Sei-ichi Kanari

An inference on the process of gas outburst from Lake Nyos, Cameroon

Abstract A large volume of lethal gas was released from Lake Nyos in the Northwest Province of Cameroon on August 21, 1986. Some 50 days after the event, a hydro-physical and -chemical survey of Lake Nyos was carried out by four Japanese scientists including the author. Vertical density distributions of lake water showed remarkably stable stratification through the whole water column. Chemical analyses showed high concentrations of dissolved CO2 and Fe which contribute to stable stratification. Assuming an initial saturation of dissolved CO2 in lake water, released gas volume was estimated to be 0.68 km3 as a difference between saturated and remaining gas contents in the present lake. Temperature drops due to quasiadiabatic expansion of CO2 gas exsolved from various water layers were estimated for every 10 m layer thickness assuming a constant temperature distribution and a complete thermal exchange between expanding gas and lake water. The calculated temperature profile agreed well with the profile measured by CTD (conductivity-temperature-depth profiler). The height of the gas cloud over the lake and the flow velocity of draining gas from the outlet of the rim were evaluated using a simple geometrical flux model. Assuming a sinusoidal variation of ejection with equal volume mixing with air and head loss coefficient of Cd=0.3, the calculated maximum gas height of 110 m and the corresponding drain velocity of 19 m s−1 were obtained.

Vertical mixing in the bottom mixed layer near the continental shelf break in the East China Sea

The strength of the vertical mixing in the bottom mixed layer near the continental shelf break in the East China Sea was directly measured with the Micro-Scale Profiler (MSP). It has been shown that there is no significant statistical relation between the turbulent energy dissipation ε and the degree of the stratificationN2. It seems that the vigorous turbulence occurs not constantly but intermittently in the bottom mixed layer so that a large variation of ε is found depending on the time. In contrast to ε, the coefficient of the vertical eddy diffusivityKz is mostly determined byN such thatKz is large in the bottom mixed layer and small in the thermocline. Large value ofKz in the bottom mixed layer is also found in the time series ofKz estimated in terms of Richardson number calculated from the data obtained with electromagnetic current meters. The value ofKz more than 10 cm2s−1 frequently occur in the layer of 20–25 m thick just above the bottom.

Micro-scale profiler (MSP) for measurement of small-scale turbulence in the ocean

This paper describes a free-fall micro-scale profiler (MSP) which can measure vertical profiles of microscale velocity shear, temperature gradient, and conductivity gradient of both fine and microscale ranges, together with the vertical profiles of temperature and conductivity. The measurable vertical scales of the microscale shear range from about 50 cm to 2 cm.The MSP was designed to perform profiling to depths of 500 m with a nominal fall speed of about 70 cm sec−1. Retrieval of the instrument is accomplished by a 700 m Kevlar string of 1.5 mm in diameter wound on a portable winch, after ballast has been released at a present depth by a mechanical rupture disk.The results of sea trials show that the microscale shear spectrum nearly corresponds with the theoretical one derived on the assumption of homogeneous turbulence.

Turbulent Structure in the Upper Layer of the Western Equatorial Pacific Ocean

The Richardson number dependence of vertical eddy diffusion coefficients in the western Equatorial Pacific Ocean was examined on the basis of a Microstructure Profiler (MSP) observations during the cruise of Natsushima (JAPACS-89). The Richardson numberRi was estimated by using the mean shear of velocity profile measured by an Acoustic Doppler Current Profiler (ADCP) with the vertical interval of 15 meters within one or two hours of the each MSP cast. The raw data plot of the vertical eddy diffusion coefficientKp shows a large scatter with increasing tendency belowRi=0.5. The relation between the mean vertical eddy diffusion coefficientKp and the Richardson numberRi, averaged over every 0.025 in theRi, supports the model of Pacanowski and Philander (1981) in the range ofRi>0.5, but coincides with the result of Peterset al. (1988) in the range ofRi<0.5.

Surface Mixed Layer Observation Using a Meteo-Oceanographic Spar Buoy, XTGP and SEMVP System

Surface mixed layer observation using a meteo-oceanographic spar buoy, XTGP and SEMVP systems are now in progress at a site 6 km off Yoichi coast of Hokkaido.

Bashi channel, Luzon strait: A hydraulic model experiment of the tidal current

Bottom current conditions due to tidal currents in the area of the Bashi Channel, Luzon Strait, are examined using a scaled hydraulic model in a uni-directional flow channel. The geometrical model is scaled in both the similitude of Froudes Number and the similitude of the four-thirds power law of eddy viscosity.The currents obtained from the experiment seem to be consistent in order of magnitude with the estimated currents based on observations made by Offshore Environmental System Ltd. In a quasi-steady tidal phase, formation of a strong core flow is found in the Bashi Channel. It can be explained by a simple theoretical model in which the formation of the strong current is attributed to the submarine topography of the Luzon Strait.