Wednesday, March 16, 2011

Detectable Radiation Could Come To U.S. But Not Likely Significant Concern For Human Health

Posted by: JeffMasters, 2:16 PM GMT on March 16, 2011
If there is going to be a major nuclear disaster with massive release of radioactivity into the atmosphere from Japan's stricken Fukushima nuclear plant, today would be the best day meteorologically for this to occur. The low pressure system that brought rain and several inches of snow to Japan yesterday has moved northeastwards out to sea, and high pressure is building in. The clockwise flow of air around the high pressure system approaching Japan from the southwest is driving strong northwesterly winds of 10 - 20 mph over the region. These winds will continue through Thursday, and will take radiation particles emitted by the stricken reactors immediately out to sea, without lingering over Japan. Since high pressure systems are regions of sinking air, the radiation will stay close to the ocean surface as the air spirals clockwise over the Pacific. The contaminated air will remain over the ocean for at least five days, which is plenty of time for the radiation to settle out to the surface.

Figure 1. Surface weather map for 8am EDT today, taken from the 6-hour forecast from this morning's 6 UTC run of the GFS model. A high pressure system to the southwest of Japan, in combination with a low pressure system to the northeast are driving strong northwesterly surface winds over the country. Image is from our wundermap with the "Model" layer turned on. The lines are sea-level pressure (blue contours, 4 mb interval) and 1000 to 500 mb thickness (yellow contours, 60 m interval). Thickness is a measure of the temperature of the lower atmosphere, and a thickness of 5400 meters is usually close to where the dividing line between rain and snow occurs.

Thursday night and Friday morning (U.S. time), the high pressure system moves over Japan, allowing winds to weaken and potentially grow calm, increasing the danger of radioactivity building up over regions near and to the north of the nuclear plant. On Friday, the high departs and a moist southwesterly flow of air will affect Japan. These southwesterly winds will blow most of the radiation out to sea, away from Tokyo. Southwesterly winds will continue through Sunday, when the next major low pressure system is expected to bring heavy precipitation to the country. Beginning Thursday night, the sinking airmass over Japan will be replaced a large-scale area of rising air, and any radiation emitted late Thursday through Friday will be carried aloft towards Alaska and eastern Russia by this southwesterly flow of rising air.

Ground-level releases of radioactivity are typically not able to be transported long distances in significant quantities, since most 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 hundred meters 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. I've made trajectory plots for the next three days assuming two possible release altitudes--a surface-based release near 10 meters, which should be the predominant altitude in the current situation, and a higher release altitude of 300 meters, which might occur from an explosion and fire from a Chernobyl-style incident. Given that the radioactivity has to travel 3000 miles to reach Anchorage, Alaska, and 5000 miles to reach California, a very large amount of dilution will occur, along with potential loss due to rain-out. Any radiation at current levels of emission that might reach these places may not even be detectable, much less be a threat to human health. A Chernobyl-level disaster in Japan would certainly be able to produce detectable levels of radiation over North America, but I strongly doubt it would be a significant concern for human health. The Chernobyl disaster only caused dangerous human health impacts within a few hundred miles of the disaster site, and the distance from Japan to North America is ten times farther than that.

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