Area-averaged concentrations, height-scales and mass balances

Abstract

The concentration data obtained in the trials without obstructions at Thorney Island are analysed to produce quantities relevant to box models of heavy gas dispersion. Mean concentrations are calculated by averaging horizontally over the cloud area. Results are plotted against a dimensionless time scaled using the initial area and buoyancy content. At early times, ground-level concentrations match a power-law dependence with index equal to the edge entrainment coefficient in a simple generic box model. Later the concentration decreases more rapidly as expected from the influence of top entrainment: this depends on a Richardson number involving the atmospheric turbulence. Simple functions are fitted to vertical concentration profiles to permit an integration over height. The time-variation of cloud height-scale and Richardson number are presented. Finally, the cloud mass balance is computed: the results appear reasonably self-consistent though various sources of systematic error can be detected particularly near the end of each trial.