The earthquake of March 11, 2011, off the coast of Sendai, Japan, brought enormous suffering to a nation that is likely more accustomed and better prepared than any other to the threat of earthquakes. But more unfortunate and destructive than the quake itself, however, was the massive tsunami following the quake. As it has been established, the tsunami knocked out the emergency cooling system in the Fukushima Daiichi nuclear power plant. The incident culminated in three explosions, on March 12, 14, and 15, that caused substantial damage to the containment buildings of reactor units 1, 3, and 2, correspondingly. The reactor vessels, though, are considered not catastrophically compromised (e.g. exposed reactor cores).
Under the current worst case scenario, it is likely that some radioactive particles from the reactor's cores, or from the spent fuel storage pools, or both, were released into the atmosphere. In such a case, rain and snow would be major components contributing to the sedimentation of these particles back to the surface, into the soil, and potentially into agricultural produce.
Figure 1. Precipitation retrival from AIRS (left), and from GEOS-5 assimilation (right), over Japan.
However, NASA’s Earth Observing System precipitation and model wind data leaves hope that the worst of this scenario have not occurred over land for at least two reasons: the prevailing westerly winds and the light precipitation over Japan during the period subsequent to the quake and tsunami. Figure 1 shows images of the precipitation data retrieval from the Atmospheric Infrared Sounder (AIRS, left column) on the Aqua satellite, and from the Goddard Earth Observing System Data Assimilation System (GEOS-5, right column). Wind vectors at 850 mb (approximately 1.5 km altitude) from GEOS-5 are also overlaid in the right column.
For these images, we used the AIRS Level 2 support product (AIRSX2SUP) gridded to a 0.5° x 0.5° map, and the GEOS-5 G5.2.0 products tavg2d_met_x (precipitation) and inst3d_met_p (850-mb winds). Each image represents a 5-day average for the period shown above the image. Despite expected differences between the satellite observations and the assimilated data, both agree that only light precipitation prevailed over Japan, with the bulk of the intense precipitation fortunately occurring downwind and offshore.
Perhaps the worst period was around March 18-22, where AIRS and GEOS-5 show most of Japan under light precipitation. GEOS-5 indicates that most of the accumulation most likely appeared during this period, peaking west from Tokyo (see KMZ). Also, on March 18, smoke remained visible over unit 4, which may have helped inject radioactive materials higher into the atmosphere (above the boundary layer, into the entrainment zone).
Indeed, the International Atomic Energy Agency first reported of agricultural food contamination around Fukushima on March 19. Radiation readings were also taken by IAEA around Tokyo, who reported dose rates, although elevated, well below those which are dangerous to human health.
The prevailing winds over the entire period were consistently from the west (westerlies), which must have helped a great deal to transport the bulk of any material from the power plant offshore, into the Pacific Ocean. Meanwhile, AIRS and GEOS-5 unambiguously show large areas of intensive precipitation downwind from Japan. Thus, there exists a good likelihood that substantial portion of this material was captured by precipitation processes (e.g. condensation nuclei and downdrafts) and precipitated over the ocean.