Effects of bed perturbation and velocity asymmetry on ripple initiation: wave-flume experiments

Laboratory experiments using a wave flume were designed to examine the threshold condition for ripple formation under asymmetrical oscillatory flows on an artificially roughened bed. Three types of sand beds were prepared in the experiments: they were flat, notched, and notch-mounded beds with bed roughness increasing in this order. The beds were constructed with three kinds of well-sorted sand with similar density, but different diameters. Data analyses were made using the two dimensionless parameters: the mobility number, M, a simplified form of the Shields number, and the Ursell number, U, a surrogate for asymmetry of flow field. The result confirmed that the threshold for ripple initiation is decreased with increasing bed perturbation and that as the bed perturbation increases, the dependency of this threshold on the flow asymmetry becomes less and finally null for the notch-mounded bed. This relationship is quantified by the following equations: M=17−14.5e^−0.03U on the flat bed, M=5.0−2.5e^−0.1U for the notched bed, and M=2.5 for the notch-mounded bed. A comparison between the previous field data and the present laboratory findings indicates that the threshold in the notch-mounded bed experiment, M=2.5, seems to provide a critical condition for rippling in the natural environment.     

Availability of an equation to evaluate error by optical path discretization in radiative transfer computation based on the successive order of scattering method

The availability of an equation to evaluate the influence of multiple scattering in the single scattering process corresponding to a layer of arbitrary optical thickness was established. In order to confirm the validity of this equation, the radiance distribution in this layer was computed using a plane–parallel layer model based on the successive order of scattering method. The relative errors in a radiance distribution computed were evaluated as a function of optical thickness by the derived equation. It was shown that this equation provides a theoretical background for determining layer thickness using the plane–parallel layer model.     

Congruence between euphausiid community and water region in the northwestern Pacific: particularly in the Oyashio–Kuroshio Mixed Water Region

The relationship between euphausiid community structure and water region was studied during a 2-year seasonal survey in the northwestern (NW) Pacific Ocean. The euphausiid community structure and its associated species were analyzed from 38 micronekton samples collected during eight cruises. The euphausiid community structure and its distribution patterns clearly corresponded to physical oceanographic features in the Oyashio region, Oyashio–Kuroshio Mixed Water Region (OKMWR), and Kuroshio region. In contrast, community structure was unrelated to seasonality. The 19 species out of 40 identified in this area were grouped and named after their habitats. The six cold-water species were grouped into three regional types: two coastal Oyashio species, three Oyashio–OKMWR species, and one Oyashio–Kuroshio species. The four species dominating in the OKMWR were categorized into each specific types: Nematoscelis difficilis as OKMWR–Oyashio species, Euphausia gibboides as OKMWR species, Euphausia similis as OKMWR–Oyashio & OKMWR–Kuroshio species, and Euphausia recurva as OKMWR–Kuroshio species. The seven warm-water species were categorized as Kuroshio–OKMWR species or Kuroshio species. The other two species were categorized as cosmopolitan species. In particular, regarding the result in the OKMWR, our study suggest that (1) the OKMWR has high species diversity, and (2) the dominant species, such as Euphausia pacifica, N. difficilis, E. similis, and E. gibboides, are considered to be key species in the food webs in this region.     

Reconstruction of the global lightning distribution from ELF electromagnetic background signals

The global lightning distribution has been obtained for the first time as the result of an inverse problem for the natural extremely low-frequency (ELF) electromagnetic background signals measured simultaneously at three stations: Lekhta (Karelia, Russia), Moshiri (Hokkaido, Japan), and West Greenwich (Rhode Island, USA). We employed a two-step technique consisting of: (1) inversion of ELF field spectra to a source distance distribution (distance profile) for each station of the network and (2) treating the obtained distance profiles as projections in the tomographic method. The structure of the source distance profiles reconstructed from experimental field spectra, varies during a day in a manner specific for each station, which can be explained generally by the temporal distribution of the sources in the tropical continental areas, so-called world thunderstorm centres (WTCs). The comparison of the obtained source distributions with the satellite data on lightning collected by Optical Transient Detector (OTD) and Lightning Imaging Sensor (LIS) shows satisfactory agreement, which proves that our newly developed inversion technique is a suitable method for mapping global lightning.     

Transient three‐dimensional‐patterned ripples appear during deformation of a two‐dimensional ripple field under wave‐induced oscillatory flow

Wave‐flume experiments on the process of deformation of pre‐existing two‐dimensional ripples revealed that one of two distinct types of three‐dimensional patterned ripple, the rhombic or the barrel type, emerged in the early stages of the experimental runs. Rhombic‐type ripples were characterized by a staggered pattern of similar‐sized mounds, whereas the barrel‐type ripples had a barrel‐like plan shape arranged in echelons. The former ripples were formed under strongly asymmetrical flow conditions with large excursion distances of water particle motion near the pre‐existing rippled bed, whereas the latter were formed under relatively symmetrical flow conditions with small excursion distances. The emergence of both types of ripples was also highly dependent on sediment grain size. Copyright © 2009 John Wiley & Sons, Ltd.     

Revisiting the Basin-edge Effect at Kobe During the 1995 Hyogo-Ken Nanbu Earthquake

The basin edge effect, i.e., the interference of the direct S wave with the surface wave diffracted off the basin edge has been invoked by many authors to explain the damage distribution during the January 17, 1995 Hyogo-Ken Nanbu (Kobe) earthquake. Here we present the results of numerical experiments obtained with the spectral element method in 2-D geometry. Our results confirm that the amplification of horizontal motion close to the basin edge can be twice as large as the one measured in the center of the basin. This additional amplification is shown to depend strongly on the edge geometry and on frequency, due to physical dispersion of diffracted surface waves. In particular we obtain maximal amplification around 3 Hz, at frequencies critical for buildings.     

A laboratory study of formative conditions for characteristic ripple patterns associated with a change in wave conditions

A wave‐?ume experiment was conducted to examine the formative condition for three types of distinctive bedforms that emerged through deformation of existing ripples due to waning wave power. They were ripple marks with: (1) a single secondary crest, (2) double secondary crests, and (3) a rounded crest. Data were analysed using two parameters, kh and d₀/λ_*, where k is the wave number, h is the water depth, d₀ is the near‐bottom orbital diameter, and λ_* is the spacing of existing ripples. The former quantity, kh, was employed as a surrogate for the degree of ?ow asymmetry. The result showed that ripples with secondary crests developed under a rather symmetrical ?ow ?eld with kh ? 0·7, if d₀/λ_* ? 1·2, whereas rounded‐crest ripples emerged under asymmetrical ?ow ?eld with kh ? 0·7, if d₀/λ_* ? ?2·9 kh + 3·2. The number of secondary crests, which initially occurred in each trough, was single if d₀/λ_* ? 0·8, or double if d₀/λ_* ? 0·8. Copyright © 2004 John Wiley & Sons, Ltd.     

A photometric and polarimetric study of the moon's surface

The results of the photometric and polarimetric observation of the Moon's surface with high resolution from October 1969 to March 1971 are discussed. It is found that, for a wide part of the Moon's illuminated surface, the brightnessm, expressed by visual magnitude per square second of arc, can be expressed as $$m = 5.0 - 2.5\log H + 2.5\log \{ {{P_v } \mathord{\left/ {\vphantom {{P_v } P}} \right. \kern-\nulldelimiterspace} P}(\alpha )\} ,$$ whereH is the variable part of Hapke's formula,P _v is the observed polarization degree, andP(α) is a function of the phase angleα. The color shows a tendency to reddening at the enhancement of the lunar brightness, and this is expressed by the relation $$B - V = - 0.2\Delta m + 1.7,$$ where Δm =m + 2.5 logH which is a sort of residual from the brightness calculated by Hapke's formula. Remarkable enhancements of the lunar brightness correspond to solar flares which appeared just before the observations. Because of this the above formula for the observed brightness can be interpreted by assuming the luminescence on the lunar surface stimulated by the solar activity.     

Recent Advances in Larval Recruitment Processes of Scyllarid and Palinurid Lobsters in Japanese Waters

Phyllosoma larvae collected to date in Japanese and Taiwanese waters have been classified into two genera (Linuparus, Panulirus) of the Palinuridae, four genera (Ibacus, Parribacus, Scyllarides, Scyllarus) of the Scyllaridae, and one genus (Palinurellus) of the Synaxidae. However, phyllosoma larvae of three Scyllarus species (S. bicuspidatus, S. cultrifer, S. kitanoviriosus) are absolutely dominant among the larvae collected in the waters. Scyllarus larvae are abundant in coastal waters while those of Panulirus are often collected in offshore/oceanic waters. Based on previous and ongoing studies dealing with spatial distributions of phyllosoma larvae in Japanese and Taiwanese waters, it appears that phyllosoma and nisto larvae of the Scyllarus are retained within coastal waters north of the Kuroshio Current. On the other hand, the life history of the Panulirus (particularly P. japonicus) may be completed within the Kuroshio Subgyre: their phyllosoma larvae may be flushed out from coastal waters into the Kuroshio, then transported through the Counter Current south of the Kuroshio into the water east of Ryukyu Archipelago and Taiwan where they attain the subfinal/final phyllosoma or puerulus stages, once again entering the Kuroshio and dispersing into coastal waters. This revised version was published online in July 2006 with corrections to the Cover Date. An erratum to this article is available at .