A low pressure system is located over Japan near Tokyo today, and the counterclockwise flow of air around this low is bringing easterly winds over the stricken Fukushima Daiichi nuclear plant, which lies to the north-northeast of Tokyo. These easterly winds are blowing radioactivity inland over Japan. As the low tracks northeastward along the coast of Japan today, winds at the Fukushima Daiichi nuclear plant
will gradually shift to northeast and then northwest, which will move radiation towards Tokyo for several hours, which may be long enough for some radiation to reach the city. NOAA's HYSPLIT trajectory model
shows that for a release of radioactivity at 50 meters altitude beginning at 21 GMT on Monday (when an explosion at the #2 reactor was recorded), with repeat releases simulated to occur every 2 hours thereafter, the plumes will stay to the north of Tokyo (Figure 1.) However, a more detailed dispersion model being run by the Austrian weather service
shows that the plumes may affect much of the Tokyo area today. Both models predict that by 18 GMT today (2pm EDT), the threat to Tokyo will be over, with more westerly winds blowing the radioactive cloud out to sea.Figure 1.
Forecast movement of a plume of radioactive plume of air emitted at 50 meters altitude at 21 UTC Monday, March 14, 2011 from the Fukushima Daiichi nuclear plant. Radioactivity is similated to be released every 2 hours thereafter, going out 24 hours. Images created using NOAA's HYSPLIT trajectory model.
As the low pressure system moves through Japan today, it will bring rain. Current radar loops
from the Japan Meteorological Agency show a wide area of rain approaching Tokyo and the Fukushima nuclear plant. Rain is very efficient at removing radioactive particles from the air, and there is the threat of surface and ground water contamination where significant concentrations of radioactive material get rained out. By Wednesday, most of the rain will be gone, and predominately northwesterly winds will build in behind the departing low pressure system. This flow regime will stay in place for the remainder of the week, keeping radioactive emissions from the nuclear plant away from Tokyo, and headed out to sea at low altitudes near the surface.
Ground level releases of radioactivity are typically not able to be transported long distances in significant quantities, since much of the material settles to the ground a few kilometers from the source. If there is a major explosion with hot gases that shoots radioactivity several kilometers high, that would increase the chances for long range transport, since now the ground is farther away, and the particles that start settling out will stay in the air longer before encountering the ground. Additionally, winds are stronger away from ground, due to reduced friction and presence of the jet stream aloft. These stronger winds will transport radioactivity greater distances.Figure 2.
Seven-day forecast movement of a plume of radioactive plume of air emitted at 8am EDT (12 UTC) today at 50 meters altitude from the Fukushima Daiichi nuclear plant. Flow of air in the warm and cold conveyor belts of the low pressure system affecting Japan are expected to loft radioactivity to 4 - 5 km altitude, where it will be transported thousands of miles over the coming week. Images created using NOAA's HYSPLIT trajectory model.
One case where a ground level release might get lofted to high altitudes is when the source region is located near an approaching low pressure system (extratropical cyclone), as is the case today. On the cold side of the approaching warm front, where the Fukushima nuclear plant is located today, lies a broad band of ascending air called the "cold conveyor belt." This conveyor belt can loft surface air to an altitude of several kilometers in a day, as seen in the trajectory plot in Figure 2. In addition, the "warm sector" of a low pressure system in front of the approaching cold front features a ribbon of ascending air about 100 - 200 km wide called a "warm conveyor belt", which is also capable of lofting surface air several kilometers high in a day. However, there is often considerable precipitation in both of these conveyor belts, which will tend to remove large quantities of radiation before it can be transported long distances. There will be some radiation from Japan lofted to high altitudes today by the low pressure system affecting the region, and if the radiation manages to escape being rained out, it could potentially be transported thousands of miles over the next week. A run of the HYSPLIT model following the path of a radioactive cloud emitted at 12 UTC (8am EDT) this morning shows the radioactivity being lofted 4 - 5 km in altitude and being transported over Alaska over the coming week. After a week of transport, this cloud will be considerably diluted, and I strongly doubt the radioactivity would be harmful to human health if rain or snow were to carry it to the ground over Alaska or Canada, assuming that the radiation levels currently being advertised at ground level in Japan are correct.