Rain in convective downdraughts

This study presents results from the simulation of idealised, rain-laden downdraughts using a large-eddy model. Each downdraught is initialised as a “cold, wet bubble”, that is, a spherical/spheroidal region of the lower atmosphere with a negative potential-temperature perturbation and non-zero rain mixing ratio. The bubbles are statically unstable and evolve into downdraughts. Results are compared for two- and three-moment microphysics parametrisations. A tendency for larger raindrops to descend ahead of the cold air is observed, which is significant in some two-moment runs. The effects of this are interpreted in the context of fluid-dynamical studies of related phenomena. It is found that a drier environment has the effect of increasing cooling in the downdraught, but reduces the peak in downdraught speed. Both these effects stem from increased rain evaporation. A relatively heavy rain load leads to a narrower downdraught, through the suppression of internal (thermal-like) circulations. Downdraughts intensify, in both cooling and vertical velocity, in correlation with the initial bubble height-to-width ratio, being greatest for vertically stretched bubbles.