A note on interpreting EURAD and NILU plume forecasts

A lot of people have been bringing up the NILU and EURAD plume forecast maps over the last couple weeks, often using them as proof for saying something like "high levels of radiation are heading right for California." I just wanted to point out the disclaimers that both of those organizations give on their webpages about interpreting their results so that everyone can put these forecasts in their appropriate informational and qualitative context. This is the disclaimer from the EURAD website http://www.eurad.uni-koeln.de/:

These animations display a potential dispersion of the radioactive cloud (Caesium 137 Isotope) after a nuclear accident in reactor Fukushima I. The continuous release rate is very uncertain, thus the calculations have to be interpreted qualitatively. Dispersion in the near surface level (Level 1), in appr. 2500 m height (Level 12) and in appr. 5000 m height (Level 16). The release rate is estimated as 10^15 Bq/d. This is appr. one tenth of the Chernobyl release. This simulation is a so called "worst case scenario" with continuous release rate. The value of 0.001 Bq/m3 correspond to appr. one millionth of the concentration at the source. At distances more than appr. 2000 km away from the source, the concentrations are not harmful to health. The simulation starts fictitious at 15.03. 00 UTC and will continue to run in order to demonstrate the intercontinental transport. When exact relaese rates are published we will restart the simulation with reliable values.

This is the warning from the NILU website http://transport.nilu.no/products/fukushima:

ATTENTION: These products are highly uncertain based on limited information for the source terms. Please use with caution and understand that the values are likely to change once we obtain more information on the overall nature of the accident. The products should be considered informational and only indicate 'worst case scenario' releases. From what we've learned recently, it seems releases of this magnitude have not yet occurred. Furthermore, these modeling products are based on global meteorological data, which are too coarse to provide reliable details of the transport of the plume across Japan. Currently we are using a daily releases distributed evenly of 0.1E18 Bq I-131, 0.1E17 Cs-137, and 0.1E19 Xe-133 per day.

There are a few things in common between these two disclaimers:

1. The maps are only "informational" and should be used "qualitatively"
2. Results are preliminary and "very uncertain": Both organizations will update their simulations once the "source term" is known. Also, weather forecasting -- which is essentially what they are doing -- is notoriously difficult.
3. "Worst case scenario": There is no evidence of sustained, constant releases of the magnitudes they are using in their calculations.

So please keep in mind that the main purpose of both the EURAD and NILU maps is to show "qualitatively" how the plume would spread across the Earth, were it an absolute "worst-case scenario." Interpreting their exact numbers, such as saying that a huge concentration is headed for California, is outside of their stated purpose for making these maps. And please also note what the EURAD disclaimer says about health: "At distances more than appr. 2000 km away from the source, the concentrations are not harmful to health." If we see any large concentrations of these isotopes arriving in California, we will definitely report it here in the BRAWM air sampling page. Mark [BRAWM Team Member] Added 5/13/2011: Here are our current air plots for I-131 and Cs-137: