The direction taken by the group was to develop an ensemble of dispersion model calculations with different atmospheric transport and dispersion models (ATDM) using the same meteorological data and the same model using different meteorological data. Each group would run their ATDM with the meteorological data they normally use for operational dispersion predictions and also, if technically possible, with the 4D-VAR mesoscale analysis provided by the Japan Meteorological Agency (JMA 3-hourly, 5-km horizontal, and 50-hybrid level vertical resolution). It was recognized that perhaps the most critical element in the deposition calculations was getting the precipitation correct. In this aspect, JMA agreed to provide their Radar/Rain Gauge analyzed precipitation fields available (every 30 min at 1-km resolution) and the participants would incorporate those fields in their calculations.
The WMO group agreed to standardize their ATDM modeling procedure and output fields to use a regular latitude-longitude grid with output averaged at 3-hourly intervals at 0.05 degree (5 km) horizontal resolution using a model computational format that treated each 3 hour period of the release as an independent calculation using a unit source emission rate. The simulations are conducted for the period 11 through 31 March 2011. The computations were made for three generic species tracked as surrogates for the radionuclides: a gas with no wet or dry scavenging (non-depositing gas), a gas with a relatively large dry deposition velocity and wet removal to represent gaseous I-131 (depositing gas), and a particle with a small deposition velocity (light particle). In this way the calculations could be post-processed for the air concentration and deposition of each radionuclide required for the UNSCEAR assessment once the final source term was determined.
The ATDM's in the ensemble were NAME, HYSPLIT, FLEXPART, MLDP0, and JMA's ATM. Meteorological analysis data were obtained from CMC's GEM-GDPS (0.30 deg), NOAA's GDAS (0.50 deg), ECMWF (0.20 deg), UKMET's UM (0.35x0.23 deg), and JMA's mesoscale analyses (5 km). Approximately 20 ensemble members are expected to be available when all calculations have been completed. The ATDM results will be post-processed to create an ensemble median and a measure of variability (box plots) which is to be provided to the UNSCEAR dose assessment groups for their calculations.
As part of the WMO evaluation, predictions from each of the ATDM member calculations as well as the ensemble median will be compared to the observed Cs-137 deposition pattern and the time series of Cs-137 and I-131 air concentrations at the two locations where these data were available during the critical phases of the accident. The preliminary modeling results are posted to a web page where the WMO group participants could evaluate various source term options, compare their model results to measurements, as well as to each other's calculations.