A quasi-similarity between wind waves and solid surfaces in their roughness characteristics

A formulation for the aerodynamic roughness length of air flow over wind waves $$z_0 = \gamma {\text{ }}u_* /\sigma p$$ which was proposed by Toba (1979) and Toba and Koga (1986) from dimensional considerations with some data analysis, is shown to correspond with a formulation for irregular solid surfaces $$(z_0 /h) = a(h/l)^{1 + \beta } $$ which resulted from work by Woodinget al. (1973) and Kustas and Brutsaert (1986);u _* is the friction velocity,σ _p the spectral peak frequency of wind waves,h the mean height of the solid obstacles,l the mean distance between their crests, andα,Β, andγ are constants. This correspondence is reached by the existence of a statistical 3/2-power law and an effective dispersion relationship for wind waves. Because both approaches of parameterizingz ₀ were arrived at independently, they provide each other mutual reinforcement.     

Prediction of ocean waves based on the single-parameter growth equation of wind waves

A method for the prediction of ocean waves was developed on the basis of the single-parameter growth equation of wind waves, proposed byToba (1978) on the basis of similarity in growing wind waves. The applicability of the method to actual problems was tested by hindcasting the wave characteristics with the method, for two cases with differing time and space scales, one in Kii Channel Approach, Japan, and the other in the North Atlantic Ocean. The results showed that the present method can predict waves within an error of 1.3 m in wave heights, which ranged from 3 to 12 m.     

Turbulent structure in water under laboratory wind waves

The structure of the turbulent boundary layer underneath laboratory wind waves was studied by using a combination of a high-sensitivity thermometer array with a two-component sonic flowmeter. The temperature fluctuations are used to detect movements of water parcels, with temperature as a passive quantity. The turbulence energy was dominant in the frequency range (0.01 0.1 Hz), which was much smaller than the wind-wave frequency (2 5 Hz), and in which the turbulence was anisotropic. There was a frequency range (0.2 2 Hz for velocity, 0.2 5 Hz for temperature fluctuation) where the turbulence was isotropic and had a –5/3 slope in the energy spectrum. These points are the same as those in previous works. However, by analyses of the time series by using a variable-interval time-averaging technique (VITA), it has been found that conspicuous events in this main turbulence energy band are the downward bursting from the vicinity of the water surface. Thus the structure of the water layer underneath the wind waves has characters which are similar to the familiar turbulent boundary layer over a rough solid wall, as already conceived. It has been found that, at the same time, the turbulence energy can be related to quantities of the wind waves (the root mean squared water level fluctuation and the wave peak frequency), for different wind and wave conditions. That is, the turbulence underneath the wind waves develops under a close coupling with the wind waves.     

Velocity variation of the Kuroshio during formation of the small meander south of Kyûshû

Historical GEK data provided by JODC is analyzed to investigate the characteristic variation in velocity of the Kuroshio, with special reference to the formation of small meanders south of Kyûshû. It is found that, during or prior to the period of small meander formation, there is a tendency for an abrupt increase in the current velocity west of Yaku-Shima (Yaku-Island), representing an increase in the main current intensity upstream. Also, there are apparent time lags in the variation in current velocity along the path of the Kuroshio between the upstream and the downstream regions of the small meander area. Namely, it is apparent that the increase in Kuroshio velocity in the Satsunan Strait procedes that offshore of Shikoku during the period of the small meander formation, by the order of one month. These results indicate that a nonlinear effect due to the increase in current velocity is a possible cause of the generation of small meanders.     

Northward cooling of the Kuncha nappe and downward heating of the Lesser Himalayan autochthon distributed to the south of Mt. Annapurna,western central Nepal

We investigated the tectonothermal history of the Lesser Himalayan sediments (LHS), which are tectonically overlain by the Higher Himalayan Crystalline. Fission‐track dating and the track length measurement of detrital zircons obtained from the Kuncha nappe and the Lesser Himalayan autochthonous sediments in western central Nepal revealed northward cooling of the nappe and possible downward heating of the autochthon by the overlying hot nappe. Nine zircon fission‐track (ZFT) ages of the nappe showed northward‐younging linear distribution from 11.6 Ma in the front at Tamghas, 6 Ma in the central at Naudanda, and 1.6 Ma in the northernmost point at Tatopani. Thermochronological invert calculation of the ZFT length elucidated that the Kuncha nappe gradually cooled down (30 °C/Myr) at the front and rapidly cooled down (120 °C/Myr) at the root zone. In contrast, the ZFT age of the Chappani Formation, located just beneath the Kuncha nappe in the central part, demonstrated a totally reset age of 6.8 Ma, whereas the Virkot Formation, structurally far from the nappe, yielded a partially reset age of 457.3 Ma. This suggests that the LHS underwent downward heating, resulting in a thermal print on the upper part of the LHS; however, the thermal effect was not sufficient to anneal ZFT totally in the deeper part. Presently, the nappe cover is eroded and denuded from this area. Detrital zircons from the Chappani Formation in Tansen area to the south of the Bari Gad Fault did not show any evidence of annealing, suggesting that nappe never covered the LHS distributed to the south of the fault.     

Discrimination of spurious self-correlation in nondimensionalized analyses of fluid dynamical data

Nondimensionalization of variables enables us to treat experiment data much more simply and efficiently by decreasing the number of variables. In some cases, trivial conclusions (which Kenney, 1982, called spurious self-correlation) result from a formal application of dimensional analyses. In contrast, in some cases fully significant conclusions can be derived. We first discuss how to construct nondimensional variables retaining the physical meanings of variables. We then propose simple and efficient methods, especially the use of “spurious triangle (SpT)”, to discriminate between significant conclusions and spurious self-correlations in the analysis of nondimensionalized variables.     

Climato-limnological signals during the past 260 000 years in physical properties of bottom sediments from Lake Baikal

The St.16 core obtained from the Academician Ridge of Lake Baikal in eastern Siberia may span about 260 000 years, and some physical properties of the core samples are closely related to aquatic paleoproductivity and climatic change. The median of grain size, grain density, and water content fluctuate synchronously. They also are connected with change in the abundance of biogenic silica (diatoms). The physical parameters indicate that there were high aquatic productivity periods around interglacial periods (MIS 5 and 7; 70 000-125 000 yr B.P. and 180 000-250 000 yr B.P.). Comparatively large clastics were transported from outside of the lake through various routes (ice rafting, etc.) in addition to fluvial routes during the glacials or 'stadials. There are ca. 20 000 yr, 40 000 yr and 100 000 yr periods in the variations of physical properties. These are related to the three Milankovitch parameters of solar insolation.     

Radiative Heat Transfer and Hydrostatic Stability in Nocturnal Fog

We have performed a one-dimensional and transient radiative heat transfer analysis in order to investigate interaction between atmospheric radiation and convective instability within a nocturnal fog. The radiation element method using the Ray Emission Model (REM²), which is a generalized numerical method, in conjunction with a line-by-line (LBL) method, is employed to attain high spectral resolution calculations for anisotropically scattering fog. The results show that the convective instability has a strong dependence on radiative properties of the fog. For the condition of a 20-m droplet diameter and liquid water content of 0.1 × 10^–3 kg m^–3;, the temperature profile within the fog becomes S shaped, and a convective instability layer forms in the middle or lower level of the fog. However, for the same water content and a 40-m diameter droplet, no strong convective instability layer forms, whereas for a 10-m diameter droplet a strong convective instability is observed.