Entrainment in Simulated Supercell Thunderstorms
Supercell thunderstorms can produce heavy precipitation, and are associated with significant weather hazards such as hail and tornadoes which can lead to deaths and destruction of property. Entrainment is one process that can modulate the intensity of supercells, and a better understanding could help improve thunderstorm forecasts and especially the precipitation they produce. In a series of studies, for the first time various mechanisms of entrainment during the mature stage of idealized supercell thunderstorms have been examined, including within different thermodynamic environments and within environments with varying unidirectional and quarter-circle vertical wind profiles. Entrainment is calculated directly as fluxes of air over the surface of the updraft core; passive fluid tracers are used to assess the resulting dilution of the air ingested by the storm. Model microphysical rates are used to compare the impacts of entrainment on the efficiency of condensation/deposition of water vapor on hydrometeors within the core, and ultimately, upon precipitation production. Results show that two entrainment mechanisms on the core surface reduce the ability of the storms to produce precipitation within the updrafts: overturning “ribbons” of horizontal vorticity which translate vertically with time (and heretofore never before recognized), and less organized turbulent eddies. A third mechanism, the storm-relative airstream, actually supplies additional moisture into the storm updraft to enhance precipitation production. Although the total entrainment increases with increasing vertical wind shear, the fractional entrainment decreases, leading to an increase in undiluted air within the storm core and an increase in condensational efficiency. However, due to the increase in hydrometeors detrained aloft and the enhanced evaporation during their fall, the precipitation efficiency as gauged by the surface rainfall decreases with increasing vertical wind shear, as suggested in past studies. Ongoing work includes examining the overall effect of modifying tropospheric relative humidity on precipitation production through the different entrainment mechanisms. These results are a start toward a more comprehensive quantification of the impact of entrainment on precipitation in supercell thunderstorms, which may in turn increase the accuracy of numerical modeling forecasts of their hazards (e.g., flash flooding).
https://illinois.zoom.us/j/88283770092?pwd=bHB0WjM2Znc1cUdvYnY3dVREbWxGUT09
