Performance of a 250-m Grid Eulerian Dispersion Simulation Evaluated at Two Coastal Monitoring Stations in the Vicinity of the Fukushima Daiichi Nuclear Power Plant

Abstract

Using two horizontal resolutions (3 km and 250 m), this study examined the performance of Eulerian models in simulating dispersion fields at two coastal monitoring stations in the vicinity of a pollutant source (3.2 km and 17.5 km distant) under the situation of the Fukushima 2011 nuclear accident. A 250-m grid simulation was newly conducted for the examination and was able to reproduce the wind and concentration fields in detail over complex terrain. The 3-km grid model could not reproduce the details of the winds and plumes around the Fukushima Daiichi Nuclear Power Plant but occasionally yielded a higher performance with a lower undetected error rate compared with the 250-m grid model due to the large numerical diffusion of the former. A disadvantage of Eulerian dispersion models is expected to be the artificial numerical diffusion in the advection process near emission sources. The artificial numerical diffusion increases the false alarm ratio (number of strikeouts while swinging) but fortunately decreases the undetected error rate (number of strikeouts while looking). This characteristic is appropriate for environmental emergency response (EER) systems. Furthermore, the 250-m grid model result was improved by a plume augmentation (i.e., max pooling) process, which enlarged the plume widths and masked short time lags and small plume drifts. Plume augmentation was advantageous to the high-resolution model for improving statistical scores, which is beneficial for EER systems.