the four-dimensional variational data assimilation (4D-VAR) technique as a tool to forecast floods. This discussion will be limited to hydrological forecast. We assume that the weather, here a large rainstorm, had already been forecasted by the meteorological services. In the 4D-VAR technique, we need to minimize, in the sense of Lagrange, a cost function which measures the difference between the forecast and the observations. The physical equations acts as a set of constraints. Here, the model is the shallow-water equations modified to include sediment transport. The minimum was found by using the steepest descent algorithm. This is made possible because the gradient of the cost function can be calculated analytically by using the adjoint equations of the model. To illustrate the 4D-VAR technique, the bypass of a simple theoretical dam as well as the more complex overflowing of the Chicoutimi River at the Chute-Garneau dam (during the 1996 flood) are investigated.     

The Kaidun meteorite: Clasts of alkaline‐rich fractionated materials

Abstract— Clasts of alkaline (the second find in meteorites) and subalkaline rocks were found in the Kaidun meteorite. One of them (#d4A) is a large crystal of albite with inclusions of fluorapatite, arfvedsonite, aenigmatite, and wilkinsonite. The two latter minerals were previously unknown in meteorites. Another clast (#d[3–5]D) has a melt crystallization texture of mainly feldspar (oligoclase) composition and contains relict grains of both high‐Ca and low‐Ca pyroxene and fluorapatite. The mineralogical characteristics of these clasts suggest a genetic relationship and an origin from the same parent body. The textural and mineralogical characteristics of the clasts indicate origin by extensive igneous differentiation. Such processes most likely took place in a rather large differentiated body. The material of clast #d(3–5)D is similar in some mineralogical respects to basaltic shergottites.     

Differential absorption measurements of mesospheric and lower thermospheric electron densities using the Buckland Park MF radar

The differential absorption experiment (DAE) was first proposed in the 1950s for the estimation of mesospheric and lower thermospheric electron density using MF/HF radars. The technique was used extensively until the late 1970s, when interest in the technique declined, due to experimental limitations and questions regarding the assumptions of the technique. This paper describes the application of the DAE within the online observations of the Buckland Park MF (BPMF) radar. The experimental limitations of the technique for the BPMF radar are discussed, with particular attention paid to effects of complex gain differences between receiving channels used to decompose the linearly received signals into circular components. Hourly and monthly averaged midday DAE electron densities are presented, revealing good agreement with IRI model estimates. Monthly averaged midnight DAE electron densities are also presented, revealing good qualitative agreement with the IRI model estimates.