An offshore plume loop and subsequent excursion inland to the south could transport any airborne material toward land in the early hours of Wednesday April 13 Japan Standard Time (JST). Northward plume transport in the morning, followed by offshore flow dominates through the afternoon of Thursday April 14 JST. Late Thursday afternoon and evening the plume is expected to turn north and inland.
The government has suggested that low-level airborne release of radiation continues, with levels drastically reduced since the peak. No documented substantial releases have occurred recently, although officials note there is still the potential for significant releases at the site. U.S. NRC chair:
“We describe the situation as static but not yet stable,” Jaczko said. “It hasn’t really changed too much in the last few days,” he said, adding it will be weeks or even months before the plant is stabilized.
The Naval Research Laboratory’s skillful data-assimilating triply nested (45 km/15 km/5 km) mesoscale meteorological model COAMPS produced the fields above, using a passive tracer to map the expected plume trajectory. Lighter gray contours show more concentrated material. Each shade of gray represents a factor of 10 difference in tracer concentration. [Plume images are a research tool; contours are arbitrary units for visualization purposes.]
The modeled “plume” is a tracer that is released at a constant concentration, simply follows the air flow, and (a) does not settle due to gravity, (b) does not get deposited with rain, and (c) does not have any radioactive decay. These neglected factors typically dramatically reduce the effects of radiation when it travels over distances of hundreds of miles. There is much uncertainty in the source term, as the releases of radiation that occurred in the past were highly intermittent. (So although modeled as such, the plume in actuality was not a continuous release.)
We have utilized COAMPS to make detailed and accurate forecasts of various coastal regions, including large-scale (synoptic) weather patterns. It also is skillful at simulating local coastal processes such as the influence of cold upwelled ocean waters, or sea breezes, when there is no strong synoptic forcing. We have previously used the model to simulate the movement of hypothetical releases along the east coast of Japan.