Abstract View
Super-droplet Method to Simulate Lagrangian Microphysics of Fallout Particles
DANA MCGUFFIN, Donald Lucas, Joe Morris, Kim Knight, Lawrence Livermore National Laboratory
Abstract Number: 69
Working Group: Aerosol Physics
Abstract
Nuclear detonations produce hot clouds that quickly cool and rise, forming dangerous ultrafine to coarse aerosol (fallout) that can potentially disperse over long distances and deposit. Therefore, the ability to predict aerosol size, chemical components, and location is necessary to inform public officials and determine guidance for the public. Quickly predicting these quantities is vital to protecting the public in these scenarios, so this work presents a computationally efficient way to model particle microphysics in a hot, turbulent cloud.
The super-droplet method (SDM) is an efficient way to model cloud microphysics without losing accuracy. SDM represents the particle size distribution by sampling it with a set of computational particles called “super-droplets”. Each super-droplet represents several real particles, and its size is adjusted as condensation and coagulation evolve. In the Lagrangian framework, each super-droplet acts as a computational particle with aerosol-specific attributes (size, composition, shape) in addition to the typical Lagrangian attributes of particle position, velocity, and mass.
Previously, SDM has been developed and applied to ice nucleation and cloud microphysics simulating precipitation. In this talk, we present the preliminary results of particle nucleation and growth in a hot cloud that represents a homogeneous cooling fireball. We apply SDM to evolve fallout microphysics in a zero-dimensional model, with plans to incorporate SDM into high-resolution large eddy simulations. The simulated processes include homogeneous nucleation, condensation, and coagulation.
Between 1945 and 1970, about two hundred atmospheric nuclear tests were performed by the United States. Several of these tests were airbursts that did not interact with the ground, which are ideal to validate the SDM model. We compare size distributions for airburst nuclear tests from the SDM simulation, previous theoretical calculations, and field measurements taken during the tests.
Prepared by LLNL under Contract DE-AC52-07NA27344.