Numerical Simulations of an Isolated Microburst. Part II: Sensitivity Experiments

Okayama Downbursts on 27 June 1991

“…This fact suggests that Downburst-IV was accelerated by the evaporation and melting of hail. Hail is believed to be effective in producing wet microbursts (Srivastava, 1987;Wakimoto and Bringi, 1988;Proctor, 1989). …”

“…Hjelmfelt et al (1986) numerically showed that the loading of rain and graupel and the cooling effect of rain evaporation and graupel melting are all important in microburst production. Proctor (1989) demonstrated, through sensitivity experiments, that snow and graupel are most effective in producing microbursts within typical dry microburst environments, while hail is most effective in producing wet microbursts within more stable environments. Wakimoto and Bringi (1988) observed a strong localized downdraft composed of melting hail.…”

Investigating the impact of atmospheric stability on thunderstorm outflow winds and turbulence

“…For example, in a typical CBL or NBL regime, the pocket of air would remain negatively buoyant throughout its descent to the surface. However, in the SBL where a low-level inversion is present, the downburst air could become less negatively buoyant (or even positively buoyant) 25 as it descends through the low-level stable layer (Proctor, 1989). Since the resulting horizontal wind speeds are a function of the maximum downdraft velocity, w min , if w min is smaller (larger), then the outflow wind speeds will in turn be smaller (larger) (Proctor, 1989;Mason et al, 2009).…”

“…However, in the SBL where a low-level inversion is present, the downburst air could become less negatively buoyant (or even positively buoyant) 25 as it descends through the low-level stable layer (Proctor, 1989). Since the resulting horizontal wind speeds are a function of the maximum downdraft velocity, w min , if w min is smaller (larger), then the outflow wind speeds will in turn be smaller (larger) (Proctor, 1989;Mason et al, 2009). Thus, if the downdraft encounters a highly turbulent environment, the mixing and entrainment of ambient environmental air can weaken the negative buoyancy of the parcel and, in turn, decrease the outflow wind speeds.…”

“…This feature is characterized by the rotation (roll-up) about a horizontal axis located towards the edge of the outflow and commonly associated with the strongest outflow winds. Further, the influence of various other factors such as terrain (Mason et al, 2010b), the interacting collision of two downburst outflows (Vermeire et al, 2011b), the microphysics and ambient humidity (Proctor, 1989(Proctor, , 1988Srivastava, 1985Srivastava, , 1987 have all been explored. Full-storm simulations utilizing advanced parameterizations (Orf et al, 2012(Orf et al, , 2014 are now able to help us advance our knowledge of downburst dynamics 25 and study these various influences.…”