A virtual tornado that kicks up debris

Adding debris to a numerical tornado model can enhance the twister’s damage potential even as it reduces its peak winds. David Lewellen (West Virginia University) presented initial findings in October at the American Meteorological Society’s 22nd Conference on Severe Local Storms.

Although post-storm surveys have shown that debris can boost the damaging effects of a strong tornado in some cases, few researchers have incorporated debris in computer models of tornadoes. Lewellen and his group considered the effects on near-surface tornado dynamics produced by several classes of debris size (0.1, 0.5, and 2.0 millimeters, corresponding to variously sized grains of sand or dirt). The model treated the debris collectively as a fluid coexisting with the atmosphere, while it tracked a few individual grains as a check on the model’s overall validity.

In the case of a strong tornado, the addition of 0.5-mm debris reduced the peak low-level winds from 110 meters per second (about 250 miles per hour) to 70 m/s (160 mph). The location of the strongest winds also shifted outward and upward from the tornado core. Adding the debris “completely changes the vertical structure of this tornado,” says Lewellen. However, he notes, “it would be premature to conclude that this weakens the tornado at low levels.” In fact, the total momentum packed by both air and debris roughly doubles from the air-only case, implying a significant increase in the tornado’s destructive force. Smaller 0.1-mm debris, in contrast, appears to reduce the tornado’s overall momentum.

Lewellen plans to introduce a mix of debris sizes in future modeling studies. The realistic-looking visualizations produced by the model (see graphic at right) may have an added benefit. Comparing these animations to videotaped twisters could help researchers find real-world analogs to numerically modeled tornadoes; the model could then shed light on the actual tornadoes’ winds and dynamics.

West Virginia University