A numerical model of cumulonimbus convection generating a protected core

Abstract

SUMMARYThe theory of buoyant motion in a turbulent environment due to Priestley (1953) is applied to steadystate Cb updraught. An axisymmetric numerical model of columnar convection is modified to allow for radial cloud structure and internal turbulence. In this model the net vertical body force on a fluid element is the buoyancy relative to adjacent elements, and represents a residual of environment-referred buoyancy and non-hydrostatic pressure gradient.Moisture exchange and lateral transfer of momentum and heat by mass exchange with the environment on the scale of the cloud are disregarded, yet the model gives realistic updraught velocities and cloud depth. It simulates such features as a dome rising from a protected core of the cloud, oscillating motion of cloud tops and penetration of the tropopause. In some respects the predicted behaviour compares favourably with actual measurements.An incidental result concerns the generating gross buoyancy which is determined by the ambient and pseudo-adiabatic temperature lapse rates. It appears that the values of the latter rate published in Smithonian Meteorological Tables and incorporated in pseudo-adiabats on aerological diagrams, are in error.

Entrainment and structure of negatively buoyant jets

“…It will continually decelerate until its mean momentum is reduced to zero and the fluid reverses direction, returning towards the source while mixing with the opposing fluid. These occur in various industrial applications, such as building ventilation and brine discharge in desalination plants (Pincince & List 1973; Baines, Turner & Campbell 1990), as well as natural phenomena, including explosive volcanic jets and cumulonimbus convection in the atmosphere (Berson & Baird 1975; Carazzo, Kaminski & Tait 2008). Some time after the initial rise of a negatively buoyant jet, the flow reaches a quasi-steady state, where it oscillates around its steady state height, , which is lower than the maximum height reached during its initial rise, the initial rise height, .…”