Supercomputers Resolve Stormy Issues

By Staff
04/07/2008 - 12:00am

This visualization of the 62-meter experiment puts the computer model's winds in motion over 1:28 minutes and shows vertical velocity at about 600-meters above the sea surface. Only the winds are moving, while the hurricane itself remains stationary in this idealized experiment. The model reveals both the turbulent eddies in horizontal winds and the punctuated pattern of vertical winds. The raggedness of the vertical winds is caused by convection, as heat rises through the storm's cumulus clouds in violent updrafts. (Illustrations courtesy Yongsheng Chen and Richard Rotunno, NCAR.)

Supercomputers Resolve Stormy Issues

Using a special allocation of computing power on CISL’s blueice supercomputer, scientists have discovered turbulent eddies swirling through a simplified tropical cyclone. Such turbulence, which occurs on too small a scale to be directly depicted in global or regional weather models, was detected by ESSL/MMM’s Yongsheng Chen and Rich Rotunno in some of the finest-scale hurricane modeling ever conducted.

The two researchers carried out the modeling from late 2006 through May of 2007, in collaboration with MMM’s Wei Wang, Chris Davis, Jimy Dudhia, and Greg Holland. The computing time was awarded to the project through a competitive program designed by NSF and NCAR to accelerate scientific discovery through very large computing resource allocations.

In a typical hurricane, bands of thunderstorms spiral into a storm-girdled eyewall, surrounding an eye that can range in width from less than five to more than 50 kilometers (3–30 miles). To capture such features, NCAR in recent years has operated an Advanced Hurricane WRF (Weather Research and Forecasting) model with a resolution as high as 1.33 km (0.8 mi). This model produces realistic-looking spiral bands that wrap around a distinct eye, much like the pictures painted by radar and satellite data. However, field studies show that a great deal of turbulence is hidden in and near these bands. The bumpy air contributes to the rough rides often experienced by reconnaissance aircraft, and it could also influence the larger-scale evolution of hurricanes.

The scientists set out to see how fine a resolution would be needed for signs of turbulence to appear in the model. They found that as the resolution tightens below 1 km (0.62 mi), the eyewall remains smooth and the peak 1-minute sustained wind speed increases. However, at the smallest resolution of 62 meters (68 yards), the eyewall breaks into short, ragged segments and the peak minute-long wind actually drops.

Rich Rotunno speculates that turbulence serves as a brake on the overall storm intensity. The results, soon to be published, should help scientists better assess the factors that determine hurricane strength in ever-sharpening forecast models.

Adapted from February 2008 Staff Notes article.

In a hurricane modeled at 185-meter (202-yard) resolution, a smooth ring of strong wind appears around the eye (left). When the resolution is increased to 62 meters (68 yards), the ring breaks into a set of small, turbulent segments (right). Each image covers an area of 37x37 kilometers (23x23 miles). The colors indicate wind speeds, ranging from dark blue (very weak, at the center of the eye) to red (very strong).