Ensemble numerical forecasts of the sporadic Kuroshio water intrusion (kyucho) into shelf and coastal waters

The finite volume coastal ocean model downscaling ocean reanalysis and forecast data provided by the Japan Coastal Ocean Predictability Experiment (JCOPE2) are used to forecast sudden Kuroshio water intrusion events (kyucho) induced by frontal waves amplified south of the Bungo Channel in 2010. Two-month hindcast computations give initial conditions of the following 3-month forecasts computations which consist of ten ensemble members. The temperature time series computed by these ten members are averaged to compare with that actually observed in the Bungo Channel, where sudden temperature rises related to kyucho events are remarkable in February, August, and September. Overall, the intense kyucho events actually observed in these months are predicted successfully. However, intense kyucho events are forecasted frequently during the period of May through June even though intense kyucho events are absent during this period in the actual ocean. It is suggested that the present downscaling forecast model requires reliable lateral boundary conditions provided by JCOPE2 data to which numerous Argo data are assimilated to enhance the accuracy. In addition, it seems likely that the model accuracy is reduced by small eddies moving along the shelf break.     

A non-linear rheological analysis of deeply located tunnels

A rheological model which is able to explain the delayed failure phenomenon has been proposed and applied to the analysis of the time dependent behaviour of deep tunnels. The model is the three element Voigt type before yielding and becomes the five element Voigt type after yielding. This means that the stiffness of the model is decreased by yielding. It is assumed that the yielding takes place when the stored elastic energy density of distortion reaches a certain maximum value. Some experiments to determine material constants are proposed. The numerical results clearly exhibited a delayed failure state of the rock masses around the tunnel surface. It was found that the extension of the yielding zone due to tunnel excavation depends on the opening method.     

Fundamental concepts of exchange and transport time scales in a coastal sea

Concepts of age, residence time, transit time, and turn-over time are summarized which are useful for describing the exchange and transport of water or materials in a coastal sea. The age of a particle is defined as a time which has elapsed since it entered the reservoir, and the residence time is defined as a time which will be taken for a particle to reach the outlet. Time scales based on the age are simply related with those based on the residence time. It is shown that a suitable time scale for representing the exchange characteristics is the average residence time and not the turnover time, which has often been used as the exchange time scale. Further, the ‘remnant function’ which describes the phenomena of exchange or transport is introduced, and is related to the residence time. Exchange and transport time scales in a coastal sea are discussed on the basis of the residence time which can be applied to not only steady-state cases, but also the cases where material is injected instantaneously. The average residence time in a one-dimensional channel and bay is obtained from the solutions of the advection-diffusion equation. If we know a flow speed and diffusion coefficient in a channel or bay regarded as one-dimensional, we can translate them into the average residence time. As an example, the average residence time of the Seto Inland Sea is discussed.     

Estimation of urban heat island intensity using biases in surface air temperature simulated by a nonhydrostatic regional climate model

This study demonstrates that urban heat island (UHI) intensity can be estimated by comparing observational data and the outputs of a well-developed high-resolution regional climate model. Such an estimate is possible because the observations include the effects of UHI, whereas the model used does not include urban effects. Therefore, the errors in the simulated surface air temperature, defined as the difference between simulated and observed temperatures (simulated minus observed), are negative in urban areas but 0 in rural areas. UHI intensity is estimated by calculating the difference in temperature error between urban and rural areas. Our results indicate that overall UHI intensity in Japan is 1.5 K and that the intensity is greater in nighttime than in daytime, consistent with the previous studies. This study also shows that root mean square error and the magnitude of systematic error for the annual mean temperature are small (within 1.0 K).