Descent and Near-Surface Evolution of Atmospheric Downdraughts

Evaporation and drag from precipitation in convective clouds induce regions of negative buoyancy and momentum, generating downward motion known as downdraughts. The impact and near-surface evolution of strong downdraughts creates hazardous gust fronts that are often responsible for the peak surface winds associated with convective storms. A combination of theory, laboratory experiments and numerical simulations are used to analyse the descent and near-surface evolution of idealised downdraughts. Downdraughts are represented as releases of negatively buoyant fluid in i) saline releases in laboratory experiments and ii) cold bubbles in large eddy simulations using the Met Office and NERC Collaboration model. Both methodologies reproduce the salient features of atmospheric downdraughts. Existing theory from Rooney (2015) is compared to both methodologies of downdraught model, finding good agreement subject to changes in empirical constants. Scalings from Lundgren, Yao, and Mansour (1992) are used to compare the experimental results with the numerical simulations and demonstrate the radial propagation of idealised downdraughts can be related to their initial properties at source. Similarity solutions from Rooney (2015) are extended to include expressions for the kinetic energy and potential energy. Previous field experiments and observations have indicated that downdraughts from elevated convection can impact stable boundary layers and generate gravity currents, waves and bores. Numerical simulations are used to examine the novel scenario of a downdraught impacting a stable boundary layer. The vertical descent and radial propagation of the downdraught, and resulting disturbance of the stable boundary are mapped out for this flow regime. Entrainment and mixing of ambient air above the SBL is suggested as an explanation for atmospheric observations of downdraughts, where the cold downdraught actually results in a warm surface signal. The initial buoyancy of the downdraught relative to stable boundary layer is shown to determine whether the regime is i) current drive or ii) wave dominated. An assessment of the resulting wave dynamics is made and compared to the relevant internal gravity wave theory. The results of this study demonstrate how the properties of the resulting outflow (gravity current, bore or wave) can be related to the initial conditions of an elevated source of negative buoyancy or momentum.