ScienceDaily (Jan. 8, 2009) — One of the largest sources of uncertainty in weather prediction involves how microscale structures influence larger-scale phenomena. For instance, previous studies have demonstrated that the structure, dynamics, and evolution of thunderstorms are very sensitive to cloud microphysical parameters.

However, those studies used resolutions too coarse to resolve tornadoes or tornado-like circulations and were therefore not able to study the sensitivity of tornadogenesis to microphysics. Snook and Xue have now conducted simulations of severe tornadic thunderstorms using a grid of 100-meter (328-feet) spacing.

They find that when the sizes of rain and hail drops are large, weaker cold pools due to reduced evaporative cooling/melting over smaller geographic regions result. Such weak cold pools are found to produce conditions that enhance low-level rotation.

The authors' simulations show that strong, sustained vertical updrafts are positioned near and above the low-level circulation centers, providing strong dynamic lifting and vertical stretching to the air at the lower levels, which favors the creation of tornadoes.

Journal reference:

1. Nathan Snook. Effects of microphysical drop size distribution on tornadogenesis in supercell thunderstorms. Geophysical Research Letters, DOI: 10.1029/2008GL035866

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