Simulating nuclear cloud rise within a realistic atmosphere using the Weather Research and Forecasting model

Abstract Models of nuclear detonation cloud rise used by emergency planning and response teams are usually simplified to enable fast run times. They also contain parameters which are tuned to historical nuclear tests. Thus, they do not fully account for the complex environments that may be encountered in emergency scenarios. In this work, a multiscale framework, spanning numerical weather prediction to large-eddy simulation, is used to simulate nuclear cloud rise within the Weather Research and Forecasting (WRF) model. By employing this approach, cloud rise is modeled with higher fidelity, including time-varying three-dimensional weather fields and complexities such as atmospheric moisture and terrain. Using modified initialization routines that allow for the inclusion of a high temperature fireball in WRF, cloud rise is first simulated for three U.S. nuclear tests over the Nevada desert. Grid nesting is used to dynamically downscale historical reanalysis data to a large-eddy simulation domain, where cloud rise is simulated at 20-30 m resolution. The downscaled atmospheric states agree reasonably well with observations from the time of the tests, and turbulent mixing induced by the cloud rise is captured. Simulated nuclear clouds show good overall agreement with available cloud rise observations, especially for high-air bursts with limited surface interaction. To demonstrate WRF’s ability to capture aerosol-microphysics interactions during cloud rise, a semi-idealized large-eddy simulation of the wartime Nagasaki detonation is also performed. Self-induced rainout caused by the condensation of rising moist air during cloud rise is captured, and wet deposition of aerosol particles is quantified. Future work will better account for surface interactions, including particulate lofting and shockwave reflection, which should improve cloud rise predictions. Additional development of a multiscale simulation framework could permit seamless fireball-to-fallout simulations to study the relationship between cloud rise dynamics and fallout risk.