Click to enlarge. 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). (Illustrations courtesy Yongsheng Chen and Richard Rotunno.)
Scientists have discovered signs of 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 carried out the initial modeling through a special allocation of computing power provided late in 2006 with the arrival of CISL’s Blue Ice supercomputer. Also contributing were MMM’s Wei Wang, Chris Davis, Jimy Dudhia, and Greg Holland.
Click image, above to view this Quicktime animation; or download the AVI format. 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. Yongsheng and Rich 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 (see left image), 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 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.
