Model behavior regarding in- and below-cloud 137Cs wet scavenging following the Fukushima accident using 1-km-resolution meteorological field data

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Wet deposition remains an important source of uncertainty in modeling of the atmospheric transport of 137Cs following the Fukushima Daiichi Nuclear Power Plant accident. Its behavior is often difficult to investigate owing to the limited resolution of meteorological field data and inconsistent model implementations. This study investigated the detailed behavior of 25 combinations of in- and below-cloud wet scavenging models using high-resolution (1 km × 1 km) meteorological input. These combinations were all implemented in the Weather Research and Forecasting-Chemistry model, thereby enabling consistent evaluation. The 1-km-resolution simulations were compared with simulations obtained previously using 3-km-resolution meteorological field data. Results revealed that rainfall of <1 mm/h is critical for simulation accuracy. The 1-km results revealed better representation of rainfall than that revealed by the 3-km results, but with spatiotemporal variability in accuracy. Owing to their sensitivity to rainfall, single-parameter wet deposition models showed improvements in performance in the 1-km simulations relative to that in the 3-km simulations. The multiparameter models showed more robust performance in terms of both simulations, and the Roselle–Mircea model presented the best performance among the 25 models considered. Wind transport showed substantial influence on the removal of atmospheric 137Cs, and it was nonnegligible even during periods in which wet deposition was dominant. The 1-km-resolution simulations effectively reproduced local-scale 137Cs concentrations but with deviations in timing, mainly because of biased wind direction. These findings indicate the necessity for a refined wind and dispersion model for local-scale simulation of 137Cs concentration.