Simulating Wet Deposition of Radiocesium from the Chernobyl Accident

In response to the Chernobyl nuclear power plant accident of 1986, cesium-137 deposition was measured in Europe at sites equipped to do so. The resulting deposition dataset is uniquely applicable to atmospheric transport model validation. Most of the airborne Chernobyl cesium was wet deposited, i.e., either via interception by falling raindrops (below-cloud scavenging) or via absorption into cloud droplets destined to become raindrops (in-cloud scavenging). The model used in this work is the Hybrid Single-Particle Lagrangian Integrated Transport (HySPLIT) model developed at Air Resources Laboratory. A cloud base modification is tested and appears to slightly improve the accuracy of one HySPLIT simulation of daily Chernobyl cesium-137 deposition over the course of the accident at isolated European sites, and degrades the accuracy of another HySPLIT simulation of deposition in Germany and Austria accumulated in the month of April, 1986. Large uncertainties in the emission specifications, model precipitation fields, and deposition measurements prevent designating the results as conclusive, but most evidence points to improved performance within 500km of the emission source. Trial and error lessons learned from hundreds of preliminary model runs are documented, and the exact HySPLIT settings of successful and meaningful simulations are appended.