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This report compares the predictions of the Health Protection Agency (HPA) application of the ‘R91â€� model with those of the UK Met Office NAME III model. »Ê¹ÚÌåÓýapp study considers a simplified application of NAME and R91 to enable a fair model comparison.

»Ê¹ÚÌåÓýapp comparison is centred upon analysis of model output generated from a single baseline run for a short duration release of the type often considered in emergency response assessments. Subsequent model runs are performed, scoping a range of model scenarios and commonly modified model input parameters. Differences in the predictions of the two models are investigated and explained. »Ê¹ÚÌåÓýapp quantitative assessment of differences in the baseline model output is used as part of a qualitative assessment of observed differences across a range of model runs and their associated output.

»Ê¹ÚÌåÓýappre is a disparity (of up to a factor of approximately 3) between time-integrated activity concentrations in air derived using NAME and those derived using R91, most notably in the near-field. R91 is more conservative in its approach, and estimates made by R91 are typically greater than those made by NAME. »Ê¹ÚÌåÓýapp primary sources of the observed differences between the model outputs are identified as:

• cross-wind spread of the plume
• vertical spread of the plume
• wind-driven advection of the plume